One of the strongest predictor of a rise in obesity in the US is the decline in cigarette smoking!

Obesity is a national problem in the US. Some estimates say 30% of Americans are currently obese (defined by having a body mass index of 30 or more). This is a huge increase from just 25 years ago. So what is causing this increase? People gain weight by consuming more calories than they burn--what is influencing them to do this? Fast food? An increasingly sedentary lifestyle?

The National Bureau of Economic Research studied a number of socio-economic factors related to weight gain to see which had the most effect. Some of the factors they considered were food prices, employment, physical activity at work, urbanization, cigarette smoking, and prevalence of restaurants.

Surprisingly, they found that cigarette smoking had the largest effect. A decline in cigarette smoking, which can hinder weight gain, accounts for about 2% of the increase in obesity.

(Source)