- AA
amino acids
- EAA
essential amino acids
- mTOR
mammalian target of rapamycin
- mTORCI
mTOR complex I
- S6K1
S6 protein kinase
The preservation of muscle function is crucial for maintaining an independent lifestyle and the capacity to perform the activities of daily living in the elderly. One of the important factors in the loss of functional performance is the progressive loss of skeletal muscle mass with ageing, called ‘sarcopenia’(Reference Baumgartner, Koehler and Gallagher1–Reference Melton, Khosla and Crowson3). This apparent muscle wasting in elderly human subjects occurs at a rate of about 0·5–1·0% per year starting at about 40 years of age. Lean muscle mass contributes up to about 50% of the total body mass of young adults but can decline to 25% by 75–80 years of age(Reference Short and Nair4, Reference Short, Vittone and Bigelow5). The loss of muscle mass is most notable in the lower limb muscles, with the cross-sectional area of the vastus lateralis reduced by as much as 40% at the age of 80 years(Reference Lexell6). Sarcopenia is associated with a three- to fourfold increased likelihood of disabilities and the loss of muscle mass especially in the lower limbs is associated with an increased risk of falls and impairment in the ability to perform routine activities.
The loss of muscle mass is viewed as a largely inevitable and undesirable consequence of ageing(Reference Paddon-Jones and Rasmussen7), with muscle loss estimated to affect 30% of people older than 60 years and >50% of those older than 80 years(Reference Baumgartner, Koehler and Gallagher1). Demographic studies indicate that the world's population aged 60 years and above will triple within the next 50 years, and the subpopulation of older adults aged 80 years and above represents the fastest-growing subpopulation in the developed world(8). It is therefore not surprising that the global ageing will have a major impact on our health-care system, as the number of frail elderly requiring hospitalisation and/or institutionalisation increases. Good health is essential for maintaining independence and to continue to actively enjoy family and community life. As such, life-long health promotion is warranted to prevent or delay the onset of non-communicable and chronic (metabolic) diseases such as heart disease and stroke, cancer and diabetes. Preventing, attenuating and/or reversing the decline in skeletal muscle mass should be the main goal for interventional strategies to promote healthy ageing.
Ageing and protein turnover in skeletal muscle
The loss of skeletal muscle mass in the elderly is characterised by atrophy of type-II (fast) muscle fibres (Fig. 1(A)), fibre necrosis, fibre-type grouping and a reduction in satellite cell content in type-II muscle fibres(Reference Lexell6, Reference Kadi, Charifi and Denis9–Reference Verdijk, Koopman and Schaart14). The loss of skeletal muscle mass is accompanied by the loss of muscle strength (Fig. 1(B)), a decline in functional capacity(Reference Bassey, Fiatarone and O'Neill15–Reference Wolfson, Judge and Whipple22) and a reduction in whole-body and muscle oxidative capacity(Reference Short and Nair4, Reference Nair23, Reference Nair24). Together, these alterations at a muscle level have substantial health consequences, since they contribute to the greater risk of developing insulin resistance due to the reduced capacity for blood glucose disposal and a greater likelihood of excess lipid deposition in liver and skeletal muscle tissue leading to hyperlipidaemia, hypertension and cardiovascular co-morbidities.
