The Impact of Protein Quality on Muscle Recovery
Sarcopenia, the progressive loss of muscle mass with age, is associated with increased risks for falls, fractures, hospitalization, poor mobility, and disability.1 It’s been estimated that sarcopenia-related complications were responsible for $18.5 billion in health care costs in 2004.2 In the United States and Canada, older adults are one of the fastest-growing populations, making sarcopenia an important public health concern.3
Sarcopenia is measurable at approximately the fifth decade of life, with losses in muscle proceeding at a rate of 0.8% per year and losses in strength slightly more precipitous at roughly 2% to 5% per year.4,5 These losses, however, are not linear and can be accelerated by periods of marked reductions in physical activity (eg, bed rest, limb immobilization) from which older adults (>60 years), as compared with younger adults, have a difficult time recovering due in part to declines in sensitivity to anabolic stimuli.6
Anabolic resistance, the term used to describe the attenuated response of skeletal muscle to anabolic stimuli (eg, protein ingestion or resistance exercise), may be a key determinant in the progression of sarcopenia in the elderly. For instance, for a given bout of resistance exercise, older adults require a three times greater volume of exercise than do younger adults to achieve a similar anabolic response.7 Additionally, older adults require a postexercise protein dose that is greater (0.4 g/kg) than those younger adults require (0.24 g/kg) to maximally stimulate muscle protein synthesis (rate of muscle growth).8 For example, a 75-kg young adult would require an 18-g dose of protein compared with a 30-g dose required by an older adult of the same mass in order to overcome anabolic resistance. Thus, there’s undoubtedly substantial difficulty for older adults to take measures to overcome anabolic resistance to maintain skeletal muscle with age.
Meeting Proposed Protein Requirements With Supplements
A possible solution may be the use of protein supplementation to achieve recommended elevated protein intakes. It’s important, however, to consider that protein sources are not all of equal quality. Therefore, it’s necessary to be mindful about the selection of protein source when recommending a supplement, particularly for older adults for whom increasing dietary protein is already a challenge. Ideally, the supplement should be high quality, meaning that it’s a complete protein (ie, it contains all essential amino acids) and is easy to digest and absorb.
Another important factor to consider when recommending a protein supplement is its leucine content. Leucine, an essential amino acid, is the only known amino acid to independently stimulate a muscle building response in healthy individuals; therefore, when consuming a protein with low leucine content, individuals may require a greater absolute or total amount of protein to achieve a maximal rate of muscle protein synthesis.15 Whey protein is one of the highest-quality proteins, according to the protein digestibility-corrected amino acid score (PDCAAS),16 the official method to determine protein quality, as well as a newly proposed method for the determination of protein quality by the Food and Agriculture Organization of the United Nations, the digestible indispensable amino acid score (DIAAS).17 Additionally, whey protein has the highest leucine content of the available protein supplements and is rapidly digestible, meaning that amino acids become readily and quickly available in the blood.
Aging and the Cycle of Disuse
Moreover, during periods of recovery from illness or surgery, older adults typically consume fewer calories, especially those from protein, resulting in weight loss, of which roughly 25% can be lost as muscle mass.23 Results have also shown that a hypocaloric state induces a significant reduction in rates of muscle protein synthesis, resulting in the loss of skeletal muscle.13 Furthermore, a previous study has shown that a drastic reduction in physical activity in the form of an 85% reduction in daily steps resulted in decreased rates of muscle protein synthesis and development of insulin resistance that were not recovered with a two-week monitoring period during which participants returned to habitual levels of physical activity.22 No study to date had examined the effects of any nutritional intervention on losses in muscle mass or muscle protein synthesis in response to abruptly reduced daily stepping in any population.
Thus, to address this scientific question, a study published recently in The American Journal of Clinical Nutrition was designed to better understand how the combination of physical inactivity, energy restriction, and protein supplementation and quality would affect skeletal muscle mass and function in healthy older adults. The main objective was to determine how elevated protein intake through the ingestion of a high-quality protein supplement (ie, whey protein) or a low-quality protein supplement might affect the loss of skeletal muscle and aid in recovery upon return to habitual activity.24
During Inactivity, Does Protein Quality Matter?
