Introduction
Conceptually, obesity can be defined as a level of body weight and
adiposity that is sufficiently excessive to damage health,
demonstrated by an increased risk of various chronic diseases,
including hypertension, stroke, type 2 diabetes, cardiovascular
disease and some forms of cancer (Danaei et al., 2009). Although
both the mechanisms and the magnitude of effect of these
associations remain incompletely understood, there is substantial
evidence that secular increases in obesity herald complementary
increases in such diseases. For example, migration studies
demonstrate linked increases in central adiposity and
cardiovascular risk following novel exposure to the industrialized
niche (Kinra et al., 2011; Poston and Foreyt, 1999). Obesity is now
considered a major public health burden in both industrialized and
modernizing countries, stimulating a coordinated strategy for
prevention by the World Health Organization.
Despite seemingly compelling evidence that obesity is bad for
human health, both public health efforts to reduce its prevalence
and clinical efforts to treat it have had only modest success. This
scenario can be attributed in part to the persisting poor
understanding of what exactly obesity is. Obesity confounds the
standard medical model of disease, which developed historically
to address biological pathogens and can be more difficult to apply
to other forms of ill health. Obesity is a nebulous concept, defined
using statistical criteria rather than explicit diagnosis of a metabolic
pathology (Cole et al., 2000; Garrow and Webster, 1985). Its causes
remain subject to vigorous debate, owing to the fact that the energy
balance equation is a ‘truism’ and hence offers no explanatory
framework for why excess weight gain occurs (Wells and Siervo,
2011). Although diverse phenotypic traits differ between obese and
non-obese individuals, it remains unclear which differences are
causal in its development.
The fact that high levels of adiposity are associated with poor
health has led to many considering adipose tissue a ‘toxic’ tissue,
a perspective that is supported by growing evidence of obesity as
a chronic inflammatory state (Bays et al., 2008; Berg and Scherer,
2005; Fernandez-Real and Ricart, 2003; Laclaustra et al., 2007).
According to this view, adipose tissue is the disease, and the less
body fat, the better health. Yet most research on adiposity is
conducted in individuals who are already overweight, thus telling
us primarily about a physiological tissue in a pathological state.
Much less attention has been directed to understanding the
contributions of adipose tissue to health and function. For this, an
evolutionary approach is required (Wells, 2006; Wells, 2009c).
Storing energy as fat is a characteristic of many organisms (Pond,
1998). It is best considered a strategy – a strategy that humans use
in general, but one in which they also demonstrate substantial
variability. The aim of this article is to discuss adipose tissue and
its associated metabolism as a flexible ‘risk management’ strategy,
to understand how it is sensitive to a wide variety of ecological
cues, and why it is an active endocrine tissue. This then helps to
understand why we are changing from a species that is well
SPECIAL ARTICLE
Disease Models & Mechanisms 595
Disease Models & Mechanisms 5, 595-607 (2012) doi:10.1242/dmm.009613
The evolution of human adiposity and obesity:
where did it all go wrong?
Jonathan C. K. Wells1
1
Childhood Nutrition Research Centre, UCL Institute of Child Health, 30 Guilford
Street, London, WC1N 1EH, UK
e-mail: Jonathan.Wells@ucl.ac.uk
© 2012. Published by The Company of Biologists Ltd
This is an Open Access article distributed under the terms of the Creative Commons Attribution
Non-Commercial Share Alike License (http://creativecommons.org/licenses/by-nc-sa/3.0), which
permits unrestricted non-commercial use, distribution and reproduction in any medium provided
that the original work is properly cited and all further distributions of the work or adaptation are
subject to the same Creative Commons License terms.
Because obesity is associated with diverse chronic diseases, little attention has been directed to the multiple beneficial
functions of adipose tissue. Adipose tissue not only provides energy for growth, reproduction and immune function,
but also secretes and receives diverse signaling molecules that coordinate energy allocation between these functions
in response to ecological conditions. Importantly, many relevant ecological cues act on growth and physique, with
adiposity responding as a counterbalancing risk management strategy. The large number of individual alleles
associated with adipose tissue illustrates its integration with diverse metabolic pathways. However, phenotypic
variation in age, sex, ethnicity and social status is further associated with different strategies for storing and using
energy. Adiposity therefore represents a key means of phenotypic flexibility within and across generations, enabling a
coherent life-history strategy in the face of ecological stochasticity. The sensitivity of numerous metabolic pathways to
ecological cues makes our species vulnerable to manipulative globalized economic forces. The aim of this article is to
understand how human adipose tissue biology interacts with modern environmental pressures to generate excess
weight gain and obesity. The disease component of obesity might lie not in adipose tissue itself, but in its perturbation
by our modern industrialized niche. Efforts to combat obesity could be more effective if they prioritized ‘external’
environmental change rather than attempting to manipulate ‘internal’biology through pharmaceutical or behavioral
means.
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adapted to diverse ecological niches into a species in which a large
proportion of individuals are characterized by excess body weight
that impacts adversely on health.
Influential but insufficient approaches
Previous evolutionary approaches to human obesity, hence relevant
to adiposity in general, have been dominated by two hypotheses,
each focusing on the concept of thrift. In the 1960s, the ‘thrifty
genotype’ hypothesis suggested that populations varied genetically
in their predisposition to store energy, on account of differential
ancestral exposure to ‘cycles of feast and famine’ (Neel, 1962). Those
experiencing more frequent famines were assumed to have
undergone selection for thriftier genes. This hypothesis was
somewhat abstract, with no discussion of when such cycles of feast
and famine might have occurred. Nevertheless, it became extremely
influential, and is still widely cited today. An alternative hypothesis,
focusing on life-course plasticity, proposed that low birth weight
babies responded to their low level of nutritional intake in early
life through alterations in growth and metabolism, which impact
on subsequent obesity risk (Hales and Barker, 1992). According to
this ‘thrifty phenotype’ hypothesis, an increased risk of diabetes
following low birth weight could be attributed to poor pancreatic
development interacting with excess body weight in later life. Again,
this hypothesis has been extremely influential; however, although
low birth weight babies are widely assumed to have an increased
risk of adult obesity, a recent meta-analysis failed to support this
assumption (Yu et al., 2011).
In both of these hypotheses, fat and thrift were considered two
sides of the same coin. Thrift remains a very useful concept in
evolutionary approaches to human adiposity and metabolism;
however, it is essential to emphasize from the outset that there is
much more to thrift than fat, and much more to fat than thrift
(Wells, 2011).
