Cancer study estimates BMI and mortality risk

by Gail Marchessault, RD, PHEc, PhD (Cand)
 
 

   Another new large study of weight and mortality confirms that the lowest mortality appears to be at moderate body weights at all ages for both men and women, but findings by race are labeled "enigmatic" in an accompanying editorial.1
   Results come from the Cancer Prevention Study II, a prospective US study that followed over a million people from 1982-19896. They are reported by epidemiologist Eugenia E. Calle, PhD, and her colleagues at the American Cancer Society in the October 1999 issue of the New England Journal of Medicine.2
   The results took into account many variables, including age (average age at the beginning of the study was 57), education (but not income), physical activity, alcohol use, marital status, use of estrogen supplements (in women), and crude consumption of fats and vegetables.
   Subjects were divided into four groups according to whether they had ever smoked and whether they reported a history of disease (including any current illness or weight loss of 10 lb. [4.5 kg] or more in the previous year). This subgroup of healthy non-smokers gives a better estimate of the association between body mass index (BMI) and mortality because it has removed the influence of smoking and disease. There were 201,622 deaths.
   The association between mortality and high BMI was strongest in healthy people who had never smoked (see Figure 1 [to be added at a later date]). The heaviest men, presumably with BMIs over 40, had 2.68 times the risk of men with BMIs between 23.5 and 24.9 (the reference category). The heaviest women had 1.89 times the risk of their reference group.
   The relationship between mortality and low BMI was strongest for current and former smokers with a history of disease. This increased mortality in lean subjects was explained primarily by cerebrovascular disease, pneumonia, and diseases of the central nervous system.
   The effects of race were further explored in the healthy non-smoking subgroup.
   White men with a BMI between 23.5 and 24.9 had the lowest death rate. For white women, the death rate was lowest at a BMI of 22.0 to 23.4.
   A small but statistically significant increase in the risk of death began at BMIs of 26.5 to 27.9 for men and 25.0 to 26.4 for women. This association appears at lower weights than is reported in other studies, and may be due to the use of self-reported data. The investigators suggest BMI may be underestimated by one unit because of people's tendency to over-report height and under-report weight. There is evidence that heavier people understate body weight more than lighter people, which would result in a further overstatement of risk of mortality in these groups.3
   In black men and women, the risk of weight-related mortality was non-significant for every category except black women with BMIs under 18.5. They had a relative risk of 1.7.
   If the black sample is large enough to detect differences, and the investigators say other reports are similar, then these findings suggest either that biology varies by race, or that the correlation is due to other factors than weight. Some evidence is presented suggestive of racial differences, but there are also many potential confounding factors. For example, differences between black and white people could be due to differences in the prevalence of dieting,4 other lifestyle behaviors,5,6 societal prejudice, and standard of living,7 among others. Attempts were made to control for some, but not all of these variables.
   The association between high BMI and mortality was consistent across three age groups, but weakened with age. The investigators point out that because mortality increases with age, more older people are affected, even though their individual risk is lower. The oldest and heaviest men (over age 75, BMI >35) had a relative risk of 1.5. The risk for the leanest category (BMI <20.5) decreased slightly with age and was 1.2 for the oldest men.
   Older women had the same relative risk (1.4) whether their BMI was above 40 or below 18.5. Younger women with BMIs this low also had a relative risk of 1.4.
   Arguably, it's death at an earlier age that concerns most people, and so the youngest age group (30-64 years) is particularly important. There were only 5,317 deaths in this group. This is important in interpreting the relative risk. If the risk of death is low in the first place, then double the risk will still be low.
   The risk for the heaviest men, with BMIs over 35, increased by only 0.41 percent even though the relative risk was 2.3. It increased by 0.35 percent for the heaviest women (with BMIs over 40 and a relative risk of 2.7). The increase is low because we are talking about a small number of deaths per 100,000 person-years. (Deaths in the reference group and the heaviest group were 250 and 659 per 100,000 person-years for men, and 163 and 513 per 100,000 person-years for women.)
   It is statistically significant, but is it important? Those involved with population health will be concerned, but individual people may decide not to worry about dying young from being fat.
 

References
1.  Williamson DF. The prevention of obesity (editorial). N Engl J Med 1999;341:1140-1141.
2.  Call EE, Thun MJ, Petrelli JM, et al. Body-Mass Index and mortality in a prospective cohort of US adults. N Engl J Med 1999;341:1097-1105.
3.  Rowland ML. Self-reported weight and height. Am J Clin Nutr 1990;52:1125-33.
4.  Ernsberger P, Koletsky RJ. Biomedical rationale for a wellness approach to obesity: an alternative to a focus on weight loss. Journal of Social Issues 1999;55:221-260.
5.   Barlow CE, Kohl, III HW, Gibbons LW, et al. Physical fitness, mortality and obesity. Int J Obes 1995;19:S41-S44.
6.   Appel LJ, Moore TJ, Obarzanek E, et al. A clinical trial of the effects of dietary patterns on blood pressure. N Engl J Med 1997;336:117-1124.
7.   Krieger N, Fee E. Social class: the missing link in US health data. Int J Health Serv 1994;24:25-44.