The loss of muscle mass with ageing is rooted in an imbalance between the rates of muscle protein synthesis and muscle protein breakdown, with the former playing the dominant role in the regulation of muscle mass in healthy individuals(Reference Koopman and van Loon1). A ‘resistance’ of muscle protein synthesis to dietary protein, which describes a situation of a blunted muscle protein synthetic response to a given dietary protein load, in otherwise healthy elderly individuals is thought by some to be a major contributing factor to age-related muscle loss(Reference Cuthbertson, Smith and Babraj2, Reference Katsanos, Kobayashi and Sheffield-Moore3). There is also evidence suggesting an attenuated myofibrillar protein synthetic response after resistance exercise in the elderly v. young(Reference Kumar, Selby and Rankin4); however, adequate nutrition in the form of dietary amino acids can overcome this difference(Reference Pennings, Koopman and Beelen5).
Recent evidence has shown that whey protein has a superior capacity, by comparison with other proteins such as soya and casein, to stimulate rates of mixed muscle protein synthesis at rest in both young(Reference Tang, Moore and Kujbida6) and elderly individuals(Reference Pennings, Boirie and Senden7), as well as after exercise in the young(Reference Tang, Moore and Kujbida6). Moreover, even when compared with an isonitrogenous mixture of equivalent constituent amino acids, whey protein stimulates a greater anabolic response(Reference Katsanos, Chinkes and Paddon-Jones8). The greater stimulatory effect of whey has been attributed to its high leucine content combined with a rapid rate of digestion, both of which lead to a rapid leucinaemia and aminoacidaemia, in general, for an optimal stimulation of muscle protein synthesis(Reference Tang, Moore and Kujbida6, Reference Pennings, Boirie and Senden7). While these studies support the use of whey protein for stimulating mixed muscle protein synthesis, they do not provide insight into the synthesis of specific muscle protein subfractions. For example, myofibrillar (i.e. contractile) and non-myofibrillar muscle protein fractions do not always demonstrate concordant responses to nutrition and exercise(Reference Moore, Tang and Burd9). Given the importance of maintaining an adequate quantity and quality of contractile proteins with age, we aimed to determine the effectiveness of rapidly digested isolated whey protein and slowly digested isolated micellar casein to stimulate rates of myofibrillar protein synthesis at rest (FED) and after resistance exercise (EX-FED) in elderly skeletal muscle. Since the postprandial rise in circulating amino acids in the blood appears to dictate the extent of stimulation of myofibrillar protein synthesis(Reference Tang, Moore and Kujbida6), we hypothesised that the myofibrillar protein synthetic response would be greater, regardless of prior contractile activity, after ingestion of whey when compared with slowly digested micellar casein.
A total of fourteen older men (72 (sem 1) years; BMI 26·4 (sem 0·7) kg/m2) were recruited to participate in the present study. Participants were non-smokers, non-diabetic and light-to-moderately active. All participants were deemed healthy based on their response to a routine medical screening questionnaire and examination of their baseline characteristics. All participants were informed of the purpose of the study, the experimental procedures involved and all the potential risks involved before obtaining written consent. The study was approved by the local Health Sciences Research Ethics Board of McMaster University and conformed to standards for the use of human subjects in research as outlined in the fifth Declaration of Helsinki and with current Canadian funding agency guidelines for use of human subjects in research(10).
Participants reported to the laboratory approximately 1 week before the experimental infusion trial to undergo preliminary testing. After familiarisation with the equipment, participants underwent strength testing on a leg extension machine to determine their unilateral ten repetition maximum. Body composition was assessed by dual-energy X-ray absorptiometry and participants were randomly assigned to the whey or micellar casein groups but counterbalanced for lean body mass. Participants were required to record all food or drink consumed in a diet log during a 3 d period (i.e. two weekdays and one weekend) before the start of the study to provide an estimate of habitual macronutrient intake as analysed by a commercially available software program (Nutritionist V, First; Data Bank, San Bruno, CA, USA). Reference lists for portion size estimates were provided to the participants. Based on the responses, the average daily energy and protein intakes were calculated (9·5 MJ/d and 1·0 g/kg per d, respectively). Moreover, 2 d preceding the infusion trial, participants were provided a standardised diet that contained a moderate protein intake (1·0 g/kg per d) and energy at a level estimated according to the Harris–Benedict equation(Reference Roza and Shizgal11), with individual activity factors according to individual self-report logs (mean activity factor 1·5 (sem 0·1)).
