The Slightly Overweight Live Longest
Warning: uncontrolled correlation. Do not act upon in any way. Married people live longer and are heavier. Sick, smokers, and drug addicted people lose weight. Body-image conscious dieters may eat unhealthily. Bicyclists are killed in droves. Muscle shows up as higher body mass index just the same as deadly beer-belly flab. Women (who live a few years longer than men) are fatter than men.
Nonetheless, a meta-analysis shows that it’s the overweight (BMI 25-30) who have the lowest all-cause mortality. Hazard ratios (1 is more risk):
0.94 (95% CI, 0.91-0.96) for overweight, 1.18 (95% CI, 1.12-1.25) for obesity (all grades combined), 0.95 (95% CI, 0.88-1.01) for grade 1 obesity, and 1.29 (95% CI, 1.18-1.41) for grades 2 and 3 obesity.
If you happen to be not-fat and are pursuing safe diet+exercise without obsessive stress and self-denial, you certainly shouldn’t try to put on weight after hearing about this correlation (which is statistically robust but, probably explained away by the average causes of being slightly overweight, not a consequence of being overweight).
I do find it plausible that being slightly overweight has both harms and benefits, though. Supposedly it’s when you have a fat belly (especially visceral fat under the abs) that you get most of the harms.