Thrift implies a degree of prosperity deriving from earlier
frugality and careful management of resources (Wells, 2009b). It
refers generically to the efficiency with which energy is used, and
can derive either from reducing energy expenditure, or storing
energy. Fat is only one strategy for storing energy, and organisms
show a wide variety of other forms of thrift, including hibernation,
torpor and a reduced activity level, along with manipulations of
growth rate, body size and relative investment in expensive organs
such as brains or secondary sexual characteristics (Wells, 2009b).
Storing energy as fat allows some organisms to tolerate ecological
uncertainty; however, many organisms respond to such uncertainty
in a rather different way – for example, through variation in
reproductive success. Boom-bust population dynamics characterize
species such as rabbits, which respond to increased energy supply
by increasing reproductive rate rather than depositing fat (Wells,
2009b).
For species with energy stores, the allocation of energy between
competing functions is potentially subject to greater control,
allowing more complex life-history strategies (see Box 1) (Pond,
2003). Indeed, a recent study found that adipose tissue is negatively
associated with brain size across mammals (Navarrete et al., 2011),
highlighting that storing energy and processing information are
alternative ways of dealing with ecological uncertainty. Humans,
however, have both large brains and high adiposity (Wells, 2009b),
making both strategies possible.
Adiposity and risk management
This review emphasizes adiposity and associated components of
metabolism as a generic metabolic form of risk management,
contrasting with behavioral risk management by the brain (Fig. 1).
It is well established that adiposity can fluctuate within individuals
in response to a variety of life-course or ecological factors, in
addition to accommodating genetic influences. Such fluctuations
represent the balance of two competing strategies: on the one hand,
individuals have a tendency to maintain a broad profile of adiposity
over time, reflecting genetic, developmental and environmental
influences; on the other hand, individuals have the capacity to gain
or lose fat over short time periods in order to buffer other
components of phenotype, and hence manage perturbations in
energy balance. Genetic variability in adiposity might likewise be
considered to represent ancestral influence on the overall risk
management strategy.
Until recently, there was a widespread tendency to consider
adipose tissue an inert fuel dump, simply storing energy for it to
be drawn upon by other tissues as required. It is now recognized
that adipose tissue secretes and responds to a wide variety of
signaling molecules (Ahima, 2006). Rather than being supplied by
large arteries, adipose tissue depots are maintained by many
smaller blood vessels from adjacent tissues, which generate close
connections with these tissues (Pond, 1998). Various receptors for
the molecular signals that control uptake and discharge of
triglycerides, located on the cell membrane and in the internal
cytoplasm, provide integration with several regulatory systems
(Pond, 1998). Through these complex and multiple signaling
systems, adipose tissue is intricately involved in a wide variety of
metabolic pathways. Far from being inert, therefore, the risk
management strategy represented by adipose tissue plays a
proactive role in allocating energy between competing functions
(Wells, 2009c), thereby contributing to life-history strategy and
promoting reproductive fitness (Hill, 1993).
The genetics of adiposity
Recent studies have emphasized a strong heritable component of
variability in adiposity. Heritability estimates from a selection of large
classical twin studies from industrialized populations, comparing
monozygous and dizygous twins, are provided in Table 1;
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SPECIAL ARTICLE The evolution of human obesity
Box 1. Life-history theory
Life-history theory addresses variability in the ‘pace’ of organisms’ life cycles
(Stearns, 1992; Hill, 1993). Through the life-course, a series of strategic
‘decisions’ must be enacted, regarding how fast to grow, when to start
breeding and how many offspring to produce at a given time point. Variability
across species in life-history strategy reflects genotypic effects of natural
selection, whereas variability within species reflects both genetic variability
and phenotypic plasticity (Stearns, 1992). In each case, optimizing life-history
strategy optimizes reproductive success, or ‘fitness’.
The two primary factors influencing life-history strategy are mortality
risk and the availability of energy. One of the key tenets of life-history
theory is that a finite amount of energy must be divided (i.e. traded off)
between competing functions, of which the most important are
maintenance, growth, reproduction and immune function, and between
current versus future reproduction (Hill, 1993). Adipose tissue, by storing
energy in the body, buffers such trade-offs against short-term ecological
perturbations, allowing them to be guided by longer-term factors.
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adoption studies have also suggested high heritability (Sorensen et
al., 1989; Stunkard et al., 1986b). However, these studies have been
conducted largely in industrialized populations, where favorable
environmental conditions might maximize the contribution of
genetic factors to phenotypic variability. Equivalent data from other
ecological settings are lacking and, under greater ecological
constraint, heritability might well be lower. Genome-wide
association studies report an increasing number of individual alleles
that are associated with adiposity, although findings often fail to
be replicated across populations, a scenario that might be due to
variability in study design and sample size as well as in population
ethnicity (Elks et al., 2010a; Heid et al., 2010; Speliotes et al., 2010;
Willer et al., 2009; Yajnik et al., 2009). Adiposity therefore seems
to have the genetic profile of a continuous trait that is affected by
numerous different genes, whose number and variability across
populations remain to be established.
The reported high heritability values are currently being reevaluated,
following recent developments in our understanding of
the biology of phenotypic variability. For example, monozygous
twins are not only more similar genetically than dizygous twins,
but are also more similar epigenetically (Fraga et al., 2005). This
means that the greater similarity of monozygous twins might not
be due entirely to genetic factors, and might also reflect a similar
environmental experience, especially in early life (Armitage et al.,
2008; Yajnik, 2004). This is an extremely important point, because
experience in utero is associated with various subsequent
components of phenotype that are relevant to adiposity.
However, genetic variability within populations is not the only
important comparison. Humans also appear systematically fatter
than other primates (Altmann et al., 1993), although few data are
available except from captive primates, themselves likely to be
overweight. Even women with a low body mass index (BMI) in
foraging populations have ~20% of weight as fat (Wells, 2012c). A
genetic perspective on human adiposity therefore emphasizes two
key issues: first, that ‘pan-human’ thrift manifests as a continuous
trait with substantial plasticity (discussed in the following section),
and, second, that it is subject to within- and between-population
variability.
Several evolutionary approaches to contemporary genetic
variability in adiposity have been proposed. In his classic paper,
Neel emphasized differential exposure to ancestral famine (Neel,
1962). Over the last decade, some have argued that there is no
evidence for such thrifty genes (Speakman, 2006; Speakman, 2008).
