Anabolic properties of animal-derived protein sources

Nutrition Society Scottish Section Meeting held at the University of Stirling, Stirling on 28–29 March 2017

Conference on ‘Nutrition and exercise for health and performance’ Symposium 2: Maintenance of muscle mass for healthy ageing

Characterising the muscle anabolic potential of dairy, meat and plant-based protein sources in older adults

Stefan H. M. Gorissen1* and Oliver C. Witard2 1Exercise Metabolism Research Group, Department of Kinesiology, McMaster University, Canada

2Physiology, Exercise and Nutrition Research Group, Faculty of Health Sciences and Sport, University of Stirling, Scotland, FK9 4LA, UK

The age-related loss of skeletal muscle mass and function is caused, at least in part, by a reduced muscle protein synthetic response to protein ingestion. The magnitude and duration of the postprandial muscle protein synthetic response to ingested protein is dependent on the quantity and quality of the protein consumed. This review characterises the anabolic prop- erties of animal-derived and plant-based dietary protein sources in older adults. While approximately 60 % of dietary protein consumed worldwide is derived from plant sources, plant-based proteins generally exhibit lower digestibility, lower leucine content and deficien- cies in certain essential amino acids such as lysine and methionine, which compromise the availability of a complete amino acid profile required for muscle protein synthesis. Based on currently available scientific evidence, animal-derived proteins may be considered more anabolic than plant-based protein sources. However, the production and consumption of animal-derived protein sources is associated with higher greenhouse gas emissions, while plant-based protein sources may be considered more environmentally sustainable. Theoretically, the lower anabolic capacity of plant-based proteins can be compensated for by ingesting a greater dose of protein or by combining various plant-based proteins to provide a more favourable amino acid profile. In addition, leucine co-ingestion can fur- ther augment the postprandial muscle protein synthetic response. Finally, prior exercise or n-3 fatty acid supplementation have been shown to sensitise skeletal muscle to the ana- bolic properties of dietary protein. Applying one or more of these strategies may support the maintenance of muscle mass with ageing when diets rich in plant-based protein are consumed.

Plant-based protein source: Animal-derived protein source: Muscle protein synthesis: Healthy musculoskeletal ageing

Ageing is accompanied by a decline in muscle mass and function, termed sarcopenia(1). Sarcopenia increases the risk for falls and fractures, dependence, morbidity and mortality(2). The underlying cause of sarcopenia is multi- factorial and complex in nature. Contributing factors include, but are not limited to, reduced physical activity levels, poor diet, chronic low-grade systemic inflamma- tion, elevated levels of oxidative stress, mitochondrial dysfunction and hormonal changes(3–5). Sarcopenia imposes significant burden on healthcare systems. In

2000, the estimated annual healthcare cost of sarcopenia in the USA reached $18·5 billion, representing 1·5 % of total healthcare expenditures for that year(6). In order to treat or prevent sarcopenia, nutritional strategies must be developed to help increase or maintain skeletal muscle mass with advancing age.

Skeletal muscle mass is regulated by the balance between muscle protein synthesis and muscle protein breakdown(7). Loss of muscle mass results from a nega- tive net muscle protein balance, i.e. when muscle protein

*Corresponding author: Dr S. Gorissen, email gorisses@mcmaster.ca

Proceedings of the Nutrition Society (2018), 77, 20–31 doi:10.1017/S002966511700194X © The Authors 2017 First published online 29 August 2017

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breakdown exceeds muscle protein synthesis over a given period of time. Muscle protein synthesis and muscle pro- tein breakdown are concurrent and constant processes that are highly responsive to physical activity and protein intake(8). Muscle protein synthesis is more responsive to both stimuli than muscle protein breakdown(9). Thus, changes in muscle protein synthesis are primarily respon- sible for changes in muscle mass in response to exercise and nutrition, at least in healthy individuals(10). Dietary protein provides amino acids that can be used as precur- sors (i.e. building blocks) for muscle protein synthesis. Moreover, the essential amino acid leucine is not only a building block for muscle protein synthesis, it acts as a signalling molecule that can directly activate the muscle protein synthetic machinery. Several studies have com- pared the postprandial muscle protein synthetic response to protein intake between young and older indivi- duals(11,12). Some studies observed lower postprandial muscle protein synthesis rates in older adults compared with the young, which has resulted in a concept termed anabolic resistance(13). Not all studies have been able to detect anabolic resistance, which might be related to the limited number of participants who are generally included in these financially expensive tracer studies(14). However, a more comprehensive evaluation of the mus- cle protein synthetic response to protein intake between young and older individuals was recently conducted by Wall et al.(14), whereby data were pooled from multiple studies conducted within the same laboratory using an almost identical study design. Study findings revealed a markedly reduced muscle protein synthetic response to the ingestion of a single meal-like 20 g bolus of casein in older compared with young adults(14). These compre- hensive data support the existence of anabolic resistance with ageing.

