Protein Requirements for Master Athletes: Just Older Versions of Their Younger Selves

Dietary protein supports athlete’s training and adaptation by providing the essential amino acid building blocks for the remodeling of muscle and body proteins. While carbohydrate and fat represent the primary energy sources during exercise, amino acid oxidation may contribute up to 10% of energy during exercise when carbohydrate availability is low [1, 2]. Thus, research has clearly established that daily protein requirements are elevated above the current minimum daily recommended allowance of ~ 0.8 g/kg/day across the spectrum of sport disciplines [3]. However, the majority of research that has contributed to these increased recommendations has been performed in active young athletes (18–35 years). The aging of these younger athletes and a greater awareness of an active lifestyle to support successful aging has contributed to expansion of athletes above the age of 35 years [4], which is the typical delineation for Master athlete although this may be discipline-specific. Therefore, there is a need to clearly define the nutrient requirements of these older athletes to support their training, adaptation, and health.

Muscle mass and strength as well as aerobic capacity generally peak in early adulthood (< 30 years) after which they begin a slow, insipid decline (~ 0.5–1%/year) that can accelerate in the 8th decade [5, 6]. However, high levels of physical activity typical of Master athletes tend to spare older adults from the typical loss of muscle and fitness that besieges their sedentary peers [7]. For example, a comprehensive review and meta-analysis by McKendry et al. [8] demonstrated that Master athletes’ aerobic capacity, muscle strength, and body composition are generally on par with young, untrained adults. The benefits of chronic endurance training are not only afforded to life-long athletes as late starting (i.e. > 50 years) Master athletes have similar aerobic capacity and muscle quality as those who maintained training from early adulthood [9]. Thus, while there is an inevitable decline in exercise capacity and performance with age, Master athletes are generally regarded as models of successful aging [7, 10]. Therefore, the question of whether other sequelae of aging that may impact protein requirements are observed in Master athletes warrants discussion. The present review will summarize the available evidence on the primary factors associated with typical age-related anabolic resistance and discuss to what extent they may be present in Master athletes. In addition, although there is relatively little research in middle-aged adults, who nonetheless could still qualify categorically as Master athletes, the review will primarily focus on older (i.e. > 60 years) adults and athletes to reflect the majority of research in this discipline. This older demographic (i.e. > 60 years) is also the primary focus of research that explores the potentially greater meal protein requirements for skeletal muscle remodeling in non-exercising older as compared to young adults (e.g. [11]). Therefore, the perception that protein requirements may differ in Master athletes is arguably more relevant to these older as compared to middle-aged Master athletes due to their greater chronological age. However, middle-aged Master athletes adhering to the recommendations discussed below and largely established in younger athletes would satisfy their protein requirements as well. Finally, the review will discuss daily protein recommendations determined using (traditional) whole body methodology (e.g. nitrogen balance, stable isotopes) but will also highlight what meal protein intake would optimize muscle protein remodeling; this latter approach will leverage knowledge from contemporary stable isotope techniques and has been used to define optimal daily protein intakes when suggesting optimal meal frequencies in older untrained adults (e.g. [12]). Thus, given meal frequencies are generally greater in athletes than untrained individuals, providing recommendations based on meal protein targets can also influence daily protein intakes, as will be discussed in more detail below.

The age-related decrements in muscle size and function are multifactorial and could include decreases in neuromuscular function, motor unit/fibre loss (especially of the type II fibres), muscle stem cell dysregulation, and/or atrophy of skeletal muscle, the latter of which is largely underpinned by an “anabolic resistance” to dietary protein and will be the primary focus of the present review (for reviews on the other factors, see [13‐15]). This anabolic resistance is characterized by a blunted muscle protein synthetic response to bolus (i.e. meal) dietary amino acid ingestion at rest [16‐18] and after resistance exercise in older untrained adults [19, 20] and is most pronounced in the myofibrillar protein fraction, which is critical for the maintenance of muscle mass and function given it represents the predominate fraction (~ 60% of total muscle protein) and includes the contractile elements of the sarcomere (e.g. actin, myosin). For example, an estimated average meal protein requirement of ~ 0.4 g/kg of high-quality, leucine-enriched protein (i.e. whey) is required to maximize myofibrillar protein synthesis (MyoPS) at rest in untrained older adults, which is ~ 65% greater than for younger adults [17]. Similarly, 20 g of high-quality protein (i.e. whey) is sufficient to maximally increase MyoPS after exercise in young, trained men, whereas older, untrained adults require up to 40 g [19, 21], findings that are also evident when the protein intakes are normalized to body mass (Fig. 1). Importantly, older untrained muscle is generally able to mount an equivalent MyoPS response to that of young adults when meal protein ingestion is at or above its respective maximal threshold [17, 19, 21], although this finding is not without exceptions [16]. Thus, this anabolic resistance to suboptimal meal protein intakes has contributed to the recommendations that older untrained adults require a greater meal protein intake to maximize muscle protein synthesis at rest [12] and after resistance exercise [11]. However, indiscriminately consuming supramaximal meal protein intakes can be inefficient for optimizing muscle remodeling as excess dietary protein may be retained in nonmuscle tissue [22] or be diverted to amino acid oxidation [19, 21, 23]. Therefore, subsequent meal protein targets will be discussed in relation to optimizing the efficiency of dietary amino acid ingestion insofar as maximizing muscle protein synthesis yet minimizing amino acid oxidative losses.