Because starvation is rarely the primary cause of death in famines
(Mokyr and O Grada, 2002), Speakman argued that famine could
not have been sufficiently powerful as a source of selection to
generate variability in thrifty genes. Instead, he proposed the ‘drifty
genotype’ hypothesis, arguing that multiple alleles impacting
adiposity accumulated below the threshold for phenotypic
detection over millions of years through genetic drift, and have only
Disease Models & Mechanisms 597
SPECIAL ARTICLEThe evolution of human obesity
Energy
stores
Single acute
signal
Food intake
Disease
burden
Social
stress
Physical
activity
Predator
threat
Multiple integrated
metabolic signals
Risk management
systems
Short-term Long-term
stresses stresses
Energy
balance
Brain
Fig. 1. Adiposity as risk management. This model treats
adipose tissue as an endocrine regulatory system
integrating multiple signals of energy supply and demand
in order to optimize the allocation of energy across
competing functions. This slower-response endocrine
system contrasts with the faster-response regulation of
behavior by the neural system (the brain), in response to
acute signals. The example shows an individual
responding to negative energy balance caused by
increasing energy demand and decreasing supply.
Table 1. Heritability coefficients from a selection of large twin studies for anthropometric indices of adiposity
Population Age n Outcome Heritability (%) Reference
China Adults 1260 twin pairs BMI 61 Lee et al., 2010
Waist 75
Finland Adults ~5278 twin pairs BMI 80 Hjelmborg et al., 2008
United Kingdom 4 years 3582 twin pairs BMI 48 Haworth et al., 2008
11 years 4251 twin pairs BMI 78
United States 20 years 4071 twin pairs BMI 77 Stunkard et al., 1986a
Adults 1224 twin pairs BMI 76 Watson et al., 2010
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recently been exposed in the modern obesogenic environment
(Speakman, 2008).
However, other life-history functions (growth, maintenance of the
large Homo brain, reproduction and immune function) might have
been very important traits exposed to selection on energetics, and
this perspective becomes more valuable if the definition of thrifty
genes is expanded to those associated with any aspect of adipose
tissue biology, rather than those associated with energy stores only.
Bouchard suggested five broad types of thrifty gene: those associated
with appetite; metabolism or thermogenesis; predisposition for
physical activity; adipocyte lipid storage capacity; and lipid oxidation
rate (Bouchard, 2007). This list can now be lengthened because genes
that are associated with adult BMI have been linked with infant
growth, pubertal timing and insulin metabolism (Elks et al., 2010a;
Elks et al., 2010b; Hattersley et al., 1998).
Contemporary human genetic variability can be attributed to
several different sources. First, some variability has persisted from
Homo erectus, passing through the population bottleneck that
characterized the origin of our own species (Garrigan and Hammer,
2006). Second, there is evidence that the gene pool of Homo sapiens
received contributions through interbreeding with other hominin
species, such as Neanderthals (Green et al., 2010) and a recently
discovered type from Siberia (Abi-Rached et al., 2011). Because
Neanderthals were well adapted to cold environments and a diet
rich in protein, these genes might be very relevant to contemporary
adiposity variability (Cochran and Harpending, 2009). Third, the
main proportion of global human genetic variability is assumed to
have occurred following the exodus from Africa around 60,000
years ago, when populations migrated to most continents and
adapted to a wide range of thermal environments, ecosystems,
dietary niches and disease loads (Garrigan and Hammer, 2006).
Fourth, the rapid expanse in global population size following the
emergence of agriculture is predicted to have substantially increased
the absolute number of new mutations in the last 10,000 years
(Cochran and Harpending, 2009).
When a new mutation occurs, it can increase in frequency in
the gene pool if it is beneficial, through a ‘selective sweep’. Recent
evidence suggests that many local selective sweeps of genes
influencing metabolism and adiposity only began relatively recently,
and are still continuing (Bender et al., 2011; Pickrell et al., 2009).
The specific functions of these genes converge on several important
components of biology, including metabolism and digestion,
immune defense, reproduction, and cognition (Cochran and
Harpending, 2009; Voight et al., 2006).
Genetic evidence indicates that adiposity is a continuous trait,
much like other life-history traits such as stature and the timing of
puberty (Elks et al., 2010b; Lango Allen et al., 2010). Continuous traits
tend to have a polygenic architecture, with each associated gene
contributing a small magnitude of effect. The polygenic basis of
adiposity maintains numerous small sensitivities to diverse ecological
stresses that are relevant to life-history strategy, integrating them into
a single phenotype (Wells, 2009b). In this way, it might represent a
moderate form of bet-hedging – i.e. increasing variance in offspring
to reduce variance in fitness (Wells, 2009b). Perhaps more
importantly, the same polygenic architecture further offers a stable
platform on which mechanisms of plasticity can function, because
the trait becomes robust to individual mutations by constraining their
magnitude of effect (Wagner, 2005).
Crucially, this perspective also suggests substantial common
ground between the genetic and plastic components of adiposity
variability. The genetic component integrates multiple ancestral
influences on life-history traits such as growth, pubertal
development, metabolism, adipocyte biology, etc., whereas the
plastic component allows greater life-course flexibility in the same
traits. Next, I consider the long-term evolutionary environment that
shaped this overall profile.
What environment shaped human adiposity?
The pan-human profile of adiposity was shaped over our
evolutionary past, reflecting ecological pressures that favored a
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SPECIAL ARTICLE The evolution of human obesity
500040003000200010000
–5.5
–4.5
–3.5
–2.5
Date (ϫ1000 years BP)
Deltaunits
Fig. 2. Isotopic trends over the last 5 million
years. Oxygen-18 (18
O) isotope levels are directly
equivalent to average global temperature. As the
world became steadily cooler, shorter-term climate
stochasticity also increased substantially, indicating
less stable ecological conditions. Based on
published data (Lisiecki and Raymo, 2005). BP,
before present.
DiseaseModels&MechanismsDMM
number of unusual traits that are characteristic of our species
(Wells, 2009b). These traits include large brains and fully bipedal
locomotion, but also key social and behavioral capacities, and they
have been widely assumed to have been favored by the emerging
‘savannah’ environment in east and southern Africa.