The progressive muscle wasting with ageing must be due to a disruption in the regulation of skeletal muscle protein turnover, leading to a chronic imbalance between muscle protein synthesis and degradation. Although it was originally reported that healthy older adults had decreased rates of basal muscle protein synthesis(Reference Short, Vittone and Bigelow5, Reference Short, Vittone and Bigelow25–Reference Yarasheski, Zachwieja and Bier31), more recent studies have failed to reproduce these findings and generally show little or no differences in basal muscle protein synthesis rates between young and old adults(Reference Cuthbertson, Smith and Babraj32–Reference Kumar, Selby and Rankin39). These discrepancies may be due to the standardisation of prior physical activity(Reference Nair24), selection of subjects(Reference Yarasheski, Welle and Nair30) or the selection of different precursor pools to calculate muscle protein synthesis(Reference Tipton40). It seems unlikely that basal muscle protein fractional synthesis rates are diminished by 20–30% as reported previously(Reference Balagopal, Rooyackers and Adey25, Reference Hasten, Pak-Loduca and Obert26, Reference Welle, Thornton and Jozefowicz28, Reference Welle, Thornton and Statt29) and/or that muscle protein breakdown is elevated by as much as 50% in the elderly compared to younger adults(Reference Trappe, Williams and Carrithers41). Such opposing alterations in the rates of protein synthesis and breakdown would be accompanied by more rapid muscle wasting than what is typically observed (3–8% per decade(Reference Lindle, Metter and Lynch20, Reference Lynch, Metter and Lindle42)), and it therefore seems unlikely that basal muscle protein fractional synthesis rates could be diminished by 20–30% during ageing as reported previously(Reference Balagopal, Rooyackers and Adey25, Reference Hasten, Pak-Loduca and Obert26, Reference Welle, Thornton and Jozefowicz28, Reference Welle, Thornton and Statt29). The relatively slow rate of muscle loss during ageing must mean that the mismatch between the average diurnal rate of muscle protein synthesis and breakdown is small. It is currently accepted that basal fasting protein synthesis and/or breakdown rates are not (substantially) different between young and elderly human subjects(Reference Hasten, Pak-Loduca and Obert26, Reference Cuthbertson, Smith and Babraj32–Reference Kumar, Selby and Rankin39, Reference Rasmussen, Fujita and Wolfe43). To better understand the skeletal muscle wasting in the elderly, researchers have started to focus on the muscle anabolic response to anabolic stimuli such as physical activity, food intake and anabolic hormones such as insulin. It was well established that the protein turnover in skeletal muscle is highly responsive to exercise and nutrient intake in healthy young individuals(Reference Koopman, Saris and Wagenmakers44). Interestingly, data from recent studies suggest that the muscle protein synthetic response to resistance exercise(Reference Kumar, Selby and Rankin39) and following the ingestion of a small amount of amino acids (AA) with(Reference Volpi, Mittendorfer and Rasmussen36, Reference Guillet, Prod'homme and Balage45) or without carbohydrate(Reference Cuthbertson, Smith and Babraj32, Reference Katsanos, Kobayashi and Sheffield-Moore33) is reduced in the elderly when compared with young controls. The latter is believed to represent a key factor responsible for the age-related decline in skeletal muscle mass(Reference Rennie46).
Anabolic response to exercise
Exercise is a powerful stimulus to promote net muscle protein anabolism, resulting in specific metabolic and morphological adaptations in skeletal muscle. Endurance training can increase whole-body and muscle oxidative capacity and endurance(Reference Wilkinson, Phillips and Atherton47), whereas resistance exercise training can increase muscle mass and strength, and thus improve physical performance and functional capacity(Reference Evans48). It generally takes weeks to months before training-induced changes in skeletal muscle mass become apparent(Reference Rennie and Tipton49). The prolonged time course for hypertrophy is a reflection of the slow turnover rate of muscle proteins, i.e. about 1% per day for contractile proteins(Reference Balagopal, Rooyackers and Adey25, Reference Nair, Halliday and Griggs50, Reference Volpi, Ferrando and Yeckel51). Although muscle hypertrophy occurs at a slow rate, a single bout of resistance exercise can rapidly (within 2–4 h(Reference Phillips, Tipton and Aarsland52)) stimulate muscle protein synthesis, and increase protein synthesis rates, particularly the myofibrillar protein synthesis(Reference Welle, Thornton and Jozefowicz28, Reference Yarasheski, Zachwieja and Bier31, Reference Wilkinson, Phillips and Atherton47), which persist for up to 16 h in trained(Reference Tang, Perco and Moore53) and 24–48 h in untrained individuals(Reference Phillips, Tipton and Aarsland52–Reference MacDougall, Tarnopolsky and Chesley54). Muscle protein breakdown is also stimulated following exercise, albeit to a lesser extent than protein synthesis(Reference Phillips, Tipton and Aarsland52, Reference Biolo, Maggi and Williams55), and results in an improved net muscle protein balance that persists for up to 48 h in untrained individuals(Reference Phillips, Tipton and Aarsland52).
