Can You Be Too Thin?

It’s certainly possible to be dangerously thin. Individuals with eating disorders such as anorexia nervosa and bulimia—and those with wasting diseases such as cancer, AIDS, and heart failure—can lose so much weight that they don’t have enough energy or basic building blocks to keep themselves alive. What about people who are thin but don’t have an eating disorder?

Some studies suggest that the connection between body mass index and premature death follows a U-shaped curve. This would mean that weighing too much—or too little—isn’t as healthy as some middle weight.

The main problem with this idea is that most of these studies included smokers and individuals with early but as-yet undetected chronic and fatal diseases. Cigarette smokers as a group weigh less than nonsmokers, in part because smoking deadens the appetite. Potentially deadly chronic diseases such as cancer, emphysema, kidney failure, and heart failure can cause weight loss even before they cause symptoms and have been diagnosed. So low weights don’t necessarily cause early death. Instead, low weight is often the result of illnesses or habits that may be fatal.

One way to untangle this chicken-and-egg problem in epidemiologic studies is to look only at nonsmokers and to ignore deaths in the first few years of follow-up. One such effort comes from the American Cancer Society, which fielded two large, long-term studies. Its first and second Cancer Prevention Studies included more than one million adults who were followed for at least 12 years. (1, 2) They excluded smokers and those with a history of cancer or cardiovascular disease at baseline, or who developed a chronic disease in the first few years of follow-up. Both studies showed a clear pattern of increasing mortality with increasing weight.

These data confirm similar observations from a 27-year follow-up of more than 19,000 middle-aged men in the Harvard Alumni Health Study, (3) a 16-year follow-up of 115,000 middle-aged women in the Nurses’ Health Study, (4) a 12-year follow-up of nonsmoking Seventh-day Adventists, (5) and a 10-year follow-up of more than 500,000 middle-aged men and women in the National Institutes of Health/AARP study. (6) A more recent analysis of the Cancer Prevention Study cohort found that increased weight was tied to increased mortality from all cancers, and specific cancers;(7) in fact, study investigators estimate that overweight and obesity is responsible for 14 percent of all cancer deaths in women and 20 percent of all cancer deaths in men.

According to the current Dietary Guidelines for Americans a body mass index below 18.5 falls outside the healthy range. But some people manage to live long, healthy lives with a low body mass index. Here’s a good rule of thumb: If you’ve always had a low body mass index and your weight doesn’t change, don’t worry. But if you start losing weight and aren’t dieting or trying to lose weight, schedule a visit with your doctor to figure out why this is happening.

References

1.Calle EE, Thun MJ, Petrelli JM, Rodriguez C, Heath CW, Jr. Body mass index and mortality in a prospective cohort of U.S. adults. N Engl J Med. 1999; 341:1097–105.

2.Stevens J, Cai J, Pamuk ER, Williamson DF, Thun MJ, Wood JL. The effect of age on the association between body mass index and mortality. N Engl J Med. 1998; 338:1–7.

3.Lee IM, Manson JE, Hennekens CH, Paffenbarger RS, Jr. Body weight and mortality. A 27–year follow–up of middle–aged men. JAMA. 1993; 270:2823–28.

4.Manson JE, Willett WC, Stampfer MJ, et al. Body weight and mortality among women. N Engl J Med. 1995; 333:677–85.

5.Singh PN, Lindsted KD, Fraser GE. Body weight and mortality among adults who never smoked. Am J Epidemiol. 1999; 150:1152–64.

6.Adams KF, Schatzkin A, Harris TB, et al. Overweight, obesity, and mortality in a large prospective cohort of persons 50 to 71 years old. N Engl J Med. 2006; 355:763–78.

7.Calle EE, Rodriguez C, Walker–Thurmond K, Thun MJ. Overweight, obesity, and mortality from cancer in a prospectively studied cohort of U.S. adults. N Engl J Med. 2003; 348:1625–38.

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