Our 23-year follow-up study suggests that all-cause mortality in older adults is increased among persons who are underweight or obese during old age and among persons who were overweight or obese at age 21 years. Both weight loss between age 21 and later life (regardless of weight at age 21) and being underweight at age 21 but not gaining weight later in life were associated with increased mortality. Conversely, being of normal weight at age 21 and gaining weight by late adulthood was associated with decreased all-cause mortality.
To maintain the population-based characteristic of our sample, we included in our analyses some participants who were younger than age 65 years at entry. Although, by our doing this, some participants did not fall into the elderly category, 97 percent of the person-years in our study were for persons aged 65 years or older. In addition, results for analyses excluding persons under age 65 years at entry (not shown) were essentially unchanged.
The shape of the association between BMI and mortality in the elderly remains controversial (1
). Most studies have found the association to be U-shaped (13
) or J-shaped (15
) in all or a subset of the subjects under study. However, other studies have found a positive linear association (7
), a negative linear association (26
), or no association (25
The variations in the shape of the association between BMI and all-cause mortality among studies may be due to differences in the age composition of the cohorts, the length of follow-up, and the variables that were adjusted for in the analyses, as well as inclusion or exclusion of smokers or early deaths. Two studies with long follow-up in the elderly have found a reverse J-shaped association (2
). The results of these studies agree with our finding that persons in the lowest weight category, regardless of age, adjustment for other confounders, or exclusion of early deaths, have the highest mortality.
The reason for increased mortality among the underweight elderly is not clear. Being lean may represent a real risk because of nutrient deficiency, frailness, and reduced functional status (23
). Persons with underlying disease and increased mortality are more likely to be underweight. In our study, however, the highest mortality was observed among the underweight even after exclusion of the first 5 years of follow-up. In some studies, increased mortality with low BMI has been seen only among smokers or the very old (15
). We observed increased mortality in the underweight regardless of smoking status or age. Persons in the underweight category in this study may represent a mix of those who chose to be lean and those who were underweight because of illness (35
). It may also be a mix, especially in the elderly, of persons with low muscle mass and persons with low fat mass (30
). Adjustments for recent weight changes and other measures of assessing body composition are important in order to pinpoint the reasons for increased mortality among the underweight elderly. Nonetheless, our results are fairly robust in showing that underweight elderly have higher mortality than normal-weight elderly.
While the authors of a recent meta-analysis of the association between adiposity and all-cause mortality emphasized the importance of adjusting for physical activity (5
), the summary relative risk for an elevated BMI from the studies that included physical activity as a covariate (RR = 1.23) was almost identical to the risk from the studies that did not (RR = 1.24). Consistent with this finding, our results were similar regardless of whether or not we adjusted for physical activities.
It has been suggested that adjusting for factors such as hypertension and diabetes in regression analyses represents overadjustment and produces biased estimates (6
), since these factors are biologic intermediates on the causal pathway between obesity and mortality rather than confounders. Adjustment for the specified medical conditions changed our results little.
The strategy of excluding early follow-up has been used in many studies to account for the potential effect of underlying disease. Some simulation studies have indicated that exclusion of the first few years of follow-up is not an effective way of dealing with confounding due to occult disease and that it may, in some circumstances, exacerbate this confounding (36
). However, investigators from another simulation study concluded that under certain assumptions this method could be very effective (39
). Nonetheless, although our results showed some changes in the magnitude of the relative risk estimates with exclusion of the first 5 years of follow-up, the main conclusions were unchanged.
The influence of age on the relation between BMI and mortality has not been widely studied, especially in the elderly. The few available studies suggest that the association between increased weight and higher mortality is primarily relevant for the younger elderly. A study with over 12 years of follow-up (23
) found that greater BMI was associated with higher mortality only up to 75 years of age. Beyond age 75, increased BMI was associated with increased mortality only among men, and above age 85, low BMI was associated with higher mortality only among women. Another study with 14 years of follow-up found an association between high BMI and increased mortality at all ages, while low BMI was associated with mortality only among persons aged 75 years or more (15
). In contrast, for all age groups in our study, the highest mortality was among underweight participants. Obese participants had increased mortality only among persons aged less than 75 years.
