Body Mass Index and Survival in Men and Women Aged 70 to 75

Leon Flicker, PhD; Kieran A. McCaul, PhD; Graeme J. Hankey, MD; Konrad Jamrozik, PhD; Wendy J. Brown, PhD; Julie E. Byles, PhD; Osvaldo P. Almeida, PhD

Disclosures

J Am Geriatr Soc. 2010;58(2):234-241. 

In This Article

Results

For the 4,931 men and 5,042 women in this study, height or weight was missing for 254 men (5.2%) and 479 women (9.5%), and these were excluded from further analysis, leaving 4,677 men and 4,563 women aged 70 to 75 and resident in metropolitan areas. There was no major difference in the mean age of the two groups (women 72.1; men 72.3), but there were minor differences in the age distribution, mainly due to a paucity of 75-year-old women and a less-apparent excess of 70-year-old men. Characteristics of the men and women at baseline are shown in Table 1. At baseline, more than 80% of men reported being married, compared with only 54% of women, whereas almost 35% of women reported being widowed, compared with only 7.6% of men. More men (15.1%) than women (5.1%) reported some level of tertiary education. Women were less likely than men to report being sedentary (18.1% vs 24.9%). With regard to medical history, there were no striking differences between men and women apart from the higher proportion of men (24.1%) than women (18.3%) reporting bronchitis or emphysema. More than 60% of women reported having never smoked, compared with only 26.4% of men, and many more men (63.1%) than women (30.7%) reported being former smokers. Men (41.4%) were more likely than women (11.5) to report levels of alcohol consumption that exceeded National Health and Medical Research Council guidelines and less likely to report no alcohol consumption (men 6.0%; women 32.7%). Based on the WHO classification of BMI, 50.3% of women were classified as normal weight (BMI 18.5–24.9), compared with 43.5% of men; 44.5% of men were classified as overweight (BMI 25.0–29.9), compared with 33.5% of women. For the covariates listed in Table 1, the extent of missing data was minimal in the men and generally between 1% and 2% in the women.

The men were observed for a total of 37,896.7 person-years (mean 8.1 years) and the women for a total of 43,816.9 person-years (mean 9.6 years). Over this time, 1,369 deaths occurred in the men and 939 in the women; of these, there were 776 deaths in men and 489 in women from cardiovascular disease, 608 deaths in men and 298 in women from cancer, and 189 deaths in men and 88 in women from stroke.

Relationship between BMI and All-cause Mortality

Figure 1 shows the relative risk estimated for all-cause mortality according to BMI at baseline. For men and women, mortality risk was lowest in those who were classified as overweight according to BMI. The minimum mortality risk was found at a BMI of 26.6 kg/m2 (95% CI=25.7–27.5) in men and 26.26 kg/m2 (95% CI=25.5–26.9) in women. For men and women with BMIs that were classified as normal, the risk of death increased as BMI decreased such that the estimated risk of death for men and women at the lower end of the normal range was almost double the risk in those who were overweight. This estimated risk was similar to that observed in obese men and women (BMI≥35.0).

Figure 1.

Hazard ratios of all-cause mortality according to body mass index (BMI) in men and women aged 70 to 75 (lines are 95% confidence intervals).

Categorizing subjects into healthy (n=2,716) or nonhealthy (n=6,094) (see Methods section) did not substantially alter the pattern of association between BMI and mortality (Figure 2). Although being nonhealthy increased the risk of all-cause mortality, there was no evidence of a major difference in the overall shape of the four curves after fitting the main effects and their interactions (sex × BMI; healthy × BMI; sex × healthy, and sex × healthy × BMI).

Figure 2.

Hazard ratios of all-cause mortality according to body mass index (BMI) in healthy and nonhealthy men and women aged 70 to 75.

Examining the relationships with the covariates listed in Table 1, the only interaction effect detected was between sex and being sedentary, and only smoking was found to have a moderate confounding effect. Being sedentary increased the risk of mortality in men by 28% (HR=1.28, 95% CI=1.14–1.44) but doubled the risk in women (HR=2.08, 95% CI=1.79–2.41). Subjects who were underweight had a greater mortality risk (HR=1.76, 95% CI=1.39–2.22) than those who were normal weight, whereas those who were overweight had a lower risk (HR=0.87, 95% CI=0.78–0.94). The risk in those who were obese was little different from those who were normal weight (HR=0.98, 95% CI=0.85–1.11).

The results from this model, unadjusted and adjusted for smoking, are shown in Table 2. All risk ratios are relative to women with normal BMI who were not classified as sedentary. Adjusting for smoking attenuated all HR estimates but more so in those estimated for sedentary men. The lowest risks of all-cause mortality were observed in nonsedentary women; in every BMI category, risk estimates were lower for nonsedentary than for sedentary women. Similarly, nonsedentary men had lower risks of all-cause mortality than sedentary men in all BMI categories. In contrast, across all BMI categories, there was little difference in all-cause mortality risks between sedentary women and nonsedentary men (chi-square=2.50, 1 degree of freedom, P=.11). Nevertheless, although being sedentary had clear effects on mortality risk for men or women who were sedentary or not, the lowest risk of mortality was consistently seen in those classified as overweight.

The relationship between BMI and risk of dying from cardiovascular disease, cancer, or chronic respiratory disease is shown in Figure 3. The lowest risks for men and women were again those with a BMI in the overweight range of the WHO classification.

Figure 3.

Hazard ratios of cause-specific mortality (cardiovascular, cancer, and chronic respiratory) according to body mass index (BMI) in men and women aged 70 to 75.

A series of sensitivity analyses were conducted. For the spline models, the number and location of the knots used were varied, but this did not alter the finding that the minimum mortality risks were observed in those who were classified as overweight. The analyses were also repeated to see whether weight loss due to latent disease, which in turn could carry significant risk of early mortality, affected the results. The analysis were restricted to participants who were alive 1 year after recruitment, but no evidence was found of any alteration in the relationship between BMI and survival, nor was any evidence found when the analysis was restricted to those alive 2 years after recruitment or 3 years after; the lowest risk of mortality remained in those classified as overweight.

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