Is overweight okay?

Content provided by the Faculty of the Harvard Medical School

Is overweight okay?

The percentage of Americans who are obese has more than doubled since the 1960s. In 1960, about 13% of Americans were obese, defined as having a body-mass index (BMI) of 30 or more. (For most people, a BMI of 30 means being about 30 pounds overweight.)

Now over 30% of the population weighs enough to be considered obese. Experts agree that this is terrible news for the country's health. Obesity greatly increases the risk of diabetes, gallbladder disease, heart disease, high blood pressure, and several types of cancer (colon, esophageal, kidney, and postmenopausal breast cancer).

But what about being just a few pounds overweight? Certainly the trend line isn't as dramatic. In 1960, about 31.5% of Americans could be classified as being overweight, defined as a BMI of between 25 and 29.9. That percentage has crept up by just a few points, to about 34%. Meanwhile, research has caused some confusion about whether being overweight (as opposed to obese) is really all that bad for you.

In 2005, federal health researchers caused quite a stir when they reported that their analysis of data from national health surveys showed that being overweight didn't increase the risk of dying prematurely. (They also found that the mortality risk from obesity was lower than it had been in previous years). Better treatments for heart disease and conditions related to being overweight was offered as an explanation. But the study came under some sharp criticism for (among other things) not taking into account that the mortality rates in overweight people might look good compared to those for thinner people because some thinner people might have been skinny because they were suffering from underlying chronic illnesses.

A different group of federal health researchers reported the results of another large study of body weight and mortality in the Aug. 24, 2006, New England Journal of Medicine. They made some adjustments to deal with the confusing effects of chronic illness. They also focused on people who never smoked. Smokers tend to be thinner, so if they're included in a study of body weight, they can make staying trim seem unhealthy when it's really smoking that's responsible.

When these authors honed down their analysis to people who didn't have a chronic illness and didn't smoke, there was no free pass for the few extra pounds. People who were overweight in their early 50s were 20%–40% more likely to be among those who died during the study's 10 years of follow-up than those with lower BMIs (23.5–24.9).

All this goes to show how difficult it can be for researchers to sort out risk factors, particularly one like body weight, which is both a cause and an effect, and which is influenced by so many factors.

It may also say something about the BMI. We like our health information short and snappy, boiled down to few simple numbers, and the BMI fits the bill. But it's really just a convenient proxy for the real culprit, the metabolic effects of fat tissue, particularly the most metabolically active form that gathers in our bellies. Moreover, people with the same BMI can be carting around very different amounts of belly fat, depending on how much muscle they have and how the fat is distributed in their bodies. Several studies have suggested that the good old-fashioned waistline might be a better indication of how much unhealthy fat we have — and should get rid of.

Last updated:	September 05, 2008