Compared to people with a normal weight (a BMI less than 25), the overweight (BMI between 25 to 30) had a 6 percent lower mortality rate—and both groups had a rate about 15 percent lower than the obese, especially the very obese (BMI above 35).
The explanation for the finding is uncertain. Perhaps the pleasantly plump but not obese have an extra reserve—a literal spare tire—that confers a survival advantage should they become seriously ill, whereas the lean-iacs do not. Or maybe the thin ones were thin because of a serious illness that, in the course the various studies, killed them. Or maybe the thin ones were thin because they were chain smokers living off Scotch and potato chips. Or just maybe the occasional pig-out does soothe the soul and make for a happier, healthier individual.
Whatever the explanation, the observation—a truly startling one—stands.
Paul Campos adds:
[I]t’s true that the fattest people in this study — those with a BMI of 35 and above — had a 29% higher mortality risk than the “normal weight” (sic) reference group. But what people tend not to take into account about these sorts of statistics is that, for most demographic groups, baseline mortality rates are extremely low, which means a few extra deaths will produce an impressive-sounding spike in relative risk.
For example, if you compare the risk that a 50-year-old man will die within the next five years to that of a 50-year-old woman, you’ll find that the man’s mortality risk is 71% higher. That sounds pretty bad, especially if you happen to be a 50-year-old man, but what this actually means is that the man has a 2.51% chance of dying over that five-year span, rather than a 1.47% chance. And note that this hazard ratio is nearly two and half times higher than that found among the very fattest people. So among the middle-aged, gender correlates far more powerfully with mortality risk than even the highest levels of “obesity.”