There is a tendency to consider the savannah a relatively stable
environment, with our modern fast-changing urban environment
generating a stark sudden contrast. An increasing volume of
palaeoenvironmental data suggests that this view of past stability
is very misleading (Potts, 1996; Potts, 2011). Isotopic data indicating
trends in global temperature indicate that, as the world became
cooler over the last few million years, this was accompanied by
another trend towards increasing climatic volatility (Fig. 2) (Lisiecki
and Raymo, 2005). This volatility in turn is assumed to equate to
continual ecosystem change, because rises and falls in temperature
and precipitation alter patterns of vegetation and associated
communities of organisms (Potts, 1996). The last 10,000-year
period has paradoxically been relatively stable climatically
(Richerson et al., 2001), but has instead been increasingly perturbed
by the niche-constructing activities of humans themselves (Wells
and Stock, 2007). There has arguably never been a lengthy period
of environmental stability during the evolution of the Homo genus.
Although outright famines might have been rare in preagricultural
populations, and severe food shortages could be
addressed by nomadism, hominin populations can be assumed to
have experienced energy stress repeatedly across and within
generations, resulting from various cycles of uncertainty. These
include seasonality, longer-term systematic climate shifts, and
extreme events such as volcanic eruptions and climatic cycles (Potts,
1996), an example in the contemporary world being El Niño effects.
More recently, human activities such as over-hunting and
unsustainable agricultural strategies have maintained ecological
perturbations (Diamond, 1998; Fagan, 1999). The ability to
withstand ecological volatility would furthermore have improved
the capacity to probe new habitats and niches, and hence colonize
new territories, a process that could have ramped up through
positive feedback effects (Wells and Stock, 2007). This issue is of
particular interest given the role of energy stores both in promoting
reproduction, and in withstanding energy stress, as discussed
below.
Although further details of this ecological volatility remain to
be established, there is already sufficient evidence to assume that
hominin evolution has been strongly shaped by stochastic
environments, with regular exposure to energy stress; this
assumption underlies the arguments below. Within such
environments, adiposity can be assumed to have interacted with
other hominin traits, such as large brains, colonizing strategy,
cooperative breeding, slow growth and dietary ecology, in complex
feedback cycles.
Adipose tissue as a strategy
The notion that adiposity functions as a complex risk management
system is supported by previous evaluations of how the tissue
evolved in vertebrates. Early mammals, for example, were small
and nocturnal. Lactating mice lay down fat by night in order to
fund lactation by day (Pond, 1984). In such small species, lactation
is too expensive to fund directly from energy intake, hence storing
energy during the period of feeding makes high energy output viable
during the non-feeding period. Other ecological stresses favoring
fat stores include migration, breeding and hibernation, each of
which temporarily overloads energy demand relative to intake
(Pond, 1998). Adipose tissue stores are particularly valuable in cold
environments, which are more vulnerable to fluctuations in energy
supply (Pond, 1998). So important is fat to some hibernating
animals that if they are starved as winter approaches, they
preferentially preserve adipose tissue, and oxidize lean tissue
instead (Dark, 2005).
We can therefore consider adipose tissue as a strategy for energy
storage that responds to multiple ecological stresses, interacting
with the characteristics of the animal. As body size increases, the
energy expenditure per kg body weight decreases (Oftedal, 2000).
Metabolic processes therefore become more efficient as body size
increases. In this way, larger body size per se, along with both
absolutely and relatively greater energy stores, increases the period
of time over which adipose tissue stores can fund energy
requirements.
Storing energy as fat is by no means the only strategy for
managing the risk of uncertainty in energy supply. A highly social
organism such as humans can store energy not only in the body
but also extra-corporally (in food hoards) or in social relationships.
This ‘redundancy’ of multiple mechanisms suggests that energy risk
management was crucial in the evolution of our species; however,
although the different approaches have much in common, they also
address different situations. Storing energy in social relationships
or food hoards buffers against individual uncertainty and
misfortune, such that other group members alleviate the risk. This
scenario gives control over the social distribution of energy stores.
Storing energy in the body buffers against situations that affect all
individuals in the social group, such as major ecological stress. This
scenario increases control over the internal distribution of energy,
as discussed below. However, storing energy in the body also carries
its own risks – the historical record has repeatedly documented
widespread cannibalism during severe famine, with some
individuals plundering the somatic energy stores of others (Keys,
1999).
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SPECIAL ARTICLEThe evolution of human obesity
100806040200
Brain
Heart
Liver
Kidneys
Pancreas
Spleen
Gut
Skeletal muscle
Adipose tissue
% reduction following starvation
Fig. 3. Percentage reduction in the mass of various organs and tissues
during starvation. During starvation, the amount of adipose tissue decreases
substantially, but most other tissues also decline. Based on published data
(Rivers, 1988).
DiseaseModels&MechanismsDMM
Fitness functions of fat
Adipose tissue and metabolism are fundamental to each of the
primary life-history functions (see Box 1), as briefly described
below.
Buffering starvation
It is widely recognized that adipose tissue can buffer against
starvation. A typical adult male and female store sufficient energy
to meet daily requirements for several weeks, typically longer in
females than males due to their higher average body fat content
(Norgan, 1997). However, Fig. 3 illustrates that adipose tissue is by
no means the only tissue that is broken down during starvation,
and that, although it is the most plastic tissue, skeletal muscle, gut,
spleen, heart and liver tissue all decrease substantially during
prolonged starvation (Rivers, 1988). Fat is therefore only one of a
wider range of energy stores. Famines have been recorded regularly
throughout human history (Keys et al., 1950), but only in certain
circumstances is death from starvation (i.e. extreme exhaustion of
energy stores) common. Rather, death commonly occurs from
infectious disease (Mokyr and O Grada, 2002), suggesting that, in
addition to providing fuel for metabolism, a key role of adipose
tissue is to maintain a viable immune system and protect vital
homeostatic functions (see below).
Buffering stochasticity
Rather than tolerating absolute famine, energy stores can be
considered more important for buffering short- and long-term
fluctuations in energy balance that are caused by seasonality in
energy supply or other similar stresses. Studies of farming
populations in seasonal environments show typical average
fluctuations in body weight of 2-3 kg (Ferro-Luzi and Branca, 1993),
with 4-5 kg occurring under more extreme conditions (Singh et
al., 1989). Detailed studies of Gambian women showed that the
majority of such weight change was attributable to losses and gains
in fat (Lawrence et al., 1987), although suppression of basal
metabolic rate during pregnancy provides another dimension of
thrift in this population (Poppitt et al., 1994). Other studies have
shown the extraordinary rapidity with which weight can be gained
through voluntary over-feeding for brief periods, as demonstrated
by the Guru Walla ceremony in rural Cameroon, where daily weight
gains approaching 0.25 kg were observed in some individuals
(Pasquet et al., 1992).
