Complementary Hypotheses on Contributors to the Obesity Epidemic.

Increased rates of obesity have occurred within virtually every race, age, sex, ethnicity, and economic group. Despite substantial punditry on the issue, the exact reasons are incompletely known. The two most common factors cited as contributing to the obesity epidemic, and those whose causal influence on increasing obesity levels in the population are often presumed unequivocally, are food marketing practices and institutionally driven reductions in physical activity. These have been called "the big two." This Perspective builds on previous writings in this area to introduce additional factors that may contribute to the obesity epidemic. It is emphasized that there may be other factors working in combination with the big two, influencing body fatness through effects on energy intake, energy expenditure, and/or nutrient partitioning.