Body proteins are continually being broken down and synthesised. This process of protein turnover (about 300 g/day in adults) is many times higher than the amount of proteins consumed from the diet. This indicates that the building blocks of the proteins, the amino acids, are reutilised in protein metabolism. Yet, part of the amino acids is lost in the form of nitrogen compounds through urine, faeces, sweating and via the skin, hair and nails. The body needs to compensate for these losses through ingestion of amino acids from dietary proteins. Extra amounts of amino acids are needed during growth, pregnancy and lactation.1

Protein and muscle healthSkeletal muscles represent the largest store of body protein; their mass depends on amongst others age, gender, health condition and regular physical activity. There are ethnicity related differences in skeletal muscle mass during the lifespan.2Maintaining a healthy skeletal muscle mass is considered important for long, independent and healthy living.3

Most of the proteins in skeletal muscle are in the form of the contractile proteins; an adult male of 70 kg may have about 7 kg of muscle protein.4,5 In the young non-obese adult skeletal muscle mass may be up to 60% of the body mass. Many cross-sectional studies, using various methodologies, show that skeletal muscle mass decreases while ageing and suggest this may start already at the age of about 30 years and is progressing after 50 years of age. Longitudinal studies, on the other hand, suggest that during most of adulthood the muscle mass is fairly stable and that at the age of 70 most healthy elderly still have about 80% of the muscle mass between 20 and 30 years of age.6

Importance of protein in each life stage

It is widely recognized that consuming high quality protein is important for both muscle growth and maintenance. Quality of the protein is defined in terms of high levels of essential amino acids with each having high digestibility in the small intestine.1Recommended amino acid scoring patterns for calculating protein quality for dietary assessment have been debated in various FAO Expert panels. The most recent meeting defines protein requirement levels for 3 different age categories, although it is acknowledged that more research is needed to determine amino acid requirements in different conditions and circumstances, such as in children, pregnancy, aging and exercise.1

The reference essential amino acid patterns per gram of protein for these 3 defined age categories are listed in Table 1.

Table 1.

Note: Tables 3 and 4 in the FAO report can also be used to derive information, see below.

Protein requirements

Protein requirements are defined in grams per kg body weight per day for growth (children) as well as maintenance (children, adults). Up till now these are considered to be the same for both sexes. For adults the average protein requirement is 0.66 g/kg/d; the safe level of intake is estimated to be 0.83 g/kg per day, based on nitrogen balance studies in apparently healthy people.1 Discussions are taking place to advocate higher optimal protein intakes for frail and sarcopenic elderly in order to maintain muscle mass.7, 8, 9

Dairy protein and muscle protein synthesis

It is widely recognized that dairy products are excellent sources of essential amino acids with high digestibility. The amount of essential amino acids in milk protein is about 54% of total amino acids (see table 4 above; number is based on 9+2 = 11 EAA). This is high compared to plant derived proteins such as soy (42%), rice (41%) and wheat (34%). The capacity of milk proteins to stimulate skeletal muscle protein synthesis has been well documented.10, 11, 12, 13, 14 Relatively few studies have addressed the potential of plant based proteins in this respect; the conclusions from available comparisons between animal and plant based proteins seem to indicate that plant based proteins result in a lower muscle synthetic response.15

References

  1. Dietary protein quality evaluation in human nutrition: report of a Joint Food and Agriculture Organization (FAO) Expert Consultation. FAO Food and Nutrition Paper 92. Auckland, New Zealand; 2011.
  2. Silva AM, Shen W, Heo M, Gallagher D, et al Ethnicity-related skeletal muscle differences across the lifespan. Am J Hum Biol. 2010; 22(1):76–82.
  3. McLeod M, Breen L, Hamilton DL, et al. Live strong and prosper: the importance of skeletal muscle strength for healthy ageing. Biogerontology 2016; DOI 10.1007/s10522-015-9631-7.
  4. Scientific Opinion on Dietary Reference Values for protein. EFSA Journal 2012; 10(2):2557.
  5. Wagenmakers AJM. Tracers to investigate protein and amino acid metabolism in human subjects. Proceedings of the Nutrition Society 1999; 58:987–1000.
  6. Mitchell WK, Williams J, Atherton P, et al. Sarcopenia, dynapenia, and the impact of advancing age on human skeletal muscle size and strength; a quantitative review. Physiology 2011; doi: 10.3389/fphys.2012.00260.
  7. Bauer J, Biolo G, Cederholm T, et al. Evidence-based recommendations for optimal dietary protein intake in older people: A position paper from the PROT-AGE study group. JAMDA 2013; 14:542e559.
  8. Deutz NEP, Bauer JM, Barazzoni R, et al. Protein intake and exercise for optimal muscle function with aging: Recommendations from the ESPEN Expert Group. Clin Nutr. 2014; 33(6): 929–936. doi:10.1016/j.clnu.2014.04.007.
  9. Paddon-Jones D, Campbell WW, Jacques PJ, et al. Protein and healthy aging. Am J Clin Nutr 2015;101(Suppl):1339S–45S.
  10. Burd NA, Gorissen SH, Vliet SV, et al. Differences in postprandial protein handling after beef compared with milk ingestion during postexercise recovery: a randomized controlled trial. Am J Clin Nutr 2015; 102:828–36.
  11. Devries MC, Phillips SM. Supplemental protein in support of muscle mass and health: Advantage whey. J. Food Science 2015; 80 (S1): A8-A15.
  12. Groen BBL, Horstman AM, Hamer HM, et al. Post-prandial protein handling: You are what you just ate. PLOS ONE | DOI:10.1371/journal.pone.0141582
  13. Hartman JW, Tang JE, Wilkinson SB, et al. Consumption of fat-free fluid milk after resistance exercise promotes greater lean mass accretion than does consumption of soy or carbohydrate in young, novice, male weightlifters. Am J Clin Nutr 2007; 86:373– 81.
  14. Wilkinson SB, Tarnopolsky MA, MacDonald MJ, et al. Consumption of fluid skim milk promotes greater muscle protein accretion after resistance exercise than does consumption of an isonitrogenous and isoenergetic soy-protein beverage. Am J Clin Nutr 2007; 85:1031– 40.
  15. Vliet SV, Burd NA, Loon LJV. The skeletal muscle anabolic response to plant- versus animal-based protein consumption. J Nutr 2015; 145:1981–91.