Obesity is a cosmopolitan disease that affects all races worldwide. However, certain ethnic and racial groups appear to be particularly predisposed. The Pima Indians of Arizona and other ethnic groups native to North America have a particularly high prevalence of obesity. In addition, Pacific islanders (Polynesians, Micronesians, Maoris), African Americans, and Hispanic populations (either Mexican or Puerto Rican in origin) in North America also have particularly high predispositions to the development of obesity.

Secular trends clearly emphasize the importance of environmental factors (particularly dietary issues) in the development of obesity. In many genetically similar cohorts of high-risk ethnic and racial groups, the prevalence of obesity in their countries of origin is low but rises considerably when members of these groups emigrate to the affluent countries of the Northern Hemisphere, where they alter their dietary habits and activities. These findings form the core concept of the thrifty gene hypothesis espoused by Neel and colleagues.

The thrifty gene hypothesis posits that human evolution favored individuals who were more efficient at storing energy during times of food shortage and that this historic evolutionary advantage is now a disadvantage during a time of abundant food availability.