Don’t Believe Everything Your Read
You may have recently heard that being overweight, but not obese, actually increases your lifespan compared to people of normal weight. This is known as the "obesity paradox" because very few studies have linked obesity with longer life. This comes from a study that spread like wildfire, gracing the pages of TIME, WebMD, and NPR.
You first have to understand body mass index (BMI), a simple calculation that divides weight (kg) by height (m2). BMI is a primitive tool for assessing weight but isn’t actually useful. For example, those with excess abdominal fat and a normal BMI are actually at increased risk for heart disease. On the other hand, athletes with a lot of muscle mass may have an elevated BMI and be labeled as “overweight”. In actuality, they are healthy and fit. Some “overweight” clients often say they feel quite healthy and do not feel the need to lose weight until their BMI suggested otherwise.
Flegal and colleagues (2013) synthesized 97 studies (providing a sample size of more than 2.8 million people and 270,000 deaths) and found that being overweight (BMI between 25 and 30) is associated with significantly decreased mortality compared to being normal weight (BMI between 18.5 and 25). These results were backed by a relatively small number of studies that came before it (McGee, 2005; Janssen & Mark, 2007).
But there are serious problems with the study design which invalidate its conclusions. Among its many issues, the aforementioned Flegal study fails to separate the effects of disease on weight from the effect of weight on risk of disease (i.e. there’s a difference between people who were normal weight for decades from people who were previously heavy and lost weight after developing a disease).
Clinically, we know that obesity (all grades of BMI greater than 30) has been associated with significantly higher all-cause mortality compared to normal weight BMI, but Flegal and colleagues only address BMI data sets, not body composition (visceral fat or body fat distribution), which we know is incredibly important. This is why we cannot believe everything we read, particularly with soundbite headlines. The sensationalism surrounding absurd claims ultimately end up creating more confusion than helping the health of the public.
As one more example, other researchers found that obesity was associated with lower mortality risk compared to people with lower BMIs, but only with certain health complications, like congestive heart failure (CHF). This is quite paradoxical, yet, in those with CHF, it may simply mean that obesity is an indication of better overall health status, but is not the reason for lower mortality (Kenchaiah et al., 2002). Again, if you look hard enough to find an association, you probably will find one, but it doesn’t mean that it’s a true association.
It’s quite possible that obese individuals may be "healthier" overall if they are (1) eating a cleaner, whole foods plant-based diet, (2) getting more exercise than their lower-weight peers, (3) non-smokers, (4) consuming little to no alcohol, and (5) less stressed. None of these factors are truly captured in a BMI measurement.
In sum, BMI is a basic measurement which cannot speculate one’s overall health risk. We don’t truly know enough about a person’s health until we look at what’s going on cellularly -- how the body interacts with our environment.
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Flegal, K. M., Kit, B. K., Orpana, H., & Graubard, B. I. (2013). Association of all-cause mortality with overweight and obesity using standard body mass index categories: a systematic review and meta-analysis. Jama, 309(1), 71-82.
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Kenchaiah, S., Evans, J. C., Levy, D., Wilson, P. W. F., Benjamin, E. J., Larson, M. G., . . . & Vasan, R. S. (2002). Obesity and the risk of heart failure. N Engl J Med, 347, 305–13.
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