It has been generally accepted that the increase in protein synthesis following exercise is due to increased mRNA translation(Reference Laurent, Sparrow and Millward56). Many laboratories have shown that the signalling pathway involving a mammalian target of rapamycin (mTOR) complex I (mTORCI) plays a crucial role in the control of mRNA translation initiation and elongation(Reference Bodine, Stitt and Gonzalez57–Reference Dreyer, Fujita and Cadenas59). The activity of mTORCI determines the activity of downstream effectors such as the 70-kDa S6 protein kinase (S6K1) and the eukaryotic initiation factor 4E-binding protein(Reference Kimball, Farrell and Jefferson60). Both play key regulatory roles in modulating translation initiation, and control the binding of mRNA to the 40S ribosomal subunit(Reference Kimball, Farrell and Jefferson60). Studies have shown that the mTORCI signalling pathway is activated after acute resistance exercise in healthy human subjects(Reference Wilkinson, Phillips and Atherton47, Reference Dreyer, Fujita and Cadenas59, Reference Drummond, Dreyer and Pennings61, Reference Koopman, Zorenc and Gransier62). Moreover, Drummond et al. (Reference Drummond, Fry and Glynn63) showed elegantly that early acute contraction-induced increase in human protein synthesis in human subjects can be blocked with rapamycin treatment indicating that mTORCI signalling is crucial during the early post-exercise recovery. In addition, it was shown that the phosphorylation status of S6K1 following resistance exercise is a good marker for the long-term increase in skeletal muscle mass in rats(Reference Baar and Esser64) and human subjects(Reference Terzis, Georgiadis and Stratakos65). Moreover, significant correlations were reported between S6K1 phosphorylation/activation and muscle protein synthesis following exercise in young healthy human subjects(Reference Kumar, Selby and Rankin39), highlighting the importance of this signalling pathway in the adaptive response to resistance exercise.
Ageing and the anabolic response to exercise
Muscle protein synthesis is responsive to resistance and endurance exercise in both young and elderly human subjects(Reference Welle, Thornton and Statt29, Reference Yarasheski, Welle and Nair30, Reference Kumar, Selby and Rankin39, Reference Drummond, Dreyer and Pennings61, Reference Fujita, Rasmussen and Cadenas66, Reference Sheffield-Moore, Yeckel and Volpi67). Some studies have reported subtle differences in changes in gene expression and anabolic signalling(Reference Hameed, Orrell and Cobbold68), with early studies indicating that the protein synthetic response to resistance-type exercise did not differ considerably between the young and elderly(Reference Hasten, Pak-Loduca and Obert26, Reference Yarasheski, Zachwieja and Bier31). In contrast, an elegant study by Kumar et al. (Reference Kumar, Selby and Rankin39) showed anabolic resistance of anabolic signalling (i.e. 4E-binding protein and S6K1) and muscle protein synthesis after resistance exercise (performed in the fasted state) in elderly men compared with young controls, which became apparent especially at higher exercise intensities. This study demonstrated that the sigmoidal response of muscle protein synthesis to resistance exercise of different (increasing) intensities was shifted downward in older men compared to younger men(Reference Kumar, Selby and Rankin39). Interestingly, this study shows that the linear relationship between S6K1 phosphorylation and muscle protein synthesis after resistance exercise, which is observed in young healthy adults, was not present in the elderly, indicating that anabolic signalling regulating mRNA translation is impaired in the older human subjects(Reference Kumar, Selby and Rankin39).
Compared to protein synthesis, not many studies have actually measured muscle protein breakdown using stable isotope tracers. Most studies rely on measurements of mRNA or protein expression of proteins involved in protein degradation such as Atrogin-1, MuRF-1, calpains and their regulators. It has been suggested that mRNA expression of proteolytic regulators, such as Atrogin-1, are elevated in muscles from old compared with young adults at rest and these levels increased even further in the elderly in response to resistance exercise. These findings from Raue et al. (Reference Raue, Slivka and Jemiolo69) suggest that the regulation of ubiquitin proteasome-related genes involved in muscle atrophy might be altered in the elderly and protein breakdown may be increased in elderly human subjects. However, whether these changes in mRNA expression translate to actual changes in protein expression and altered proteasome activity has yet to be established. Thus, there is a paucity of data regarding the measurement of muscle protein breakdown in response to exercise in the elderly and it is clear that further research is needed to assess the impact of exercise and specific exercise modalities on post-exercise muscle protein synthesis and breakdown rates and associated myocellular signalling in young and elderly human subjects.