The aetiology of anabolic resistance with ageing is not entirely understood, but is proposed to be mediated by impairments in several physiological processes(13). A reduced rate of dietary protein digestion and amino acid absorption and/or a greater splanchnic amino acid retention may limit the postprandial availability of amino acids for muscle protein synthesis(15,16). In addition, a decline in insulin-mediated capillary recruit- ment, muscle tissue perfusion and the abundance or functionality of amino acid transporters may limit the delivery of amino acids to the muscle and the uptake of amino acids by the muscle(17–19). At the molecular level, an impaired activation of mechanistic target of rapamycin complex 1 and downstream signalling (e.g. p70S6 kinase, 4E-BP1) that regulates muscle protein synthesis also may contribute to anabolic resistance with ageing(20,21). Understanding the relative contribu- tion of these processes to anabolic esistance with ageing is of critical importance for the design of effective nutri- tional strategies for combatting sarcopenia. Several fac- tors are known to influence the muscle protein synthetic response to protein ingestion, most notably the quantity of protein consumed on a meal-by-meal basis(22). In addition, the quality (i.e. source) of ingested protein has been shown to modulate the postprandial muscle protein synthetic response(23). Accordingly, the primary

focus of this review is to compare the muscle anabolic capacity of animal-derived (dairy and meat-based) proteins with various plant-based proteins in older adults.

Anabolic properties of animal-derived protein sources

Several studies have demonstrated that animal-derived protein sources such as dairy (e.g. milk and eggs) and meat (e.g. beef) elicit a robust stimulation of muscle protein synthesis in older adults(23). However, not all animal-based protein sources are comparable in terms of anabolic properties that determine the amplitude and duration of the postprandial muscle protein syn- thetic response. For example, whey protein is char- acterised as a fast protein based on its rapid protein digestion and amino acid absorption kinetics, whereas casein clots in the stomach and is slowly digested and absorbed(24,25). The ingestion of 20 g fast digestible whey protein, that is particularly high in leu- cine content, has been shown to stimulate muscle pro- tein synthesis to a greater extent compared with a matched dose of slowly digestible micellar casein in older men(26). These findings are consistent with similar studies in young adults(27) and highlight the importance of a rapid rise in blood leucine concentrations for stimulating a robust increase in muscle protein synthesis(28).

Although whey protein has consistently been shown to elicit a robust stimulation of muscle protein synthesis, whey protein represents a fraction of milk and is com- monly co-ingested with casein(29). Accordingly, a recent study assessed the postprandial muscle protein synthetic response to ingesting 20 g whey protein compared with a milk protein concentrate composed of both whey protein and casein(29). Study findings revealed a more rapid appearance of circulating amino acids after whey protein ingestion; however there was no difference in the postprandial muscle protein synthetic response between whey protein and milk protein concentrate in middle-aged men. In terms of comparing the anabolic potential of animal-derived protein-rich foods, we recently assessed postprandial protein handling and the subsequent muscle protein synthetic response to the ingestion of 350 ml fluid skimmed milk compared with 160 g cooked lean minced beef (both providing 30 g protein) during recovery from resistance exercise in young men(30). Beef was more rapidly digested and absorbed, which resulted in a greater rise in plasma amino acid availability and higher peak plasma leu- cine concentrations. Skimmed milk ingestion resulted in a moderate but rapid rise in circulating plasma leu- cine and stimulated muscle protein synthesis to a greater extent during the early 0–2 h recovery period than beef. Taken together, these data suggest that milk is equally effective as whey protein and superior to beef with regard to stimulating muscle protein synthesis.

