OBJECTIVE: Whole body protein turnover (PTO) and resting energy expenditure (REE) are both correlated to fat-free mass (FFM), in young and elderly subjects, and REE is positively correlated to PTO in young adults. Thus, the aim of this study was to compare the energetic cost of PTO in young (n=39, 23.4±3.1 y) and elderly (n=41, 67.5±3.6 y) healthy volunteers.
MEASUREMENTS: REE (indirect calorimetry), PTO (13C-leucine isotopic dilution) and body composition (bioelectrical impedance analysis with age-specific equations) were measured in the postabsorptive state.
RESULTS: Elderly subjects had a higher fatness (30.5±7.1 vs 18.2±5.5%, elderly vs young, P<0.001), a similar REE (0.97±0.13 vs 1.06±0.15 kcal min−1), and a lower PTO (1.28±0.22 vs 1.44±0.18 μmol kg−1 min−1, P<0.001). PTO, REE and FFM were significantly correlated and after adjustment for FFM, REE was positively correlated to PTO (r=0.61, P<0.001). The slope of this relationship was the same in both groups, while the adjusted mean REE was lower in elderly subjects (0.97±0.09 vs 1.05±0.07 kcal min−1, P<0.01).
CONCLUSION: In comparison with young subjects, the energetic cost associated with PTO in elderly subjects is not different, but the proportion of REE not associated with PTO is lower.
Protein synthesis and protein breakdown involved in the constant remodelling processes of body proteins are biochemical pathways consuming energy. Therefore, protein turnover (PTO) contributes significantly to energy requirements in animals and humans.1,2 The correlation between PTO rate and energy expenditure (EE) has already been demonstrated in young adults.3 During catabolic disorders, the stimulation of PTO is often associated with an increased resting energy expenditure (REE),4,5,6,7,8 although it does not appear to be the case in cancer patients.9 However both PTO10 and EE11,12 are strongly correlated with fat-free mass (FFM). Thus, the question arises about the true relationship between PTO and EE, independent of FFM. Welle and Nair3 have shown that in young adults, after adjusting for whole body potassium, leucine flux was still significantly correlated with resting metabolic rate. This relationship has never been examined in elderly subjects, despite changes in the body composition of this population.
In the elderly, and in absolute values, PTO,13,14,15 REE (reviewed by Benedek et al16) and FFM17,18 are reduced. However, after accounting for differences in body composition, PTO appears to be similar in young and elderly volunteers in most cases.10,15 By contrast, an age reduction in REE independent of the changes in FFM has been reported,19,20,21 but is still debated, some studies finding no differences in REE after correction for FFM.22,23,24 These observations might suggest that the energetic cost associated with PTO (the level of REE for a given rate of PTO independently of FFM) is reduced in elderly as compared to young subjects. Therefore, the aim of the present study was to compare the relationship between PTO and REE in young and healthy elderly subjects after adjustment for differences in body composition.
Materials and methods
Thirty-nine young (Y; 14 women, 25 men; 23.4±3.1 y) and 41 elderly (E; 17 women, 24 men; 67.5±3.6 y) volunteers were included in the study. Each subject had a normal physical examination without known history of renal, cardiovascular, endocrine or digestive disease. All subjects maintained their usual physical activity and body weight. Characteristics of the subjects are presented in Table 1. The purpose and the risks of the study were fully explained, and written informed consent was obtained from each participant before the experiment.
L-[1−13C]-leucine (99 mole percent excess, MPE) was purchased from CIL (Andover, MA). A check on isotopic and chemical purity of labelled leucine was made by gas chromatography mass spectrometry (GCMS). Solutions of tracers were tested for sterility and pyrogenicity before use. They were prepared in sterile apyrogen saline, and the tracers were filtered through 0.22 μm filter membranes.
