The study objective was to evaluate relationships between mortality risk and well-defined measures of adiposity, fat distribution, and fitness in older adults. In age, gender, and examination year adjusted analyses both BMI and WC were associated with mortality risk; but percent body fat and FFM were not related to mortality. The association between total mortality with WC persisted after adjusting for baseline differences in age, gender, smoking, abnormal exercise ECG responses, and health status. Further adjustment for fitness eliminated the significant mortality risk associated with abdominal obesity. A J-shaped relationship between BMI and mortality remained significant after considering the influences of several covariables, including fitness. Fitness had a strong inverse association with mortality, and this pattern of results was changed little by adjustments for adiposity or fat distribution. Thus our primary finding is that both fitness and BMI were strong and independent predictors of all-cause mortality in adults ages 60 years or older. Other adiposity measures either did not predict mortality (percent body fat, FFM), or did not do so in models adjusted for competing risk predictors (WC).
We previously demonstrated that lower levels of fitness are strongly associated with higher risk of all-cause and CVD mortality in younger and middle-aged men with various levels of health status.12,13,39,40
The analogous relationship is clear within adiposity subgroups.12,13
Our findings from the current study are consistent with these earlier ACLS results, and expand them to the older segment of the cohort. Especially notable is our finding () that higher levels of fitness are inversely related to all-cause mortality in both normal weight and overweight BMI subgroups, in those with a normal WC and in those with abdominal obesity, and in those who have normal percent body fat and those who have excessive percent body fat. However, obese (BMI ≥30 kg/m2
) unfit individuals were at no higher risk for mortality when compared with obese fit individuals. The obese I and II group had relatively small sample sizes and fewer deaths; therefore, these results must be confirmed in larger studies. In addition, we observed that fit individuals who were obese (such as BMI, 30.0-34.9, or abdominal obesity, or excessive percent body fat) had a lower risk of all-cause mortality than did unfit, normal weight or lean individuals. Our data therefore suggest that fitness levels in older people influence the association of obesity to mortality.
There have been controversial results concerning the relationship between mortality and obesity in older adults. Some studies,18,23,25
but not all,19,21
have suggested lower mortality risk in heavier individuals. We found a J-shaped association between mortality and BMI computed from measured height and weight. The age, sex, and examination year adjusted mortality rate per 1000 person-years was the lowest in the overweight group, and the highest in the class II obesity group (). However, the multivariate (without fitness) adjusted model showed a non significant association (HR, 0.87; 95% CI, 0.70-1.07) with overweight compared with normal weight persons (, column 1). The fully adjusted model (including fitness) attenuated the quadratic trend (, column 2). Our findings are consistent with the report from Gale et al,26
who also found no evidence of increased mortality risk in mildly to moderately overweight women and men aged 65 and over after adjusting for self-reported physical activity. Further joint analysis () showed that in fit individuals the mortality risk was not significantly different across the 4 BMI categories, while in unfit individuals the mortality risk was J-shaped, with lower risk in those with BMI 25.0-34.9 and higher risk in those with BMI 18.5-24.9 and 35.0 or greater. These results support the hypothesis that moderate and higher fitness levels favorably influence mortality risk across categories of body composition. Normal weight individuals in our study had greater longevity only if they were physically fit; furthermore, obese individuals who were fit did not have elevated mortality. The quadratic trend across BMI in the unfit individuals deserves further comment. In general, unfit individuals were inactive and had low aerobic power at baseline, whereas fit individuals were highly active and had high aerobic power. In the elderly, BMI is also a marker of other factors such as fitness and muscle mass; therefore, maintaining BMI at older age is an overall marker of health.17
This may be attributable to competing causes of mortality that become important factors with increasing age. It may also reflect selection factors that have allowed survival to older age.
In older populations, abdominal obesity, assessed by WC,22-24
has been a better mortality predictor than BMI. Other indicators of adiposity, such as body fat, also have been examined for mortality associations.26,27
However, the independent association between body fat and mortality in the older population has not been adequately demonstrated.14,27
Researchers speculate that the controversial association between adiposity and mortality in older people may be attributable to selective survival, cohort effects, or unadjusted confounding.41
We found that BMI or WC, but not percent body fat, predicted overall mortality in adults at least 60 years old. From a practical perspective, these findings suggest that more complicated and expensive body fat measurement does not provide an advantage in assessing mortality risk over more readily available and less expensive obesity measures such as BMI or WC. These findings also suggest that total adiposity per se may not be the factor that increases mortality risk among the elder. Rather, fat distribution and some other factor intrinsic to BMI (e.g., frame size) may underlay mortality risk in older adults. Further investigation of effects of various measures of adiposity on mortality in other elderly populations, and on the potential role of confounding and modifying variables, would contribute usefully to this research area.