Anabolic response to food intake
Protein turnover in skeletal muscle is highly responsive to nutrient intake(Reference Rennie, Edwards and Halliday70). Ingestion of AA and/or protein strongly stimulates muscle protein synthesis(Reference Paddon-Jones, Sheffield-Moore and Zhang35, Reference Volpi, Mittendorfer and Wolf37, Reference Volpi, Ferrando and Yeckel51, Reference Rennie, Edwards and Halliday70, Reference Paddon-Jones, Sheffield-Moore and Katsanos71). Besides serving as a substrate for polypeptide biosynthesis, AA were shown to directly activate regulatory proteins in mRNA translation, while non-essential AA do not induce a substantial increase in muscle protein synthesis. In contrast, essential amino acids (EAA) increase muscle protein synthesis in the absence of increased non-essential AA availability. The branched-chain amino acid, leucine, is of particular interest since it has the unique ability to directly increase signalling through mTOR and its downstream targets 4E-binding protein and S6K1 and ribosomal S6. The EAA(Reference Tipton, Gurkin and Matin72, Reference Volpi, Kobayashi and Sheffield-Moore73), and leucine in particular(Reference Smith, Barua and Watt74, Reference Norton and Layman75), seem to represent the main anabolic signals responsible for the post-prandial increase in muscle protein synthesis. The observations that EAA show a dose-dependent stimulation of muscle protein synthesis without increasing plasma insulin(Reference Bohe, Low and Wolfe76), and that carbohydrate ingestion does not affect protein synthesis(Reference Borsheim, Cree and Tipton77), suggest that insulin is rather permissive instead of modulatory(Reference Rennie46, Reference Bohe, Low and Wolfe76, Reference Greenhaff, Karagounis and Peirce78). Greenhaff et al. (Reference Greenhaff, Karagounis and Peirce78) showed that insulin in the range of 30–150 μU/ml does not further stimulate muscle protein synthesis. In contrast to protein synthesis, muscle protein degradation seems to be very responsive to relatively small changes in insulin concentrations. Insulin levels of 15 μU/ml can almost maximally reduce muscle protein breakdown(Reference Wilkes, Selby and Atherton79) and there seems to be no further inhibition above 30 μU/ml(Reference Greenhaff, Karagounis and Peirce78). These data suggest that protein breakdown can be already maximally reduced by slightly increased insulin concentrations which can be achieved by the intake of a small breakfast in healthy young men(Reference Rennie46).
Ageing and the anabolic response to food intake
Data from recent studies suggest that the muscle protein synthetic response to the ingestion of a small amount of EAA(Reference Cuthbertson, Smith and Babraj32, Reference Katsanos, Kobayashi and Sheffield-Moore33) is attenuated in the elderly, and is now believed to represent one of the key factors responsible for the age-related decline in skeletal muscle mass. The so-called ‘anabolic resistance’ in elderly human subjects was demonstrated by a rightward and downward shift of the dose–response relationship between myofibrillar protein synthesis and the availability of leucine in the plasma(Reference Cuthbertson, Smith and Babraj32). Cuthbertson et al. (Reference Rennie46) showed that even a very large (40 g) dose of EAA is not able to bring the curve back to values for young subjects, suggesting that supplementation with extra protein, EAA or leucine will not be sufficient to restore the rate of muscle protein synthesis in older adults, relative to those found in the young.
The mechanisms responsible for the proposed anabolic resistance to protein and/or AA administration in the elderly are yet to be elucidated fully. Cuthbertson et al. (Reference Cuthbertson, Smith and Babraj32) reported decrements in amounts of signalling protein in the protein kinase B/mTORCI pathway in old muscle and showed an attenuated rise in the activation of key signalling proteins in this pathway after ingesting 10 g EAA in the elderly v. the young. These findings seem to be consistent with previous observations by Guillet et al. (Reference Guillet, Prod'homme and Balage45) who showed reduced S6K1 phosphorylation following combined AA and glucose infusions in the elderly. Combined, these data suggest that anabolic signalling is impaired in skeletal muscles of older compared to younger adults(Reference Cuthbertson, Smith and Babraj32, Reference Bohe, Low and Wolfe76), which may be in part due to insulin resistance in the elderly. Recent data suggest that muscle protein breakdown is not strongly inhibited by insulin in the elderly(Reference Wilkes, Selby and Patel80), whereas other reports suggested that muscle protein synthesis is resistant to the anabolic action of insulin in the elderly(Reference Volpi, Mittendorfer and Rasmussen36, Reference Rasmussen, Fujita and Wolfe43). It has been proposed that the anabolic resistance can be attributed to a less responsive impact of physiological hyperinsulinemia on the increase in skeletal muscle blood flow and subsequent AA availability in aged muscle(Reference Rasmussen, Fujita and Wolfe43, Reference Fujita, Rasmussen and Cadenas81), which would agree with the reduced activation of the phosphatidylinositol-3 kinase–protein kinase B–mTOR signalling pathway and with the lesser increase in the muscle protein synthetic rate after AA/protein ingestion in the elderly(Reference Cuthbertson, Smith and Babraj32).
