Donald K. Layman, PhD Nancy R. Rodriguez, PhD, RD, CSSD, FACSM

Transcription

1 Egg Protein as a Source of Power, Strength, and Energy Donald K. Layman, PhD Nancy R. Rodriguez, PhD, RD, CSSD, FACSM High-quality proteins make a valuable contribution to the synthesis and maintenance of muscle and indirectly to the regulation of blood glucose levels, thus contributing to power, strength, and energy. Eggs have traditionally been used as the standard of comparison for measuring protein quality because of their essential amino acid (EAA) profile and high digestibility. They provide a nutrient-dense source of energy from protein and fat, approximately 75 kcal per large egg, as well as several B vitamins, including thiamin, riboflavin, folate, B 12, and B 6, which are required for the production of energy by the body. Given the unique complementary relationship between the EAA leucine and glucose utilization by muscle, it would follow that a diet rich in the amino acid leucine would be advantageous to men and women undergoing endurance training. Leucine is also a critical element in regulating muscle protein synthesis and may be the key amino acid defining the increased needs for EAA to optimize skeletal muscle mass. Increased tissue levels of leucine combine with circulating insulin to allow skeletal muscles to manage protein metabolism and fuel selection in relation to diet composition. Moreover, muscle recovery from exercise, both resistance and endurance, seems to be dependent on dietary leucine. Because eggs are an excellent, nutrient-rich source of leucine, as well as other EAAs, these protein-related benefits may be important to active individuals who routinely consume eggs as part of a varied, balanced diet. Nutr Today. 2009;44(1):43 48 Power, strength, and energy are key components of a healthy and vital life. Building and maintaining muscle tissue are primary contributors to these quality-of-life end points, as muscle is central to human health and functional capacity. 1 Muscle tissue is involved in several metabolic functions, including regulating blood sugar. About 40% of body protein is found in skeletal muscle. 2 Numerous studies have clearly documented that consumption of protein from various sources and amino acid supplements stimulates muscle protein synthesis. 3Y6 Specifically, essential amino acids (EAA) are required to stimulate protein synthesis, whereas nonessential amino acids play a supportive role and serve other metabolic purposes. 7 As a source of high-quality protein, providing all of the EAA, eggs make a valuable contribution to the synthesis and maintenance of muscle and indirectly to the regulation of blood glucose levels, thus contributing to power, strength, and energy. High-Quality Protein in a Healthful Diet The primary function of dietary protein is to supply nitrogen and amino acids, both essential and nonessential, in amounts and proportions needed for the synthesis of body proteins. Thus, the amino acid content and profile of specific food proteins are a primary determinant of protein quality. 8 Animal proteins, such as those from eggs, meat, poultry, fish, and milk, are well recognized as being high-quality proteins. A primary fate of ingested amino acids is incorporation into muscle protein, which plays a central role in whole-body protein metabolism by serving as the principal reservoir for amino acids in protein synthesis. 1 Increasing protein or amino acid intakes will support the maintenance, repair, and synthesis of skeletal muscle proteins and may optimize muscle strength. 1 Research in healthy, young men shows that both muscle mass and strength are improved by increased availability of amino acids, subsequent to ingesting an amino acid supplement, even in the absence of physical activity. 5 Nutrition Today, Volume 44 Number 1 January/February,

2 Egg as a Source of Power, Strength, and Energy Increasing protein or amino acid intakes will support the maintenance, repair, and synthesis of skeletal muscle protein and may optimize muscle strength. Because the level of habitual protein intake has been shown to influence the synthesis of muscle protein after exercise, 9 it is reasonable to hypothesize that routine consumption of high-quality proteins, such as that in eggs, would impart benefits to skeletal muscle. With regard to protein source, a study of elderly women showed that there was a greater net protein synthesis when the women ate a mostly animal protein diet (14.5% of energy provided by mixed animal protein; 5.1% from mixed vegetable protein) compared with that noted when they consumed a diet consisting of mostly vegetable protein (15.1% mixed vegetable protein; 5.0% mixed animal protein) for 2 weeks. 