Our results also support the hypothesis that higher levels of fitness can reduce the risk of premature death,12,33,42,43
and expand the evidence supporting this relationship in obese older persons. In a prospective cohort of 18 750 male and 37 417 female Chinese ages 65 or older, Schooling et al.17
recently reported that both physical activity and higher BMI were strongly associated with lower mortality in a dose-response manner. Our earlier ACLS report in older persons also demonstrated that lower fitness, an objective measure of functional capacity that is related to recent physical activity habits, is associated with higher risk of all-cause mortality.37
However, neither our earlier report nor that by Schooling et al. assessed the joint associations of physical activity, BMI, and outcomes. In the current study, we found that fitness is a strong predictor of overall death among older adults, independent of body composition and other mortality risk factors. Additional studies are needed that concurrently evaluate the joint association among objective measures of fitness or activity, body size and fatness, and longevity in the rapidly growing older population.
Growing evidence suggests that skeletal muscle function (e.g., strength, power, endurance) may contribute to improved physical functioning and longevity through biological pathways that are related to but independent of aerobic fitness.44,45
FFM was not a significant predictor of mortality risk in the present study. However, it is possible that the quality of FFM (e.g., functional phenotype) rather than the absolute amount of FFM is the key factor in determining health risk. We were unable to include in the present study an objective measure of muscle function in order to examine its independent and joint relationship with adiposity, fitness, and mortality risk. More data are needed to further explore the role of muscle function in successful aging and enhanced longevity among older adults.
Our study had several strengths. We used standardized and objective measurements of fitness and adiposity and examined their associations with mortality, providing quantitative risk estimates and a lower likelihood of misclassification on the exposure variables. We are unaware of any other report in which these data are available. The extensive baseline physical examination permitted systematic evaluation of the presence or absence of baseline medical conditions. The relatively long follow-up (average 12 years) was sufficient to accrue enough fatal endpoints to allow for assessing the joint association among risk factors and mortality within adiposity strata.
Limitations of the current study include a focus on participants who were primarily white and well-educated, with middle to upper socioeconomic status. The results may not apply to other groups of older adults. However, the homogeneity of our sample strengthens the internal validity of our findings by reducing potential confounding by unmeasured factors related to socioeconomic status, such as income, education, or prestige. Residual confounding from undetected subclinical disease at baseline may exist, although it seems unlikely that it would explain all of the observed association between fitness, adiposity, and mortality, especially given the extensive medical examination performed at baseline. The primary results were not changed meaningfully when deaths in the first 2 years of follow-up were excluded. We did not have adequate information about diet or medication use to study these factors. Physical activity was assessed by self-report on the standardized medical history/health habits questionnaire, and is likely to be a crude approximation of actual physical activity habits. We focused primarily on all-cause mortality because of the relatively small number of cause-specific deaths, which prevented us from stratifying cause-specific analyses by adiposity measures. However, we did some exploratory analyses on associations between fitness, BMI, WC, percent body fat, or FFM, and cause-specific mortality (data not shown). In the current study, CVD and cancer accounted for 74% of total deaths. After adjusting for age, sex, examination year, and current smoking, fitness was significantly associated with CVD, CHD, and cancer mortality (Linear Ptrend
<.05, for each). All adiposity exposures were associated with cancer mortality (P
<.05, for each). Future studies should include these important exposures, and extend the analysis to these and other specific causes of death with particular interest to public health, such as CVD, CHD, stroke, diabetes mellitus, and cancer. Due to a limited sample of women, who contributed relatively few deaths to the analysis, we combined women and men for analyses, and adjusted the analyses for sex. In our previous reports on fitness in which we have been able to perform parallel analyses in men and women, results have generally been similar for women and men.34,46
We only have a single baseline assessment on fitness, adiposity measurements, and other exposures, thus we can not examine whether changes in any of these variables occurred during follow-up, and whether this may have influenced the study results.
In conclusion, our prospective findings in adults ages 60 and over show that low fitness predicted higher all-cause mortality risk after adjustment for potential confounding factors including adiposity. Fit individuals had greater longevity than unfit individuals regardless of their body composition or fat distribution. Our data provide further evidence regarding the complex long-term relationship among fitness, body size, and survival. We may be able to reduce all-cause death rates among older adults, including those who are obese, by promoting regular physical activity, such as brisk walking for 30 minutes or more on most days of the week (about 8-kcal/kg per week), which will keep most individuals out of the low fitness category.43
Enhancing functional capacity also should allow older adults to achieve a healthy lifestyle and to enjoy longer life in better health.