Many studies deal with the effect of smoking on the BMI-mortality relation by including it as a covariate in the analyses, while other studies analyze only nonsmokers. The authors of one report concluded that the association between BMI and all-cause mortality is present in both smokers and nonsmokers but the effects of underweight, overweight, and obesity may differ depending on smoking status (5
). Another study found a positive linear association between BMI and mortality among never smokers but no association among recent former smokers or current smokers (35
). We found increased mortality among the underweight regardless of smoking status, while mortality was increased in obese never and past smokers but not in current smokers. It is possible that the harmful effect of obesity is overshadowed by the harmful effect of smoking, so that obese smokers do not show higher mortality when compared with normal-weight smokers.
Although some studies have found differences in the BMI-mortality association for men and women (20
), our risk estimates for men and women were almost identical. The only difference was found when early follow-up was excluded, resulting in greater attenuation of the relative risk for underweight men than for underweight women.
Few studies have reported on the effects of being overweight or obese in early adulthood on all-cause mortality in later life. The available findings are consistent with our results showing increased mortality in the overweight or obese (29
). Since our cohort was 73 years of age at entry, on average, most were aged 21 years during the 1920s and 1930s. The prevalence of overweight and obesity has greatly increased since then, especially in the past few decades (42
). Even though the proportion of persons who were obese at age 21 in our study was smaller than the proportion of obese 21-year-olds today, we have no reason to believe that the observed associations should not apply to those who are aged 21 now. However, since our study included only people who survived to enter the retirement community, the effect of weight at age 21 on mortality in our study is limited to its effect in old age and does not reflect its effect on mortality between age 21 and age at entry.
While many studies of the association between weight change and all-cause mortality have been conducted, the results are difficult to compare among studies because of differences in the time period between the two weight measures, the length of follow-up, and how weight change is expressed. Increased mortality has been associated with weight loss in most studies of weight change (19
), but in other studies it has been associated with weight gain (24
), both weight loss and extreme weight gain (35
), or neither (59
). In contrast to our findings, the few studies most similar to ours in terms of weight change over a long time period and follow-up periods of more than 20 years have mostly shown increased mortality with weight gain (24
). The differences between those results and ours might be explained by the older age of our participants.
The benefit of gaining weight throughout life was reported in 1985 with the publication of tables of ideal weight (61
), where the lowest mortality occurred at progressively increasing body weight with advancing age (5 kg per decade). Although our most consistent finding was that weight loss was associated with increased mortality, we also saw evidence of benefit with weight gain among persons who were underweight or of normal weight at age 21. The statistical significance of these findings, however, depended on whether or not adjustment was made for other confounders and on whether early follow-up was excluded.
When interpreting our results regarding weight change, it is important to note that we were able to evaluate only weight change between age 21 years and the age at which the questionnaire was completed (an average period of 52 years). We were unable to determine when the weight change occurred or to distinguish between gradual weight change and rapid weight change, fluctuating weight and relatively stable weight, and intentional weight change and unintentional weight change. All of these factors may be important in analysis of the effect of weight change on mortality (12
Other limitations included the use of self-reported data on height and weight and recall of weight at age 21 years. The validity and reliability of such data are controversial. Although we previously found relatively good agreement (>90 percent) in this population between self-reported height and weight and height and weight obtained from medical records (8
), some studies suggest that bias in reporting of height and weight varies with weight, such that overweight and obese persons are more likely to underreport their weight than normal- or underweight persons (63
). This under-reporting would result in misclassification of participants when using standard weight categories. A strategy for dealing with this potential misclassification is to analyze ranked data such as quintiles. Although we did not report those results, we obtained very similar findings when the data were analyzed in quintiles: increased mortality among persons in the lowest and highest quintiles of BMI, with the highest mortality being seen among persons in the lowest quintile.
Despite its limitations, our large, population-based, long-term study of older adults with almost complete follow-up provides important information on the relations of weight and weight change to mortality in the elderly. We were able to adjust for several important confounding variables and to explore the modifying effects of smoking status, age, and gender. Our study highlights the risks on mortality in older age of being overweight or obese in young adulthood and underweight or obese in later life.