10 It has been recommended that the most practical means of promoting or increasing skeletal muscle protein anabolism is to include a highybiological-value protein at each meal. 11 High-Quality Protein From Eggs The quality of any single dietary protein source is based on its amino acid composition and the digestibility of that protein. In general, animal proteins, such as eggs, provide higher quality protein compared with vegetable-based proteins because of their higher levels of the EAA lysine, threonine, valine, isoleucine, leucine, methionine, phenylalanine, tryptophan, and histidine. 12 Protein quality can be assessed also with respect to its efficiency in supporting body protein metabolism, which takes into account the ability of a protein to provide the correct proportions of the EAA for building new proteins plus the unique roles of amino acids, such as leucine, which provide a regulatory signal for initiating protein synthesis. Eggs have traditionally been used as the standard of comparison for measuring protein quality because of their EAA profile and high digestibility. In fact, egg protein quality is sufficiently high because of surfeit levels of many EAAs, such that egg protein can be mixed with inferior-quality proteins without compromising its own quality. 8 As a low-cost source of high-quality protein, rich in EAA, eggs can improve the overall nutritional quality of the diet. There are at least 5 measures of protein quality used to compare the ability of various dietary protein sources to support human health and development. These include the protein digestibility corrected amino acid score (PDCAAS), protein efficiency ratio, biological value, net protein utilization, and protein digestibility. 13Y15 The PDCAAS is the internationally approved method for protein quality assessment, 16 although other methods are still widely used. Using the PDCAAS, eggs rank similar to cow s milk (see Table 1) for protein quality. Eggs are graded highest, alongside beef, for protein digestibility, and using the biological value, net protein utilization, and the protein efficiency ratio, eggs rank highest for protein quality among cow s milk, beef, and soy. Recognizing that there are limitations to all methods of assessment of protein quality and that none of these approaches rely on direct measurements of the stimulation of muscle or whole body protein synthesis, it is noteworthy that eggs rank consistently high across all current measures. The recent Dietary Reference Intakes for macronutrients used the PDCAAS to determine dietary protein requirements. The PDCAAS accounts for the amino acid content of a food s protein, its true digestibility, and its ability to supply EAA in amounts adequate to meet the requirements of a 2- to 5-year-old child. This age group was used as the reference standard because of the high demands for growth and development. 12 Values of PDCAAS higher than 100% are not used as such but are truncated to 100%. The values shown in Table 1 are the true PDCAAS values of several proteins, including eggs. The truncation of PDCAAS values is designed to make equal the power of high-quality proteins to balance the amino acid composition of inferior proteins. However, the limitation to this approach is the assumption that a PDCAAS value greater than 1.0 does not provide any additional nutritional benefit. 16 Based on studies providing protein with various essential and non-eaa compositions, 1 there is reason to believe that natural dietary protein sources rich in specific amino acids provide greater nutritional benefits compared with others. Therefore, the truncation of PDCAAS compromises its function and is not useful in the context of mixed diets with proteins from a variety of sources. Energy From Eggs All foods provide energy in the form of proteins, fats, and carbohydrates. Eggs provide a nutrient-dense source of energy from protein and fat, approximately 75 kcal per large egg, plus several B vitamins, including thiamin, riboflavin, folate, B 12, and B 6, which are required for the production of energy by the body. A potential metabolic advantage of eggs is that they provide energy 44 Nutrition Today, Volume 44 Number 1 January/February, 2009