Another mechanism that has been suggested to contribute to the anabolic resistance to food intake in elderly men is an impairment in dietary protein digestion and/or absorption(Reference Boirie, Dangin and Gachon82). Recent data show that the digestion rate of protein is an independent regulating factor of post-prandial protein anabolism(Reference Dangin, Boirie and Garcia-Rodenas83). As such, it seems plausible to assume that any impairment in protein digestion and/or absorption will reduce the appearance rate of dietary AA in the bloodstream, thereby reducing AA delivery to the muscle and subsequently attenuating the muscle protein synthetic response. To accurately assess the appearance rate of AA derived from dietary protein, the labelled AA need to be incorporated in the dietary protein source(Reference Beaufrere, Dangin and Boirie84–Reference Dangin, Boirie and Guillet86). As free AA and protein-derived AA exhibit a different timing and efficiency of intestinal absorption(Reference Boirie, Gachon and Corny85), simply adding labelled free AA to a drink containing protein does not provide an accurate measure of the digestion and absorption kinetics of the ingested dietary protein(Reference Boirie, Fauquant and Rulquin87). These methodological restrictions represent the main reasons why only a few researchers have investigated the differences in digestion and absorption kinetics of specific dietary protein sources and the disparity in anabolic response between young and elderly human subjects. These studies have suggested that AA utilisation in the splanchnic area is elevated in the elderly(Reference Boirie, Dangin and Gachon82), which would imply that less of the ingested AA are available for muscle protein synthesis(Reference Boirie, Dangin and Gachon82). We have recently repeated similar experiments, comparing the appearance rate of dietary L-[1-13C]phenylalanine in the circulation following the intake of 35 g intact intrinsically labelled casein protein(Reference Koopman, Walrand and Beelen88). Our data clearly show that splanchnic extraction is not altered significantly in elderly men, and that over a 3 and 6 h period the same amount of dietary phenylalanine appears in the circulation(Reference Koopman, Walrand and Beelen88). Although we did not observe any impairment in digestion and absorption in the elderly, we observed substantially (about 12%) lower rates of whole-body protein synthesis and phenylalanine hydroxylation following protein ingestion in the elderly men compared to the young men (Fig. 2), calculated over the first 3 h, subsequent 3 h or total 6 h time period after protein ingestion. Consistent with these observations, we observed a 14% difference in muscle protein synthesis rates between young and elderly men over the 6 h period, although this difference did not reach statistical significance(Reference Koopman, Walrand and Beelen88). Not all researchers have found impaired muscle protein synthetic response to protein intake in the elderly as similar protein synthetic rates were observed in young and elderly human subjects after ingestion of large amounts of carbohydrate and proteins(Reference Koopman, Verdijk and Manders89), and following ingestion of large(Reference Symons, Sheffield-Moore and Wolfe90) and small amount of beef(Reference Symons, Sheffield-Moore and Wolfe90, Reference Symons, Schutzler and Cocke91). Discrepancies may arise from differences in timing of biopsy collection, the precursor pool used to calculate muscle protein synthesis or the age of the elderly volunteers studied. Clearly, more research is warranted to determine the extent of an anabolic resistance to food (i.e. intact protein) intake that exists in elderly human subjects.
