Counteracting many of the resolutions surely made earlier this month was the release of a study showing that being overweight is linked to greater longevity. This study was actually a meta-analysis of previous studies. While meta-analysis may seem to be a great way to reach a conclusion (how can compiling the results of many other studies not be beneficial?) this is not necessarily the case. There are a number of concerns regarding meta-analyses. Flaws with this particular meta-analysis may include the inclusion of studies with very small sample sizes, potentially heterogeneous results (if the results of the studies were quite different, drawing a conclusion based on these studies is inappropriate), there may also be publication bias (if studies with conflicting results are not being published then they can’t be included in the meta-analysis).
Beyond the problems with using meta-analysis, there are additional concerns regarding the present study and the resulting news reports. Despite the study clearly demonstrating that all classes of obesity were associated with “significantly higher all-cause mortality“, reports tend to gloss over this fact. Even if the finding that lower risk of mortality is associated with overweight, there is still higher risk associated with obesity. We also know that correlation does not equal causation. There may be another common factor causing increased longevity in overweight individuals.
While there has long been an associated found between underweight and mortality causation is difficult to argue. Thus, the current study may merely be observing the fact that when people are ill they lose weight and are more likely to die.
Classification by BMI, as was done in this study, is also not particularly useful. BMI is not an accurate representation of health. It would be interesting to learn what the results would have been had the studies utilized the Edmonton Obesity Staging System.
Further to the last point, this study does not take into account health-span. That is, just because a person lives longer does not mean that they are necessarily healthy for those extra years – if it is even years, the study simply found that during the study period, those who were overweight were 6% less likely to die than were “normal” weight people this percentage is so small that it may even be due to chance. We know that overweight and obesity are associated with increased risk of a number of health problems, such as, hypertension, diabetes, heart disease, gout, and some types of cancer.
Meta-analyses like these can’t tell you what’s best for you as an individual. Everyone is going to be at their happiest and healthiest at different weights. It’s up to you to decide what that means for you.