In modern times, the prevalence of obesity has risen dramatically as a result of a multitude of factors ranging from biological to the social. Evolution has contributed not only to the genetic factors influencing fatness, but also to societal norms that shape the culture surrounding food, nutrition, and weight. Obesity levels vary both intra- and internationally based on the cultural and genetic influences acting upon various groups within and between societies. Different socioeconomic groups are disproportionately prone to obesity due to the options, or lack thereof, available to them.
Furthermore, affluent societies such as those in the US and Europe are more prone to higher levels of obesity than those in developing countries. Perspectives on fatness in societies in developing countries also differ vastly from those in heavily industrialized societies due to the different contexts of the social implications of obesity. These social contributors along with genetic influences provide an explanation for the prevalence of obesity in modern societies as compared to historical time periods. In the context of biological influences, behavioral tendencies provide a further explanation behind the rise of obesity in industrialized societies.
Largely shaped by childhood environment, behaviors surrounding food intake during adolescence and adulthood further contribute to the growing challenge of staying fit in today’s world. These behaviors have largely been shaped not only by the environment but also by historical events which have shaped food consumption habits; these historic shifts in food intake norms have shaped the genetic factors which influence obesity and aid in explaining certain individuals’ tendency to become overweight. Obesity, as well as the varying cultural perceptions surrounding it, evolved as a result of a combination of genetics, environmental, and social influences.
There are three ways in which genetics may affect obesity: monogenically, syndromically, and polygenically (Frazier-Wood, Wang 2015). The monogenic model is comprised of differences in a single gene, such as the gene controlling the presence or lack of the OB protein, which regulates body weight by increasing activity levels and basal metabolic rate (BMR) as well as promoting a reduction in food intake through increased satiety or decreased hunger. Syndromic effects result in obesity as a component of a more complex physiological disorder, usually arising from abnormalities in the endocrine system as in Prader-Willi syndrome. Syndromic obesity is usually accompanied by intellectual disability and other impairments.
Finally, polygenic obesity arises as the result of the interactions between many genes that act in conjunction with the environment to result in phenotypic obesity. Polygenic obesity is inheritable but in an unpredictable manner, unlike monogenic obesity. This is the most prevalent manner by which obesity is inherited; it is thought to be the most significant genetic factor in the rise of obesity levels in recent years (Frazier-Wood, Wang 2015). While these factors outline the etiology of obesity on an individual level, the rise of obesity on a larger scale can be explained by examining the evolutionary mechanisms that may have acted upon the gene pool.
On a larger scale, several types of selection may have shaped the way in which obesity affects human evolution as a whole; several theories exist that may explain humans’ tendency to be predisposed to obesity. The first postulates that the trait once provided an adaptive advantage but is now rendered useless in the context of modern society. “Thrifty alleles” would allow certain individuals to better conserve energy in the form of excess fat, allowing them to survive during periods of famine (Speakman 2015). However, food was never available in such excess so even individuals carrying thrifty alleles did not become so overweight to experience the disadvantages of obesity. This is not the case in industrialized societies, so those carrying these thrifty alleles are more likely to develop the health complications that accompany obesity.
The second theorizes that there is neither positive nor negative selection working on genes related to obesity; instead, the condition arose as a result of neutral evolutionary processes such as genetic drift (Speakman 2015). Finally, the third theory describes genes related to obesity as maladaptive; it theorizes that obesity has never conferred an evolutionary advantage, but rather it has arisen as a “byproduct of natural selection on some other [advantageous] trait” (Speakman 2015: 11). Each of these theories exists within the understanding that genes are not the sole contributor to the growing obesity levels in modernized societies; it is the interaction between genetics and behavioral factors that result in obesity within the context of the modern environment.
Throughout most of evolutionary history, cultural perceptions surrounding fatness were generally positive (Brown 1990). This can be evidenced through the positive depictions of obese females in ancient art; some of the oldest known sculptural pieces, such as the Venus of Willendorf, depict excess weight as a sign of fertility and wealth. In the context of the time period, during which obesity was uncommon and its negative impacts on health were unacknowledged, it makes sense that it was seen as a privilege reserved only for the select few who were able to indulge in an overabundance of food.