Early work from the laboratory of Yves Boirie(Reference Boirie, Dangin and Gachon82–Reference Beaufrere, Dangin and Boirie84, Reference Dangin, Boirie and Guillet86, Reference Dangin, Guillet and Garcia-Rodenas92) showed that ingestion of a slowly digested protein (casein) led to a more positive whole-body protein balance (averaged over a 7 h period) when compared with the ingestion of a fast digestible protein (whey) or a mixture of free AA in healthy, young subjects(Reference Dangin, Boirie and Garcia-Rodenas83). In contrast, ingestion of a fast protein resulted in greater (whole-body) net protein retention compared to a slow protein when provided to healthy, older men(Reference Boirie, Dangin and Gachon82, Reference Beaufrere, Dangin and Boirie84, Reference Dangin, Boirie and Guillet86, Reference Dangin, Guillet and Garcia-Rodenas92). The latter response might be attributed to the reported anabolic resistance of the muscle protein synthetic machinery to become activated in the elderly. In accordance with the fast v. slow protein concept, we tested the hypothesis that the ingestion of a casein protein hydrolysate, i.e. enzymatically pre-digested casein, would enhance protein digestion and the absorption rate in elderly men(Reference Koopman, Crombach and Gijsen93). We expected that this enhanced AA uptake in the gut would result in a greater increase in plasma AA availability and might improve the post-prandial muscle protein synthetic response. Elderly men ingested 35 g intrinsically L-[1-13C]phenylalanine labelled casein or casein hydrolysate and we assessed the appearance rate of dietary phenylalanine in the circulation and the subsequent muscle protein synthetic response. The ingestion of casein hydrolysate accelerated the appearance rate of dietary phenylalanine in the circulation, lowered splanchnic phenylalanine extraction, increased post-prandial plasma AA availability and tended to augment the subsequent muscle protein synthetic response in vivo in human subjects, compared to the ingestion of intact casein(Reference Koopman, Crombach and Gijsen93). The difference in the appearance rate of dietary protein between intact and hydrolysed casein was particularly evident in the first 3 h after the protein ingestion, with about 50% more dietary phenylalanine appearing in the circulation after ingestion of the casein hydrolysate(Reference Koopman, Crombach and Gijsen93). Consistent with these findings, it was reported that protein pulse feeding (providing up to 80% of daily protein intake in one meal) leads to greater protein retention than ingesting the same amount of protein provided over four meals throughout the day (spread-feeding) in elderly women(Reference Arnal, Mosoni and Boirie94, Reference Arnal, Mosoni and Boirie95). These findings may indicate that part of the proposed anabolic resistance in the elderly might be compensated for, in part, by enhancing AA availability during the post-prandial period.
Ageing and the anabolic response to combined exercise and nutrition
We have shown previously that muscle protein synthesis rates are lower in the elderly (about 75 year) compared to young controls under conditions in which resistance-type exercise is followed by food intake(Reference Koopman, Verdijk and Manders96). However, combined ingestion of carbohydrate and protein during recovery from physical activity resulted in similar increases in mixed muscle protein synthesis rates, measured over a 6-h period, in young and elderly men(Reference Koopman, Verdijk and Manders96). Consistent with our findings, Drummond et al. (Reference Drummond, Dreyer and Pennings61) reported similar post-exercise muscle protein synthesis rates over a 5-h recovery period in young v. elderly subjects following the ingestion of carbohydrate with an EAA mixture. However, their data indicated that the anabolic response to exercise and food intake was delayed in the elderly. During the first 3 h of post-exercise recovery, the young subjects showed a substantial increase in the muscle protein synthesis rate, which was not observed in the elderly. The delayed activation of muscle protein synthesis in the elderly may be attributed to a more pronounced activation of AMP-activated protein kinase and/or reduced extracellular-signal-regulated kinases1/2 activation during exercise, which seems to be consistent with an attenuated rise in 4E-binding protein phosphorylation following resistance-type exercise in older adults(Reference Kumar, Selby and Rankin39). These data highlight the importance of measuring muscle protein synthesis over different time periods (0–3 h and 3–6 h) following exercise and/or food intake to gain more information about impairments in activation of protein synthesis in the elderly. The mechanisms responsible for the delayed intracellular activation of the mTOR pathway in skeletal muscle remain unclear, but might include differences in muscle recruitment, muscle fibre-type composition, the capacity and/or sensitivity of the muscle protein synthetic machinery, the presence of an inflammatory state and/or the impact of stress on the cellular energy status of the cell between young and older adults.
