What is a healthy weight? If you use the standard cutoffs for body mass index (BMI)—a key method for evaluating body weight in relation to height—it's a BMI in the range of 18.5 to 25, categorized as "normal." BMI of 25 to 30 is overweight; above 30, obesity. (To find out your BMI, use the NIH's calculator.) There has long been debate about these specific cutoffs and about the accuracy of BMI as a way to evaluate health. Some experts believe that the cutoffs for weight categories may be too low, and a new Danish study in the Journal of the American Medical Association suggests they are right.
Researchers analyzed three decades of data from 120,000 residents of Copenhagen, looking for connections between BMI and mortality rates. They found that those with the lowest all-cause mortality risk—what you might call the "sweet spot" for longevity—had a BMI of about 24 in the 1970s, but now have a BMI of 27. That's the middle of the overweight category. Moreover, while people with a BMI that qualified as "obese" had a 30 percent higher mortality rate than people of normal weight in the 1970s, the two groups now have similar rates. The riskiest BMI ranges are still at the extremes—below about 21 (low-normal and underweight) and above 33 or so, the data show.
The researchers suggested that the trend towards better mortality rates at higher BMIs may be related to improved treatment for cardiovascular disease and its risk factors. That would be most beneficial for people at higher BMI levels, who are at greatest cardiovascular risk.
If all of this sounds vaguely familiar, that’s because this is hardly the first study to produce such findings. In 2013 we reported on a major study that pooled data on nearly 3 million people from a dozen countries. It found that overweight people had a 6 percent lower overall death rate than people with “normal” weight. It also concluded that mildly obese people were at no greater mortality risk than normal-weight people. In addition, it found that the longevity benefit of carrying some extra pounds seemed to be most notable for people over 65.
In any case, this Danish study is hardly the last word in the debate about "optimal" weight. In fact, two months later a massive analysis in The Lancet challenged the findings. It pooled data from 239 studies involving more than 10 million people and concluded that the lowest death rates are still in the traditional "normal" range of BMI (18.5 to 25). BMIs in the overweight range of 25 to 30 were found to entail a modestly higher mortality rate; underweight (BMI below 18.5) and Grade 1 obesity (BMI 30 to 35) are both associated with moderately increased mortality rates. Grade 2 or 3 obesity (BMI over 35) are linked to the highest death rates, producing a J-shaped curve rising sharply with higher BMIs.
Bottom line: Clearly, for overall health, it’s worst to be either underweight or obese. But keep in mind that all of these mortality rate estimates are just population averages, and that your optimal weight depends on many factors—notably your fitness level, where you store the extra pounds (abdominal fatis riskiest), family history, and your risk for various diseases. What’s more, mortality rates are just the tip of the iceberg. Even when they don’t kill people, obesity-related disorders increase disability and impair quality of life.
Also see Measuring Body Weight: Beyond BMI