3 Egg as a Source of Power, Strength, and Energy Food Science Table 1. A Comparison of Quality Scores for Common Food Proteins: Egg, Milk, Beef, and Soy Protein Protein Efficiency Ratio a True Digestibility, b % Protein Digestibility Corrected Amino Acid Score c Biological Value (BV), d % Net Protein Utilization e Egg white Milk Beef Soy a A reflection of a food s biological value, by measuring protein retention by body tissues (weight gain per gram of protein ingested). b Ratio of nitrogen absorbed from food to nitrogen ingested. c Takes into account the level of limiting amino acids in protein as well as true digestibility. d A measure of how efficiently food protein, once absorbed from the gastrointestinal tract, can be turned into body tissues. Egg white protein has a BV of 100, the highest BV of any single food protein. e Takes into account both true digestibility and BV. Sources: Schaafsma 16 and Hoffman and Flavo. 13 without causing a surge in blood sugar or insulin levels, thereby avoiding potential rebound reduction in blood glucose that is frequently associated with fatigue and a general lack of energy. 17,18 One group of researchers found that in 18 normal participants, including egg proteins at breakfast resulted in lower insulin responses compared with those noted with consumption of otherwise identical breakfasts containing ham proteins. 17 Reduced blood glucose and insulin response was also observed in a group of 12 healthy young men after eating a breakfast that provided whole egg or yolk. The whole egg or yolk at breakfast also increased cholecystokinin and gastric inhibitory peptide levels and delayed gastric emptying. 18 Egg white alone, as well as the whole egg or yolk, resulted in increased levels of gastric inhibitory peptide. These observations are intriguing, given the roles of these compounds in satiety and appetite regulation. A longer term (16 weeks), larger (160 participants) study evaluating a weight-loss diet showed that those individuals who ate 2 eggs at breakfast each day lost twice as much weight and had an 83% greater decrease in waist circumference compared with those who ate a bagel-based breakfast, suggesting that the inclusion of protein-rich eggs at breakfast may help promote satiety during weight loss. 19 Finally, a study of 30 overweight and obese women that evaluated whether there was any weight management benefits associated with consuming eggs as part of a healthful breakfast compared an egg breakfast to an equal-weight, bagel-based breakfast that provided the same amount of calories. When participants ate the egg breakfast, they reported greater satiety and significantly lowered their calorie intake over the next 24 hours compared with when they ate the bagel breakfast. The researchers suggested that their findings indicate the involvement of hormones, including insulin, which regulates blood glucose levels. 20 In total, this work demonstrates a role for egg proteins as a component of a balanced breakfast in stabilizing blood glucose and insulin levels and in promoting satiety in otherwise healthy, obese, overweight, and normal-weight men and women. Eggs are also rich in the EAA leucine, a nutrient that works in concert with the insulin signaling pathway to modulate glucose use by skeletal muscle. 21 Whereas total protein is important in providing substrates for gluconeogenesis, leucine seems to uniquely regulate oxidative use of glucose by skeletal muscle. 21 These leucine-specific mechanisms are believed to contribute to a stable glucose environment (leucine may help spare glucose) and to help modulate insulin responses. 22 Egg protein is second only to milk protein in its leucine content, 23 making eggs one of the most concentrated sources of leucine in the diet. Eggs are also rich in the essential branched-chain amino acid leucine, which works in concert with the insulin signaling pathway. Strength and Power From Egg Protein Strength is a measure of force, and power is the rate of doing work, which is a function of force and velocity (speed). For many consumers, increased protein intake is synonymous with increased muscle size, power, and strength. Although these beliefs were once considered myth, a body of scientific literature does exist in support of this perception. 24 More specific to this article is the Nutrition Today, Volume 44 Number 1 January/February,

4 Egg as a Source of Power, Strength, and Energy common belief that consumption of animal proteins better supports gains in muscle mass and, subsequently, muscle strength and power because they are higher quality proteins. Eggs are rich in the EAAs needed for the synthesis of muscle protein, and they are specifically rich in leucine. 7,23,25 Most research focuses on the relationship between dietary protein intake and muscle mass, 24 which is a key component of physical strength and power. 26 Muscle protein and, therefore, muscle mass, strength, and function are directly affected by protein intake in both the young and the old. 1 High dietary protein intakes increase protein synthesis by increasing systemic amino acid availability. 