After having refrained from strenuous exercise for 3 d, participants arrived at the laboratory after an overnight fast and a baseline blood sample was drawn. Subsequently, participants performed an acute bout of unilateral resistance exercise with one leg (EX-FED) on a guided-motion knee extension machine (three sets at the participant's ten repetition maximum). The contralateral leg did not perform exercise but was under the influence of nutrition (FED).
A primed (2 μmol/kg), constant infusion (0·05 μmol/kg per min) of l-[ring-13C6]phenylalanine (99 at%; Cambridge Isotopes, Andover, MA, USA) was initiated. Immediately after the start of the infusion, participants consumed a drink containing 20 g micellar casein (MCN-85; AMCO, Burlington, NJ, USA), which contained 8·2 g essential amino acids (EAA), 4·0 g branched-chain amino acids (BCAA) and 1·6 g leucine, or 20 g whey protein isolate (Alacen 352; Fonterra, Palmerston North, New Zealand), which contained 10·2 g EAA, 5·2 g BCAA and 2·8 g leucine, determined using methods described previously(Reference Wilkinson, Tarnopolsky and Macdonald12). All drinks were dissolved in 400 ml of water. Drinks were enriched with a small amount of tracer in accordance with the phenylalanine content of approximately 4 % of the micellar casein and whey protein(Reference Tang, Moore and Kujbida6). This approach is necessary to prevent dilution of tracer enrichment, and thus maintain steady-state enrichment in the intracellular amino acid pool(Reference Burd, West and Rerecich13), after introduction of unlabelled amino acids into the blood (i.e. the amino acids from the protein drink). Arterialised blood samples were collected every 0·5–1 h after the start of infusion and processed as described previously(Reference Moore, Robinson and Fry14). Biopsy samples were taken at 240 min from the vastus lateralis of the EX-FED and FED legs using a using a 5 mm Bergström needle that was custom-modified for manual suction under local anaesthesia (2 % xylocaine). Biopsy samples were immediately blotted and freed of any visible blood, fat or connective tissue before immersion in liquid N2 and storage at − 80°C until further analysis.
Plasma [ring-13C6]phenylalanine enrichments were determined as described previously(Reference Glover, Phillips and Oates15). Blood amino acid concentrations were measured by HPLC as described previously(Reference Moore, Phillips and Babraj16). Blood glucose concentrations were analysed using a blood glucose meter (OneTouch Ultra 2; Lifescan, Inc., Milpitas, CA, USA) within 5 min of blood collection. Plasma insulin was measured using a commercially available immunoassay kit (Alpco Diagnostics, Salem, NH, USA). Myofibrillar-enriched protein fractions were isolated from approximately 30 mg of wet muscle as described previously(Reference Burd, West and Staples17). A separate piece of wet muscle (approximately 15 mg) was used to extract the intracellular amino acids as described previously(Reference Burd, Holwerda and Selby18).
The fractional synthetic rates of myofibrillar proteins were calculated according to the standard precursor–product equation, with intracellular phenylalanine enrichment as a surrogate for transfer RNA labelling(Reference Burd, West and Rerecich13). The use of ‘tracer-naive’ participants allowed us to use the pre-infusion blood sample as the baseline enrichment for muscle protein; an approach we have recently validated and described in detail elsewhere(Reference Burd, West and Rerecich13).
A between-subject repeated-measures design was utilised for the present study. Data were analysed using a two-factor repeated-measures ANOVA. Tukey's post hoc test was performed to determine differences between means for all significant main effects and interactions. For all analyses, differences were considered significant at P < 0·05. Results are presented as means with their standard errors.
Fasting plasma insulin concentrations were similar in the micellar casein 39·1 (sem 7) pmol/l and whey protein (36·2 (sem 5) pmol/l) groups. Whey ingestion transiently increased plasma insulin by approximately 179 % above fasting at 60 min, whereas there was no effect of micellar casein (Table 1). Blood glucose remained stable across time for both whey and micellar casein groups (data not shown).
EAA, essential amino acids.
* Mean values were significantly different from the 0 time point (P < 0·05).
