The American Medical Association officially recognized obesity as a disease in 2013. But in the past 13 years, there's not been much of a shift in the understanding of what causes obesity — not in the general public, in people who contend with the condition or in the practice of medicine. Most people still think of obesity as a character flaw caused by too many calories and not enough physical activity. But it's much more complex than that.

A study analyzing National Health and Nutrition Examination Survey (NHANES) data found that even though US adults' BMI increased between 1988 and 2006, the amount of calories adults consumed and the energy they expended were unchanged. It also appears that the quality of calories consumed (low versus high glycemic index) is as important a consideration as the total quantity. And genetics only explains about 2.7% variation in people's weight, according to a study in Nature. That all adds up to this: The two most common explanations for obesity — calories in, calories out and family history — cannot, by themselves, explain the current epidemic.