Introduction
The older adult population is rapidly growing with over 600 million people globally being aged 65 years or older(Reference Wan, Goodkind and Kowal1). This increased ageing population is associated with an increase in the prevalence of chronic diseases such as CVD, hypertension, obesity and diabetes(2). CVD affects over 40 million adults above the age of 65 years in the USA(Reference Bell and Saraf3) and remains the number one cause of death globally(4). The global prevalence of diabetes is increasing, particularly in older adults, and the number of cases is expected to double in the next two decades(Reference Sesti, Incalzi and Bonora5). Currently, over 25 % of older adults in America are diabetic and 51 % are pre-diabetic(Reference Sesti, Incalzi and Bonora5). In Europe, the prevalence of diabetes in older adults is on average 20 %, with variations among nations ranging from 14–16 % in Denmark to 31 % in Greece(Reference Sesti, Incalzi and Bonora5). A recent European study, involving over twenty countries, reported a significantly higher incidence of overweight and obesity in older adult populations, compared with middle-age and younger adults(Reference Marques, Peralta and Naia6). On a global level, more than 650 million adults were reported as obese in 2016(7). Furthermore, a European countries report estimated that 20 million people aged >50 years have osteoporosis(8). These data confirm that these diseases are a serious worldwide public health concern and are prevalent in the older adult population. Thus, in this growing population identifying ways to promote healthy ageing, through diet and exercise, has never been more important.
Skeletal muscle mass (SMM) is largely associated with physical function and athletic performance. Preserving SMM is essential for maintaining metabolic health and can play an essential role in minimising the risk of diseases, such as CVD, obesity, diabetes and osteoporosis(Reference Wolfe9). SMM is determined by a fine balance between muscle protein synthesis (MPS) and muscle protein breakdown. Though the exact mechanisms are unknown, essential amino acids (EAA) play a role in MPS by functioning as signalling molecules to induce MPS(Reference van Vliet, Burd and van Loon10). To date, studies have primarily focused on leucine, which is recognised as the most potent amino acid (AA) to stimulate a postprandial MPS response(Reference Wilkinson, Hossain and Hill11,Reference Crozier, Kimball and Emmert12) . MPS is also influenced by the myostatin–Smad2/3 pathway, a major signalling pathway which acts as a negative regulator of protein synthesis(Reference Schiaffino, Dyar and Ciciliot13). It is important to acknowledge that MPS is not the only controlling factor in muscle mass accretion/loss. Transcription factors like SRF (serum response factor) help regulate skeletal muscle growth(Reference Schiaffino, Dyar and Ciciliot13) and androgens and b2-adrenergic agents can have anabolic effects on skeletal muscle(Reference White, Gao and Puppa14,Reference Koopman, Gehrig and Leger15) . In addition, strength and conditioning exercises can positively increase SMM.
The availability of protein, more specifically AA, is important for MPS(Reference Witard, Wardle and Macnaughton16); thus dietary protein is a key stimulus for preserving SMM. In addition, AA availability is influenced by protein source, digestibility, absorption kinetics and protein intake pattern(Reference Dideriksen, Reitelseder and Holm17,Reference Arnal, Mosoni and Boirie18) . The international RDA for protein is 0·8 g/kg body weight per d, irrespective of age(19). However, the International PROT-AGE Study Group(Reference Bauer, Biolo and Cederholm20) and the European Society for Clinical Nutrition and Metabolism (ESPEN) recommend that the daily protein requirement for healthy individuals over 65 years is 1·0–1·2 g protein/kg body weight(Reference Deutz, Bauer and Barazzoni21). Furthermore, several studies advise that consuming higher than the RDA of protein may help preserve muscle mass(Reference Dideriksen, Reitelseder and Holm17,Reference Bauer, Biolo and Cederholm20-Reference Bollwein, Diekmann and Kaiser22) . Ageing can be associated with reduced food intake(Reference Giezenaar, Chapman and Luscombe-Marsh23) and anabolic resistance (losing the ability to build muscle mass) is exacerbated with age(Reference Witard, Wardle and Macnaughton16). Thus, strategies are needed to help increase protein intake and preserve SMM in older adults. One strategy would be to consider the specific nutritional requirements and physiology of older adults and design functional foods that are a good source of dietary protein that when consumed have a positive effect on MPS and SMM.
The broad nature of muscle hypertrophy and protein metabolism means that its complexity cannot be discussed in one review. The primary focus of this review is to examine the role of dietary protein on muscle mass preservation during ageing, by exploring the role of EAA and protein intake patterns. In addition, to consider novel dietary strategies for optimising SMM preservation in older adults, factors such as protein source, food form and digestibility are also discussed. Finally, the potential use of protein blends in the design of functional foods and the benefits of ‘snack’ functional foods for older adults will be considered.