1 Hoppe and coworkers 27,28 conducted studies to characterize associations between protein intake and growth in young children and found that rates of growth were higher and increases in muscle mass were greater with increased protein intake. Low protein intake or consumption of low-quality protein in elderly persons may facilitate muscle loss, 11,29,30 which can ultimately lead to sarcopenia. Studies have shown that elderly individuals experience increases in strength with habitual consumption of increased dietary protein. In very old men and women, the use of a protein/calorie supplement was associated with greater gains in strength and muscle mass when combined with a strength training program compared with strength training alone. 31 Others have shown that supplementing the diet with as few as 22 g of EAA (11 g twice a day) without modifying physical activity increased lean body mass, function, and strength in elderly participants with impaired glucose intolerance. 3,4,32 The anabolic effect of exercise is amplified when combined with amino acids or protein intake. 32 More specifically, a dose-response relationship between EAA concentrations in the blood and muscle protein synthesis exists, such that muscle protein synthesis increases as blood levels of EAA increase. 32Y35 Studies focusing on protein intake and the muscle protein synthetic response to aerobic exercise are limited. No differences were found in lean body mass in previously unfit men and women in response to a 6-week endurance training program while consuming a weight maintenance diet that provided 10% of calories as protein with or without eggs. 36 The researchers speculated that significant changes in body composition (ie, an increase in muscle tissue and decrease in fat) might have been noted if the training period was longer and protein intake was increased above the Recommended Dietary Allowance (0.8 g/kg). The mechanism to explain the dose relationship of plasma EAA and muscle protein synthesis is unknown. However, a potential mechanism is the unique role of leucine in regulation of protein synthesis at the translation initiation stage. Increases in the intracellular leucine concentration, in combination with insulin, regulate the signaling cascade for stimulation of the translation initiation factors eif4e, rps6, and eif4g. 37 Given the unique complementary relationship between leucine and glucose utilization by muscle, it would be worthwhile to ascertain if a diet rich in leucine would be advantageous to men and women undergoing endurance training. 21,38 Leucine is a critical element in regulating muscle protein synthesis and may be the key amino acid defining the increased needs for EAA to optimize skeletal muscle mass. 21 Increased tissue levels of leucine combine with circulating insulin to allow skeletal muscles to manage protein metabolism and fuel selection in relation to diet composition. 21 Together, insulin and leucine allow skeletal muscle to coordinate protein synthesis with the existing physiological state and dietary intake. 37 Thus, when intracellular levels of leucine are increased, leucine has the ability to spare glucose and maintain blood sugar levels during exercise and periods of food restriction. This may be particularly important during periods of endurance exercise when sparing of blood glucose is important. 22 Moreover, muscle recovery from exercise, both resistance and endurance, seems to be dependent on dietary leucine. 37 Because eggs are an excellent, nutrient-rich source of leucine, as well as other EAAs, it is reasonable to ascertain that protein-related metabolic benefits would be imparted to active individuals who routinely consume eggs as part of a varied, balanced diet. Beyond the standard measurements of protein quality, the amount of EAA provided by eggs as a percentage of protein is equal to or greater than other animal proteins (see Table 2). This point is of particular significance considering that muscle protein synthesis is dependent on the level of EAA in the blood. A recent study found that diets that included eggs as a protein source provided significantly more EAA (28%) and leucine (26%) than did egg-free diets where protein came from other animal (chicken and beef) and vegetable sources (N. Rodriguez, unpublished data). For children and young individuals, consuming high-quality protein is important to get the most EAA to maximize growth, muscle gain, and strength. For elderly individuals, getting a regular source of high-quality protein is important for preventing age-related loss of muscle mass. Protein sources that are rich in EAA, inexpensive, and highly digestible and easy to consume are important for the elderly population. Summary Eggs have traditionally been used as the reference standard for measuring protein quality because of their EAA profile and high digestibility. With regard to 5 46 Nutrition Today, Volume 44 Number 1 January/February, 2009