Anabolic resistance to dietary amino acids is multifactorial and could include chronic inflammation, reduced capillarity and/or vasodilatory capacity (and hence amino acid delivery), attenuated muscle uptake (e.g. amino acid transporter expression/function), and/or reduced ribosomal protein content or activity. However, Master athletes are regarded as successful models of aging [10] and are distinguished from their nonactive peers, on which the majority of aging research is focused, by their high levels of habitual activity. Thus, it is questionable whether meal protein recommendations for older untrained adults, which typically advocate for larger protein ingestions to offset any age-related anabolic resistance [11], are relevant for Master athletes. In light of the lack of specific research investigating daily or, especially, meal protein requirements of older trained adults, the following sections will discuss whether some of the factors commonly associated with or that underpin anabolic resistance could be present in Master athletes.

Resistance exercise is an inherently anabolic exercise modality that has been consistently shown to increase muscle protein turnover and attenuate the otherwise negative net muscle protein balance in the rested fasted state [45, 83]. However, consuming an exogenous source of amino acids is necessary to support maximal rates of muscle protein synthesis and a positive net protein balance, the latter of which is further supported by an ablation of the normal exercise-induced increase in muscle protein breakdown in the fasted state [46, 84, 85]. Current recommendations for young, resistance trained males is ~ 20 g of high-quality (i.e. leucine and essential amino acid-enriched) protein to maximize post-exercise rates of mixed muscle protein synthesis and MyoPS, while minimizing irreversible amino acid oxidative losses [21, 23]. Importantly, there is little evidence that young females exhibit a differential post-exercise muscle protein synthetic response with protein ingestion [86, 87], which permits a more generalizable recommendation of ~ 0.3 g/kg as an efficient target dose for resistance trained athletes [78]. Given the position above that there is little evidence for the presence of ‘anabolic resistance’ in Master athletes, this relative protein intake would represent a suitable target for older athletes as well to enhance muscle protein remodeling after a bout of resistance exercise (Fig. 3). However, should older athletes be concerned with the sufficiency of this intake then one could apply a safety margin for the estimated average requirement of 1.24, which represents the standard approach of twice the average assumed variance of 12% common to population level assessments [88]; this would, therefore, translate into ~ 0.37 g/kg or a modest ~ 28 g for a 75 kg athlete after exercise. While the impact of immediate post-exercise protein ingestion to enhance resistance training adaptations (e.g. strength, lean mass gains) has been reported to be small-to-negligible in mostly untrained individuals [89], it is beyond question that exogenous amino acid ingestion is required to maximize muscle protein synthesis [19, 21, 23, 77] and induce a positive muscle net protein balance [46, 84]; these outcomes would be consistent with the prudent advice that athletes consume a source of protein during the early post-exercise recovery period to facilitate recovery [3, 90]. For time points beyond the 3–4 h post-exercise period and/or on rested days, a target meal protein intake of 0.3 g/kg would also be sufficient [17].