Adaptation to the cold
Body fatness tends to be greater in populations inhabiting colder
environments (Wells, 2012c). Although some have suggested that
fat is favored as insulation in such conditions, this is not well
supported by evidence, and high fatness impedes heat loss during
exercise (Pond, 1998). Instead, increased energy stores in cold
environments can ensure a supply of energy for thermogenesis,
while also providing greater buffering against negative energy
balance, which is potentially more harmful in cold environments.
Several stresses co-vary with climate, including dietary ecology,
food insecurity and disease load. A recent analysis of non-Western
populations showed that climatic variability was more strongly
associated with peripheral than central adiposity, suggesting that
central adiposity is more strongly influenced by other ecological
stresses (Wells, 2012c).
Growth
Energy stores are important in funding growth, both via the
energetics of reproduction and through the individual life-course
(Wells, 2009c). Maternal nutritional status regulates the probability
of conception (Schneider 2004; Wade et al., 1996), whereas baseline
adiposity and pregnancy weight gain contribute to growth of the
fetus (Anderson et al., 1984). In early postnatal life, storing energy
predicts the amount of lean mass accreted subsequently. This
suggests that growth strategy is sensitive to signals of energy
availability, although in early life these signals relate more strongly
to maternal nutritional status than to the external environment per
se (Wells, 2010).
Buffering the brain
Recent studies suggest that brain energy requirements are a
function of neuron number (Herculano-Houzel, 2011). Hence, the
high neuron number in the human brain [which is associated with
upregulation of many brain genes compared with non-human apes
(Caceres et al., 2003)] generates a particularly high energy
requirement, equivalent to 20% of basal metabolism even in adult
life. The high level of adiposity in early life has been attributed to
the benefits of buffering the heightened energy needs of the brain
at this age (Kuzawa, 1998). At birth, the brain accounts for ~80%
of total energy expenditure, a value which decreases to ~20% by
adulthood (Holliday, 1978). The brain is not insulin sensitive and
has obligatory energy requirements. Body fat might guarantee brain
energy supply during early life when infection risk is high, and can
provide ketones as an alternative substrate during starvation
(Kuzawa, 1998).
Reproduction
The substantial sexual dimorphism that is observable for adult
adiposity can be attributed to the primary role that females play in
the direct provision of energy for reproduction (Lassek and Gaulin,
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–130–120–110–100 –90 –80 –70 –60 –50 –40 –30 –20 –10 0 10
0
2
4
6
8
10
12
14
16
18
20
Triceps dimorphism
[(Ln male value – Ln female value)ϫ100%]
Frequency(no.populations)
Fig. 4. Histogram of sexual dimorphism in triceps skinfold thickness in 96
non-industrialized populations. These data indicate the trend for
substantially greater arm fat in females compared with males, but also
substantial variability among populations in this dimorphism. Reprinted with
permission (Wells, 2012b).
DiseaseModels&MechanismsDMM
2006; Wells, 2009c). Fig. 4 illustrates a histogram of population
variability in sexual dimorphism in triceps skinfold thickness. The
magnitude of dimorphism varies substantially, in accordance with
ecological factors such as energy availability and climate, but the
mean female triceps skinfold thickness is greater than that of males
in the vast majority of these 96 populations. Importantly, specific
fat depots such as gluteo-femoral adiposity might be particularly
important in providing essential fatty acids for offspring brain
growth (Lassek and Gaulin, 2007; Rebuffe-Scrive et al., 1985),
indicating that fat often supplies more than mere energy for
biological functions. Notably, such gluteo-femoral fat also seems
to be less detrimental to cardiovascular risk than central abdominal
fat (Snijder et al., 2004a; Snijder et al., 2004b).
Immune function
Recent work has emphasized the fundamental contribution of
adipose tissue to the immune system. Immune function is
metabolically expensive, involving a variety of costs, including the
defense and repair of specific tissues, the metabolic cost of fever,
and the production and maintenance of lymphocytes and other
immune agents (Romanyukha et al., 2006). Ironically, these costs
also include the growth and metabolism of the pathogens
themselves. Again, adipose tissue not only provides energy for
immune function, but also anatomically specific molecular
precursors for immune agents (Mattacks et al., 2004; Pond, 2003)
and a range of pro- and anti-inflammatory cytokines that play
numerous roles in both immune defense and the repair of damaged
tissues (Atanassova et al., 2007; Badman and Flier, 2007; Permana
and Reardon, 2007) (Box 2).
Psychosocial stress
Like many other mammals, humans live in social groups
characterized by varying degrees of social hierarchy. In this context,
the feeding behavior and metabolism of subordinate individuals is
sensitive to behavioral signals from dominant individuals. In
humans, as in other primates, subordinate individuals show a
different neuroendocrine profile to dominant individuals, having
increased cortisol and neuropeptide Y (NPY) levels, leptin
resistance, and a greater appetite and central adiposity (Adam and
Epel, 2007; Epel et al., 2001; Siervo et al., 2009). Patterns of fat
metabolism and deposition therefore represent an adaptive
response to the psychosocial environment.
Sexual selection
Given the multiple functions of adiposity, it is no surprise that it
represents a key trait that is subject to sexual selection in females,
who bear the direct costs of reproduction (Norgan, 1997). Across
human populations, female adiposity is generally considered
attractive up to a point. However, this association of adiposity and
attractiveness is also subject to ecogeographical and cultural
variability, indicating that males can facultatively shift their
preferences according to local ecological conditions (Tovee et al.,
2006).
The sections above have briefly demonstrated that adipose tissue
is associated with numerous different biological functions.
Importantly, they can all be integrated within the theoretical model
of life-history. Adipose tissue can provide energy for each of these
functions, and its ability to do so is enabled by two key factors: its
involvement in multiple signaling pathways through which energy
needs and energy availability interact, and its interaction with
fundamental feedback systems that proactively allocate energy
between these functions. Hormones such as leptin (secreted by
adipose tissue itself), insulin (responsive both to adiposity and
ongoing metabolic dynamics) and cortisol (sensitive to a variety of
ecological stressors) play key roles in integrating adipose tissue with
these competing functions (Harshman and Zera, 2007; Schneider,
2004; Tatar et al., 2003; Wade et al., 1996), but many other signaling
molecules are also important.
