What’s a “healthy” weight?

BMI, or body mass index, takes into account a person’s height and weight to estimate body fatness in these categories:

• Less than 18.5: Underweight
• 18.5 to 24.9: Normal weight
• 25 to 29.9: Overweight
• 30 to 34.9: Grade 1 obesity
• 35 to 39.9: Grade 2 obesity
• 40 or greater: Grade 3 obesity

Health by numbers

The purpose of the review was to compile the results of different studies that looked at BMI categories and corresponding overall mortality risk (death from all causes). Using the results from 97 studies including 2.88 million people, the investigators found these trends:

• Compared with people of normal weight, those who were overweight had a 6% lower chance of dying from any cause.
• People with grade 1 obesity had a 5% lower chance of dying from any cause compared with normal-weight people, but this result wasn’t statistically significant.
• People with grade 2 or grade 3 obesity had a 29% greater risk of dying from any cause compared with people of normal weight.

“Our findings are consistent with observations of lower mortality among overweight and moderately obese patients,” said lead study author, Dr. Katherine M. Flegal of the National Center for Health Statistics, Centers for Disease Control and Prevention. “Possible explanations have included greater likelihood of receiving optimal medical treatment, cardioprotective metabolic effects of increased body fat, and benefits of higher metabolic reserves.”

Putting the results into context

This all sounds really encouraging, especially at evening snack time. But here’s something to keep in mind: the study looked at overall mortality, but not cause-specific mortality. What this means is that the risk of certain diseases and conditions may be higher in overweight people, even though the combined risk from all causes is lower.

So if you have a strong family history of diabetes, for example, you might want pay more attention to your weight. “The evidence that obesity can cause metabolic abnormalities and many diseases is backed by decades of research,” says Harvard School of Public Health’s Nutrition Chair, Dr. Walter Willett, who cautions that the results of the new study should not be taken as license to pack on extra pounds in the hope of living longer.

(JAMA 2013;309:71–82)