Contributing Factors of Obesity

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Environmental Factors

The environment is a major determinant of overweight and obesity. Environmental influences on overweight and obesity are primarily related to food intake and physical activity behaviors.  In countries like the United States, there is an overall abundance of palatable, calorie-dense food. In addition, aggressive and sophisticated food marketing in the mass media, supermarkets, and restaurants, and the large portions of food served outside the home, promote high calorie consumption.

Many of our sociocultural traditions promote overeating and the preferential consumption of high-calorie foods. For many people, even when caloric intake is not above the recommended level, the number of calories expended in physical activity is insufficient to offset consumption. Mechanization limits the necessity of physical activity required to function in society. Many people are entrenched in sedentary daily routines consisting of sitting at work, sitting in traffic, and sitting in front of a television or a computer monitor for most of their waking hours.

In this obesity-promoting environment, individual attitudes and behaviors are critical in weight management. Many individuals may need extended treatment in clinical or community settings to enable them to cope with the complexities of long-term weight management, especially if there is a history of unsuccessful attempts at self-treatment.  When the typical daily routine is so strongly biased towards promoting and perpetuating overweight and obesity, very high levels of knowledge, motivation, personal behavioral management skill, and lifestyle flexibility are required for an overweight or obesity-prone individual to avoid becoming overweight, or progressing to moderate or severe obesity.

Although there are undoubtedly some inter- and intrapopulation variations in the genetic predisposition to become overweight or obese, several lines of evidence suggest that genetic factors alone cannot explain the demographic and ethnic variations in overweight and obesity prevalence. For example, there is a difference in obesity prevalence among low- and high-income white women in industrialized societies. Other studies of populations, including migration studies, have shown an increase in average body weight in those who move from a traditional to a Westernized environment. Culturally determined attitudes about food, physical activity, and factors that vary with income, education, and occupation may increase the level of difficulty in weight management. Body image concerns and other motivations for avoiding obesity or controlling weight within given limits also vary with ethnic background, age, socioeconomic status, and gender. Thus, the competence of practitioners in working with diverse socio-cultural perspectives can be a critical factor in the success of obesity treatment. 

Genetic Factors

Obesity is a complex multifactorial chronic disease developing from interactive influences of numerous factors—social, behavioral, physiological, metabolic, cellular, and molecular. Genetic influences are difficult to elucidate and identification of the genes is not easily achieved in familial or pedigree studies. Furthermore, whatever the influence the genotype has on the etiology of obesity, it is generally attenuated or exacerbated by non-genetic factors.

A large number of twin, adoption, and family studies have explored the level of heritability of obesity; that is, the fraction of the population variation in a trait (e.g., BMI) that can be explained by genetic transmission. Recent studies of individuals with a wide range of BMIs, together with information obtained on their parents, siblings, and spouses, suggest that about 25 to 40 percent of the individual differences in body mass or body fat may depend on genetic factors.  However, studies with identical twins reared apart suggest that the genetic contribution to BMI may be higher, i.e., about 70 percent.  There are several other studies of monozygotic twins reared apart that yielded remarkably consistent results.  The relative risk of obesity for first-degree relatives of overweight, moderately obese, or severely obese persons in comparison to the population prevalence of the condition reaches about 2 for overweight, 3 to 4 for moderate obesity, and 5 and more for more severe obesity.

Support for a role of specific genes in human obesity of body fat content has been obtained from studies of Mendelian disorders with obesity as one of the clinical features, single-gene rodent models, quantitative trait loci from crossbreeding experiments, association studies, and linkage studies. From the research currently available, several genes seem to have the capacity to cause obesity or to increase the likelihood of becoming obese.  The rodent obesity gene for leptin, a natural appetite-suppressant hormone, has been cloned, as has been its receptor.  In addition, other single gene mutants have been cloned.  However, their relationship to human disease has not been established, except for one study describing two subjects with a leptin mutation. This suggests that for most cases of human obesity, susceptibility genotypes may result from variations of several genes.

Severely or morbidly obese persons are, on the average, about 10 to 12 BMI units heavier than their parents and siblings. Several studies have reported that a single major gene for high body mass was transmitted from the parents to their children. The trend implies that a major recessive gene, accounting for about 20 to 25 percent of the variance, is influenced by age and has a frequency of about 0.2 to 0.3.  However, no genes have yet been identified. Evidence from several studies has shown that some persons are more susceptible to either weight gain or weight loss than others.  It is important for the practitioner to recognize that the phenomenon of weight gain cannot always be attributed to lack of adherence to prescribed treatment regimens.

 

Psychological Factors

Research relating obesity to psychological disorders and emotional distress is based on community studies and clinical studies of patients seeking treatment. In general, community-based studies in the United States have not found significant differences in psychological status between the obese and non-obese.  However, several recent European studies in general populations do suggest a relationship between obesity and emotional problems. Thus, it may be premature to state that there is no association between obesity and psychopathology or emotional distress in the general population. More focused, hypothesis-driven, and long-term studies are needed.

Overweight people seeking weight loss treatment may, in clinic settings, show emotional disturbances.  In a review of dieting and depression, there was a high incidence of emotional illness symptoms in outpatients treated for obesity⁷⁶.  However, several factors influenced these emotional responses; including childhood onset versus adult onset of obesity (those with childhood onset obesity appear more vulnerable).  Another study that compared different eating disorder groups found that obese patients seeking treatment showed considerable psychopathology, most prominently mild to severe depression . Sixty-two percent of the obese group seeking treatment showed clinically significant elevations on the depression subscale of the Minnesota Multiphasic Personality Inventory, and 37 percent of this same group showed a score of 20 or higher (indicating clinical depression) on the Beck Depression Inventory. Focusing on depression was considered an important component of the weight loss program. 

Another study compared obese people who had not sought treatment to an obese group that had sought treatment in a professional, hospital-based program, and to normal weight controls.  Again, obese individuals seeking treatment reported more psychopathology and binge eating compared to the other groups. Both obese groups reported more symptoms of distress than did normal weight controls. The authors suggest that the obese population is not a homogenous group, and thus, may not respond in the same way to standardized treatment programs. In particular, obese individuals seeking treatment in clinic settings are more likely than obese individuals not seeking treatment and normal controls to report more psychopathology and binge eating