† Mean values were significantly different from those of the micellar casein group (P < 0·05).
‡ EAA (sum of Leu, Ile, Val, Phe, Lys, His, Met and Thr; note that Trp was not measured).
EAA in the blood peaked at 60 min after micellar casein ingestion (P < 0·001; Table 1) before returning to fasting EAA concentrations at 120 min post-drink (P = 0·07; Table 1). A prolonged elevation in blood leucine concentrations was detected after micellar casein ingestion with a peak at 60 min post-drink before returning to baseline by 240 min (P = 0·24; Table 1). Whey protein ingestion induced an acute rise in blood EAA and leucine concentrations at 60 min post-drink that was greater in amplitude than micellar casein ingestion (both, P < 0·05; Table 1).
Linear regression analysis indicated that the slopes of the plasma enrichments were not significantly different from zero (both, P < 0·05) in the micellar casein or the whey group, indicating that an isotopic plateau was achieved (data not shown). Intracellular free phenylalanine enrichments in the casein group were 0·043 (sem 0·001) tracer:tracee (t/Tr) ratio and 0·044 (sem 0·001) t/Tr in the FED and EX-FED conditions, respectively. Intracellular free phenylalanine enrichments in the whey group were 0·040 (sem 0·003) t/Tr and 0·041 (sem 0·001) t/Tr in the FED and EX-FED conditions, respectively. Resistance exercise-induced rates of myofibrillar protein synthesis were greater (P < 0·001) than those observed after feeding alone in both micellar casein and whey protein groups (Table 1). However, whey protein ingestion stimulated myofibrillar protein synthetic rates to a greater degree than micellar casein at rest in both FED (P = 0·002) and EX-FED (P < 0·001) conditions (Fig. 1).
We report for the first time that whey protein ingestion stimulates rates of myofibrillar protein synthesis greater than the equivalent protein ingested as micellar casein in healthy elderly men not only at rest, but also after resistance exercise. The practical application of this finding, matching that of Pennings et al. (Reference Pennings, Boirie and Senden7), is that isolated whey protein is a more effective source of protein for augmenting postprandial muscle protein synthesis rates in older men. Moreover, the greater stimulatory effect on myofibrillar protein synthesis with whey was also observed during post-exercise recovery where the myofibrillar protein synthetic rate was approximately 60 % greater than that seen after ingestion of micellar casein. Collectively, these results are consistent with a greater stimulatory effect of whey protein ingestion v. other, more slowly digested high-quality dietary proteins of similar amino acid concentrations, such as micellar casein, on mixed muscle protein synthesis rates previously observed in young(Reference Tang, Moore and Kujbida6) and elderly men(Reference Pennings, Boirie and Senden7).
The present data are at odds with the results of Reitelseder et al. (Reference Reitelseder, Agergaard and Doessing19) in young men and Dideriksen et al. (Reference Dideriksen, Reitelseder and Petersen20) in older men and women demonstrating that ingestion of whey protein compared with calcium caseinate resulted in similar rates of myofibrillar protein synthesis. Certainly, methodological differences including different types of protein (i.e. micellar casein v. calcium caseinate), the timing of the synthetic measurements (i.e. 4 v. 6 h of recovery) and/or the tracer infused (i.e. [13C]leucine v. ring-[13C6]phenylalanine) may account for the contrasting findings between their work(Reference Reitelseder, Agergaard and Doessing19, Reference Dideriksen, Reitelseder and Petersen20) and ours(Reference Tang, Moore and Kujbida6). The main difference, however, probably relates to the characteristics of micellar casein, the pure native component found in milk, having different absorption kinetics from that of calcium caseinate. This point is not trivial, as it is clear that the increased solubility induced by reacting casein with an alkali of Ca increases the rate of digestion and speeds the resultant aminoacidaemia after calcium caseinate ingestion(Reference Reitelseder, Agergaard and Doessing19, Reference Dideriksen, Reitelseder and Petersen20) to a markedly faster rate of that seen with micellar casein(Reference Tang, Moore and Kujbida6). The longer tracer incorporation time (i.e. approximately 6 h) may have precluded the researchers' ability to detect the potential benefits of whey protein, since hyperaminoacidaemia after whey ingestion is rapid and relatively transient(Reference Reitelseder, Agergaard and Doessing19, Reference Dideriksen, Reitelseder and Petersen20). In fact, Reitelseder et al. (Reference Reitelseder, Agergaard and Doessing19) observed a trend towards a greater muscle protein synthetic rate up to 3 h of recovery after ingestion of whey when compared with calcium caseinate in young men(Reference Reitelseder, Agergaard and Doessing19). The same researchers also observed a sharp decline in rates of myofibrillar protein synthesis between 3 and 6 h following whey ingestion after resistance exercise, which is inconsistent with our data of a sustained elevation in myofibrillar protein synthesis rates during this time(Reference Moore, Tang and Burd9).