Protein turnover studies
At the beginning of the experiment, after an overnight fast, a venous catheter was retrogradly inserted into a dorsal vein of the hand for arterialized blood sampling after introduction of the hand in a 70° heated ventilated box. Another catheter was inserted into a vein of the controlateral arm for tracer infusions. After a prime dose (4.2 μmol kg−1) a continuous (0.07 μmol kg−1 min−1) infusion of L-[1−13C] leucine was administered and continued for 3 h. Blood samples were taken prior to any infusion and at 20 min intervals during the last hour of the plateau, ie from 120 to 180 min. After centrifugation, plasma supernatant was separated, an internal standard added, and the sample kept at −20°C until further analysis.
Resting energy expenditure
Gas exchanges were measured by indirect calorimetry (Deltatrac, Datex, Geneva, Switzerland) during the last 45 min of tracer infusion. REE was calculated with equations derived by Ferrannini.25
Body weight was measured in underwear to±0.1 kg on a SECA scale (SECA, France). Height was measured bare-foot with a SECA microtoise. FFM and fat mass were estimated by bioelectrical impedance analysis. After a 30 min rest, resistance to a 50 kHz, 800 μA alternative current was measured in volunteers through surface electrodes as described elsewhere.26 Resistance was converted to total body weight (TBW) with age specific equations which have been validated against dilution of labelled water (Vaché et al26 for elderly subjects and Segal et al27 for young). An hydration factor of 0.732 was used to calculate FFM from TBW as this factor is independent of age.28
Analytical methods and flux calculations
Plasma [13C]-leucine enrichments were measured by selected ion monitoring electron impact GCMS (Hewlett-Packard 5971A, Palo Alto, CA) using tertiary-butyldimethylsilyl derivatives as previously described.15 Leucine kinetics were calculated in steady-state conditions as follows.15 Briefly, whole body leucine flux (in μmol kg−1 min−1) was:
Leu flux=F[13C] Leu([13C] Leu MPE×0.01)−F [13C] Leu}
where F[13C]Leu is the [13C] leucine infusion rate (μmol kg−1 min−1) corrected for isotopic purity, and [13C]Leu MPE is the plasma [13C] leucine enrichment.
Finally, if the assumption is made that leucine constitutes 8% of the mean body protein by weight, 1 μmol min−1 of leucine flux corresponds to 2.36 g day−1 of PTO.3
Results are expressed as mean±s.d. Amino acid kinetics were compared between the two groups by a one-way analysis of variance, the age being the category. The significance was accepted at the 5% level, and simple correlations were sought with the least square method. Adjustment for FFM was performed according to the techniques described by Ravussin and Bogardus.11 Regression lines were compared between young and elderly subjects using an analysis of covariance.29 Computations were performed with Statview 4.0 statistical package (Abacus Concept, CA).
Physical characteristics of the volunteers are displayed in Table 1, and show that body weight, body mass index (BMI) and percentage fat were significantly higher in elderly subjects. Table 2 displays parameters of protein and energy metabolism. Expressed in μmol kg BW−1 min−1, leucine flux and oxidation were significantly lower in elderly subjects, but in absolute value, leucine turnover rate in μmol min−1 (89.2±21.9 in elderly vs 89.4±18.0 μmol min−1 in young volunteers, P=NS) and REE in kcal min−1 did not differ significantly.
As shown in Figure 1, leucine flux (in μmol min−1) was significantly correlated with FFM (r=0.82, for the whole population, P<0.001), and with REE (r=0.83, P<0.001) in both groups. REE was also significantly correlated with FFM (r=0.81, P<0.001).
By analysis of covariance, the slope of the relationship between leucine flux and FFM did not differ between young and elderly subjects (Figure 1, P=NS), nor did the adjusted mean leucine fluxes (89.2±12.4 in elderly vs 89.4±9.9 μmol min−1 in young volunteers, P=NS).
