A recent study released in this weeks JAMA has found an association between being moderately overweight and having longer life. I would like to believe that being mildly overweight conveys a mortality advantage over being skinny because so many Americans (including me until 2 years ago!) fall into that category but consider me in the group of skeptics. From reading the study it seems to me that this study has a major flaw in it's methodology that could explain these results.
This study found a relatively modest (6%) mortality benefit to being moderately overweight. As anyone who has ever worked in medical field knows often times when people are very sick they get very skinny. Many, many diseases cause profound weight loss especially at the end of the road. People who suffer from cancer, AIDS, heart disease, drug and alcohol problems, hematologic disease, renal disease, gastrointestinal disease , dementia, immunological disease and many, many medical problems can lose weight as their disease progresses. In order for this study to be meaningful the authors would have had to have provided sufficient and rigorous statistical corrections to account for this fact. Unfortunately this was not possible based on the study methods.
This study relies on data from numerous other smaller studies pooled together to make it's conclusions. Not all of the studies that the authors included in their data pool were able to correct for the presence of chronic underlying disease. Some of these studies were and some were not. And as readers we have no idea how many were and how many weren't or what if any methods were used. We are left to trust that the authors of all these individual papers sufficiently corrected for the presence of chronic disease when possible, used the right statistical methods to do so and corrected for the diseases most likely to confound the results. Obviously this is a huge area of uncertainty and a huge leap of faith for us to accept this puzzling conclusion of modest effect.
Because many of the papers did not correct at all we can be sure that at least some skinny, very sick people with chronic disease found their way into the study. Was this enough to explain all of the results? The truth is we may never know. To me it seems very plausible that the modest association observed between moderate obesity and mortality could be at least partially explained by what effectively amounts to a confounding variable that impacts both the exposure (body weight) and the outcome (mortality rate) aka the presence of chronic disease.
On the other side of the coin if even if these findings are somewhat skewed it does raise a point I often convey to patients which is the distinction between body weight (which is based on how many calories we eat and burn) vs metabolic/cardiovascular health (which is based more on what we eat and if we exercise). Even if the mortality benefit observed is "white noise" being slightly obese is hardly a death sentence. The truth is the likely effect of being slightly overweight on death risk is somewhere around mildly harmful to none to, as this study argues, slightly beneficial. The overall take home message is that slight obesity does not neccesarily mean completely unhealthy.
While I question these findings, I think it is possible to skinny and unhealthy and that it is possible to be healthy and overweight. The real trick is to exercise, eat right and to keep proven cardiovascular risk factors under control (smoking, hypertension, cholesterol, diabetes). I still maintain that being a normal body weight is probably better than being overweight even after this study, but we can add this skewed study as a reminder that when it comes to mortality the big money is in cardiovascular risk not neccesarily weight.
