Is Being Overweight Healthy?
You might have heard about the meta-analysis released January 2 in the Journal of the American Medical Association, Association of All-Cause Mortality with Overweight and Obesity (1), which found that “overweight” people, based on body mass index (BMI) had a lower risk of mortality than what is considered a healthy BMI.
Before I had a chance to read the study myself, I came across a USA Today article, Experts weigh in on the dangers of extra pounds, quoting the well-respected nutritionist, Dr. Walter Willett from Harvard School of Public Health, about the JAMA study:
“The most serious problem in the paper is that the normal-weight group included a mix of lean and active people, heavy smokers, patients with cancer (and) other conditions that cause weight loss, and frail elderly people who had lost weight due to rapidly declining health. Because the overweight and obese groups were compared to this mix of healthy and ill persons who have a very high risk of death, this led to the false conclusions that being overweight is beneficial and that grade 1 (moderate) obesity carries no extra risk. The new statistics are completely misleading for anyone interested in knowing about their optimal weight. … The paper is a pile of rubbish.”
Yikes! That’s quite a rip on the JAMA study’s authors. Not only that, but why would a research group put so much effort into a meta-analysis but fail to adjust for some of these obvious confounders, and why would JAMA publish it?! But Willett’s criticisms would explain how they could have reached such a counter-intuitive finding and all would still be good in the nutrition universe.
I read the study fully intending to find the criticisms by Dr. Willett to be valid. Alas, I did not. From my reading of the paper, they did adjust for smoking and age, and analyzed the study for any bias due to “frail elderly people.” They did some testing to make sure previous heart disease and cancer were not affecting the results.
The control groups for the studies in the meta-analyses were typically people with a BMI of from 18.5 to < 25 or from 20 to < 25. The study found that people with a BMI of 25 to < 30 had a statistically significant 8% reduced risk of mortality (.92, .88-.96). People with a BMI of 30 to < 35 had essentially the same risk of mortality. It wasn’t until you got to the group of people with a BMI of 35 or greater that risk of mortality significantly increased.
So, what could be going on here? Well, a BMI of 18.5 to 20 has often been considered to be unhealthfully thin, but from what I gleaned from the paper’s discussion, excluding such people did not appreciably affect the results.
BMI doesn’t account for muscle mass, as Willett alludes to above, but it is hard to believe that there were enough muscle-bound people in the meta-analysis to confound the results to any significant degree.
Willett went on to say in the interview:
“In the last several years, two other major analyses, involving the collaborative efforts of more than 150 scientists, have been conducted on the relation of body weight to mortality…these studies showed clearly that both overweight and all grades of obesity are associated with increased mortality.”
In contrast, the JAMA authors mention that their results are consistent with two previous meta-analyses. And they give some reasons why being overweight might be associated with lower risk of mortality:
“Possible explanations have included earlier presentation of heavier patients, greater likelihood of receiving optimal medical treatment, cardioprotective metabolic effects of increased body fat, and benefits of higher metabolic reserves.”
Occam’s razor would dictate that the best explanation is simply that it’s healthier to be “overweight,” and I don’t like doing backwards somersaults to wish away findings that, had they gone the intuitive way, would be unquestioned as solid. That said, given the prevalence of type 2 diabetes and the fact that vegans have been shown to have a much lower rate of type 2 diabetes and much lower average BMIs, I’m not going to try to gain weight. But it’s a little demoralizing to have one of the most basic ideas in nutrition ̵ that being overweight is not better than being a normal weight ̵ significantly questioned.
In conclusion: Rubbish? Not from what I can tell. Food for thought? Yes. As is often the case, this isn’t the last word.
Flegal KM, Kit BK, Orpana H, Graubard BI. Association of all-cause mortality
with overweight and obesity using standard body mass index categories: a
systematic review and meta-analysis. JAMA. 2013 Jan 2;309(1):71-82. |