McFreedom

Via Jane Galt, I note that FuturePundit is commenting on two recent studies that show that "being overweight shortens life expectancy." While that may not sound new, the interesting point of these studies that BMI (body mass index, the rough-and-ready approximation of how heavy you are relative to your height) outside the normal ranges - both high and low - has long been known to increase early mortality. There's a number of reasons for this, but part of the issue is that if you have something really wrong with you (like cancer), you often lose a lot of weight right before you die. So, presumably, in some of the people with a low BMI, that BMI is really a symptom and not a cause.

He quotes Dr. Meir Stampfer, in the New York Times as concluding the case is closed. Being overweight or obese "raise[s] the risk of mortality...It really should be the final word...as to whether [being] overweight is actually bad for you or not." And, yet, the author of another recent study, Dr. David M. Mannino, says that "diabetes is really the driving factor among early death from criticial illness among people who are overwieght or obese." In this study, he controlled for diabetes among the overweight and obese, and discovered that having a high BMI is not in an of itself risky, if you don't get diabetes.

The trick, of course, is that having a high BMI is a major risk factor for diabetes. There's no question that, if you change your diet and exercise, and that drops your BMI, you'll reduce your risk for adult onset diabetes substantially. There's a study from 2001 that showed that, among those high-risk for diabetes, doing 150 minutes a week of light exercise (walking) and losing 5-7% of body weight cut risk of the disease by 58%.

To sum up what we know so far: Having a high BMI is bad because it's correlated with getting diabetes. Regular excercise and diet control are correlated with a normal BMI. The conclusion coming from these correlations is "you can substantially reduce your risk of dying early by lowering your BMI." One question I've had after reading these studies is, "Does being fat in and of itself increase your risk for diabetes? Or is it that excercise protects you, and since regular excercise is correlated with lower BMI, the BMI correlation is a red herring?" To a certain extent, this is an academic question, as "fat but fit" individuals are fairly uncommon. Most people, if they excercise regularly, trim out (I have no study to back this assertion up; it's just my own personal observation).

Interestingly, a study in 2004 set out "to examine the relative contributions and joint association of physical activity and BMI with diabetes." This study only included women, and defined "active" as "expending more than 1000 kcal on recreational activities per week," which, I'll note, isn't much. Their conclusion was that, although both BMI and activity level are "independent predictors," "the magnitude of the association with BMI was greater than with physical activity." In other words, exercising is good for preventing diabetes, having a normal BMI is better, and best is to do both.