After adjustement for FFM, there remained a significant correlation between adjusted REE and adjusted leucine flux (r=0.61, P<0.001). However, the adjusted mean REE was lower in elderly than in young subjects (0.97±0.09 vs 1.05±0.07 kcal min−1, P<0.01). Figure 2 shows that the slope of the relationship between leucine flux and REE was not different between young and elderly subjects (P=NS), so the energy expended in association with PTO was not different in the two groups (487±54 vs 619±86 kcal/24 h, young vs elderly, P=NS; Figure 3). However, for any given adjusted leucine flux, REE was systematically lower in elderly (P<0.001, Figure 2), so that EE not associated with PTO was significantly lower in elderly (1022±94 vs 778±130 kcal/ 24 h, young vs elderly, P<0.05; Figure 3).
Changes in body composition associated with aging, particularly the decline in FFM, have been reported to be responsible for a reduction in both REE and PTO.10,12 However, the interrelationship of energy and protein metabolism is less often studied after changes in body composition are taken into consideration.3 Thus, this work is the first study trying to determine whether the relationship between energy and protein metabolism, adjusted for FFM, is modified with aging. The main results are that, expressed in kg body weight, PTO (as reflected by leucine flux and oxidation) is reduced in healthy elderly subjects, and that after adjustment for FFM, PTO and also REE did not differ in the two groups. In many studies, PTO is normalized between populations after dividing by the FFM. When differences in FFM are observed between categories, statistical adjustment has been proposed to be better than the calculation of a ratio, which can lead to spurious conclusions.30 Therefore, the residual relationship between energy and protein metabolism was analysed, and we found a positive and statistically significant relationship between REE and PTO.
These results suggest that the relationship between REE and PTO is independent of FFM quantity in the young as well as in the elderly subjects. However, PTO was associated with a systematically lower REE in elderly individuals, as indicated in Figure 3. This difference in REE between young and elderly subjects could have been foreseen since (i) PTO corrected for FFM appears to be similar between young and elderly subjects,10,15 and (ii) REE might be reduced, even after correction for FFM, in elderly volunteers.19,20,21 This latter point is, however, a matter for debate.22,23,24
This could either mean that PTO has a lower energy cost in the elderly or that the non-protein components of REE have a lower contribution to REE in elderly subjects. The former is unlikely since PTO is not biochemically more efficient, as indicated by the identical slopes of the regression lines for the two groups. Therefore, the energetic cost of increasing PTO by 1 g protein is not different, ie 2.96±0.53 kcal in elderly and 2.36±0.58 kcal in young subjects. Changes in energetic cost of protein metabolism might have also been expected in relation with the modifications of the composition of FFM. Indeed, muscle tissue is the main contributor to the age-associated decrease in FFM.17,18 The changes in liver, brain, kidney and heart masses associated with aging are of a small magnitude.31 Therefore, adjustment per kg FFM means different tissue and organ composition in young and elderly subjects. This should not affect PTO. Indeed, although Ward and Richardson32 argue in favour of a reduced protein synthesis associated with aging animals, the evidence is poor except for specific muscle proteins33 in humans. Fu and Nair34 and Boirie et al35 showed that both albumin and fibrinogen synthesis are not changed in elderly subjects, in the postabsorptive and in the fed state. Furthermore, muscle protein synthesis and breakdown are not dramatically altered in aging rats when muscle atrophy itself is considered.36
The latter hypothesis of a reduced non-protein-associated EE in elderly subjects is suggested by the observation that, for each kcal min−1 expended at rest, the fraction of REE not attributed to PTO is lower in elderly subjects. The difference in REE might reflect a genuine difference in energy metabolism at rest. There is no convincing explanation for this decreased REE, adjusted for FFM, associated with aging apart from a possible decreased liver respiration rate37,38 and a decline in catecholamine sensitivity.39
In conclusion, PTO and REE are significantly correlated to each other and to FFM. The energetic cost associated with differences in PTO is the same in healthy young and elderly subjects. However, the proportion of REE accounted for by PTO is higher in elderly volunteers, since the energy associated with non-protein metabolism is significantly reduced.
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