To approach the problem of obesity scientifically, we must first define and measure it.  Dr. Friedman describes various methods for estimating obesity, including the body mass index, or BMI. He then explains the body’s mechanisms for counting calories and accurately balancing food intake and energy consumption. Results from adoption and twin studies indicate a significant genetic contribution to human obesity. Dr. Friedman discusses his identification of the hormone leptin and its critical role in the control of body fat, further strengthening the case that obesity and weight control are largely a function of biology. From the Series:HHMI's Understanding Biomedical Research Series