With the goal of maximizing muscle mass and quality as a priority of most strength athletes, nutrition practices for these individuals would ideally support enhanced tissue remodeling (i.e. breakdown and synthesis of muscle protein) and a positive net protein balance. Thus, the repeated consumption of mixed macronutrient meals with an adequate quantity of protein to support MyoPS and carbohydrate to support training demands would be the cornerstone of their nutrition plan, which generally mirrors the practices of many younger elite athletes who consume 4–5 meals per day [91]. Incidentally, consuming four moderate (i.e. ~ 0.25 g/kg) whey protein doses supports greater rates of MyoPS [92] and whole body anabolism [93] than the same quantity of protein as two large (i.e. ~ 0.50 g/kg) or eight small (i.e. ~ 0.13 g/kg) meals during a prolonged 12 h post-exercise recovery period in young trained males. Moreover, a balanced distribution of protein intake as compared to a skewed intake has been reported to support moderately greater gains in lean body mass during resistance training in younger adults [94]. Thus, general recommendations for the repeated ingestion of moderate protein-containing meals every ~ 4 h (~ 4 to 5 meals) have been suggested to optimize muscle mass in trained populations [90, 95] (Fig. 4). This recommendation would likely also extend to Master athletes given the emerging evidence that meeting optimal meal protein targets is generally associated with greater lean mass and strength in their older untrained peers [96, 97], although additional research is still warranted in this field [98, 99]. However, 4–5 protein-containing meals at ~ 0.3 to 0.37 g/kg (designed to maximize muscle protein remodeling) could be manipulated to provide a daily intake (i.e. ~ 1.5 to 1.6 g/kg/day) that would be close to what has been reported to support maximal post-exercise rates of whole body protein remodeling and net protein balance [100, 101] as well as training-induced gains in fat free mass [102]. While lean body or muscle mass does not necessarily translate to greater muscle strength, it is notable that even small differences in daily protein intake (i.e. ~ 1.21 vs. ~ 1.34 g/kg/day) may be associated with detectable differences in muscle quality (total strength normalized to fat-free mass) in Master athletes [103]. It should be noted, however, that studies that specifically investigate daily protein targets typically do not provide the daily intake in a pattern that acute, lab-based studies would suggest would support an efficient muscle protein remodeling (i.e. maximal muscle protein synthesis with minimal amino acid oxidation), and therefore, might result in a slightly greater estimate. Therefore, a meal-protein approach to the dietary protein target has been advocated previously [78, 90, 104] and may be integrated more easily into most athletes’ optimal habitual meal frequency pattern (i.e. ~ 5 meals/eating occasions per day) [91] but generally skewed daily intake (i.e. the majority in the evening) [105].

Endurance training has consistently been shown to increase daily protein requirements due in part to the requirement to replenish exercise-induced amino acid oxidative losses [1, 106]. Although there are discrepancies between methodologies used to estimate requirements in athletes, safe intakes (i.e. 2 × standard deviation or upper 95% confidence interval of the estimated average requirement) during training are ~ 50% greater than the general recommended dietary allowance by nitrogen balance (i.e. ~ 1.2 to 1.4 g/kg/day) [1, 107] and tracer-derived indicator amino acid oxidation (i.e. ~ 1.7 to 1.8 g/kg/day) [108, 109]. Incidentally, seminal research using nitrogen balance that included both younger (~ 27 years) and middle aged (~ 52 years) athletes reported identical requirements of ~ 1.2 g/kg/day [107], suggesting protein requirements in younger Master athletes are not substantially different from athletes half their age. However, the relevance of merely maintaining nitrogen balance for athletes aiming to optimize training, recovery, and/or performance has been questioned previously [110, 111]. In support, young endurance athletes who consume 1.2 g/kg/day protein and adequate carbohydrate are able to maintain protein balance during 4 days of higher volume training despite experiencing apparent losses of muscle strength and 5 km time-trial performance [112]. In contrast, when these athletes consumed 1.75 g/kg/day, which was established to be sufficient to maximize whole body protein synthesis during recovery [108], whole body net protein balance remained positive during training and was concomitant with small-to-moderate benefits (relative to 1.2 g/kg/day) in muscle strength and time trial performance [112]. Collectively, these highlight that protein requirements are towards the upper range of the current sports nutrition guidelines [3]. An intake of ~ 1.7 to 1.8 g/kg/day, which represents a modest ~ 15% of total energy, is generally met in younger athletes provided they consume adequate energy [105] but may not be reached in some Master athletes [103, 113].

