We find that both changes in smoking and in obesity are expected to have large effects on mortality. For males, the reductions in smoking have larger effects than the rise in obesity throughout the projection period. By 2040, male life expectancy at age 40 is expected to have gained 0.92 years from the combined effects. Among women, however, the two sets of effects largely offset one another throughout the projection period, with a small gain of 0.26 years expected by 2040.
Source: NBER paper.