Essential amino acids and muscle protein synthesis
Maintenance of muscle mass is achieved through a balance between MPS and muscle protein breakdown rates. To gain SMM, MPS rate must exceed the rate of muscle protein breakdown and when the converse happens skeletal muscle wasting can occur(Reference Koopman and van Loon24). The availability of AA, in particular EAA, is important in the stimulation of MPS(Reference Zizza, Tayie and Lino25). EAA activate the mammalian target of rapamycin (mTOR) signalling pathway in muscle cells, which serves to integrate intracellular and extracellular signals that regulate cell growth and metabolism(Reference Wang and Proud26). Increased plasma aminoacidaemia means more AA are available to stimulate mTOR for increased MPS(Reference Bollwein, Diekmann and Kaiser22), and therefore preservation of muscle mass. The exact mechanism by which EAA induce the mTOR pathway (Fig. 1) has been explored in several studies(Reference Hsu, Kang and Rameseder27–Reference Yoon32). The mTOR protein is a 289-kDa serine-threonine kinase that exists in two complexes, mTOR complex 1 (mTORC1) and mTORC2, which are important modulators of cell growth and proliferation(Reference Hsu, Kang and Rameseder27,Reference Rebsamen, Pochini and Stasyk28) . Findings from a recent study in cultured SH-SY5Y cells and mouse embryonic fibroblasts indicate that tyrosine kinase Src plays a crucial role in the AA mediation of mTORC1(Reference Pal, Palmieri and Chaudhury29). Src regulates mTORC1 activity via Rag GTPases(Reference Bar-Peled, Chantranupong and Cherniack30), by causing the separation of Gap Activity TOward Rags 1 (GATOR1), a Rag GTPase-activating protein, from the Rag proteins(Reference Pal, Palmieri and Chaudhury29). The Rag GTPases are composed of RagA or RagB bound to RagC or RagD(Reference Pal, Palmieri and Chaudhury29). In addition, Sestrin2 has been identified as one of the proteins involved in sensing and signalling AA, in particular leucine, to the Rag GTPases(Reference Chantranupong, Scaria and Saxton31).
Overview of mammalian target of rapamycin (mTOR) signalling pathway showing the basic features of mTOR involved in regulating muscle protein synthesis. Amino acids enter the muscle from the bloodstream. Sestrin2 is involved in sensing and signalling the amino acids to the Rag GTPases(Reference Chantranupong, Scaria and Saxton31). The interaction of Sestrin2 with Gap Activity TOward Rags 2 (GATOR2) inhibits mTOR complex 1 (mTORC1) signalling in the absence of leucine(Reference Chantranupong, Scaria and Saxton31). GATOR1 is a Rag GTPase-activating protein which causes RagA/B to switch to an inactive form containing GDP, which inactivates mTORC1 in the absence of amino acids. GATOR1 inhibits mTORC1(Reference Pal, Palmieri and Chaudhury29). In the presence of amino acids, GATOR2 activates RagA/B and inhibits GATOR1, which promotes activation of mTORC1. Rheb is an essential and potent kinase activator of mTORC1(Reference Pal, Palmieri and Chaudhury29). Activation of mTORC1 brings about phosphorylation of 70 kDa ribosomal protein S6 kinase 1 (S6K1) and eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1), thereby promoting protein synthesis by activating ribosomal protein S6 and by causing the release of eIF-4E, the translation initiation factor(Reference Yoon32).
The role of leucine
Leucine plays an important role in stimulating MPS, with several in vivo studies reporting its effects on MPS in both animal(Reference Crozier, Kimball and Emmert12) and human(Reference Wilkinson, Hossain and Hill11) skeletal muscle. The mechanism by which leucine improves muscle mass is by acting as a potent activator of the mTOR pathway(Reference Olza, Mesa and Poyatos33). Several studies have been conducted in older adults investigating the impact of leucine on MPS and its role in muscle mass preservation in this cohort (Table 1). Katsanos et al. (Reference Katsanos, Kobayashi and Sheffield-Moore34) reported that increasing dietary leucine from 1·7 g to 2·8 g caused a comparable increase in postprandial MPS rates in elderly participants (average age 66·6 years) compared with young adult participants (average age 29·7 years). The beneficial effects of leucine were further demonstrated in a study involving elderly men, where ingestion of leucine (2·5 g) with pure dietary protein (casein) increased postprandial MPS rates(Reference Wall, Hamer and de Lange35). Even in the case of hyperaminoacidaemia, the addition of leucine to complex meals can efficiently increase MPS, as observed in a study where supplementation of mixed meals with 0·052 g leucine/kg body weight significantly increased MPS rates in healthy elderly men(Reference Rieu, Balage and Sornet36). Furthermore, co-ingestion of leucine (5 g) three times per d (with each meal) can increase MPS under resting conditions, irrespective of total protein intake(Reference Murphy, Saddler and Devries37), suggesting that leucine makes an impact on MPS in older adults without the need to alter food intake. The authors proposed, however, that the increase in MPS over 3–5 d may only have been an initial response to additional leucine and may not have been sustained over a longer period of time(Reference Murphy, Saddler and Devries37). Therefore, future studies should have a longer duration to determine if MPS is sustained. Overall, these studies suggest that the addition of free leucine to meals represents an effective strategy to augment postprandial MPS in older adults.
