What is responsible for this sudden increase in the number of obese people in the U.S.? Obesity most likely results from an interplay between genes and the environment.  However, the recent explosion in the prevalence of obesity has occurred too quickly to be explained solely by changes in our genes. One hypothesis is that, in the past, humans who were able to accumulate fat when food was plentiful were better able to survive during times of food scarcity. Now, however, in the U.S. and many other countries, food is plentiful and foods that are high in calories but often low in nutritional value tend to be relatively inexpensive.  Furthermore, most people engage in far less physical activity than did previous generations.  Ironically, the ability to accumulate fat during times of plenty – which was originally thought to confer a benefit – has become a detriment to our health.  Research supported by the NIDDK has led to improved understanding of the factors contributing to this complex condition.