Epidemiology
It is estimated that 34% of adults in the United States are obese. The prevalence increases with age up to age 50, at which point it falls sharply in accordance with the increased mortality rate. Obesity is more common in women, particularly after age 50, because of the higher mortality rate among obese men after that age. Obesity is also more common among minorities and low-income populations. It has been estimated that about 25% of children and 20% of adolescents are significantly overweight.

Family studies of obesity show that 40% of adolescents studied at age 15 who had one obese parent were obese, whereas 80% of those with two obese parents were obese. This compares to only a 10% incidence of obesity among adolescents whose parents are of normal weight. Studies of monozygotic and dizygotic twins suggest genetic factors, but environmental influences are also present. Adoption studies have shown conflicting evidence for genetic transmission. Evidence for the heritability of somatotypes is stronger than for obesity. This fact may be significant in that even a moderate degree of ectomorphic body habitus may protect against the development of obesity.

Etiology
Although there is great variability in weight among humans, individuals show remarkable consistency over time. Humans who agree to increase or decrease their weights for experimental purposes generally return to their starting weights when allowed to eat freely. Such observations have led to the theory that there is a biological set point for body weight in humans. This is supported by animal studies in which lesions of the ventromedial hypothalamus cause hypo- and hyperphagia, respectively. To the extent that the “set point theory” is applicable to humans, many obese individuals may be dieting in opposition to biological factors that make dieting far more difficult for them than for other people.

Weight gain can occur as a result of an increase in either the number or the size of fat cells. The fat cells of adults with juvenile-onset obesity may be about the same size as those of normal-weight persons, but there may be up to five times as many. Persons with adult-onset obesity may have a normal number of larger-than-normal fat cells. In studies in which fat cell number and size were determined, individuals tended to stop losing weight when fat cell size returned to normal. Since fat cells once formed do not disappear, fat cell number may determine the lower limit of weight for persons who by dieting have worked to reduce cell size to normal. There are two periods of cellular proliferation in normal-weight children: birth to 2 years of age and 10-14 years of age. In obese children, the period may extend well past 2 years of age, with consequent hypercellularity of fat tissue early in life. Although this may be partly under genetic control, the cellular theory of obesity thus has important implications regarding nutritional practices and weight regulation for children.

The gene governing the storage and breakdown of fat, and leptin, the protein it codes for, have been identified. Leptin has been shown to resolve obesity in genetically deficient mice, however, human trials have failed to consistently produce weight loss. There are multiple central nervous system chemical regulators of appetite, including several neuropeptides, the endogenous opiates, serotonin, dopamine, and norepinephrine. Their role in obesity, however, has not been established.

Early psychoanalytic theories of obesity held that obese individuals had unresolved dependency needs and were fixated at the oral level of psychosexual development. The symptoms of obesity were viewed as depressive equivalents, attempts to regain “lost” or frustrated nurturance and care. Recent studies have failed to demonstrate an increased incidence of psychopathological disorders in obese compared to normal-weight individuals. However, a subgroup of juvenile-onset obese subjects has gross disturbances in body image, ie, they view their bodies as hideous and loathsome and feel that others view them with contempt. They have a negative self-concept, are very self-conscious, and have impaired social functioning. Such experiences may contribute to the development and maintenance of obesity. Furthermore, since obese individuals are often discriminated against socially and are perhaps less often the object of sexual desire than normal-weight individuals, the maintenance of obesity may in some cases reflect an unconscious wish to remain isolated to avoid conflicts relating to sexuality or emotional intimacy.

Although there is no specific family constellation that predisposes to obesity, members of families lacking in warmth and love may use food and overeating as a substitute for love. The mothers in such families are often lonely individuals whose own childhoods were marked by social, economic, or emotional dep-rivation. Such mothers may unconsciously wish to have fat children. Identification with their “well-fed, well-cared-for” children may compensate for earlier deprivation. Such families may also equate physical size and the state of being “well fed” with physical and emotional strength. Obese children in such families may thus actually fear weight loss by concretely interpreting it as a loss of physical strength and emotional well-being.

The higher incidence of obesity among lower socioeconomic classes has been noted. In some societies in which food is scarce, obesity may be valued as a symbol of prosperity. In affluent countries such as the United States, value is instead placed on thinness, perhaps because foods low in calories but of high nutritional value are more expensive and unaffordable to the poor.

The definition of obesity may itself be culturally determined. Since 1943, revisions in standard height and weight charts have steadily lowered the ideal weights for women. The ideal weight for an average 5 foot 4 inch woman in 1943 was approximately 130 pounds; in 1980 it was under 120 pounds. Ideal weights for men have also been lowered, though not as much, and in 1974 the ideal weight for an average 5 foot 10 inch man was actually higher than the corresponding standard in 1943. These revisions have not been based on morbidity or mortality statistics but on measurements of the heights and weights of 25-year-old graduate students. Such standards do not take into account the fact that the percentage of body fat increases with age but instead reflect the fashion trends of the youthful, affluent college populations. For women, the steady decline in ideal weight reflects the upper-class emphasis on fashion model thinness as the standard of beauty. For men, there is greater acceptance of a wider variety of body types. Attractive men may be thin, eg, long-distance runners and basketball players; or bulky, eg, weight lifters and football players. This broader range of acceptability may account for the less consistent downward trend in ideal weights for men listed in standard charts.

If the 1980 standards for ideal weights are accepted, and if obesity is defined as weight at least 20% above ideal, then the average American woman is by definition obese, and the average American man is on the verge of obesity.