It is only in recent years that fatness has developed a generally negative connotation, demonstrated by the growing influence of diet culture and negative media portrayals of obesity. However, while obesity is viewed most negatively in more industrialized societies, these are the groups in which it is most prevalent. Yet, although this is not currently the case in most industrialized societies, many developing societies continue to view overweight individuals as healthy and affluent. This is likely due to the cultural belief that obesity is an indicator of wealth in less industrialized societies, as there is a positive association between obesity and social class in developing countries. Conversely, there exists an inverse correlation between wealth and obesity within affluent societies (Brown 1990). The discrepancy is likely due to the accessibility of food, or lack thereof, as distributed among the different socioeconomic classes within a society.
Distribution of obesity among members of a population is not random; it affects genders, ethnic groups, and socioeconomic classes disproportionately. It has been shown that females generally demonstrate a higher predisposition to obesity than males, as there exists a greater proportion of peripheral fat (Brown 1990). Furthermore, industrialization is associated with a trend towards smaller family size, meaning a reduction in number and frequency of pregnancies and breastfeeding in women, thus decreasing expenditure of peripheral fat stores and putting the modern woman at great risk of obesity (Brown 1990).
Additionally, while obesity may be an accurate indicator of wealth in developing countries since access to excess food is a privilege generally reserved for those of higher social status, this is not the case in many industrialized societies. Although for most, there is easy access to an excess of food, there exists a discrepancy between what options are made available to different members of society. In general, those of lower socioeconomic status have less access to nutrient dense, healthy foods and are instead exposed more frequently to less healthy options such as fast and processed food. This disproportionate lack of health food options in lower-income areas is referred to as a “food desert” (Poland 2014).
In addition to this lack of exposure, the healthy options that do exist are generally more expensive, creating a further barrier for low-income families. Because there exists a disproportionate distribution of ethnic groups among social classes, obesity also is correlated to ethnicity in this manner. It is erroneous to assume that particular ethnic groups are more genetically predisposed to obesity than others, but instead certain ethnic groups display a higher proportion of overweight individuals as a result of their socioeconomic positions. Furthermore, the tendency to choose partners within one’s own social class— “associative mating”— also contributes to the concentration of obesity within certain subgroups (Brown 1990).
The rise of the modern economy is positively correlated with the prevalence of obesity in heavily industrialized societies. The rise of obesity lends itself particularly well to economic modernization, which is associated with a decline in energy expenditure due to labor mechanization as well as an increase in caloric consumption due to an increase in accessibility to processed, calorie-dense, and nutrient sparse foods. The modernization of the economy has wide-reaching impacts on food consumption and energy expenditure. The industrialization has lent itself to the minimization of food scarcity and famine through methods such as increased production and more efficient systems of food storage. This in conjunction with a rapid decline in the need for physical labor and a trend towards a more sedentary lifestyle increases the surplus between caloric intake and energy expenditure, making it more difficult for many to maintain a healthy weight.
Historically, several major events contributed significantly to the severe uptick in the percentage of the population classified as overweight or obese. First, the shift from a plant-based to an omnivorous diet lent itself to larger brain size, followed by the rise of agricultural societies which introduced a more sedentary lifestyle that was later expedited by the Industrial Revolution. Four to six million years ago, primate ancestors of humans survived predominantly on a diet rich in plant-based foods, with the most common macronutrient being carbohydrates. However, once other animals became a more viable option for food sources, primitive hominoids became much more reliant on proteins and fatty acids in their dietary intake.
This change in consumption composition allowed for the evolution of a larger brain, as neural development is reliant on fatty acids. As humans began to establish more developed societies, the rise of agriculture provided a second shift in dietary norms, leading to the reestablishment of carbohydrates as the primary macronutrient. Settlement in lands surrounding the Tigris and Euphrates rivers— the “Fertile Crescent”— inspired the beginning of agricultural societies, which quickly spread, allowing humans to abandon a nomadic lifestyle (Pijl 2011). The intensive labor of agricultural work, however, combined with a moderate production of food, tempered the effects of the loss of a traditional hunter-gatherer society for which humans are best adapted. Thus, obesity remained generally uncommon until the dawn of the Industrial Revolution.
In the recent years following the Industrial Revolution, food production did not reach a maximum; in fact, the era of the Great Depression resulted in food shortages. However, because this was accompanied by the mechanization of labor and a decrease in energy expenditure, the average weight in the United States fluctuated little. However, in the years immediately following the Second World War, food production dramatically increased resulting in easy access to calorie-dense diets; simultaneously, energy expenditure remained low.
