In a recent online article in Nature, science writer Virginia Hughes explores the ongoing controversy about the so-called obesity paradox — the finding from a growing number of studies that being overweight (but not obese) is sometimes associated with a longer rather than a shortened life.
She opens the article with a description of last January’s headline-grabbing meta-analysis in the Journal of the American Medical Association (JAMA). It reported that people who were “overweight” (generally defined as a body mass index, or BMI, of 25 to 29.9) were 6 percent less likely to die than those of “normal” weight (a BMI of 18.5 to 24.9) over a similar time period.
Among people who were obese (BMI of 30 or higher), the study found an 18 percent increased risk of death. Most of that risk, however, was for people with BMIs at or above 35.
These findings spurred a quick and angry backlash from other researchers, particularly Dr. Walter Willet, a professor of epidemiology and nutrition at Harvard University, who has long argued that only a “normal” BMI lowers the risk of death — at least, among people who don’t smoke. In an interview with National Public Radio, Willet angrily called the JAMA study “a pile of rubbish.” He claimed that its authors, most notably Katherine Flegal, an epidemiologist at the National Center for Health Statistics, had not controlled for weight loss caused by sickness and smoking, as well as other factors.
Flegal defended her data and countered with her own criticisms of Willet’s research, including the fact that much of it relies on people self-reporting their height and weight, a data-collecting method that isn’t completely trustworthy. (Most people tend to underestimate what they weigh.)
Support for the paradox
Many researchers see merit in Flegal’s overall finding, largely because it appears to be supported by other recent studies. Writes Hughes:
One hint [of that support] lies in the growing number of studies over the past decade showing that in people with serious illnesses such as heart disease, emphysema and type 2 diabetes, those who are overweight have the lowest death rates. A common explanation is that people who are overweight have more energy reserves to fight off illness. They are like contestants on the television show Survivor, says Gregg Fonarow, a cardiologist at the University of California, Los Angeles: “Those that started off pretty thin often don’t come out successful.”
Metabolic reserves could also be important as people age. “Survival is a balance of risks,” says Stefan Anker, a cardiology researcher at Charité Medical University in Berlin. “If you are young and healthy, then obesity, which causes problems in 15 or 20 years, is relevant,” he says. With age, though, the balance may tip in favour of extra weight.
Genetic and metabolic factors may also be at play. Last year, Mercedes Carnethon, a preventive-medicine researcher at Northwestern University in Chicago, Illinois, reported that adults who develop type 2 diabetes while they are of normal weight are twice as likely to die over a given period as those who are overweight or obese. Carnethon says that the trend is probably driven by a subset of people who are thin yet ‘metabolically obese’: they have high levels of insulin and triglycerides in their blood, which puts them at a higher risk for developing diabetes and heart disease.
All this suggests that BMI is a crude measure for evaluating the health of individuals. Some researchers contend that what really matters is the distribution of fat tissue on the body, with excess abdominal fat being most dangerous; others say that cardiovascular fitness predicts mortality regardless of BMI or abdominal fat.
Many shades of grey
The issue of the “obesity paradox” is, of course, far from resolved. But the controversy surrounding it is a good example of how health issues almost always turn out to be much more complicated and nuanced than we originally believed (hoped?) them to be. And scientists and clinicians are just as likely as the rest of us to want health messages to be unambiguous.
“Scientists like to believe that they can operate in shades of grey,” write the editors of Nature in an editorial that accompanies Hughes’ article. “But simple messages and themes are seductive. In the public-health and nutrition fields, the idea that excess weight might benefit health is called the ‘obesity paradox’ despite the fact that it does not focus on obesity, which everyone agrees is bad, and so is not a paradox. The ‘mildly overweight paradox,’ presumably, sends mixed messages. When Willett dismissed the Flegal study as a ‘pile of rubbish’ there were no shades of grey evident.”
‘The yellow brick road to truth’
So what should the public make of this now very public debate? “If the obesity-paradox studies are correct, the issue then becomes how to convey their nuances,” writes Hughes. “A lot of excess weight, in the form of obesity, is clearly bad for health, and most young people are better off keeping trim. But that may change as they age and develop illnesses.”
In other words, the issue is an evolving one, and much, much more complicated than just maintaining the “right” BMI.
And trying to disparage and dismiss findings that point to a more complicated message about weight and health due to a fear that the public might get confused and misinterpret the message is, well, unhelpful.
“One study may not necessarily tell you the truth, but a bulk of studies saying the same thing and being consistent, that really is reinforcing,” Dr. Samel Klein, an obesity expert at Washington University in St. Louis, Mo., told Hughes. “We need to follow the data just like the yellow brick road, to the truth.”