Long-term interventions
The clinical relevance of nutritional and/or exercise intervention in the elderly stems from the long-term impact on skeletal muscle mass and strength, and the implications for functional capacity. In accordance with the previously discussed findings, the muscle protein synthetic machinery is able to respond to anabolic stimuli, albeit maybe to a lesser extent(Reference Rennie46), until very old age(Reference Fiatarone, Marks and Ryan97, Reference Frontera, Meredith and O'Reilly98). Although it was suggested previously that elderly human subjects need more protein(Reference Campbell, Trappe and Jozsi99), more recent studies by Campbell et al.(Reference Campbell, Johnson and McCabe100), who performed very comprehensive nitrogen balance experiments, clearly showed that dietary protein requirements did not increase with age, and that a dietary protein allowance of 0·85 g/kg per day is adequate. Some researchers believe that the attenuated muscle protein synthetic response to food intake in the elderly can, at least partly, be compensated for by increasing the leucine content of a meal(Reference Katsanos, Kobayashi and Sheffield-Moore34, Reference Rieu, Balage and Sornet101). However, we have shown previously that additional leucine intake does not further increase muscle protein synthesis after resistance exercise when ample protein is ingested by elderly men(Reference Koopman, Verdijk and Beelen102). In addition, we investigated the effect of 3 months of leucine supplementation with each main meal (7·5 g/d) on skeletal muscle mass and strength and on glycemic control in healthy elderly men(Reference Verhoeven, Vanschoonbeek and Verdijk103). Consistent with our observations from our acute post-exercise study, we did not observe any effect of leucine supplementation on skeletal muscle mass and strength. In addition, no improvements in indices of whole-body insulin sensitivity blood-glycated Hb content, or the plasma lipid profile were observed. We concluded that long-term leucine supplementation (7·5 g/d) does not augment skeletal muscle mass or strength and does not improve glycaemic control or the blood lipid profile in healthy elderly men.
Resistance exercise training interventions were shown effective in augmenting skeletal muscle mass, increasing muscle strength and/or improving functional capacity in the elderly(Reference Fiatarone, Marks and Ryan97, Reference Frontera, Meredith and O'Reilly98, Reference Ades, Ballor and Ashikaga104–Reference Verdijk, Gleeson and Jonkers119). In addition, endurance(Reference Fiatarone, Marks and Ryan97, Reference Frontera, Meredith and O'Reilly98, Reference Ades, Ballor and Ashikaga104–Reference Vincent, Braith and Feldman110) exercise was shown to enhance the skeletal muscle oxidative capacity, resulting in greater endurance capacity(Reference Short, Vittone and Bigelow5, Reference Short, Vittone and Bigelow120). Although the muscle regenerative capacity seems to decline at a more advanced age, the reduced satellite cell pool size(Reference Verdijk, Gleeson and Jonkers119) does not compromise the capacity for muscle hypertrophy to occur even at an advanced age(Reference Dedkov, Borisov and Wernig121–Reference Thornell, Lindstrom and Renault123) and resistance exercise training was shown to increase muscle fibre size(Reference Kadi, Schjerling and Andersen124–Reference Olsen, Aagaard and Kadi127). Recently, Verdijk et al.(Reference Verdijk, Gleeson and Jonkers119) assessed the effects of 12 weeks of leg resistance exercise training on fibre-type specific hypertrophy and satellite cell content in healthy, elderly men. Prolonged training resulted in a 28% increase in the size of type-II muscle fibres and a concomitant 76% increase in type-II muscle fibre satellite cell content in elderly males(Reference Verdijk, Gleeson and Jonkers119). The apparent differences in fibre size and/or satellite cell content between type-I and type-II muscle fibres prior to intervention were no longer evident after 12 weeks of training. Overall, these findings suggest that satellite cells are instrumental in the generation of new myonuclei to facilitate muscle fibre hypertrophy(Reference Snijders, Verdijk and van Loon128).