Due to different methodological approaches and inherent assumptions of nitrogen balance and stable isotope tracer methodology, it may be an academic debate as to which represents the ‘true’ requirement for these endurance athletes. Thus, a meal protein intake that maximizes muscle protein remodeling may also be explored for how to parse athlete protein recommendations. Recently, Churchward-Venne et al. [114] performed an elegant study examining the protein dose–response for muscle and whole body protein synthesis after endurance exercise in trained young adults and revealed that myofibrillar, but not mitochondrial, protein synthesis was nutritionally responsive and was maximized with the equivalent of ~ 0.5 g/kg of milk protein. Whole body protein synthesis did not plateau over the range of meal protein intakes studied (i.e. up to ~ 0.65 g/kg), which confirms the greater capacity to assimilate dietary amino acids into non-muscle protein pools [22, 115] that may nevertheless still be important for an endurance athlete’s recovery (e.g. plasma volume expansion, red blood cell remodeling, bone collagen turnover, etc.) [116]. Interestingly, the relative difference (~ 0.2 g/kg) between this post-endurance exercise maximal effective dose and the previously estimated maximal dose for resistance exercise [78] is similar to the protein equivalent of what could be mobilized from exercise-induced muscle catabolism [117] and subsequently oxidized [31]. As amino acid oxidative losses (especially of the branched chain amino acids including leucine) have been suggested to underpin the greater acute [31] and daily [1, 106] protein requirements of endurance athletes, it is possible that any factor that minimizes amino acid oxidation during exercise could have the reciprocal effect of a relative reduction in the acute post-exercise protein requirement. For example, estrogen has been shown to have a protein-sparing effect with endurance exercise due to a greater ability to mobilize and oxidize fatty acids [118, 119]. Middle aged (~ 52 years) [120] and older (~ 65 years) [121] trained adults have been reported to have a greater contribution of fat to total energy expenditure during intensity-matched exercise than younger athletes. Thus, inasmuch as this greater relative fat oxidation would also be accompanied by a reduction in amino acid oxidative losses, this would further suggest Master athletes would be unlikely to have requirements that are greater than younger athletes.

From a practical standpoint, it has been suggested that protein requirements may be greater in Master athletes after strenuous bouts of exercise that may elicit some degree of acute muscle damage given the reported delay in the restoration of muscle function relative to younger athletes [122]. This recommendation is based on some of the most relevant research on Master athlete protein intakes and muscle remodeling, which demonstrated that free-living rates of MyoPS in older athletes was ~ 16% of their younger athletic peers during 3 days of recovery from a bout of downhill running despite both groups consuming an equivalent 1.6–1.7 g/kg/day protein [123]. In a follow-up study, Master athletes completed a similar bout of damaging exercise (i.e. 30 min downhill running) in the morning before consuming protein supplements (3 × 0.3 g/kg or 3 × 0.6 g/kg every 2 h) during acute recovery prior to assessing performance through a battery of tests in the same afternoon [124]. This research revealed that Master athletes consuming the high-protein intake, which would exceed current recommendations in young athletes after exercise [78] and older untrained adults at rest [17] to maximize MyoPS, had a slightly attenuated decrease in muscle strength (~ 3.6 vs ~ 8.6%, respectively) compared to the lower protein condition [124]. While this research is of potential interest to Master athletes who might train multiple times per day, it may represent a practically challenging dietary approach to adhere to given the satiating effect of this macronutrient and the requirement to achieve appropriate carbohydrate targets for optimal performance. Nevertheless, these data could align somewhat with cross-sectional research suggesting that higher, but still suboptimal (i.e. ~ 1.35 g/kg/day), daily intakes are generally associated with greater muscle function in a small cohort of mixed discipline Master athletes [103]. However, it should be noted that optimizing rates of MyoPS does not always align with enhanced recovery of muscle function [125, 126], highlighting the need for additional research that may promote the restoration of exercise performance and/or optimize training adaptations outside of what is currently recommended for muscle protein remodeling [127].

Despite the reportedly greater relative participation rates of females than males in some Master events (e.g. marathon) [164], there is an unfortunate under-representation of female participants in sport science research [73]. Thus, much of our sports nutrition guidelines has been derived from research in males, which has forced the field to consider how the presence or absence of sex hormones may influence protein metabolism and nutritional requirements. Premenopausal female athletes are unlikely to have substantial differences in their protein requirements after resistance exercise given that muscle protein synthesis is general consistent across the menstrual phase [165] and available evidence is lacking to suggest female athletes’ response to dietary protein is different from their male counterparts [78]. Where requirements may differ slightly is with endurance training when the estrogen:progesterone ratio is high as the former has been shown to have a protein sparing effect during aerobic exercise [118, 119]. Thus, while direct evidence across the menstrual phase is lacking, protein requirements may be a conservative ~ 10 to 15% lower during the follicular as compared to luteal phase. Additional research is required to more clearly identify whether oral contraception may be a modifying influence on protein requirements [166]. Therefore, a practical approach to ensure protein needs are met by premenopausal woman across the menstrual phase could be to adhere to general guidelines derived primarily in males, as summarized above.