Fig. 5 presents a schematic diagram of energy metabolism as an
‘allocation game’, whereby energy is directed to competing
functional or storage ends (Wells, 2009a). For example, reductions
in adiposity drive reductions in leptin, which in animal studies
curtails reproductive function and components of immune function
(Drazen et al., 2001; Prentice et al., 2002; Schneider, 2004). Similar
signaling systems are expected in humans, although the details are
still being established, and the importance of leptin in human
adipose tissue biology might have been overemphasized. In addition
to the signaling molecules described above, various molecules
secreted by adipose tissue influence the relative allocation of
energy between these ends. For example, the insulin-sensitizing
hormone adiponectin moderates many aspects of reproduction
through receptors in the ovaries, oviduct, endometrium and testes,
as well as in the central nervous system (Campos et al., 2008;
Michalakis and Segars, 2010); adiponectin levels are also sensitive
to early-life experience. Although additional work remains to be
done, it is already clear that adipose tissue plays a fundamental
regulatory role on the allocation of energy between competing lifehistory
functions.
Risk management in operation: the long view
Although humans have a common risk management strategy in
their adipose tissue, its characteristics also differ between
populations, and between individuals within populations. Unlike
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SPECIAL ARTICLEThe evolution of human obesity
Box 2. Selected cytokines and growth factors
secreted by adipose tissue
Pro-inflammatory cytokines:
•Interleukin-1 (IL-1), IL-6, IL-18
•Tumor necrosis factor-a
•Eotaxin
•Resistin
•Leptin
Anti-inflammatory cytokines:
•Adiponectin
•IL-10
•IL-1 receptor antagonist
•Prohibitin
Growth factors:
•Angiopoietin-1
•Fibroblast growth factors
•Epidermal growth factor-like growth factor
•Hepatocyte growth factor
•Insulin-like growth factor
•Nerve growth factor
•Platelet-derived growth factor
DiseaseModels&MechanismsDMM
the brain, which represents another strategy for risk management,
adipose tissue biology seems to be somewhat tailored to local
ecological conditions. The scenarios described below illustrate how
the risk management strategy might be modified by long-term or
short-term experience.
Numerous studies have highlighted systematic differences
between populations in adiposity and metabolism. For example,
compared with Europeans, south Asians have a higher mean body
fat content for a given BMI value, whereas West African populations
tend to have lower body fat content and greater lean mass. These
differences in adiposity are associated with variability in metabolic
traits such as insulin sensitivity and blood pressure. Most attention
has been paid to these particular ethnic groups, because south
Asians and Africans or Afro-Caribbeans comprise significant
minority populations in some industrialized populations. Beyond
this, a continuum of ethnic variability in adiposity is apparent
(Wells, 2012c). What ecological pressures can account for such
ethnic variability?
Some of the variability can be attributed to non-genetic factors,
potentially incorporating immediate effects of diet and
transgenerational growth patterns. However, some of the variability
derives from genetic factors, indicating adaptation to longer-term
stresses. For example, climate has been associated with physique
and adiposity (Wells, 2012c), basal metabolism, and low-density
lipoprotein (LDL) concentrations (Snodgrass et al., 2007).
Similarly, I have suggested that adaptation to local disease loads
is an important source of such genetic variability. The ‘variable
disease selection’ hypothesis assumes that specific infectious
diseases impose different metabolic burdens, favoring variable
responses in terms of fat distribution and cytokine biology (Wells,
2009a). For example, a number of infectious diseases have been
described as ‘unconquerable’, in that the pathogen and the immune
system develop a ‘precarious standoff’ in which the immune system
remains activated without completely defeating the pathogen (Roth
et al., 2011). Such diseases include tuberculosis, malaria, hepatitis
B, schistosomiasis, African trypanosomiasis, Chagas disease and
syphilis (Roth et al., 2011).
For each disease, the availability of increased energy stores might
boost the immune system, but the optimal depot for storing the
energy, and the optimal range of associated cytokine signaling,
might vary between the diseases (Wells, 2009a). For example, I have
suggested that intramuscular adipose tissue in African populations
might have been favored as an adaptation to malaria, providing
local fuel for muscle tissue in this disease, for which fever is a key
component of immune defense, whereas visceral adiposity might
have been favored in south Asian populations as an adaptation to
gut-borne infections, which can be promoted by regular monsoon
rainfall (Wells, 2009a).
This hypothesis can be placed alongside other hypotheses for
ethnic genetic variability in traits relevant to adiposity, such as
dietary ecology (Kagawa et al., 2002) and thermal stress (Hancock
et al., 2008). However, the reason why the variable disease selection
hypothesis merits particular attention is that it offers an explanation
for variability in cytokine biology – and it is the cytokine load
imposed by obesity that seems to be particularly detrimental in
terms of chronic degenerative disease risk (Alvehus et al., 2010;
Fernandez-Real and Ricart, 2003; Hotamisligil, 2006; Tilg and
Moschen, 2006). The immune system is well established to be under
selection in humans, and evidence for ethnic variability in cytokine
levels is now emerging (Drazen et al., 2001; Ivanova et al., 2011;
Paalani et al., 2011). Although these hypotheses are intriguing,
supporting evidence is currently limited and this is an important
area for further work.
Risk management in operation: the short view
Within the life-course, experience might also predispose to
variability in the adipose risk management system. Recent studies
have shown how experience in early life generates differences in
life-history strategy, with multiple effects on adiposity. For example,
babies born small tend to undergo rapid infant growth (Ong et al.,
2000). These traits are then associated with the timing of puberty,
and with adiposity both in adolescence (Ong et al., 2009; Ong et
al., 2007) and adulthood (Pierce and Leon, 2005). A life-history
perspective attributes these associations to different reproductive
strategies (Nettle et al., 2010), and therefore different relative
allocations of energy to maintenance (longevity), growth, immune
function and reproduction. Exposure to psychosocial stress likewise
generates metabolic responses, some of which influence adiposity
(Brunner et al., 2007).
