Causes of obesity
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- Wright, S.M. & Aronne, L.J. Abdom Radiol (2012) 37: 730. doi:10.1007/s00261-012-9862-x
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The prevalence of obesity has been rising steadily over the last several decades and is currently at unprecedented levels: more than 68% of US adults are considered overweight, and 35% are obese (Flegal et al., JAMA 303:235–241, 2010). This increase has occurred across every age, sex, race, and smoking status, and data indicate that segments of individuals in the highest weight categories (i.e., BMI > 40 kg/m2) have increased proportionately more than those in lower BMI categories (BMI < 35 kg/m2). The dramatic rise in obesity has also occurred in many other countries, and the causes of this increase are not fully understood (Hill and Melanson, Med Sci Sports Exerc 31:S515–S521, 1999).
KeywordsObesityEtiologyWeight managementCauses of obesityBariatric surgery
The prevalence of obesity has been rising steadily over the last several decades and is currently at unprecedented levels: more than 68% of US adults are considered overweight, and 35% are obese . This increase has occurred across every age, sex, race, and smoking status, and data indicate that segments of individuals in the highest weight categories (i.e., BMI > 40 kg/m2) have increased proportionately more than those in lower BMI categories (BMI < 35 kg/m2). The dramatic rise in obesity has also occurred in many other countries, and the causes of this increase are not fully understood .
Although obesity is most commonly caused by excess energy consumption (dietary intake) relative to energy expenditure (energy loss via metabolic and physical activity), the etiology of obesity is highly complex and includes genetic, physiologic, environmental, psychological, social, economic, and even political factors that interact in varying degrees to promote the development of obesity .
The food, or “built” environment has shifted in ways that promote overeating: highly caloric and fat-laden foods are not only affordable but also easily accessible (i.e., numerous fast food restaurants, vending machines of energy dense items in schools and offices, etc.). These highly palatable foods are frequently available in large portions, which contribute to increased daily caloric intake . Not only have commercial portion sizes increased, the number of processed food items (typically high in sugar, fat, and sodium) available in grocery stores, mini-marts, and convenience stores has skyrocketed. Today, the majority of products in grocery stores are non-perishable, highly processed, and pre-packaged foods. These products are heavily marketed not only to adults but also to children as well. Convenient, easy to prepare, and inexpensive, these high calorie products are frequently consumed by millions of families who are struggling to meet the economic and scheduling demands of today’s fast paced lifestyle.
Physical activity levels have also dramatically decreased in the past several decades. It has been estimated that less than half of US adults engaged in recommended levels of physical activity in 2005 . Levels of physical activity have also decreased in adolescents . There is less access to physical activity (fewer sidewalks), less physical education in schools , and more time is spent on sedentary behaviors such as television watching, surfing the internet, and playing video games . The myriad advances in technology developed over the past few decades have made many tasks more efficient, but in the process have ultimately decreased the number of calories expended (i.e., TV remote controls, automatic garage door opener, etc.).
Contributing Factors to the Obesity Epidemic
The food environment
Decreases in physical activity
Drug-induced weight gain
Decline in cigarette smoking
Reduction in variability of ambient temperature
Changes in distribution of ethnicity and age
Increasing gravida age
Greater reproductive fitness of higher BMI individuals yielding the selection for obesity-predisposing genotypes
Assortative mating and floor effects
Changes in policy
Sleep debt has also been linked to increased body weight. Some studies have shown that hours of sleep per night are negatively correlated with BMI , and sleep restriction has been shown to increase hunger and appetite . The relatively recent decline in cigarette smoking may also be a factor that has contributed to the obesity epidemic, since studies have shown that weight gain is a common sequelae of smoking cessation. Furthermore, smokers typically weigh less than non-smokers.
Endocrine disruptors, industrially produced substances that can affect endocrine function, may also be a contributing factor to the etiology of obesity. They include dichlorodiphenyltrichloroethane, some polychlorinated biphenols and some alkylphenols, that may act by disturbing endogenous hormonal regulation . Other factors (Table 1) that can possibly contribute to the obesity epidemic include a reduction in the variability of ambient temperature due to central cooling and heating, changes in population distribution of ethnicity and age, increasing gravida age, intrauterine effects, and greater reproductive fitness of higher BMI individuals yielding the selection for obesity-predisposing genotypes. Although the genetics of obesity is a highly researched area, just a small number of rare single genetic abnormalities have been discovered. Current research on the epigenetics of obesity is investigating the influence of behavioral and environmental factors on genetic expression .
It has been suggested that policies put into place by the US government to increase the food supply in the 1970s contributed to an abundance of food and therefore an increase in population energy intake . For instance, US farm subsidy policies may have caused certain foods to be more abundant and cheaper, potentially contributing to lower relative prices and increased consumption of fattening foods . Infections may also play a role in the etiology of obesity. Adenovirus-36 (Ad-36) infection has been shown to cause obesity in animals , and studies have shown that obese individuals are more likely to have been infected with Ad-36 .
Social networks may also contribute to the increasing prevalence of obesity. One study showed that a person’s chance of becoming obese increased by 57% if he or she had a friend who became obese in a given interval . A similar phenomenon was observed among adult siblings and married couples. Among adult siblings, the chance of one sibling becoming obese increased by 40% if the other sibling had become obese, and among married couples, the likelihood of one spouse becoming obese increased by 37% if the other had become obese.
Thus, the dramatic rise in the incidence of obesity in many countries appears to be due to the complex interaction of a variety of factors including genetic, physiologic, environmental, psychological, social, economic, and political. Given the numerous and significant deleterious health consequences associated with obesity, there is an urgent need for the development of highly effective interventions that aim to reverse these “obesogenic” drivers, including both government policies as well as health education and promotion programs.