The number of obese Americans has increased dramatically over the last 40 years, and in 2001 the nation's surgeon general went so far as to call obesity an epidemic. Since that landmark declaration, efforts to combat obesity have slowly grown, and although no one knows exactly why, the obesity rate among children and adolescents has leveled off. It is still, however, alarmingly high. One-third of children and two-thirds of adults are overweight or obese, posing a daunting public-health challenge. Those adults are more likely to develop serious illnesses like heart disease, diabetes and hypertension. While diet programs emphasize personal responsibility, public-health experts blame a multitude of factors -- many beyond individuals' control -- for the societywide epidemic. Prominent among those factors are low consumption of fruits and vegetables and America's "obesogenic" environment, which promotes increased portion size, non-healthful foods and physical inactivity.