Mechanistic studies increasingly demonstrate that such lifehistory
plasticity is enabled by several dimensions of physiology,
involving hormonal adaptations and epigenetic variability in gene
expression. For example, babies born small have low levels of the
hormone insulin-like growth factor-1 (IGF1) (Ibanez et al., 2009;
Randhawa and Cohen, 2005) but, if nutrition is adequate to support
catch-up growth, the expression of insulin receptors and the IGF1
receptor in muscle and related tissues is upregulated (Muhlhausler
et al., 2009), with long-term metabolic effects. Other epigenetic
effects demonstrated in animal studies target regulatory systems
controlling appetite (Coupe et al., 2009). The primary target of such
plasticity might be growth variability; however, the metabolic effects
often impact the amount, distribution and activity of adipose tissue.
In contemporary populations, therefore, each of these early-life
dmm.biologists.org602
SPECIAL ARTICLE The evolution of human obesity
Store
Expend
Tissues
Functions
Functional
Brain
Fuel depot
Maintenance
Organs
Adipose
Reproduction
Synthesis
Activity
Glycogen
Muscle
Energy
intake
Losses
Signals
Insulin
Leptin
Resistin
Adiponectin
Growth factors
Interleukins
Peripheral
Deep
Fig. 5. Energy metabolism as an ‘allocation game’, with incoming energy
allocated to ongoing biological functions (energy expenditure) or a range
of energy stores. The allocation ‘decisions’are assumed to be orchestrated by
various signaling molecules. Adapted with permission (Wells, 2009a).
DiseaseModels&MechanismsDMM
adaptations might interact with the subsequent environment to
moderate obesity risk (Wells, 2012a). Importantly, however, the
mechanisms underlying such physiological plasticity seem to differ
between populations, such that both early-life and subsequent risk
factors can vary (Wells, 2012a).
Where is the disease of obesity located?
So far, this article has described adipose tissue as a flexible risk
management strategy for allocating energy across competing lifehistory
functions under stochastic conditions. This system is
assumed to have evolved under conditions of regular energy stress,
and indicates that adipose tissue biology in humans is fundamental
to our species’ persistence (Wells, 2009c). In modern environments,
this adaptive system is paradoxically associated with ill health,
through chronic excess weight gain and the metabolic comorbidities
of obesity (Hankey, 2012; Lustig, 2008; Prasad et al.,
2011).
This scenario can be attributed to the way in which multiple
components of modern urban environments interact with the
sensitive physiological pathways that confer adaptive plasticity.
There is a widespread assumption that factors that are specific to
human biology render our species unusually vulnerable to the state
of obesity. Yet this leads to the question: at what level of biology is
the disease of obesity located? Is obesity ‘internal’ to the individual
– a characteristic of genome and physiology that is amenable to
molecular or pharmaceutical manipulation – or is it external and
a consequence of environmental factors? Answering this question
is relevant to strategies for both prevention and treatment.
Research into the molecular basis of body weight regulation has
increased exponentially in recent years, following the discovery of
an increasing number of relevant hormones, peptides and other
signaling molecules, allowing numerous regulatory pathways to be
elucidated (Huda et al., 2006; O’Rahilly et al., 2003; Schwartz and
Morton, 2002). Much of this effort has been directed to the
development of novel pharmaceutical products that curb appetite
or reduce energy absorption. Although a number of such products
have been developed, few have been classified safe for human use
(Wilding, 2004; Rodgers et al., 2012). The complexity and
redundancy of molecular pathways of weight regulation seem to
hinder the identification of safe ‘entry points’ that are still capable
of generating substantial effects. Unlike the case of cholesterol, there
is no obvious rate-limiting step in energy metabolism and, because
adiposity is a continuous trait, no individual gene generates a large
magnitude of impact on phenotype. Although further progress,
potentially adopting a polytherapeutic strategy whereby multiple
pharmaceutical agents are combined (Rodgers et al., 2012), is likely,
the extent of potential success remains unknown. Bariatric surgery
has proven very effective in reducing type 2 diabetes, one of the
primary co-morbidities of obesity (Buchwald et al., 2009), but is
clearly an aggressive option with significant effects on quality of
life. Furthermore, neither pharmaceutical nor surgical approaches
are currently relevant to obesity prevention.
The situation for behavior is more complex. Humans likewise
show much behavioral diversity, yet, in non-obesogenic
environments, behavioral variability does not typically lead to excess
weight. However, humans can deliberately fatten themselves, as
reported in various cultures (Pasquet et al., 1992). Obesity can
therefore be said to have a specific behavioral component, and
behavior might therefore seem the obvious target for both
treatment and prevention. Nonetheless, weight management
programs typically achieve only modest weight loss on average
(although some individuals perform better) and many who initially
lose weight subsequently regain it (Taubes, 2008). The lack of
success in behavioral prevention programs is indicated by upward
trends in obesity prevalence in many industrialized countries, and
even more so in countries undergoing rapid economic development
(Misra and Khurana, 2008; Popkin, 2007).
Because obesity is rare in traditional societies, it is clear that
environmental factors are essential for its occurrence in the vast
majority of individuals, and that these factors drive behavioral
change at the level of the individual (Poston and Foreyt, 1999).
Molecular variability might account for differential susceptibility
to the obesogenic niche, but the niche itself is a prerequisite for
obesity except in the case of very rare genetic disorders (O’Rahilly
et al., 2003). Significantly, evidence increasingly suggests that many
non-human species become overweight if subjected to the human
industrialized niche. Many primate species become obese in
captivity (Kemnitz and Francken, 1986), and diverse species of pet
animals also gain excess weight (German, 2006). These findings
remind us that expression of human obesity is dependent on
exposure to certain conditions, which are conventionally termed
the ‘obesogenic niche’. One key element of such environmental
effects is their role in eliciting differential susceptibility to obesity.
Recently, life-history traits have received attention as being central
to the life-course emergence of variability in obesity susceptibility
(Dulloo, 2008; Dunger et al., 2006; Wells, 2011; Yajnik, 2004).
The dominant role played by the energy balance equation in
obesity research has resulted in particular emphasis on high energy
intake (‘gluttony’) and low energy expenditure on physical activity
(‘sloth’) as the primary determinants of the condition (Prentice and
Jebb, 1995). Many studies have sought evidence for gluttony or sloth
in those prone to weight gain, and the obesity epidemic is also
widely attributed to secular trends in population energy intake or
physical activity level (Briefel and Johnson, 2004; Dollman et al.,
2005). In popular wisdom, obesity is the product of too much food
and too little activity, seemingly implicating personal responsibility.
