A new study published in the Journal of the American Medical Association (JAMA) indicates that a body mass index or BMI of 25-29.9 (overweight) is associated with the lowest risk of death and that class 1 obesity (BMI 30-34.9) is not associated with an increased risk of mortality. As this study hit the presses January 2nd (and I’m sure no editorial thought was given by JAMA to such a study coming out at the first of the year) when many people are thinking about weight loss resolutions, it was covered widely in the press and I read several op-eds claiming vindication for obesity. One op-ed on a major news site was indignant that CT scanners couldn’t accommodate a friend (some CT scanners have difficulty accommodating patients over 300 lbs). The author’s solution? Build bigger CT scanners because obesity isn’t bad at all. This new study proves it.
My answer? Back the truck up.
First of all the study doesn’t say that being overweight is good for you and that being an ideal weight is bad. What the study does tell us is that people who have a BMI of 35 or greater are more likely to die. This is not new information. A BMI of 35 is a lot of extra weight, depending on your height it could easily mean 70 extra pounds or more. 15% of Americans have a BMI of 35 or greater. Only people with a BMI over 35, way over 35, need bigger CT scanners. I’m not saying that severely obese people shouldn’t have access to imaging studies, but the answer to the epidemic of severe obesity is not to claim vindication based on the inaccurate interpretation of one study and simply build bigger equipment.
What about the lower risk of death in the overweight and class 1 obesity groups compared with the normal BMI group? Well, this can be explained by a variety of factors:
- The wrong control group. Many researchers question whether the control group should really be a BMI of 22-24.9, not the wider range of 18.5-24.9 used in this study. The reason, many people at the thinner end of the scale are thin because of illness and this obviously skews mortality statistics.
- BMI is an imperfect tool with which to predict mortality when the result isn’t one extreme (< 18.5) or the other (>34.9). This is not a new finding. BMI just looks at weight, not the proportion of weight that is muscle mass vs. fatty tissue. Many people with a normal BMI have very little muscle mass and thus are carrying around excess fat and are less healthy than their BMI suggests. There are better metrics to look at mortality risk for people who have a BMI in the 18.5-34.9 range, such as waist circumference, resting heart rate, fasting glucose, leptin levels, and even DXA scans (just to name a few). The problem is that not all these measurement tools are practical on a large-scale.
- A small amount of fat may provide an extra energy reserve for someone who becomes chronically ill, thus skewing the survival stats. For example, consider the dramatic weight loss associated with chemo…if you can’t eat due to extreme nausea and you have a little extra fat then you burn fat, but if you have no fat and can’t eat then you start breaking down muscle. This is a phenomenon has popped up in a few studies and definitely requires more research, because obesity is definitely associated with worse outcomes in many cancers.
- Not all fat is created equal. Belly fat, the metabolically active muffin top, is what contributes to diabetes and other inflammatory conditions. Having a few extra pounds around the middle is far worse than having a few extra pounds on the hips. Again, not new information. BMI doesn’t distinguish between belly fat and thigh fat.
What is very important is that we don’t take erroneous messages from this study (hello, health reporters for major news outlets looking for attention-grabbing headlines). This study says nothing more than we need better tools than BMI to assess mortality risk for people who have a body mass index between 18.5 and 34.9 and that BMI doesn’t predict “ideal weight,” it only tells us that extremes are bad. This study also confirms that the 15% of Americans with a BMI of 35 are at increased risk of dying prematurely, a point sadly missed by many.
Body mass index simply doesn’t convey enough information to assess mortality risk for 85% of the population, but that fact (which isn’t new) shouldn’t stop each and every one of us from striving everyday to be the healthiest that we can be.
And with that, I’m off for my run.