Studies investigating the impact of various essential amino acids on muscle protein synthesis (MPS) and muscle protein breakdown in the elderly
n, Sample size; EAA, essential amino acids; Leu, leucine; Arg, arginine; ↑, increase; LBM, lean body mass; SMM, skeletal muscle mass.
Current data suggest that giving doses above 7 g EAA, containing at least 2·5 g leucine per meal, is sufficient to increase MPS rates, with potential to attenuate muscle wasting in older adults (Table 1). However, it is important to note that in these studies muscle atrophy was not measured following leucine supplementation; therefore it remains inconclusive as to whether leucine supplementation can benefit muscle wasting conditions. As well as improving postprandial MPS, increasing leucine in the diet can improve muscle mass and strength in the elderly. Dillon et al. (Reference Dillon, Sheffield-Moore and Paddon-Jones38) reported increases in lean muscle mass (3·9 %) in elderly women following consumption, twice daily for 12 weeks, of a leucine-rich EAA supplement containing 1·39 g leucine (Table 1). In an additional study, comprised of a similar age group, a leucine-rich EAA supplement containing 3·95 g leucine, taken twice daily for 16 weeks (Table 1), resulted in a significant (P<0·05) increase in lean body mass and muscle strength even in the absence of exercise(Reference Børsheim, Bui and Tissier39). However, contrary to this, some studies report that leucine supplementation does not have an impact on MPS and muscle mass preservation, even when total protein intake was greater than the RDA(Reference Rieu, Sornet and Bayle40,Reference Vianna, Resende and Torres-Leal41) . Verhoeven et al. (Reference Verhoeven, Vanschoonbeek and Verdijk42) and Leenders et al. (Reference Leenders, Verdijk and van der Hoeven43) reported that chronic leucine supplementation caused no changes to SMM or strength in healthy and diabetic elderly men. One review article hypothesised that leucine supplementation had no effect on muscle mass when the leucine signal and the rise in AA no longer function in sync. This can occur due to rapid absorption of free leucine following digestion, while other AA are released at a later stage after digestion(Reference Dardevet, Remond and Peyron44). The delayed release of AA means that protein synthesis was only stimulated for a short period postprandially; therefore a significant increase in muscle mass was not observed(Reference Dardevet, Remond and Peyron44). Overall, the evidence implies that anabolic resistance may be overcome by increasing the leucine content of a meal. Further studies are warranted and, in particular, should focus on whether long-term leucine supplementation helps reduce loss of muscle mass during ageing.
Table 1 shows studies investigating the impact of various EAA on MPS and muscle protein breakdown in the elderly(Reference Katsanos, Kobayashi and Sheffield-Moore34,Reference Wall, Hamer and de Lange35,Reference Murphy, Saddler and Devries37–Reference Børsheim, Bui and Tissier39,Reference Vianna, Resende and Torres-Leal41–Reference Leenders, Verdijk and van der Hoeven43,Reference Ferrando, Paddon-Jones and Hays45–Reference Symons, Schutzler and Cocke47) .
The role of other amino acids
Research on the role of other AA in MPS during ageing is limited, with the majority of data reported being extrapolated from in vitro cell culture models and studies in young pigs. Studies on isoleucine show that it can enhance the fractional synthetic rate of protein synthesis in bovine mammary cells(Reference Appuhamy, Knoebel and Nayananjalie48), which has important consequences for muscle contraction(Reference Richter and Hargreaves49). Data from a study in pigs, whose diet was supplemented with 7·8 g/kg crystalline lysine, suggest that lysine may potentially function as a regulator of the protein synthesis process, as an increase in total muscle weight and muscle protein accumulation was observed(Reference Morales, García and Arce50). Lysine was shown to promote protein synthesis at a translational level by helping to construct proteins, and also at a transcriptional level by affecting the expression of myosin in muscle(Reference Morales, García and Arce50). Tryptophan has been shown to attenuate muscle loss, by directly impacting molecular signalling in skeletal muscle(Reference Dukes, Davis and Refaey51) and studies by Wang et al. (Reference Wang, Qiao and Yin52) demonstrated that threonine was associated with decreased MPS in young pigs. However, the authors highlighted that the young age of the pigs (25 d) may have contributed to this finding, because the small intestine of young pigs is more sensitive (regarding protein synthesis rates) to dietary threonine levels, and so there may have been a reduced uptake of AA by the intestinal mucosa, even in the case of excess threonine. While these studies suggest a role for other AA in MPS and preserving muscle, it is important to acknowledge that none of these studies involved aged animals, highlighting the need for further studies in older models to confirm if the effects are similar.