Following the two weeks of step reduction and hypocaloric diet, participants resumed habitual levels of physical activity for one week and were provided with calories to maintain energy balance but were still at an elevated protein intake, as part of a “recovery” phase. The increased protein intake from weeks two to five was achieved through twice-daily supplementation with either 30 g of whey protein or 30 g of collagen peptides. Collagen peptides were chosen as the comparator protein because they are an incomplete protein (lacking the essential amino acid tryptophan), rendering them a low-quality protein with a PDCAAS and DIAAS score of 0. (Any protein lacking an essential amino acid is considered incomplete and cannot be ranked as part of protein quality scores.) Importantly, collagen peptides have substantially less (68% less) leucine than does whey protein. Furthermore, collagen peptides are marketed toward older adults as a supplement meant to increase skeletal muscle mass; however, there’s little to no molecular evidence to underpin its efficacy.25-27 Conversely, whey protein has been shown to robustly stimulate rates of muscle building in both younger28 and older29,30 participants and to preserve muscle loss during periods of energy restriction.31
As hypothesized, fat- and bone-free mass (FBFM), as evaluated by dual-energy X-ray absorptiometry, was reduced following two weeks of reduced daily stepping in combination with energy restriction in both groups, regardless of supplement type. Upon resumption of physical activity, both groups regained FBFM, but the whey protein–supplemented group regained a greater amount of lean tissue. Researchers found that consumption of an energy-restricted diet resulted in reductions in rates of muscle protein synthesis that were not further exacerbated by the two weeks of step reduction. Furthermore, after participants returned to habitual physical activity, whey protein facilitated an increase in rates of muscle protein synthesis, while rates in the collagen peptides group remained attenuated.
Previous literature32,33 has shown that, despite ingestion of a whey protein supplement during single leg immobilization or consumption of essential amino acids during bed rest,26 individuals lose skeletal muscle with disuse. The findings from the current study showing that skeletal muscle was lost despite consumption of a high-protein, supplemented diet corroborate that nutrition alone may not protect against inactivity-induced losses in muscle mass. To date, only neuromuscular stimulation and resistance exercise have shown promise in the ability to offset the loss of skeletal muscle during bed rest and step reduction, though future research is required in order to determine optimal strategies for the offset of muscle loss with disuse.34-36 These results highlight that, regardless of elevated protein intake and protein quality, a severe reduction in physical activity is a potent catabolic stimulus despite that participants were not confined to bed rest.
Additionally, results demonstrate that energy restriction and step reduction do not act synergistically to alter skeletal muscle loss and point to a potential theoretical threshold at which healthy older adults are able to mitigate catabolism such that consecutive catabolic stimuli do not impact muscle protein synthesis additively. Lastly, it appears as though the role of protein quality may be increasingly important during rehabilitation or convalescence from illness given the augmentation of muscle protein synthesis in the current study with whey protein; therefore, interventions should focus on nutrition during postoperative recovery or following acute illness, as protein supplementation has been shown to increase muscle mass in combination with resistance exercise in older adults.37 Few studies have examined protein supplementation during rehabilitation, but previous studies have shown that protein supplementation increased knee extensor strength in patients following a hip fracture38 and increased knee extensor strength and distal vastus laterialis muscle mass in individuals with anterior cruciate ligament injuries.39
Improving patient outcomes following acute disuse is a multifactorial effort. Ultimately, limiting periods of disuse and encouraging physical activity are key to regulating the pace of sarcopenia and the maintenance of skeletal muscle mass and function. Skeletal muscle losses and alterations in muscle protein synthesis from the present study should be taken into consideration given the population analyzed (ie, healthy older adults free of chronic conditions and living independently) and the disuse model invoked (step reduction as opposed to bed rest) as improvements in FBFM and rates of muscle protein synthesis may not be as robust in compromised populations or those who are frail at the onset of disuse.
Whey Protein: A Solution to Meet the Protein Nutritional Needs for the Geriatric Population
As this article demonstrates, whey protein is one of the highest quality proteins with the highest leucine content. It’s rapidly digestible and is found naturally in dairy. Because of its functionality as a protein supplement, whey protein easily can be incorporated into different foods and beverages and therefore could be used to increase protein intake in different circumstances/needs older adults face. The nutritional characteristics of whey protein position it as the protein supplement with potential unique benefits for older adults—to maintain muscle health and allow them to function in situations such as recovery after acute hospitalization.24 In conclusion, the scientific information provided herein suggests that the dramatic loss of muscle mass and strength by older people may be efficiently treated by including a high-quality protein, such as whey protein, as part of nutrition strategies to reduce the burden of sarcopenia-related issues associated with acute hospitalization.
— Sara Y. Oikawa, MSc, is a PhD candidate in the department of kinesiology in the Exercise Metabolism Research Group at McMaster University in Hamilton, Ontario, Canada.
— Moises Torres-Gonzalez, PhD, is chair of the Scientific Sub-Committee of the Whey Protein Research Consortium and director of nutrition research at the National Dairy Council in Rosemont, Illinois.
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