Obesity Article Summary

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Department
Anthropology
Course
ANTC41H3
Professor
Larry Sawchuk
Semester
Winter

Description
The evolution of human fatness and susceptibility to obesity: an ethological approach Jonathan C. K. Wells Abstract - This article considers the proximate causes, ontogeny, fitness value and evolutionary history of human fat deposition o Proximate causes include diet composition, physical activity level, feeding behaviour, endocrine and genetic factors, psychological traits, and exposure to broader environmental factors o Fat deposition peaks during late gestation and early infancy, and again during adolescence in females o Human 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 o The 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 life o Introduction of agriculture has had three significant effects:  Exposure to regular famine  Adaptation to a variety of local niches favouring population-specific adaptations  The development of social hierarchies which predispose to differential exposure to environmental pressures o Thus, 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 Introduction - 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 o To 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 organisms - 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: o Fat 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 turnover • 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 o Obesity is the result of interplay between multiple genetic and environmental factors o Genes cannot explain secular trends in body mass within a population o Paternal 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 o Protein 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 o High fat intakes have been linked to greater body mass and fatness o A 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 o Dietary 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 requirements - Physical activity o Physical activity could be considered merely a proxy for energy expenditure o Physical activity may not promote weight loss but restricts weight gain - Feeding behaviour o Human 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 signals - Endocrine factors o A wide variety of hormones influence, and are influenced by, body fat, operating at different levels o Individually 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 o Insulin 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 o Amylin, linked also to gastric emptying, plays similar role, such that insulin and amylin co-signal adiposity to the brain o Leptin function as a ‘starvation signal’, as many energy- requiring physiological processes do not operate in its absence  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) - Inflammation o Adipose 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  Insulin resistance also contributes to inflammatory response by influencing the availability of energetic substrates - Psychological factors o Changes in social psychology, for example in attitudes to infant behaviour, may alter the dynamics of signaling sy
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