Other factors that influence muscle protein synthesis
Protein intake pattern
While there is no working definition for protein intake pattern, it has been described as the spread of protein over a number of meals (protein spread-feeding), or the consumption of a large amount of protein in a single meal (protein pulse-feeding; PPF)(Reference Deutz, Bauer and Barazzoni21). There is conflicting evidence as to whether protein intake pattern is relevant to protein synthesis and SMM preservation. Kim et al. (Reference Kim, Schutzler and Schrader53) investigated the effects of ingesting protein in older adults as mixed meals of two doses of protein (0·8 g/kg per d or 1·5 g/kg per d) and two intake patterns (uneven; 15, 20 and 65 % protein at each meal: v. even; 33 % protein distribution at each meal) over 4 d. The results showed that protein intake above the RDA (1·5 g/kg per d) increased whole-body net protein balance through higher rates of protein synthesis but protein intake pattern had no effect on net protein balance. While there were a number of limitations with this study, namely small sample size (four or five subjects per group), short study duration (4 d) and variable sex ratio, a more recent longitudinal study, involving a larger number of participants (seven subjects per group), also reported that protein intake pattern had no effect on lean body mass, muscle strength or functional outcomes in older adults(Reference Kim, Schutzler and Schrader54). As the minimal RDA for protein was provided in all of these studies it would be expected that maximal MPS response would be achieved and thus not affected by protein intake pattern. However, earlier research by Arnal et al. (Reference Arnal, Mosoni and Boirie18) tested the efficiency of PPF v. protein spread-feeding in improving protein anabolism in healthy elderly women. After 14 d, the PPF group had higher N retention compared with the protein spread-feeding group, due to higher whole-body protein synthesis rates. A more recent study by Bouillanne et al. (Reference Bouillanne, Neveux and Nicolis55) supports this finding, demonstrating that PPF improved MPS in hospitalised (malnourished) elderly patients by increasing plasma postprandial EAA. Additionally, the authors reported that postprandial AA bioavailability persisted after 6 weeks of PPF. While results in undernourished patients(Reference Bouillanne, Neveux and Nicolis55) cannot be directly extrapolated to healthy adults, these studies suggest that a single high-protein meal may be better at stimulating MPS than protein spreading throughout the day. It is repeatedly emphasised that the MPS response to food intake lasts for 4–5 h after ingestion in adults(Reference Arnal, Mosoni and Boirie18,Reference Kumar, Atherton and Smith56,Reference Moore, Robinson and Fry57) . Norton et al. (Reference Norton, Toomey and McCormack58) recently reported that a twice-daily supplementation (at breakfast and lunch) of a milk-based protein matrix (containing 106 (sd 20) g protein per d) increased lean tissue mass in older adults. This may be a more favourable approach for older adults who suffer from reduced appetite and gait speed and poor nutritional status. However, as protein synthesis is not the only controlling factor in muscle protein accretion, it is not possible to conclude that protein consumption at regular intervals throughout the day will maximise daily muscle protein accretion. The current evidence available makes it difficult to confirm that PPF is the superior protein intake pattern. Further studies on the impact of protein intake pattern on MPS in older adults are warranted.
Protein quality
Protein quality, defined by its AA composition and digestibility, has a significant impact on protein metabolism(Reference Dideriksen, Reitelseder and Holm17) and affects the potential of a protein to contribute to increased MPS following consumption(Reference van Vliet, Burd and van Loon10). Studies suggest that animal protein sources are superior to plant sources in their ability to stimulate MPS(Reference Phillips59,Reference Hartman, Tang and Wilkinson60) and increase SMM. Phillips(Reference Phillips59) reported that protein from beef was superior in its ability to stimulate postprandial MPS in middle-aged men (approximately aged 55 years) compared with an isonitrogenous amount of soya-based protein. Another study in younger men compared the habitual consumption of dairy protein with soya protein, in conjunction with similar resistance exercise regimens, for a period of 12 weeks, and reported an increase in lean muscle mass in the dairy product group(Reference Hartman, Tang and Wilkinson60). Whey protein is distinct from other proteins because it is rapidly digested and contains a high proportion of EAA(Reference Volek, Volk and Gomez61). Volek et al. (Reference Volek, Volk and Gomez61) demonstrated that whey was significantly (P<0·05) more effective than an isoenergetic amount of soya protein in increasing lean body mass in young men following a 36-week study that included defined exercising conditions. Overall, studies assessing the ability of soya protein to increase MPS have consistently shown that soya protein is incapable of increasing MPS to the same extent as an isonitrogenous amount of whey protein(Reference Tang, Moore and Kujbida62), skimmed milk(Reference Wilkinson, Tarnopolsky and MacDonald63) or beef(Reference Morales, García and Arce50).
