In a recent essay in Scientific American Paul Raeburn asked a good question: “Can Fat Be Fit?” In his attempt to answer his question he referred to an article we published in 2005 about the excess mortality associated with different levels of body mass index (BMI) in the U.S. Mr. Raeburn did not try to contact any of us before he wrote his essay, and he may have misunderstood some aspects of our article. We would like to clarify some of these issues for Scientific American readers.
Why did we do this research? Not to be “critics.” Our article came out of a joint research project by scientists from the National Cancer Institute and the Centers for Disease Control and Prevention. We wanted to make improved estimates of the numbers of excess deaths associated with different BMI levels, using better methods and newer national data. Our goal was not to present a point of view, but rather to provide a factual account of our statistical results.
We did not say that excess fat carries “no health risks.” As our article explains, we looked only at mortality and not at other health measures. Nor does our article say that our work refutes the “conventional wisdom that Americans carrying excess fat are at increased risk of death from heart disease, diabetes and various kinds of cancer.” In our 2005 article, we looked at mortality from all causes combined. We did not try to separate heart disease, diabetes and cancer mortality from other causes, which amount to almost half of total mortality.
We distinguished carefully between people classified as normal weight (BMI of 18.5 to 24.9), people classified as overweight (BMI of 25 to 29.9) and people classified as obese (BMI of 30 or above). We found that the risk of death was higher among the obese than among those of normal weight. In a follow-up article that used the same data, we found that the increased risk of death among the obese was primarily due to heart disease, diabetes and certain forms of cancer. We also found, however, that overall overweight adults (BMI of 25 to 29.9) had no higher risk of dying than those of normal weight and perhaps a lower risk. Although Raeburn called this a “startling conclusion,” it is not an unusual finding. Similar results have been reported by many other large studies.
For example, Al Snih and colleagues studied over 8,000 adults over age 65 from the Established Populations for the Epidemiologic Studies of the Elderly cohort and found the lowest mortality among the overweight and modestly obese groups. Dolan and colleagues studied over 8,000 women ages 65 from the Study of Osteoporotic Fractures and similarly found the lowest mortality rates among the modestly overweight; they reported that these findings were not attributable to smoking or measures of preexisting illness. Janssen reported results for almost 5,000 older adults from the Cardiovascular Health Study who were followed for up to 9 years and found that mortality in the overweight group was 11% lower than in those of normal weight. McGee combined data for over 350,000 people from 26 studies and once again found slightly lower mortality among the overweight than among those of normal weight. In a 7-year follow-up of more than 90,000 women who participated in the Womens Health Initiative Observational Study, McTigue and colleagues found no association between overweight and mortality. Farrell and colleagues studied over 9,000 women followed for 11 years and found no increased mortality in the overweight.
In a large national sample in China of almost 170,000 adults who were followed for up to 9 years, Gu and colleagues found lower mortality in the overweight category than in the normal weight category; their results were not affected by extensive exclusions for smoking and pre-existing illness. In Finland, Haapenen-Niemi and colleagues followed over 2,000 men and women ages 35-63 for 16 years. The researchers found the lowest all-cause mortality rates among the overweight men and women. Laara’s study of 12,000 Finnish women who were followed for 22 years found that “… moderately overweight women (BMI of 25 to < 29) had a consistently lower mortality than women of normal weight.” Another study of almost 50,000 Finns followed for an average of 17 years by Hu and colleagues also found no increased mortality among overweight men and women. Arndt and colleagues followed almost 20,000 German construction workers for 10 years and found the lowest mortality in the overweight category. Among almost 5,000 middle-aged European men from the Seven Countries Study who were followed for 25 years Visscher and colleagues found that “a BMI of 25-30 was not related to increased mortality.”