5 Egg as a Source of Power, Strength, and Energy Food Science Table 2. Protein and Amino Acid Density of Animal Proteins Eggs 1% Milk Beef a Protein density, % of calories as protein EAA density, % of protein as EAA Abbreviation: EAA, essential amino acid. a Beef values represent a composite of trimmed retail cuts, separable lean, and fat trimmed to one-fourthyinch fat, choice, cooked. Source: US Department of Agriculture Nutrient Database for Standard Reference, Release well-accepted measures of protein quality, eggs rank highest in three, rank first alongside beef in another, and rank second to cow s milk with the current internationally approved method PDCAAS. For people who avoid milk for any reason, eggs become the highest quality protein source in their diets. Studies have shown that eggs promote satiety and that the mechanism for this effect involves hormones known to regulate blood glucose levels. Eggs are also rich in EAAs needed for synthesis, maintenance, and repair of muscle proteins. They are a rich source of EAA, including leucine, a key element in regulating muscle protein synthesis, particularly during recovery from exercise, as well as in regulating the use of glucose by muscle tissue. As a source of energy, as well as several nutrients required for energy metabolism and a source of high-quality protein, eggs help fuel the body and assist in the building and maintaining of muscle tissue essential for strength and power. The protein found in eggs is superior to most and equal to other high-quality protein sources routinely found in the diet. There is no higher quality protein than that provided in eggs. The protein found in eggs is superior to most and equal to other high-quality protein sources routinely found in the diet. There is no higher quality protein than that provided in eggs. Donald K. Layman, PhD, is a professor emeritus at the Department of Food Science and Human Nutrition at the University of Illinois. Dr Layman s research focuses on protein and amino acid requirements, and his work has helped to define roles of the branched-chain amino acids in skeletal muscle metabolism. Nancy R. Rodriguez, PhD, RD, CSSD, FACSM, is an associate professor of Nutritional Sciences, University of Connecticut Sports Nutrition Programs at the University of Connecticut. Dr Rodriguez is actively involved in research on the relationships between exercise, protein intake, and protein utilizations in different population groups. Corresponding author: Donald K. Layman, PhD, University of Illinois, 437 Bevier Hall, 905 South Goodwin, Urbana, IL (dlayman@uiuc.edu). REFERENCES 1. Wolfe R. The underappreciated role of muscle in health and disease. Am J Clin Nutr. 2006;84:475Y Rennie M, Tipton K. Protein and amino acid metabolism during and after exercise and the effects of nutrition. Ann Rev Nutr. 2000;20:457Y Tipton K, Elliott T, Cree M, Wolf S, Sanford A, Wolfe R. Ingestion of casein and whey proteins result in muscle anabolism after resistance exercise. Med Sci Sports Exerc. 2004;36(12):2073Y BLrsheim E, Quynh-Uyen B, Tissier S, Kobayashi H, Ferrando A, Wolfe R. Amino acid intake increases leg muscle mass, function and strength in elderly. Presented at: American College of Sports Medicine Annual Meeting [board 143]; June 1Y4, 2005; Nashville, TN. 5. Paddon-Jones D, Sheffield-Moore M, Aarsland A, Wolfe R, Ferrando A. Exogenous amino acids stimulate human muscle anabolism without interfering with the response to mixed meal ingestion. Am J Physiol Endocrinol Metab. 2005;288:E761YE Wilkinson S, Tarnopolsky M, MacDonald M, MacDonald J, Armstrong, Phillips S. 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(4):1031Y Volpi E, Kobayashi H, Sheffield-Moore M, Mittendorfer, Wolfe R. Essential amino acids are primarily responsible for the amino acid stimulation of muscle protein anabolism in healthy elderly adults. Am J Clin Nutr. 2003;78:250Y The use of biological value of a protein in evaluating its quality for human requirements. Joint FAO/WHO/UNU Expert Consultation on Energy and Protein Requirements, m2835e00.htm. Accessed December Rodriguez N, Fislocky L, Gaine P. Dietary protein, endurance exercise, and human skeletal-muscle protein turnover. Curr Opin Clin Nutr Metab Care. 2007;10:40Y Pannemans D, Wagenmakers A, Westerterp K, Schaafsma G, Halliday D. Effect of protein source and quantity on protein metabolism in elderly women. Am J Clin Nutr. 1998;68:1228Y Symons T, Schutzler S, Cocke T, Chinkes D, Wolfe R, Paddon-Jones D. Aging does not impair the anabolic response to a protein-rich meal. Am J Clin Nutr. 2007;86: 451Y456. Nutrition Today, Volume 44 Number 1 January/February,