Studies involving other plant-based proteins indicate that they can stimulate MPS to a comparable level as dairy proteins but only when consumed in amounts that largely exceed the RDA. This is probably due to the fact that most plant-based proteins have a lower leucine content of 6–8 % compared with animal proteins which have 8·5–10 %, and thus when consumed in low doses, do not increase MPS to a comparable extent as animal proteins(Reference van Vliet, Burd and van Loon10,Reference Joy, Lowery and Wilson64) . However, when matched for leucine content or protein quantity, MPS rates in response to plant proteins do not differ significantly from animal proteins. This was demonstrated in a study by Joy et al. (Reference Joy, Lowery and Wilson64) who observed that 48 g rice protein decreased fat mass, increased lean body mass, skeletal muscle hypertrophy and muscle strength to a comparable degree as an equivalent amount of whey protein during an 8-week intervention study in healthy young men. In agreement, Babault et al. (Reference Babault, Païzis and Deley65) reported that 25 g pea protein resulted in a comparable increase in SMM compared with 25 g whey protein, in a 12-week study involving 137 young males. Data from these studies make it reasonable to suggest that plant proteins can be used as an alternative to animal proteins, but only when the amount consumed is greater than the RDA for protein. To date there have been limited studies performed to extrapolate these findings to older adults. Gorissen et al. (Reference Gorissen, Horstman and Franssen66) compared the effects of wheat protein to whey and casein proteins on MPS rates in older adults. Findings revealed that ingestion of 35 g wheat protein hydrolysate, providing 2·5 g leucine, increased MPS rates to a comparable level as intact casein, despite casein resulting in a more prolonged aminoacidaemia. The same study found that although intact whey protein caused a notable increase in plasma EAA concentrations, ingestion of wheat protein hydrolysate caused more sustained levels of AA in the circulation, which resulted in greater stimulation of postprandial MPS rates(Reference Gorissen, Horstman and Franssen66). The authors suggested that the sustained levels of AA could improve anabolic resistance in an older population, leading to greater MPS response(Reference Gorissen, Horstman and Franssen66). Interestingly, a recent study in aged rats fed a meal mixed of soya and whey proteins (70 % soya, 30 % whey) reported that plant proteins can counteract anabolic resistance as efficiently as animal proteins, but only if the protein and leucine content are increased(Reference Jarzaguet, Polakof and David67). In general, these studies indicate that to achieve comparable effects to animal proteins, plant-based proteins need to be incorporated in the diet at high daily doses. However, further studies in older adults are warranted to confirm the effects in this cohort.
Protein digestibility, absorption kinetics and older adult physiology
Protein digestibility and absorption kinetics are independent predictors of postprandial MPS(Reference Pennings, Boirie and Senden68,Reference Dangin, Boirie and Garcia-Rodenas69) . Protein digestibility relates to the amount of dietary protein that is effectively digested and absorbed in a form acceptable for body protein synthesis(Reference Rutherfurd and Moughan70). It is influenced by protein structure, the food matrix in which a protein is contained, the presence of anti-nutritional factors and the processing method applied to a food(Reference Joye71). Digestion speed of a protein can also influence postprandial protein retention and accretion(Reference Dangin, Boirie and Garcia-Rodenas69,Reference Rutherfurd and Moughan70) . Slowly digested proteins, such as casein, are associated with slower AA absorption kinetics and this affects whole-body protein anabolism by promoting protein deposition(Reference Boirie, Dangin and Gachon72). Casein promotes postprandial protein deposition by inhibiting proteolysis without excessive increase in AA concentration in the plasma(Reference Boirie, Dangin and Gachon72). Further, the importance of digestion rate on protein deposition in young men has been investigated(Reference Dangin, Boirie and Garcia-Rodenas69). It was demonstrated that in this cohort, slow-digesting proteins are better utilised postprandially than fast-digesting proteins, as slow-digesting proteins cause higher postprandial protein gain than fast-digesting proteins(Reference Dangin, Boirie and Garcia-Rodenas69). Interestingly, later studies by these authors revealed that fast-digesting proteins may be more beneficial for older adults, as it limits the loss of protein during ageing by affecting protein turnover(Reference Dangin, Boirie and Guillet73) and protein gain(Reference Dangin, Guillet and Garcia-Rodenas74). This is because fast-digesting proteins are associated with increased AA absorption and fast protein turnover(Reference Dangin, Boirie and Guillet73). The authors reported a more favourable postprandial leucine balance in elderly subjects with fast-digesting proteins(Reference Dangin, Boirie and Guillet73). Data from these studies suggest that older adults should aim to consume more fast-digesting proteins than slow-digesting proteins, in order to limit protein loss during ageing, which may favourably make an impact on the maintenance of muscle mass and strength. Further studies are warranted to elucidate the role that protein type may play in muscle mass preservation.
It has been reported that protein digestion rates and subsequent MPS in healthy older adults is comparable with younger adults(Reference Koopman, Walrand and Beelen75). However, protein digestion efficiency may be a key determinant of AA bioavailability for protein synthesis(Reference Gorissen, Rémond and van Loon76). A study of older adults, with reduced ability to chew, reported lower whole-body protein synthesis rates compared with dentate subjects following consumption of meat(Reference Rémond, Machebeuf and Yven77). Impaired chewing efficiency not only delayed absorption of meat proteins, but also lowered the amount of AA entering peripheral blood. While an increase in plasma AA was observed, there were insufficient levels to stimulate whole-body protein synthesis. These findings highlight the importance of considering chewing efficiency when developing protein-rich snacks and meals for older adults. The food form must be adapted for chewing ability to allow for maximum AA absorption and postprandial protein synthesis. Future research should investigate how protein digestion and absorption kinetics of different proteins sources influence aminoacidaemia and MPS rates while taking into account the chewing efficiency of its subjects.
