License to Eat? Obesity and Lower Mortality

January 30, 2013

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Hi. I am Dr. Henry Black, Clinical Professor of Internal Medicine at the New York University Langone Medical Center, former President of the American Society of Hypertension, and a member of the Center for the Prevention of Cardiovascular Disease at New York University.

I want to discuss a paper that was published in the Journal of the American Medical Association in January,[1] which questioned some things that we thought we understood. This is a paper from the National Center for Health Statistics at the Centers for Disease Control and Prevention that looked at the categorization of weight and mortality.

We categorize weight arbitrarily based on body mass index (BMI). BMI is weight in kilograms divided by height in meters squared (kg/m2). Anyone with a BMI < 18.5 is considered underweight, anyone from 18.5 to 24.9 is considered normal, and 25-29.9 is called overweight. A BMI > 30 is considered obese. In fact, obesity has now been parsed a bit further and is graded from grade 1, which is BMI 30-34.9; grade 2, which is 35-39.9; and grade 3, which is > 40.

These are arbitrary categorizations in a continuous variable, and that often gets us into trouble. What Flegal and her colleagues did was look at enormous numbers of studies that reported BMI and outcomes. Over 4000 studies were looked at. They got rid of studies that were not accurately conducted. Some were overadjusting for things that could have led to mortality. Some were underadjusted. They ended up with about 90 studies that they considered accurate.

What they found -- to some people's surprise and to some people's horror -- was that having grade 1 obesity, a body mass index between 30 and 34.9, did not increase mortality. In fact, people who were overweight (ie, BMI 25-29.9) had less of a problem than people who were considered to be of normal weight (BMI 18.5-24.9). If you had grade 2 or grade 3 obesity with a BMI > 35, there was a clear increasing mortality. However, this was not so for grade 1 obesity.

What was surprising about this finding in our national campaign about obesity is that it seemed to give some people confidence that it's okay to be overweight. But that is not exactly what they said. What they said is that grade 2 and 3 obesity with a BMI > 35, which is common in the United States and elsewhere, was associated with an increase in mortality. They didn't look at specific-cause mortality, just all-cause mortality. The important thing is that this is not a license to go eat whatever you want. This is an attempt to be a little more rational about what we do, what we recommend, and how much we torture our patients sometimes and ourselves to try to lose weight, when in fact our weight is not necessarily causing us a problem. They looked at diabetics, smoking, and many things that had confused what BMI is.

I think the important thing here is that we need to be very careful about what we say to patients, what we say to each other, and be a little more reasonable sometimes, because if we are pushing something that isn't scientifically based, we are making a mistake. Thank you very much.