Less smoking, more obesity. Net loss: Eight months of life

Anyone who's ever quit smoking (or tried to) knows that once the puffing stops, the eating begins. And as individual smokers go, so goes the nation: The U.S. has seen smoking rates fall and obesity rates rise in recent decades, and the combination has cut Americans' life expectancy.

A recent New England Journal of Medicine study of trends in rising obesity and falling smoking rates found that the former has "overwhelmed the positive effects of declines in smoking in multiple scenarios." The study predicts: "If past obesity trends continue unchecked, the negative effects on the health of the U.S. population will increasingly outweigh the positive effects gained from declining smoking rates."