The present results in elderly men are entirely consistent with our recent observation of a greater post-exercise rise in myofibrillar protein synthesis with a bolus ingestion of whey when compared with small repeated feedings mimicking that of casein ingestion(Reference West, Burd and Coffey21) and with those of Pennings et al. (Reference Pennings, Boirie and Senden7). However, determining myofibrillar protein synthesis rates, as we did in the present study, will generally result in rates that are approximately 2–3-fold lower than mixed muscle protein synthesis rates, which was the protein fraction studied by Pennings et al. (Reference Pennings, Boirie and Senden7). It is noteworthy that the degree of insulinaemia was greater after whey protein consumption (Table 1). Although this result may have had little influence on the observed myofibrillar protein synthesis rates(Reference Wilkes, Selby and Atherton22), a higher insulinaemia after meal consumption may be favourable in terms of suppressing proteolysis in ageing muscle(Reference Wilkes, Selby and Atherton22). Regardless, we propose that the ingestion of a leucine-rich and rapidly digested protein, such as whey, has the best potential to maximally stimulate rates of myofibrillar protein synthesis during the acute recovery period after exercise in aged muscle. It should also be highlighted that the present results are consistent with recent reports in elderly men(Reference Pennings, Koopman and Beelen5) demonstrating that casein protein is still able to support increased rates of muscle protein synthesis, albeit at lower rate than whey, when ingested after exercise.
In the ‘real world’, it is common for individuals, regardless of age, to consume mixed meals that contain all macronutrients. Indeed, insulin resistance of muscle protein metabolism in ageing muscle is a recurring theme(Reference Wilkes, Selby and Atherton22–Reference Fujita, Glynn and Timmerman24). Thus, the co-ingestion of carbohydrate, and the associated greater insulinaemia, with dietary protein may be beneficial for the feeding-induced stimulation of muscle protein synthesis rates in the elderly. However, we would expect little additional benefit in terms of stimulating muscle protein synthesis with co-ingestion of carbohydrate with a saturating dose of whey protein in older participants, which is what we observed in younger men(Reference Staples, Burd and West25). Indeed, the optimal dose of whey protein to consume remains to be defined in ageing muscle, either at rest or after resistance exercise. Moreover, carbohydrate co-ingestion may also be beneficial for the stimulation of muscle protein synthesis rates after ingestion of a protein source which results in a prolonged aminoacidaemia, such as micellar casein. Given all this, it is clear that further research is necessary to confirm any speculations.
The present study reveals that in elderly men, a rapid increase in EAA and leucine after ingestion of isolated whey protein stimulated greater FED and EX-FED when compared with a prolonged leucinaemia that occurs with micellar casein ingestion. This finding continues to support a growing body of evidence suggesting that the amplitude and possibly the rate of postprandial rise in circulating amino acids in the blood dictates the extent of stimulation of myofibrillar protein synthesis. The present data may have a practical ramification for formulations of protein feedings designed to maximise muscle protein synthesis rates, which would appear to be most beneficial when consisting of whey protein.
The present study was funded by the Natural Science and Engineering Research Council (NSERC) of Canada (awarded to S. M. P.). The authors would like to thank Todd Prior and Tracy Rerecich for technical expertise and analytical assistance. The authors' responsibilities were as follows: Y. Y., D. R. M., J. E. T. and S. M. P. conceived and designed the experiment; Y. Y., N. A. B., J. E. T., M. A. T. and S. M. P. collected and processed the samples; N. A. B. analysed the samples; N. A. B. and S. M. P. conducted the statistical analysis. N. A. B., D. R. M., M. A. T. and S. M. P. interpreted the data and wrote the manuscript. None of the authors has personal or financial conflicts of interest to declare.