Menopause is generally characterized by a temporary but accelerated loss of muscle mass that is not attenuated by hormone replacement therapy [167] but can be partially offset by resistance exercise [168, 169], highlighting the importance of muscle contraction to offset ‘normal’ age-related muscular alterations. However, post-menopause has also been reported to represent a divergence in rested and postprandial muscle protein metabolism in untrained older adults (for review, see [170]). For example, while both untrained older females and males present a general anabolic resistance to exogenous amino acids at rest, females may be able to compensate for this with a greater basal rate of muscle protein synthesis [171, 172]. Studies are currently lacking that have compared both older males and females after exercise in response to differences in dietary protein ingestion, which would be more relevant to Master athletes. When evaluating the increase in MyoPS after resistance exercise with whey protein ingestion normalized to body mass performed by a single laboratory [19, 53, 161], it was equivocal whether the response in healthy older untrained females mapped on to the response in older untrained males. For example, older males consuming ~ 0.25 or ~ 0.49 g/kg bolus of whey protein demonstrated a ~ 116% and ~ 187%, respectively, increase in MyoPS after resistance exercise. Using a similar methodology and protein source, older untrained females consuming an intermediate dose of ~ 0.37 g/kg reported an intermediate (~ 144%) [161] or a lower (~ 43%) [53] increase in MyoPS compared to the males. Thus, while much research is required to more clearly determine the specific protein requirements in postmenopausal female athletes there is currently insufficient evidence to suggest they would be substantially different from the available research in males.

Aging is associated with a gradual loss of bone density and strength that can predispose bones to increased fracture risk. There is evidence that chronic physical activity associated with the training of many Master athletes (especially those performing strength and power training) helps maintain bone mineral density [9, 173], which is consistent with the osteogenic nature of bone loading. However, there is also evidence that a daily protein intake greater than the current RDA of 0.8 g/kg/day is associated with greater bone mineral density [174], especially when consumed with adequate calcium and potentially vitamin D intake [175]. Given the recommendations for protein discussed above would translate into intakes that would be in excess of the current RDA, Master athletes adhering to the current guidelines would meet their protein needs for maintaining bone health. Potential caveats could be to ensure Master athletes also consume adequate calcium (i.e. > 600 mg/day), vitamin D (> 800 IU/day), and alkaline salts, the latter of which would help regulate acid–base balance, a risk factor for bone loss with age [176], and can be found in a balanced diet enriched with fruits and vegetables. Readers interested in additional nutritional recommendations to maintain bone health are referred to other reviews on this topic (e.g. [177]).

It has been suggested that performance in Master triathletes has yet to peak [178] and may be aided by an advanced understanding of training, recovery, therapeutics, and sports nutrition. However, Master athletes self-report lower post-exercise meal protein intakes than their younger counterparts [179] and may generally consume suboptimal daily intakes (i.e. < 1.31 g/kg/day) [103], although one must be mindful of the general propensity for under-reporting in a range of dietary assessment strategies [180]. These lower intakes may also reflect a lack of knowledge on the part of some athletes as 50% of older triathletes responded “I don’t know” when asked about an optimal post-exercise protein dose, whereas only 22% responded with (at the time of its publication) the recommended 20–25 g protein [179]. Importantly, in light of a recent post-endurance exercise protein dose–response study [114], younger athletes on average consumed close to the recommended intake of ~ 0.5 g/kg, whereas older adults were below this new target (i.e. ~ 0.4 vs. 0.3 g/kg, respectively) [179]. Therefore, increasing total daily protein intake and/or meal protein during the early post-exercise recovery period may represent a safe and feasible means for Master athletes to enhance their recovery, adaptation, and performance.

Similar to their younger peers, Master athletes should ensure their nutrition plans include adequate dietary protein intake to support their training and recovery. Fortunately, current evidence does not support the need for higher protein intakes than what is currently recommended for and developed in younger athletes as much of the discussion on increased protein requirements in older adults to offset any age-related anabolic resistance is of little relevance to highly active Master athletes. Athletes who apply a dietary approach that targets an optimal meal protein intake and frequency to efficiently stimulate muscle protein remodeling (in addition to other important aspects of recovery such as optimal carbohydrate and energy intake) would likely reach a daily protein intake that would approximate ‘optimal’ daily protein estimates from studies that do not specifically control for meal timing or pattern (summarized in Table 1); however, it is argued that a meal protein approach, which may yield a slightly lower daily estimate due to a more efficient protein intake, would be more practical for dietary planning while also ensuring a balanced daily protein distribution for optimal muscle remodeling. While additional research could confirm whether current recommendations for Master athletes are indeed optimal, the ability of Master athletes to age gracefully and successfully allows them to look back at what supports their younger contemporaries as a strong foundation on which to build their success going forward.