This perspective increasingly seems to be over-simplistic.
The energy balance equation is based on fundamental physics
and cannot itself be wrong, but its application in obesity research
is increasingly considered problematic (Lustig, 2006; Taubes, 2008;
Wells and Siervo, 2011). Attention has begun to turn to a wider
range of possible environmental or demographic risk factors, such
as central heating, shifts in sleep patterns, chronic psychosocial
stress, exposure to television screens, later age at first birth,
environmental pollutants, etc. (Brunner et al., 2007; Keith et al.,
2006). Although all of these proximate factors merit attention and
might prove to be associated with variability in adiposity, almost
all researchers pay little attention to why these behavioral trends
are taking place.
Risk management meets political economy
A historical perspective, rarely adopted by scientists, offers ample
evidence that food supplies fluctuate substantially through the
influence of market forces. Importantly, these forces drive both
under-nutrition as well as excessive energy intake (Albritton, 2009).
A large proportion of the global population remains underDisease
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SPECIAL ARTICLEThe evolution of human obesityDiseaseModels&MechanismsDMM
nourished, often as a direct consequence of exploitation as cheap
labor in agricultural production, industry or new urban economies.
Another large and increasing proportion of the population has
become overweight, owing to various manipulations that maximize
profit for the food retail and associated industries (Albritton, 2009).
Recent work on the role of refined-carbohydrate diets in perturbing
insulin metabolism highlights how the content of foods might drive
changes in human behavior, rather than vice versa (Taubes, 2008;
Wells and Siervo, 2011).
Capitalism therefore drives both under-nutrition and excessive
energy intake. However, in recent decades, a further effect is for
capitalism to drive the transition from one nutritional state to the
other (Fig. 6). In ‘emerging market’ modernizing countries such as
India, obesogenic food products interact with metabolic
adaptations to chronic malnutrition to produce a high prevalence
of obesity that can approach 50% in urban populations (Pandey et
al., 2011). A recent study showed, for example, that the transmission
of obesity from mother to offspring is much more likely if the
mother was herself born small (Cnattingius et al., 2011),
demonstrating the impact of prior under-nutrition on susceptibility
to the obesogenic niche. These rapid secular trends are driven by
the aim of the food industry to target new markets (Stuckler et al.,
2012), in the absence of any efforts to address persisting undernutrition.
This is illustrated by a persisting high prevalence of
under-nutrition in the same populations in which obesity is rapidly
escalating, a situation known as the dual burden (Doak et al., 2000;
Doak et al., 2005).
Capitalism therefore stands out from other politico-economic
systems in its capacity to drive both under-nutrition, through the
demand for cheap labor, and excessive energy intake, through the
demand for ever-increasing levels of consumption to boost
profits. In the midst of this economic environment, the risk
management system represented by adipose tissue swings
between extremes of chronic energy deficiency in some, and
obesity in others. Chronic psychosocial stress is no longer a signal
of uncertainty in energy availability, but is instead a prevalent
component of the modern working environment in which energydense
foods are readily available, and psychosocial manipulations
of appetite are promoted by the food industry (Brunner et al.,
2007; Nestle, 2007; Shell, 2002).
The notion that obesity is a disease ‘inside’ the body, a function
of each individual’s genotype and phenotypic plasticity, is not well
supported by this perspective. As this review has argued, adipose
tissue itself is a valuable organ with many beneficial effects on
diverse biological functions. In environments of limited food
availability, almost no combination of genotype and plasticity will
lead to obesity, the exception being very rare genetic conditions
such as Prader-Willi syndrome. Instead, obesity can be considered
a disease ‘outside’ the body, deriving from an inappropriate food
supply and marketing system, producing a niche to which
individuals then vary in their susceptibility.
Efforts to address the global obesity epidemic have been
singularly unsuccessful, owing to their focusing at the level of the
individual. Most obesity prevention campaigns are characterized
by denial at many levels of the fundamental role played by the global
economy. As capitalism draws ever more of the global population
into its multinational marketplace, the prevalence of obesity rises
in concert. A major shift in strategy, from individual-based science
to population-based economics, will probably be required to
reduce the global health burden of excessive energy intake and
obesity. If the disease component of obesity lies not in adipose tissue
itself, but in the interaction between adipose tissue biology and our
modern industrialized environment, efforts to combat obesity
would be much more effective if they prioritized ‘external’
environmental change rather than attempting to manipulate
‘internal’ biology through pharmaceutical or behavioral means.
Conclusion
Adipose tissue is found in many vertebrates, where it represents a
sophisticated means of accommodating uncertainty in energy
supply. The fact that adipose tissue is the source of numerous
signaling molecules highlights its role in orchestrating life-history
decisions. This risk management system is, however, increasingly
destabilized in many human populations. Prevailing economic
policies cause individuals to be subjected to a range of ‘invasive’
cues favoring fat accumulation, in environments in which actual
energy availability has high stability. The combination of
insulinogenic diets and psychosocial stress on the one hand, and
low energy demand for physical exertion, reproduction and
immune function on the other, stimulates chronic lipogenesis but
reduces lipolysis. At this point, high levels of adiposity become toxic
and harmful to health. It is these socio-environmental cues,
collectively orchestrated by our capitalist economic system, that
are the optimal target for obesity prevention.
This article is part of a special issue on obesity: see related
articles in Vol. 5, issue 5 of Dis. Model. Mech. at
http://dmm.biologists.org/content/5/5.toc.
COMPETING INTERESTS
The author declares that he does not have any competing or financial interests.
FUNDING
This research received no specific grant from any funding agency in the public,
commercial, or not-for-profit sectors.
dmm.biologists.org604
SPECIAL ARTICLE The evolution of human obesity
Capitalism
Cheap labor:
under-nutrition
Consumerism:
over-nutrition
‘Emerging markets’:
driving the
dual burden
Fig. 6. Schematic diagram of the role of capitalism in global undernutrition
and over-nutrition. In recent decades, the exponential growth of
the globalized economy has led to rapid economic development in many
countries, resulting in populations further undergoing rapid shifts from
chronic under-nutrition to chronic energy excess. In each case, powerful
commercial companies privatize the profits arising from manipulating the
global supply of food, but socialize the health costs (Albritton, 2009).
DiseaseModels&MechanismsDMM
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