The Age-Related Decline in Resting Energy Expenditure in Humans Is Due to the Loss of Fat-Free Mass and to Alterations in Its Metabolically Active Components

Abstract

There is conflicting evidence as to whether the age-related decline in resting energy expenditure (REE) can be attributed to i) absolute changes in fat-free mass (FFM), ii) alterations in the composition of FFM or iii) decreasing organ metabolic rates. This study directly addressed the first and second hypotheses by quantification of metabolically active components of FFM assuming constant tissue respiration rates to calculate REE (REEc). REE was measured (REEm) in 26 young (13 females, 13 males, age 22-31 y) and 26 elderly subjects (15 females, 11 males, age 60-82 y) by indirect calorimetry and detailed body composition analysis was obtained using bioelectrical impedance analysis (BIA), dual energy X-ray absorptiometry (DXA), and MRI. Specific organ metabolic rates were taken from the literature. REEm adjusted for differences in FFM was lower in older subjects than in younger control subjects (5.43 +/- 0.61 MJ/d compared with 6.37 +/- 0.48 MJ/d; P < 0.001). Skeletal muscle mass plus liver mass accounted for 86% and 48% of the variance in REE in young and elderly subjects, respectively. The difference between REEm and REEc was 0.03 +/- 0.40 MJ/d and -0.36 +/- 0.70 MJ/d in young and elderly subjects, respectively. In the elderly 58% of the difference in variance was attributed to heart mass. REEm - REEc was -1.40 +/- 0.44 MJ/d in subjects with hypertensive cardiac hypertrophy, i.e., heart mass > 500 g, suggesting a decrease in heart metabolic rate with increasing heart mass. Excluding five elderly subjects with cardiac hypertrophy resulted in agreement between REEm and REEc in the elderly (-0.10 +/- 0.48 MJ/d). We concluded that the age-related decline in REE is attributed to a reduction in FFM as well as in proportional changes in its metabolically active components. There is no evidence for a decreasing organ metabolic rate in healthy aging.

Lower resting metabolic rate in the elderly may not be entirely due to changes in body composition

“…This correspondence of the results is even more remarkable in that by contrast to the study of Gallagher et al (2000) organ masses were not measured in our study but derived from the regression equations developed by Garby et al (1993). Bosy-Westphal et al (2003) also measured RMR and body composition including several organ masses in 26 young (13 females, 13 males) and 26 elderly subjects (15 females, 11 males) and specific organ metabolic rates were taken from the literature. RMR adjusted for differences in FFM according to Ravussin and Bogardus (1989) was significantly lower in elderly relative to young subjects and differences between measured and calculated RMR were 30 and À360 kJ/day in young and elderly subjects, respectively.…”

“…This was discussed in some recent studies (Gallagher et al, 2000;Bosy-Westphal et al, 2003). In our second approach, we therefore considered the detailed composition of the FFM and investigated whether age-related differences in the proportions of the miscellaneous organ masses on FFM could account for the lower RMR in elderly subjects.…”

“…These data from Gallagher et al (2000) suggest that the decline of RMR with advancing age cannot be explained totally by changes in body composition. Bosy-Westphal et al (2003) measured RMR in 26 young and 26 elderly subjects and also obtained detailed body composition analysis. In contrast to the results from Gallagher et al (2000), in this study no differences between measured and calculated RMR were reported in young and elderly subjects, respectively.…”

Greater than predicted decrease in resting energy expenditure with age: cross-sectional and longitudinal evidence

“…On the other hand, when FFM was quantified by the total body potassium technique, the age related decline in REE was no longer apparent. Hence, the decrease in body cell mass with age could be responsible for the age-related decline in REE, independently of changes in FFM (Bosy-Westphal et al, 2003). Since it has been previously demonstrated that body cell mass can vary with age, adiposity and sex, independently of FFM (Roubenoff, 2000;Roubenoff and VA, 2000), it is possible that a decreased body cell mass could account for our findings.…”

“…The decline in age-related changes in FFM composition, especially in relative masses of metabolic active organs like heart, liver, kidneys or brain and of metabolic less active tissues like muscle, bone or skin could also be responsible for the decline in REE during aging as it was discussed in some recent studies (Gallagher et al, 2000;BosyWestphal et al, 2003;Krems et al, 2005) and could also help to explain the differences between measured and predicted values (Gallagher et al, 2000;Roubenoff, 2000;Roubenoff and VA, 2000;Bosy-Westphal et al, 2003;Krems et al, 2005). A lower than predicted REE in aging individuals is not due to changes in physical activity participation.…”