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

Discussion

This study has demonstrated that, for people who have survived to the age of 70, mortality risk is lowest in those with a BMI classified as overweight according to the WHO. People who were classified as normal weight according to their BMI had a higher risk of death than the overweight group. This association remained for the common categories of mortality in the Australian population, including cardiovascular disease and cancer. Even after removing the effects of early mortality, those who were overweight were still at lowest risk, a finding consistent with the observation that weight loss in older age groups is associated with greater mortality.[15]

These results further recent work of a preformed meta-analysis of 32 previous studies over 30 years.[10] The results suggested that people aged 65 and older who were classified as overweight had mortality similar to that of those who were classified as normal. This meta-analysis included individuals aged 65 and older, but those who reach the age of 70, as in the current study, may exhibit a more-pronounced selection effect, having survived an additional 5 years. The current study has shown that older people with a BMI within the normal weight range may have higher mortality than those who are overweight. Reasons for these differences are uncertain, but lower mortality in those who are overweight has also been observed in older people with chronic conditions. In a meta-analysis of patients with existing coronary artery disease, overweight and obesity were associated with better survival, potentially because of the importance of metabolic and nutritional reserves for sick older people.[16] It may also be possible that there is less association between being overweight and mortality with time, as evidenced by a recent report of successive National Health and Nutrition Examination Surveys.[17]

In this study, sex did not alter the relationship between BMI and mortality. Although women experienced lower mortality for all categories of BMI than men, the relative effect of BMI within each sex was similar. The effect on mortality of being sedentary was different for men and women. The protective effect of participating in any exercise was much greater in women than in men. Although previous work[18] has found protective effects for men and women, the differences in effect between men and women that the were found in the current are difficult to explain. Whether this is due to a discrepancy between ability and performance in this age group is unknown. For example, women who are sedentary might be unable to exercise because of ill health, whereas men who are sedentary might be capable of exercising but choose not to do so.

One of the major problems in observational studies of this type is reverse causality (i.e., older people who become unwell for any reason often lose weight before death). This study attempted to mitigate this by contrasting subjects who were relatively healthy with those who had major chronic diseases or smoked. The analyses were also performed conditional on 1-, 2-, and 3-year survival. These further analyses did not reveal any apparent differences in the relationship between BMI and mortality. Other studies have estimated the association between BMI and mortality after adjusting for the effects of conditions such as hypertension, diabetes mellitus, and hypercholesterolaemia, but this practice has been criticized because they are sequelae of obesity and hence in the causal pathway from obesity to mortality. Controlling for the biomedical variables removes some of the effect of being overweight and leads to biased estimates of the mortality risk associated with BMI.[19]

The current study has some limitations. Height and weight were collected only once, at study entry, and there were no data pertaining to weight fluctuations (gain or loss) before or during the follow-up period, whether such changes in weight were intentional or unintentional (e.g., due to underlying disease), and whether they had occurred recently or over a prolonged period. A second limitation was the use of BMI, which although well accepted as a surrogate measure of body fat, is also known to be imperfect. Contrary to its design assumption, BMI is age- and sex-dependent.[20] It is also insensitive to changes in body fat distribution, which commonly occur with aging as bone mineral density and fat-free mass (e.g., muscle) diminish while fat mass increases.[21] Additionally, only self-reported height and weight were available for the men and women, and self-report and measured BMI have been shown to differ. In adult populations (≥18), the overall trend with self-reported data has been to underestimate weight and overestimate height, thus underestimating BMI[22] and biasing the results to underestimate any inverse association between being overweight and mortality. Although correction equations for self-reported BMI have been proposed, there is no consensus on the most-appropriate method.[23–25] Arguably, large waist circumference, which is an equally safe, inexpensive, and practical measure of obesity in large samples, would have been a better measure of visceral and total fat, but these data were not available for the ALSWH cohort.

The results from this study come from cohorts in which mortality was lower than observed in the general population from which they were recruited, as is observed frequently in cohorts of older people.[26] This suggests that, in these cohorts, a principal reason for nonresponse was ill health, and the results need to be interpreted with this in mind. The relationship between BMI and mortality described may not apply to older people who are frail and at risk of death.

A greater risk was found for extreme obesity. A recent report has also described such a U-shaped pattern[27] but also described a lower mortality risk for overweight nondisabled adults aged 75 to 84. However, mortality risk must be balanced by the potential loss of physical function associated with obesity.[28]

In conclusion, these results add further credence to claims that the WHO BMI thresholds for overweight and obese are overly restrictive for older people. Overweight older people are not at greater mortality risk, and there is little evidence that dieting in this age group confers any benefit; these findings are consistent with the hypothesis that weight loss is harmful.

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