Protein source
Animal proteins, such as those sourced from eggs, dairy products and meat, are highly digestible (>90 %)(78). Studies have reported a fast and transient increase in plasma AA levels following consumption of whey protein by healthy adults(Reference Dangin, Boirie and Garcia-Rodenas69,Reference Boirie, Dangin and Gachon72) and additional studies revealed that 50–70 % of protein-derived AA from milk and beef become bioavailable 5–6 h after consumption(Reference Dangin, Boirie and Guillet73,Reference Gorissen, Rémond and van Loon76) . Once bioavailable, these AA can help increase postprandial MPS. Plant protein sources (such as oat, pea, potato) exhibit digestibility values ranging from 45 to 80 %(78). The lower digestibility rates associated with plant proteins may be due to the presence of anti-nutrients, such as phytates, tannins and saponins(78). In vivo studies with pigs demonstrated that polyphenols (tannins in particular) decreased ileal protein digestibility by 5 %(Reference Myrie, Bertolo and Sauer79). More recently Dufour et al. (Reference Dufour, Loonis and Delosière80) reported that the release of polyphenols from a food matrix had a negative impact on protein digestibility. In the study, when proteins were consumed with phenolic extracts protein digestibility was decreased by 15 % compared with when the proteins were consumed with whole fruit and vegetables that contained polyphenols(Reference Dufour, Loonis and Delosière80). These studies indicate that the composition of a food matrix, in particular the polyphenol content, has an impact on plant protein digestibility. Future strategies to improve the digestibility and absorption kinetics of plant proteins, and subsequently MPS, could be selective plant breeding(Reference van Vliet, Burd and van Loon10) that considers polyphenol composition or foods designed based on specific protein:polyphenol content. Further studies, however, are needed to identify optimal protein:phenol ratios.
It has been reported that plant proteins, particularly soya and wheat, have a lower potential to stimulate MPS because their AA are readily converted to urea(Reference Joy, Lowery and Wilson64,Reference Fouillet, Juillet and Gaudichon81–Reference Yang, Churchward-Venne and Burd83) . It is thought that the rapid conversion of plant proteins to urea is due to a lack of one or more EAA(Reference van Vliet, Burd and van Loon10). This unbalanced AA profile may lead to a lower retention of AA by the digestive tract and thus cause more AA to enter the portal vein to hepatic tissue, which signals the liver to increase ureagenesis(Reference van Vliet, Burd and van Loon10). The increased conversion of AA to urea means that less plant-derived AA become available in the systemic circulation, resulting in lower postprandial availability of AA to stimulate MPS(Reference van Vliet, Burd and van Loon10). Currently there is a lack of research on the digestion kinetics of plants other than soya and wheat, and it is not fully understood why consuming excessive amounts of plant proteins, which do have a complete AA profile, increases urea production; therefore future research in this area would be of interest.
Food processing and protein digestibility
Food processing also makes an impact on the digestibility and absorption kinetics of proteins. Glycation, also known as the Maillard reaction, is the chemical reaction that occurs between AA and reducing sugars, usually in the presence of heat. A recent systematic review on the effect of processing on milk protein digestibility concluded that glycation decreases digestibility(Reference van Lieshout, Lambers and Bragt84). Both in vitro and in vivo studies have reported that AA glycation can have an impact on protein digestibility and the bioavailability and functionality of AA(Reference Gilani and Sepehr85–Reference Hellwig and Henle88). It is hypothesised that glycation decreases protein digestibility by: (1) directly blocking lysine and preventing it from cleavage by digestive proteases; (2) indirectly blocking lysine residues which are near the enzyme cleaving sites; and (3) causing cross-linking which hinders the accessibility of the cleavage sites by proteases(Reference van Lieshout, Lambers and Bragt84). The high temperature used in processing can cause protein denaturation. However, this does not negatively make an impact on digestibility, but rather alters the gastric emptying of proteins, affecting digestion kinetics(Reference van Lieshout, Lambers and Bragt84). Other processing reactions can also affect digestibility; for example, oxidation has been shown to modify EAA such as methionine, tyrosine and tryptophan(Reference Rutherfurd, Montoya and Moughan89). Furthermore, racemisation, which transform l-amino acids into their d-form, is thought to affect ileal protein digestibility by reducing recognition of d-amino acids and free d-amino acids by peptidases(Reference de Vrese, Frik and Roos90). Cooking significantly increases the protein digestibility of chickpeas, beans, lentils, peas and kidney beans(Reference Drulyte and Orlien91). Microwave cooking, using a very low amount of energy (500 J), significantly increased the protein digestibility of faba bean varieties from 46·0, 52·2 and 51·5 %, to 57·1, 68·0 and 53·2 %(Reference Pysz, Polaszczyk and Leszczynska92). Pressure cooking and autoclaving caused significant improvement in protein digestibility in moth beans(Reference Negi, Boora and Khetarpaul93), soyabeans(Reference Piecyk, Wołosiak and Druzynska94) and peas(Reference Frikha, Valencia and de Coca-Sinova95). In light of all this evidence, the metabolic impact of food processing methods on SMM remains to be elucidated. It is clear that processing affects protein digestibility, but most studies do not report the impact of food processing on MPS or SMM; thus further research is warranted in this area.
