A Second Look at the Link Between Obesity and Mortality

A controversial study published earlier this year in the Journal of the American Medical Association shows that overweight people have significantly lower mortality risk than normal weight individuals, and slightly obese people have the same mortality risk as normal weight individuals.

This meta-analysis, headed by statistician Katherine Flegal, Ph.D., at the National Center for Health Statistics, looked at almost 100 studies that included 3 million people and over 270,000 deaths. They concluded that while overweight and slightly obese appears protective against early mortality, those with a body mass index (BMI) over 35 have a clear increase in risk of early death. The conclusions of this meta-analysis are consistent with other observations of lower mortality among overweight and moderately obese patients.

Many public health practitioners are concerned with the ways these findings are being presented to the public. Virginia Hughes in Nature explains “some public-health experts fear…that people could take that message as a general endorsement of weight gain.” Health practitioners are understandably in disagreement how best to translate these findings into policy, bringing up the utility of BMI in assessing risk in the first place.

Walter Willett, chair of the nutrition department at the Harvard School of Public Health, told National Public Radio that “this study is really a pile of rubbish, and no one should waste their time reading it.” He argues that weight and BMI remain only one measure of health risk, and that practitioners need to look at the individual’s habits and lifestyle taken as a whole.

In the same JAMA issue, authors Steven Heymsfield and William Cefalu, both physicians, also argue that “not all patients classified as being overweight or having grade 1 obesity, particularly those with chronic diseases, can be assumed to require weight loss treatment. Establishing BMI is only the first step toward a more comprehensive risk evaluation.”

BMI is a comprehensive tool that measures height-to-weight ratio and was originally devised in 1832 by Adolphe Quetelet. Insurance companies, who performed the first large-scale studies on the correlation between health and death, found that overweight individuals tended to die earlier than those of normal weight, and used BMI as their metric.

Subsequent studies have shown as an individual’s BMI goes up, so does the risk of developing type 2 diabetes, hypertension and heart disease. In 1972, Ancel Keys published the “Indices of Relative Weight and Obesity,” and BMI began to be used as the best predictor for health by public health agencies and practitioners. In 1998, the NIH categorized BMI arbitrarily to the easy-to-round numbers that we are familiar with today.

While the advantage of using BMI is that it is cheap and easy, the downside is that it fails to distinguish between lean and fatty body mass. Many studies depend on self-reported height and weight, yet people tend to overestimate their height, and underestimate their weight, giving them a lower BMI than they actually have. So while BMI is publicity friendly and something we are all generally familiar with, it is an imprecise measurement of health.

Taking a waist circumference may be a better way than BMI to assess health risk (waist-to-hip ratios can also be used, although waste circumference is easier to measure and interpret, making it a better choice). Abdominal obesity, or having an “apple shaped” body, is associated with a higher risk of death from heart disease even at normal BMI’s. In fact, multiple studies show that waist circumference is a more effective predictor than BMI of risk of death from heart disease and type 2 diabetes.

Since the publication of the JAMA article, the American Medical Association has released a resolution defining obesity as a disease. In this declaration, the AMA discusses the merits of using BMI along with waist circumference as a metric for diagnoses (H-440.866). It remains unclear, however, how many practitioners are even familiar with what a waist circumference is or how to measure it correctly.

Epidemiologists are hesitant to jump to the conclusion that being overweight or slightly obese may be “good” for you. There may be explanations for the protective effect that do not have to do with weight at all. For example, as Flegal and her co-authors point out, heavier individuals may present with illness earlier and seek out care earlier, leading to better outcomes. They also may have a greater likelihood of receiving optimal medical care by conscientious practitioners who have learned to be hyper-vigilant about connections between weight and health.

There are also explanations that fat does have a protective metabolic effect in older individuals, which may bestow a benefit of more metabolic reserves. Researchers also argue that the weight-death relationship found in Flegal’s study may be confounded by smoking and sickness, where smokers and those who are chronically ill tend to be thinner but die earlier. These issues are addressed in the JAMA article, but some researchers are unsatisfied to the extent of analysis.

Although death was the main metric in this study, it is important to remember that morbidities such as diabetes and cardiovascular disease continue to have strong evidence to support a correlation with overweight. This makes clarifying messages to the public even more challenging. In an editorial in Nature, the controversy surrounding this argument boils down to how to effectively communicate these studies to the public.

The editors maintain that “the political mantra on public-health advice is clear: don’t send mixed messages.” How best to present these studies to an American public where 40% of men and 30% of women are overweight and costs associated with obesity are $150 billion dollars annually, remains a major public health challenge.

Lua Wilkinson, MA, RD, CDN is a PhD student in Nutrition at Cornell University, where she studies digital social networks in the promotion of maternal and child nutrition globally. Her work has appeared in The Atlantic, Tea Leaf Nation, The China Beat, Savage Minds, and Asia Healthcare Blog.