By 1980, a dramatic increase in obesity levels was observed, predictably accompanied by a higher average caloric intake. Also, in recent years— the past two decades— there has been observed a slowing in the rise of adult obesity, likely due to a more comprehensive public knowledge of dietary needs (Lewis, Basu 2015). Extrapolating from this data, it is likely that as people become more informed regarding healthy relationships with food and exercise, the rate of increase in obesity levels will continue to decrease, any the trend towards high obesity percentages may eventually be reversed. One way in which this may be accomplished is through the improvement of the childhood environment, the growth period within which lifelong dietary habits are formed, and education surrounding healthy relationships with food.
There are five major contributors that shape obesity in childhood, including diet and food environment, physical activity and sedentary behaviors, feeding practices, parenting style, and family routine (Hughes et al. 2015). Children’s relationship with food is shaped largely by the environments to which they are exposed, ranging from that of their schools, neighborhoods, and homes. Food preferences developed at home as a child often lasts into adulthood, which is especially detrimental to children of low-income families, who may not be exposed to healthy options as a result of the convenience of lower-cost but less nutritional items. In schools, while meal programs are highly regulated by local school boards and national standards, items outside the meal programs, such as those sold in vending machines and school stores, are associated with poorer nutritional quality.
Furthermore, neighborhoods in low-income areas are more likely to harbor a greater quantity of fast-food stores and a minimal number of health food options or full-service grocery stores. This disproportionately affects children of low-income families, who are thus more likely to develop a dependence on processed food consumption, a habit that is not easily broken even in adulthood. The attitudes surrounding sedentary behavior and exercise also play a role; children who are encouraged to partake in physical activity are less likely to grow obese.
Thirdly, feeding practices among parents, including the foods they provide and methods by which they feed their children, also greatly affect childhood relationships with food. Children are more likely to respond to “external cues,” such as the expectations of a parent or amount of food left on a plate, rather than to “internal cues” such as feelings of hunger and satiety (Hughes et al. 2015: 8). I expected to clear their plates or to eat a certain volume of food, children are less likely to be attuned to personal feelings of satiety and more likely to eat more in the absence of hunger. Furthermore, authoritative styles of parenting have been shown to be most effective in resulting in positive health outcomes, as authoritative parents “place reasonable nutritional demands… as well as being sensitive to the child’s need” (Hughes et al. 2015: 9-10). The frequency of meals eaten together as a family has also been correlated with lower rates of childhood obesity; the organization of these meals as well as rules and limits on consumption are more easily monitored in a family setting and thus result in an overall healthier diet. These behaviors developed in childhood set the tone for a lifelong dietary structure.
Modern behavioral habits including increased consumption of sweetened beverages and decreased consumption of fiber contribute to the growing epidemic of obesity. The determining factor in weight gain, loss, or maintenance is the caloric surplus or deficit; a diet that is nutrient-dense will likely result in an appropriate caloric intake as nutrient-dense foods promote satiety. Consumption of “empty” foods and beverages such as processed foods and sugar-sweetened beverages result in decreased satiety and thus the consumption of excess calories due to a lack of fullness. Sugar-sweetened beverages not only do little to promote satiety but also contain a high glycemic load, causing increased insulin production and thus resulting in hunger in the hours following consumption of the beverage. Additionally, calorically dense and nutrient-lacking beverages and processed foods are generally low in fiber.
Fiber promotes satiety while containing a relatively small caloric content (Lewis, Basu 2015). Furthermore, low levels of dietary fiber are associated with obesity as well as gastrointestinal, cardiovascular, and other health complications. Behavioral tendencies such as the inclination to choose easily accessible and cheap processed foods, which are usually high in calories and low in fiber, contribute to the lack of satiety and later increased hunger, thus resulting in a tendency to consume excess calories and subsequent weight gain.
The obesity epidemic runs most rampant in industrialized societies due to the modernization of machinery, which minimizes physical labor, and the overproduction of food, which leads to easily accessible excess caloric intake. While the genes linked to obesity may have once been beneficial in traditional hunter-gatherer societies, the advent of agriculture, and later the dawn of the Industrial Revolution, has caused the detrimental effects of obesity to outweigh the positives. Furthermore, obesity affects populations differ based on factors such as socioeconomic status, gender, and genetic predisposition. In addition to the genetic components influencing obesity, environmental factors such as behavioral choices also interact to expedite its development. In recent years, a more comprehensive public education regarding food intake and dietary needs has slowed the rate of increase of obesity levels, especially among more affluent members of society. By becoming more conscientious of relationships with food and dietary practices, a healthier outlook may help to diminish the tendency to become obese and assuage the health detriments associated with excess weight.
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