Food strategies to improve muscle protein synthesis in older adults
Protein blends
A protein blend is the combination of two or more protein sources and the potential of protein blends to increase MPS has been the focus of recent research(Reference Reidy, Walker and Dickinson96–Reference Butteiger, Cope and Liu98). Witard et al. (Reference Witard, Wardle and Macnaughton16) suggested that protein blending has the potential to allow for optimal plasma aminoacidaemia to increase and lengthen the MPS response(Reference Witard, Wardle and Macnaughton16). AA in a protein blend can potentially act synergistically to bring about increased MPS response and biological activity. Blending different protein sources could also potentially improve protein digestion rates to a comparable level to that of fast-digesting proteins, which is beneficial for maximum MPS.
The efficacy of a protein blend for MPS stimulation in young adults has been evaluated, and it was found that 19·3 g of a protein blend that consisted of 25 % soya, 25 % whey and 50 % casein prolonged postprandial plasma aminoacidaemia to a level comparable with 17·7 g whey protein(Reference Reidy, Walker and Dickinson96). Likewise, Borack et al. (Reference Borack, Reidy and Husaini97) used the same protein blend in a study with elderly male subjects and reported similar responses in hyperaminoacidaemia, mTORC1 signalling and MPS stimulation. In both studies the protein diets (protein blend v. whey) were matched for total EAA but not leucine content, although both diets still contained enough leucine to stimulate MPS(Reference Pennings, Groen and van Dijk99). While both protein diets elevated hyperaminoacidaemia and mTORC1 signalling, fractional synthetic rate elevation was sustained to a significantly (P<0·05) greater extent 2–4 h postprandially in the whey protein group(Reference Borack, Reidy and Husaini97). In addition, the study reported that the protein blend did not extend the duration of MPS stimulation more than one individual protein source (whey)(Reference Borack, Reidy and Husaini97). This re-emphasises the idea that whey protein is more efficient at promoting positive N balance in skeletal muscle after exercise than soya–dairy blends given at the same dose. An in vivo study by Butteiger et al. (Reference Butteiger, Cope and Liu98) reported that a soya–dairy blend meal with different ratios of dairy:soya proteins (25 % whey, 50 % casein, 25 % soya) significantly (P<0·05) increased MPS rates to a greater extent compared with whey or soya protein alone. The results were attributed to the increased casein level used in the blend, and the slow-digesting nature of this protein which delays the appearance of plasma AA. It is important to acknowledge that the study was performed in young rats; thus trials in older models should be conducted to see if a similar response would be observed. Also, it remains to be explored if these findings would influence muscle wasting conditions.
Collectively, the data from these studies demonstrate that protein blends can be designed to increase MPS and potentially attenuate muscle wasting conditions. However, all of these studies utilised a mixture of plant and animal proteins, and all contained dairy proteins. There is limited research on the biological impact of plant-only protein blends. Van Vliet et al. (Reference van Vliet, Burd and van Loon10) suggested that the consumption of plant-based protein blends (regardless of leucine content), with no limiting AA, may have a more positive impact on postprandial MPS response than ingestion of an individual plant protein source in older adults and compromised populations, but additional studies are warranted. For example, in vitro synergy tests to compare the effects of plant protein blends to individual plant or animal protein sources are required, and additional in vivo studies investigating the impact of plant-based protein blends on muscle mass gain and muscle loss are also lacking. It is also necessary to explore what benefits could be achieved from using protein blends with high leucine content(Reference Jarzaguet, Polakof and David67) and what quantities of plant-based protein blends should be consumed by older adults with anabolic resistance.
Food form
Liquid- and solid-food matrices can elicit different postprandial aminoacidaemia responses. The impact of food form on protein delivery to the muscles to enhance MPS rates has been the focus of recent research. Conley et al. (Reference Conley, Apolzan and Leidy100) provided beverage and solid-form (macronutrient-matched) meal replacements to older adults and reported that the liquid form resulted in higher initial (30 min) and sustained (4 h) plasma AA concentration compared with the solid-form supplement. Burke et al. (Reference Burke, Hawley and Ross101) reported that liquid forms of protein (soya milk, skimmed milk) achieved higher peak concentrations of plasma AA compared with ingestion of solid protein-rich foods (protein bar, beefsteak, eggs); however, this study was carried out in young adults. This limited evidence suggests that liquid-form protein-rich meals elicit more rapid and higher plasma AA concentration, provided they contain sufficient amounts of EAA, but further studies in older adults are needed. Furthermore, Stull et al. (Reference Stull, Apolzan and Thalacker-Mercer102) found that liquid meal replacement products were less satiating than solid meal replacements in older adults, which implies that liquids may be better to increase protein intake and increase MPS rates in undernourished individuals, where an increase in overall food intake is required(Reference Giezenaar, Chapman and Luscombe-Marsh23), but solid meals with more satiating properties may be preferable when food intake is more restricted.