Food matrix and texture may represent another important factor that modulates the muscle protein

Characterising the muscle anabolic potential of dairy, meat and plant-based protein sources in older adults 21

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synthetic response to protein ingestion in older adults(23). A recent study demonstrated that minced beef is more rapidly digested and absorbed than beef steak, resulting in a greater availability of protein-derived amino acids in the circulation and a more positive whole-body net protein balance after the ingestion of minced beef com- pared with beef steak(31). The 135 g portion of beef admi- nistered in this study provided about 20 g protein, which was unable to stimulate muscle protein synthesis. Consistent with this observation, a previous dose– response study demonstrated that 113 g minced beef (24 g protein) was not sufficient to stimulate muscle pro- tein synthesis under both rested and post-exercise condi- tions in middle-aged men(32). Instead, 170 g beef, providing 36 g protein, was required to stimulate muscle protein synthesis. Taken together, these data suggest that minced beef may be more effective in stimulating muscle protein synthesis compared with beef steak at higher doses of protein intake.

Global differences in food sources constituting daily protein intake

To date, most studies in older adults that have assessed the postprandial muscle protein synthetic response to protein intake have administered an animal-derived pro- tein source(26,31,33–37). Relatively few studies have charac- terised the muscle protein synthetic response to ingestion of a plant-based protein source(10,38,39). This gap in knowledge may be considered surprising given that a greater variety of plant-based protein-rich foods are readily available compared with animal-derived protein foods and most dietary protein consumed worldwide is in fact derived from plant (60 %) rather than animal (40 %) sources (Table 1)(40). An estimated 4 billion peo- ple live primarily on a plant-based diet, while an esti- mated 2 billion people worldwide live primarily on a meat-based diet(41). Of the plant-based food sources, cer- eals provide the greatest contribution, responsible for approximately 65 % of plant protein intake and 40 % of total protein intake. Pulses, nuts, seeds and vegetables provide a moderate contribution (2·2–10·4 g/d) to daily protein intake, whereas lower amounts of protein are provided by potatoes and fruit (about 3 g/d). Importantly, the contribution of plant and animal-based protein sources to total daily protein intake is specific to the continent or country of interest. In Africa and Asia, plant-based foods provide 77 and 66 % of total protein intake, respectively, whereas the contribution of animal- derived food sources to total dietary protein intake is greater in the USA (56 %), Europe (57 %) and Oceania (65 %). Across the USA, Europe and Oceania, meat and dairy sources provide the greatest contribution (about 80 %) to daily animal protein intake, whereas as little as 7 g meat and 4 g milk is consumed per capita per day in Africa. It may be argued that future research designed to assess the muscle protein synthetic response to an ingested protein source should focus on the most commonly consumed protein source in any given country.

Sustainability of commonly consumed dietary protein sources

A more advanced understanding of the anabolic poten- tial of various plant-based protein sources also may be considered critical given concerns regarding the global sustainability of animal-based protein diets. A sustain- able diet may be defined as ‘a diet with low environmen- tal impact that contributes to food and nutrition security and to healthy life for present and future generations’(42). The food supply chain accounts for about 20 % of all annual greenhouse gas emissions attributed to the UK(43). In the UK, the consumption of animal-derived protein foods is increasing at a rate 2-fold greater than plant-based protein foods(44). Diets that primarily con- tain animal-derived food sources (e.g. meat and dairy products) are associated with high greenhouse gas emissions (>4 kg carbon dioxide equivalents (CO2e)/kg edible weight; Fig. 1)(42). According to recent UK esti- mates, the production and consumption of beef (about 70 kg CO2e/kg) and pork (8 kg CO2e/kg edible weight) contributes most of all food sources to greenhouse gas emissions(45). Interestingly, unlike most other dairy pro- ducts (e.g. cheese and eggs), milk is associated with only moderate greenhouse gas emissions. The production and consumption of most plant-based protein foods, including wheat, oat and potato are associated with low greenhouse gas emissions (<1 kg CO2e/kg edible weight)(42). Notable exceptions include rice (4 kg CO2e/kg edible weight) and to a lesser extent soya (2 kg CO2e/kg edible weight)

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