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ANTC41H3 Chapter Notes -Human Fat, Amylin, Insulin Resistance

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Larry Sawchuk

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The evolution of human fatness and susceptibility to obesity: an ethological approach
Jonathan C. K. Wells
- This article considers the proximate causes, ontogeny, fitness value and evolutionary
history of human fat deposition
oProximate causes include diet composition, physical activity level, feeding
behaviour, endocrine and genetic factors, psychological traits, and exposure to
broader environmental factors
oFat deposition peaks during late gestation and early infancy, and again during
adolescence in females
oHuman fat stores not only buffer malnutrition, but also regulate reproduction and
immune function, and are subject to sexual selection
- Early hominid evolution was characterized by adaptation to a more seasonal
environment, when selection would have favoured general thriftiness
oThe evolution of the large expensive brain in the genus Homo then favoured
increased energy stores in the reproducing female, and in the offspring in early
oIntroduction of agriculture has had three significant effects:
Exposure to regular famine
Adaptation to a variety of local niches favouring population-specific
The development of social hierarchies which predispose to differential
exposure to environmental pressures
oThus, humans have persistently encountered greater energy stress
- The capacity to accumulate fat has therefore been a major adaptive feature of our species,
but is now increasingly maladaptive in the modern environment
- Alterations to the obesogenic environment are predicted to play a key role in reducing the
prevalence of obesity
- Mammalian body mass can be divided into fat and fat free components
- Traditionally, obesity has been regarded as a physiological trait, and has been interpreted
using biochemical or physiological models
oTo the evolutionary biologist, however, energy stores represent a strategy, and
comprise an important component of the way in which organisms adapt
themselves and their descendants to the environment
Life-history theory: considers how factors such as size and growth rate
contribute to variability in the ‘pace’ of life and reproductive schedule of
- There’s difficulty of adopting evolutionary approaches to the phenomena of obesity that
are not directly preserved in the fossil record
How fat are Humans?
- Humans have been described as being among the fattest of all mammals
- Prolonged energy stress is potentially common to all kinds of animals, yet obesity in the
animal kingdom is rarely encountered
- Most fat mammals inhabit very different environments to humans
Proximate Causes
-The energy balance equation:
oFat deposition can be attributed to positive energy balance, on the basis of the
principles of thermodynamics
But has limited ability to explain the causation of fat deposition
Unless weight gain is extremely rapid, the level of positive energy balance
required for fat deposition is negligible in proportion to daily energy
Thus making it impossible to identify which of the two
components is ‘responsible’
It is only by measuring aspects of energy intake and expenditure
other than their caloric quantity that the causes of weight gain may
be identified
-Genetic factors
oObesity is the result of interplay between multiple genetic and environmental
oGenes cannot explain secular trends in body mass within a population
oPaternal genes enhance offspring growth whereas maternal genes suppress
In accordance with parent-offspring conflict theory which predicts a
conflict of interest between parental genomes over maternal investment in
each offspring
-Diet composition
oProtein intake is generally assumed not to act as a proximal cause of weight gain
in most age groups, but in early life higher protein intakes have been associated
with higher growth rates that in turn have been linked to subsequent obesity
oHigh fat intakes have been linked to greater body mass and fatness
oA high-carbohydrate diet is assumed to be beneficial as it implies a reduction in
dietary fat content.
However, the form in which carbohydrates are ingested may influence
mass status
Carbohydrate-containing foods can be categorized in terms of glycemic
index and glycemic load
Referring to variability in blood-glucose levels following
ingestion, and the consequent production of insulin
oDietary composition therefore influences weight gain through one of two
pathways: stimulating appetite, or inducing alterations in metabolism
Each of which can lead to over-consumption of energy relative to
-Physical activity
oPhysical activity could be considered merely a proxy for energy expenditure
oPhysical activity may not promote weight loss but restricts weight gain
-Feeding behaviour
oHuman feeding behaviour is characterized by complex regulatory mechanisms
and their interaction with the foods being consumed
Positive drives to initiate or continue feeding, based on sensual cues such
as the sight, smell or palatability of food, interact with negative feedback
signals derived from learned associations and gastrointestinal or metabolic
-Endocrine factors
oA wide variety of hormones influence, and are influenced by, body fat, operating
at different levels
oIndividually such hormones have little long-term influence on fat stores
Satiety signals, cholecystokinin and ghrelin, are produced by the gut in
response to food ingestion
Combines with nonhormonal factors, gastric distension, to
terminate eating
Adiposity signals which are transported into the brain
These include leptin, secreted directly by adipose tissue, and
insulin and amylin, secreted by b cells in the pancreas
oInsulin is the main hormone controlling blood glucose
levels, and its secretion by the pancreas is partly
determined by ambient glucose concentration
Fatter individuals secrete proportionally more
insulin for a given increase in blood glucose levels
oAmylin, linked also to gastric emptying, plays similar role,
such that insulin and amylin co-signal adiposity to the brain
oLeptin function as a ‘starvation signal’, as many energy-
requiring physiological processes do not operate in its
When plasma insulin concentration is increased, it causes a rapid decrease
in blood glucose levels, and stimulates rather than reduces appetite
Diabetics treated with insulin therefore tend to gain weight
including fat
Obese individuals are characterized by alterations in insulin
sensitivity, and by insulin resistance, where abnormally high levels
of insulin are required to control blood glucose levels
Insulin resistance is associated with hyperinsulinaemia (increased
insulin production)
oAdipose tissue, in particular visceral adipose tissue, secretes a range of
inflammatory factors
Fat plays an important role in providing energy for immune function
The inflammatory response to infection is mediated by certain cytokines,
known to induce fat synthesis in the liver