"Overweight" is unhealthy –but over what weight?

Michael Pollan, author of the best sellers The Omnivore's Dilemma and In Defense of Food, has coined the term "our national eating disorder." Pollan refers to the state of disarray into which our eating habits have fallen in the last two or three decades due to both the increased variety of foods available and the barrage of conflicting advice about what to eat. Adding to the confusion are mixed messages we receive about how much extra weight is actually dangerous to our health.

Experts agree that being obese (currently defined as having a body mass index or BMI over 30, or being roughly 30 pounds overweight) is unhealthy. 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). Currently, about 30% of Americans are obese – a clear public health problem.

But what about the 30% or so of us who are overweight, but not obese? Interestingly, though obesity rates have jumped dramatically since the 1960's, the incidence of overweight (currently defined as having a BMI between 25 and 29) has increased only slightly since then. Much research in the past few years has focused on the question of whether obesity and overweight may, in fact, be qualitatively as well as quantitatively different conditions—and that being overweight might, in fact, not be so bad.

According to a report in the Harvard Health Letter: "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)."

More recently, researchers have considered measurements other than BMI, such as waist-hip ratio (an estimate of "belly fat"), to identify mildly overweight people at increased risk for diabetes, cardiovascular disease, and other conditions.

It seems to me that, as with many phenomena in science and medicine, there will turn out to be a bell shaped curve regarding the relationship between weight and health. Very obese people and very underweight people likely have unequivocal health risks and the rest of us fall on a spectrum in which heredity, diet, exercise, and stress, may all increase or lessen the health impact of our weight. Until the issue of how much extra weight is clearly unhealthy is resolved (if it ever is), it seems a good idea to follow Michael Pollan's pithy advice from his most recent book: "Eat food. Not too much. Mostly plants."

Do you have questions about what being overweight means to your health? Post them here and I'll respond to as many as I can.

Dr. Suzanne Koven practices internal medicine with a special interest in weight issues at Massachusetts General Hospital in Boston, and teaches at Harvard Medical School.

Healthy Eating
As research continues to show links between diet and diseases like cancer, diabetes, and heart disease, it's clear that "eating a balanced diet" is no longer the most important nutrition goal. With Healthy Eating: A guide to the new nutrition, a special report from Harvard Medical School, learn how the latest research has resulted in a new healthy eating pyramid, a new concept of good fats and bad fats, and a greater understanding of how foods  influence health and longevity. Use this research to make the perfect healthy diet plan for you.

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