Snack foods
The term ‘snack’ lacks a consistent definition. ‘Snacking’ has been described as an eating occasion involving the consumption of energy and nutrients, regardless of whether healthy options are chosen(Reference Hess, Jonnalagadda and Slavin103). Other definitions refer to ‘snacks’ as typically high-energy, sugary, salty or fatty food or beverage items(Reference Hess, Jonnalagadda and Slavin103) and define a ‘snack’ based on the time it is consumed during the day(Reference Duffey, Pereira and Popkin104–Reference Wang, Zhai and Zhang107) or based on the amount consumed(Reference Nicklas, Yang and Baranowski108). The lack of a concise definition for snack foods makes it difficult to classify ‘snacking’ as beneficial or harmful to health. Indeed, many studies tend to focus on the detrimental effects of snack foods and fewer studies focus on the potential positive impact. For older adults the use of ‘snack foods’ could play a role in promoting healthy ageing, especially for undernourished subjects. Inadequate protein and energy consumption in older adults can contribute to decreased functional ability, resulting from increased loss of muscle mass, insufficient energy stores, and decreased immune function(Reference Chernoff109). Snacking can increase daily protein intake in older adults(Reference Zizza110), and subsequently potentially improve muscle mass preservation and/or muscle mass gain. Zizza et al. (Reference Zizza, Tayie and Lino25) reported a significant increase in energy, protein, carbohydrates and total fat in ‘snacking’ older adults (≥ 65 years) compared with non-snacking older adults(Reference Zizza, Tayie and Lino25). The authors reported that ‘snacking’ older adults consumed 6 g more protein than ‘non-snacking’ older adults(Reference Zizza, Tayie and Lino25). While the study did not assess the relationship between increased protein intake and MPS or muscle mass preservation in the test subjects, it is reasonable to extrapolate that the additional dietary protein positively influenced MPS and muscle mass gain. Snacking is also positively associated with gait speed(Reference Xu, Yu and Zizza111) and recent research has shown that an even distribution of protein intake throughout the day can positively influence frailty in older adults and quality of life(Reference Bollwein, Diekmann and Kaiser22). Physical function is important in this age group, as increased frailty increases the risk of falls, which in turn increases the risk of mortality and morbidity(19).
Other benefits associated with snacking include the fact that increased meal frequency has the potential to improve lipid profiles and subsequently decrease the risk of CVD, by decreasing total and LDL-cholesterol(Reference Hess, Jonnalagadda and Slavin103). Furthermore, an inverse relationship between snacking and body weight has been reported(Reference Hess, Jonnalagadda and Slavin103), suggesting a potential role for snacking in maintaining a healthy body weight in older adults. These studies suggest that snacking has the potential to improve nutritional status and physical function; however, knowledge on the role of snacks in preserving muscle mass is limited. Further research is warranted to evaluate the role of snacking in overcoming anabolic resistance in the elderly and in muscle mass preservation.
Conclusion
To date, leucine remains the only EAA that has been extensively studied for its role in MPS in older adults, with the majority of studies indicating that incorporating leucine in sufficient doses in the diet increases MPS and improves SMM in this cohort. Additional studies are needed to investigate the role of other EAA, in particular isoleucine, lysine, threonine and tryptophan. There is still no consensus as to whether protein intake pattern can significantly affect MPS, but studies relating to older adults suggest that health and nutritional status are important considerations when deciding which intake pattern is most appropriate. The digestibility and absorption kinetics reported for plant proteins suggests that they have inferior anabolic properties for stimulating MPS, compared with animal protein sources. However, blending different protein sources has had favourable effects on MPS; thus, this strategy has potential. Studies are needed to identify optimal protein blends, including plant protein blends that stimulate MPS and investigate their potential to preserve SMM in older adults. Liquid-form foods have a more favourable effect on postprandial aminoacidaemia than solid-form foods; however, the effects of food form on MPS and SMM preservation need to be further elucidated. The evidence supporting snack foods as a strategy to improve MPS and SMM is currently limited; however, snacking may be beneficial as a strategy to increase protein intake and maintain a healthy body weight for undernourished older adults, and thus would help to promote healthy ageing.
Acknowledgements
The work conducted in the present review was supported by a RÍSAM PhD scholarship to E. T. O. from Cork Institute of Technology.
All authors contributed equally to this manuscript.
There are no conflicts of interest.
