In older men, mortality was related to body composition, whereas in women, no corresponding relationship could be revealed except in WHR. The insufficient number of mortality events in women (242 deaths in men vs 78 deaths in women) may account for the absence of relationship. That remains to be elucidated by future studies with longer follow-up period.
Our observation suggests that overweight is favorable for survival in older men. The 4th BMI quintile (24.16–25.91), which had the lowest mortality, actually corresponded to the overweight to obesity category according to the World Health Organization Asia-Pacific cutoff (overweight BMI > 23 and obesity BMI > 25) (33
). The beneficial effect of high BMI for survival was only slightly attenuated at the highest quintile (5th vs 4th quintile: 18.4 vs 17.8 per 1,000 person-years and statistically insignificant, p
.866). On the other side, the mortality in the lowest weight quintile (BMI < 21.01) was 1.96 times that of the 4th BMI quintile (). Our results are consistent with several previous reports in other populations, which concluded that underweight was hazardous and overweight advantageous (7
Our cohort, although of a fair size, did not include too many very obese participants: the highest BMI quintile was 25.91 and higher. It is uncertain whether the inclusion of more obese participants might alter the shape of the mortality curve. Nevertheless, sufficient number of our participants already belonged to the obesity range according to Asia-Pacific cutoff for obesity; hence, this mortality pattern could be considered representative of the Chinese population (34
The DXA measurements in the present study offered direct assessment of body fat mass and enabled scrutiny of the relationship between mortality and total body adiposity. The analysis of BFI, however, resulted in a pattern similar to that of BMI, with the higher adiposity being protective rather than hazardous. In fact, the reduction in HR for every quintile increment was nearly identical among BMI (HR = 0.85, p < .005), BFI (HR = 0.86, p < .005), and BMMI (HR = 0.86, p < .005), which may suggest that overweight is advantageous for survival through increasing adiposity and muscle mass.
Nevertheless, the apparent U-shaped relationship between BMI and mortality has been explained by the opposing effect of fat and muscle mass, with increasing obesity being harmful but increasing muscle mass beneficial for survival (15
). Following this hypothesis, we have examined the independent effect of fat and muscle mass on mortality by placing them in the same multivariate model (Model 2, ). Although a higher muscle mass, as expressed by the BMMI favored survival (), it became insignificant after adjusting for BFI, which represented body fatness (Model 2, ). Yet high body fat (BFI) persisted to favor survival after adjustment for muscle mass (BMMI), which was in contrary to previous works (28
), which reported that increasing adiposity was hazardous. Our data, however, suggested that increasing adiposity, independent of muscle mass, might be beneficial in older adults, at least in the range of adiposity that we examined. We therefore have to postulate that adiposity, and not muscle mass, favors survival in older men and that we did not observe any opposing effect of muscle loss and fat gain on mortality, as both favored survival in the same direction ().
Reaffirming the unfavorable effect of central obesity, we have also observed that the mortality in the highest WHR quintile (WHR > 0.98) was 1.58 times that of the middle quintile (WHR, 0.92–0.94), which had the lowest mortality. Such WHR values, nevertheless, were already considered central obesity. Again, similar to general adiposity and overweight, older men were likely to be more resistive to the hazard of central obesity or might even benefit from mild-grade central obesity.
The other measure of central obesity, RTF, on the other hand, demonstrated a very robust J-shaped relationship with mortality (). The central accumulation of fat appeared again to be advantageous for survival until it reached the 4th quintile, after which it became disadvantageous. The lowest mortality RTF quintile corresponded to a mean WHR of 0.94. Such WHR again is considered central obesity, reinforcing the notion that older men may benefit from mild-grade central obesity.
Based on the favorable effect of general adiposity on mortality that we had found, we postulated that the total amount of fat (general adiposity) and the distribution of fat (central adiposity) might have opposing effects on mortality, with the former being favorable and the latter unfavorable. On examining the relative importance of truncal versus general adiposity in relation to mortality, we found that the protective effect of general adiposity (BFI) ceased and the J-shaped relationship between truncal adiposity and mortality remained robust (Model 1, ). This analysis suggested that it was the distribution of adiposity rather than total adiposity that determined mortality and that survival would be affected adversely by having either too little or too much central fat. It is uncertain how this observation contributes to the U-shaped relationship between BMI and mortality.
We have used RTF and WHR separately to represent central obesity. RTF, being a more sophisticated measurement, was able to discriminate mortality difference among all quintiles, whereas WHR, the other parameter of central obesity, could only demonstrate excess risk in the highest quintile in men ( and Model 3, ). Albeit this finding should not underscore the validity of WHR, which is a simpler and more readily available measurement.
Our study has several limitations. The participants were healthy community dwellers who were free from any disability, and the relationship between body composition and mortality may differ in the disabled or frail (13
). Our results, therefore, cannot be generalized to the disabled or institutionalized populations. DXA-measured adiposity, although is reliable, may not relate to mortality as closely as computerized tomography-measured or DXA-derived visceral adiposity. The adiposity mortality curve could be more accurately described if the latter two modalities were used. Lastly, the effect of central adiposity may vary across different ethnic groups. Our participants were all ethnic Chinese, and as there may be ethnic differences in fat distribution, our observations may not be extrapolated to the Western population.