An examination of national health survey records shows Americans are exercising less, which could be driving the rising obesity rates.

Lack of exercise, not diet, linked to rise in obesity, Stanford research shows

Inactivity rather than overeating could be driving the surge in Americans' obesity, according to a study by a team of Stanford researchers. Examining national health survey results from 1988 through 2010, the researchers found huge increases in both obesity and inactivity, but not in the overall number of calories consumed.