6 Egg as a Source of Power, Strength, and Energy 12. Food and Nutrition Board, Institute of Medicine of the National Academies. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: The National Academies Press; Hoffman J, Flavo M. ProteinVwhich is best? J Sports Sci Med. 2004;3:118Y FAO/WHO/UNU. Protein and amino acid requirements in human nutrition: report of a joint WHO/FAO/UNU expert consultation. World Health Organ Tech Rep Ser #935. Geneva, Switzerland: FAO/WHO/UNU, 2007: Schaafsma G. Evaluation of the nutritional value of proteins. In: The Western Diet With a Special Focus on Dairy Products. Belgium: Institute Danone. 1997:21Y28. ISDN X. 16. Schaafsma G. The protein digestibility-corrected amino acid score. J Nutr. 2000;130:1865SY1867S. 17. Villaume C, Beck B, Rohr R, Pointel J, Debry G. Effect of exchange of ham for boiled egg on plasma glucose and insulin responses to breakfast in normal subjects. Diabetes Care. 1986;9(1):46Y Pelletier X, Thouvenot P, Belbraouet S, et al. Effect of egg consumption in healthy volunteers: influence of yolk, white or whole-egg on gastric emptying and on glycemic and hormonal responses. Ann Nutr Metab. 1996;40(2): 109Y Vander JS, Gupta A, Khosla P, Dhurandhar NV. Egg breakfast enhances weight loss. Int J Obes. 2008;32: 1545Y Vander Wal J, Marth J, Khosia P, Jen K, Dhurandhar N. Short-term effect of eggs on satiety in overweight and obese subjects. J Am Coll Nutr. 2005;24(6):510Y Layman D, Baum J. Dietary protein impact on glycemic control during weight loss. J Nutr. 2004;134:968SY973S. 22. Layman D, Walker D. Potential importance of leucine in treatment of obesity and the metabolic syndrome. J Nutr. 2006;136:319SY323S. 23. US Department of Agriculture, Agricultural Research Service, USDA Nutrient Data Laboratory, USDA Nutrient Database for Standard Reference, Release Lambert C, Frank L, Evans W. Macronutrient considerations for the sport of bodybuilding. Sports Med. 2004;34(5):317Y Smith K, Reynolds N, Downie S, Patel A, Rennie M. Effects of flooding amino acids on incorporation of labeled amino acids into human muscle protein. Am J Physiol Endocrinol Metab. 1998;275:73Y McArdle W, Katch F, Katch V. Exercise Physiology: Energy, Nutrition, and Human Performance. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; Hoppe C, Udam T, Lauritzen L, Molgaard C, Juul A, Michaelsen K. Animal protein intake, serum insulin-like growth factor I, and growth in healthy 2.5-y-old Danish children. Am J Clin Nutr. 2004;80:447Y Hoppe C, Molgaard C, Thomsen B, Juul A, Michaelsen K. Protein intake at 9 mo of age is associated with body size but not with body fat in 10-y-old Danish children. Am J Clin Nutr. 2004;79:494Y Castaneda C, Charnley J, Evans W, Crim M. Elderly women accommodate to a low-protein diet with losses of body cell mass, muscle function, and immune response. Am J Clin Nutr. 1995;62:20Y Castaneda C, Dolnikowski G, Dallal G, Evans W, Crim M. Protein turnover and energy metabolism of elderly women fed a low-protein diet. Am J Clin Nutr. 1995; 62:40Y Evans W. Protein nutrition, exercise and aging. J Am Coll Nutr. 2004;23(6):601SY609S. 32. Biolo G, Tipton K, Klein S, Wolfe R. An abundant supply of amino acids enhances the metabolic effect of exercise on muscle protein. Am J Physiol. 1997;273:E122YE Volpi E, Ferrando A, Yeckel C, Tipton K, Wolfe R. Exogenous amino acids stimulate net muscle protein synthesis in the elderly. J Clin Invest. 1998;101:2000Y Volpi E, Mittendorfer B, Rasmussen B, Wolfe R. The response of muscle protein anabolism to combined hyperaminoacidemia and glucose-induced hyperinsulinemia is impaired in the elderly. J Clin Endocrinol Metab. 2000; 85:4481Y Bohé J, Low A, Wolfe R, Rennie M. Human muscle protein synthesis is modulated by extracellular, not intramuscular amino acid availability: a dose-response study. J Physiol. 2003;552:315Y Pikosky M, Gaine P, Martin W, et al. Aerobic exercise training increases skeletal muscle protein turnover in healthy adults at rest. J Nutr. 2006;136: 379Y Norton L, Layman D. Leucine regulates translation initiation of protein synthesis in skeletal muscle after exercise. J Nutr. 2006;136:533SY537S. 38. Kimball S, Jefferson L. Signaling pathways and molecular mechanisms through which branched-chain amino acids mediate translational control of protein synthesis. J Nutr. 2006;136:227SY231S. 48 Nutrition Today, Volume 44 Number 1 January/February, 2009