Scientific American presents Nutrition Diva by Quick & Dirty Tips. Scientific American and Quick & Dirty Tips are both Macmillan companies.

There’s been a lot of buzz lately over a new study which seems to suggest that people who are overweight (but not obese) may actually have a higher life expectancy than those who are considered normal or healthy weight. In this analysis, researchers categorized people as normal weight, overweight, or obese using Body Mass Index (BMI), a number that takes into consideration your weight and your height. Many people chalked these surprising results up to the limitations of the BMI as an indicator.

The BMI doesn’t account for body composition; that is, the relative proportion of fat and muscle tissue. People often point out that a bodybuilder with lots of muscle and very little fat could easily have a BMI in the “obese” category. But let’s be honest, bodybuilders account for an extremely small fraction of the general population. As a simple tool for assessing the general population, the BMI is actually fairly reliable.

Others took these new results to mean that being overweight simply isn’t as dangerous to your health as everyone claims. I even saw bloggers suggesting that the obesity epidemic is simply a hoax dreamed up by the dieting industry to sell books and weight loss drugs. I think that’s a rather silly, not to mention dangerous, position.

Do Overweight People Live Longer?

In this latest analysis, people who were obese had the highest risk of death. But people who were in the overweight category actually had a lower risk of death than those who fell into the normal BMI category. That may seem surprising, until you consider the fact that the study lumped together people of all ages and in all states of health.

In a general population like this one, many people will end up dying of cancer, which is the second leading cause of death in most Western countries. Although obesity can increase your risk of getting cancer, people who are sick with cancer almost always lose a great deal of their body mass. People who die of old age also typically experience a loss of body mass in the last years of life. In other words, what this study really shows is that people who are very sick or very close to the end of their lives are likely to have a lower BMI. But that doesn’t mean that being heavy is a way to extend your lifespan.

In fact, when you filter out people who are underweight due to advanced age or serious illness, it’s pretty clear that being overweight increases your risk of many diseases and ultimately reduces life expectancy. The more overweight you are, the shorter your life expectancy. Statistically speaking, your long-term risks are lowest if your BMI is between 18.5 and 25. (Click here to calculate your BMI.)

Continue reading on