In this large prospective study involving black women, the risk of death from any cause among women who had never smoked was lowest among women with a BMI of 20.0 to 24.9, with an increased risk of death for all categories of BMI in the overweight and obesity range. A larger waist circumference was associated with an increase in the risk of death from any cause only among nonobese nonsmokers. For both BMI and waist circumference, the positive association with the risk of death was stronger for deaths from cardiovascular disease than for deaths from cancer or other causes. Weaker associations were observed among current and former smokers than among women who had never smoked, even after adjustment for pack-years of smoking.
The results of the present study are similar to those of a pooled analysis involving white men and women, in which the risk of death from any cause was lowest for a BMI of 20.0 to 24.9 among subjects who had never smoked and all categories of overweight and obesity were associated with an increased risk of death from any cause.7
The hazard ratios are also similar in magnitude. Previous studies involving black women have shown weaker associations of BMI with the risk of death and a higher nadir of risk, but some of these studies were small, had limited follow-up, or did not present results separately for women who had never smoked and had no history of cancer or cardiovascular disease.2,3,9-15
In a pooled analysis involving Asians, the risk of death was lowest for a BMI of 22.6 to 27.5 among East Asians who had never smoked, with an increased risk of death below and above this range, whereas among South Asians, the risk of death did not increase with increasing BMI.8
For a given BMI, Asians are reported to have a higher percentage of body fat and blacks a lower percentage of body fat than whites.23
Despite these differences, our findings in blacks for the optimal BMI range with respect to the risk of death are generally consistent with those in both whites and East Asians.7,8
Waist circumference has been positively associated with the risk of death independently of BMI in several large cohort studies that mostly included white participants.4,16-18
In two of these studies, the association was stronger among women with a lower BMI than among heavier women,4,16
but in two other studies, the association was similar across categories of BMI.17,18
In the present study, a larger waist circumference was associated with an increased risk of death only among nonobese women. In the three previous studies that have reported on waist circumference and the risk of death among blacks, associations were weaker than those in other racial or ethnic groups18
or were absent.11,13
We observed increases in the risk of death among women with a low BMI or a small waist circumference. The stronger associations during the first few years of follow-up suggest that the increases may reflect, at least in part, illness-related weight loss.
A higher BMI and a larger waist circumference were each strongly associated with an increased risk of death from cardiovascular conditions. Only a very high BMI was significantly associated with an increased risk of death from other causes, and we observed no significant associations for the risk of death from cancer. Previous studies that comprised mostly white participants showed increases in the risk of death from both cardiovascular conditions and cancer in association with a high BMI2,7
or a large waist circumference,4,16,17
with stronger associations for death from cardiovascular causes. Our study may have had insufficient power to detect associations with the risk of death from cancer.
We found that the positive association between BMI and the risk of death was stronger among women with more years of education than among those with fewer years of education, but there was not a significant interaction according to educational level. These results are compatible with those of the Cancer Prevention Study I, which showed a significant positive association between BMI and the risk of death among black women with at least a high-school education and no significant association among those with less education.12
Other studies involving blacks have not reported results according to level of education. An association between body size and risk of death may be less evident among less-educated persons because they have higher absolute rates of death24
and because factors associated with low socioeconomic status (e.g., psychosocial stress and limited access to care) may play a greater role.25,26
In the Black Women’s Health Study, 97% of participants had at least a high-school education, as compared with 83% of the general population of black women of the same ages.27
Thus, we were able to informatively assess body size and risk of death among women with educational levels achieved by most black women but not among those with less than a high-school education. Previous studies of BMI and risk of death involving blacks have included a greater proportion of less-educated participants. This difference may account in part for the stronger relationship in the present study than that previously reported among blacks.2,3,9-13
Strengths of our study include its large size, the detailed data on potential confounders, and the long duration of follow-up. Restricting the primary analyses to participants who had never smoked and had no history of cancer or cardiovascular disease at baseline reduced the potential for confounding by smoking and preexisting illness. A limitation of the study was the use of self-reported measures of body size. However, data from a validation study in our cohort indicated a high correlation between self-reported and measured anthropometric variables.20,21
Whether these results are generalizable to men is unknown. In the pooled analyses of BMI and risk of death involving whites7
the results were similar for men and women. The largest studies that have reported results according to race included insufficient numbers of blacks to establish whether the associations of obesity with the risk of death are similar in black men and women.2,3,18
In conclusion, overweight as well as obesity was associated with an increased risk of death from any cause among black women who had never smoked. In addition, a large waist circumference was associated with an increased risk of death among women who were not obese.