Flawed Obesity Study Minimizes Health Risks of Excess Weight
U.S. government researchers made the news in late April with two startling findings: They found that overweight people have a lower mortality risk than normal weight people, and that obesity and overweight cause far fewer U.S. deaths each year than previously thought. (1)
Newspapers around the world quickly jumped upon the story with headlines such as, "Some extra heft may be helpful," "Being fat ‘lessens risk of early death,’" and, "A bit over and worth the weight." Regrettably, much of the coverage failed to note that there are serious flaws in the government researchers’ work. In fact, this study is hardly the final word on the relationship between weight and health-and it appears to have grossly underestimated the link between being overweight and mortality risk.
Here’s what the federal researchers did: They classified U.S adults from three separate surveys based on their body-mass index-a ratio of weight to height that is used as an indirect indicator of healthy (or unhealthy) weight. The researchers grouped people into standard BMI categories corresponding to underweight (BMI below 18.5), normal-weight (BMI of 18.5 to 24.9), overweight (BMI of 25.0 to 29.9), and obese (BMI higher). And then they tracked them to determine which group was most likely to die.
Sounds simple enough. But determining the precise range of BMI associated with lowest mortality can be difficult, because the approach that researchers use to conduct their analyses can bias their findings. One such problem is a phenomenon that researchers call "reverse causation": Low body weight often results from chronic disease, rather than being a cause of chronic disease. The weight loss may have been unintentional as a result of the underlying disease process; or the weight loss may have been intentional, because patients with serious conditions often become motivated for the first time to lose weight. Regardless, because of this phenomenon, people with a BMI below 25 are a mix of healthy individuals and those who are ill and have lost weight due to their disease. Leaner people are also more likely to smoke than their heavier counterparts. If researchers fail to account for both reverse causation and the adverse effects of smoking, they will find artificially inflated mortality rates among lean people, thus diminishing the harmful impact of overweight and obesity.
There is no perfect way to sweep away these biases, but this study falls far short. The researchers did not exclude people with known chronic disease from their study. Although the researchers did conduct other analyses to address some of these concerns, such as excluding people with a history of weight loss and excluding smokers, they did not use these approaches simultaneously, probably because their dataset was too small to do so. However, larger studies have clearly shown that when these approaches are combined, and the study only includes people who are healthy at the start, the association between increasing BMI and mortality becomes substantially stronger.
It is also noteworthy that the vast majority of the deaths in this study occurred among people who were aged 70 and older at the time their BMI was measured. Weight loss due to chronic disease and loss of muscle mass is common in the elderly, making it problematic to estimate excess mortality due to overweight and obesity. For example, overweight or obesity at age 45 may cause diabetes to develop at age 55, which can lead to a heart attack at age 65, heart failure and weight loss due to incapacity and muscle wasting at age 70, and finally death at age 75. If one looks only at the relationship between BMI and mortality starting at age 70, there would appear to be a link between underweight and mortality, when in fact the earlier overweight or obesity led to the death. Thus, guidelines for healthy weights are most reliable when based on BMI measures in persons younger than age 70 or 75.
Even if one accepts the study’s estimates of obesity-related mortality, people cannot afford to become complacent about the obesity epidemic. The health impact of excess body weight is enormous and extends far beyond higher mortality. Currently, 2 in 3 U.S. adults and 1 in 6 children are overweight or obese. Obesity and overweight is a major risk factor for many chronic health conditions, including hypertension, diabetes, heart disease, stroke, and even certain types of cancer. Obesity also leads to disability, impairs quality of life, and contributes to skyrocketing healthcare costs. Americans have not yet witnessed the full consequences of the current obesity epidemic, especially of obesity starting in childhood or adolescence. Public health initiatives to promote healthy nutrition, regular physical activity, and healthy weight in childhood and throughout adulthood remain critical.
The aim of the Harvard T.H. Chan of Public Health Nutrition Source is to provide timely information on diet and nutrition for clinicians, allied health professionals, and the public. The contents of this Web site are not intended to offer personal medical advice. You should seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this Web site. The information does not mention brand names, nor does it endorse any particular products.