Protein/AA ingestion before, during and/or after exercise acutely stimulates muscle protein synthesis and reduces muscle protein breakdown to facilitate muscle fibre hypertrophy. Remarkably, little evidence exists that dietary interventions can further augment the adaptive response to prolonged exercise training in the elderly. The proposed importance of ample dietary protein intake in the long-term adaptive response to resistance training in the elderly has been a topic of intense debate(Reference Campbell and Evans129–Reference Campbell and Leidy131). Some researchers suggest that the current RDA for habitual protein intake of 0·8 g/kg per day(Reference Rand, Pellett and Young132, Reference Trumbo, Schlicker and Yates133) is marginal to allow lean mass accretion following resistance exercise training(Reference Campbell, Trappe and Jozsi99) or even insufficient for long-term maintenance of skeletal muscle mass in sedentary elderly human subjects(Reference Campbell, Trappe and Wolfe134). However, others have shown that when habitual dietary protein intake is standardised at 0·9 g/kg per day, exercise-induced increases in muscle mass become apparent and further increases in protein intake does not provide any additional effect(Reference Iglay, Thyfault and Apolzan114). In addition, data from Walrand et al. (Reference Walrand, Short and Bigelow135) indicated that although increased protein intake in the elderly further improved nitrogen balance (by increasing AA oxidation), no beneficial effects on muscle protein synthesis and muscle function were observed. These observations might explain why most studies fail to observe any additional benefit of nutritional co-intervention on the skeletal muscle adaptive response to prolonged resistance exercise training in the elderly(Reference Fiatarone, Marks and Ryan97, Reference Frontera, Meredith and O'Reilly98, Reference Fiatarone, O'Neill and Ryan106, Reference Godard, Williamson and Trappe113, Reference Iglay, Thyfault and Apolzan114, Reference Verdijk, Jonkers and Gleeson117, Reference Haub, Wells and Tarnopolsky118, Reference Campbell, Crim and Young136–Reference Welle and Thornton139). However, it has been suggested that it is not the total protein amount per se, but the timing of protein intake that is crucial for its stimulatory effect on muscle protein synthesis and muscle fibre hypertrophy. Esmarck et al. (Reference Esmarck, Andersen and Olsen140) concluded that the intake of a protein supplement immediately after each bout of resistance-type exercise was required for skeletal muscle hypertrophy to occur with a 12-week intervention in the elderly. Although the absence of any hypertrophy in the control group seems to conflict with previous studies that show muscle hypertrophy following resistance training without any dietary intervention, the proposed importance of nutrient timing is supported by more recent studies investigating the impact of AA or protein co-ingestion prior to, during and/or after exercise on the acute muscle protein synthetic response(Reference Beelen, Koopman and Gijsen141, Reference Tipton, Rasmussen and Miller142). Verdijk et al.(Reference Verdijk, Jonkers and Gleeson117) compared increases in skeletal muscle mass and strength following 3 months of resistance exercise training with or without protein ingestion prior to and immediately after each exercise session in elderly males. Timed protein supplementation prior to and after each exercise bout did not further increase skeletal muscle hypertrophy in healthy, elderly men who habitually consumed about 1·0 g protein/kg per day. Taken together, the available data suggest that sufficient habitual protein intake (about 0·9 g/kg per day) combined with a normal meal pattern (i.e. providing ample protein three times daily) will allow for substantial gains in muscle mass and strength with resistance exercise training in the elderly. Additional protein supplementation does not seem to provide large surplus benefits to the exercise intervention in healthy, elderly males. Additional protein intake may reduce subsequent voluntary food consumption in the elderly(Reference Fiatarone Singh, Bernstein and Ryan143) and consequently some have suggested that supplementation with EAA would be more efficient(Reference Timmerman and Volpi144). Clearly, acute studies have shown benefits of timed supplementation with small (7–15 g) amounts of EAA on muscle protein synthesis(Reference Katsanos, Kobayashi and Sheffield-Moore33, Reference Paddon-Jones, Sheffield-Moore and Zhang35, Reference Paddon-Jones, Sheffield-Moore and Katsanos71). However, well-designed, double-blind, placebo-controlled long-term studies to investigate beneficial and adverse effects of long-term EAA supplementation in the elderly are yet to be performed(Reference Henderson, Irving and Nair145).
Conclusions
The loss of skeletal muscle mass with ageing is associated with reduced muscle strength, the loss of functional capacity and an increased risk for developing chronic metabolic disease. The progressive loss of skeletal muscle mass does not appear to be attributed to age-related changes in basal muscle protein synthesis and/or rates of protein breakdown. Recent studies suggest that the muscle protein synthetic response to the main anabolic stimuli, i.e. food intake and/or physical activity, is blunted in the elderly. Despite this potential anabolic resistance to food intake and/or physical activity, resistance exercise training can stimulate net muscle protein accretion significantly. Prolonged resistance exercise training has proved to be an effective intervention for attenuating and/or treating the loss of muscle mass and strength in the elderly. Further research is warranted to provide insight into the interactions between nutrition, exercise and skeletal muscle adaptations in order to define more effective nutritional, exercise and/or pharmaceutical interventional strategies to prevent and/or treat sarcopenia.
Acknowledgements
R. K. is a C.R. Roper Senior Research Fellow in the Faculty of Medicine, Dentistry and Health Sciences at The University of Melbourne and his research is currently funded by grants/fellowships from the Ajinomoto Amino Acid Research Program (3ARP, Ajinomoto, Japan) and the European Society for Clinical Nutrition and Metabolism (ESPEN). The author declares no conflict of interest.