While the effect of the consumption of isolated protein sources dissolved in liquid beverages has been documented, the impact of consuming a food such as whole eggs on postprandial protein metabolism has rarely been studied.
Recent research put this question squarely in the forefront. A key goal of the study was to determine the effect of the consumption of nutrient-dense protein on muscle protein metabolism, with the added healthful factor of exercise. Study parameters, methods and conclusions were outlined in an article entitled, “Study on Postprandial Muscle Metabolism Shows Whole Eggs Stimulate Postexercise Muscle Protein Synthesis more so than Isonitrogenous Egg White,” as published in the American Society for Nutrition (S van Vliet et al. Am J Clin Nutr 106 (6), 1401-1412. 2017 Oct 04.)
Part of the study focused on the importance of consuming nutrient-dense protein sources, such as whole eggs. In recent years, dietary recommendations pushed toward the consumption of egg whites, for unsubstantiated health reasons involving fat and cholesterol, noted the study’s authors. The egg yolk, however, is nutrient-dense. For example, it contains 40 percent of the egg’s protein. Oftentimes whole nutrient-dense foods are recommended as a protein source. This raises the question; which part of the egg is better as a source for muscle protein synthesis-the nutrient-dense whole egg or egg whites?
In the study, the diet of egg-laying hens was supplemented with isotopically labeled leucine (0.3% L-[5,5,5-2H3]). This permits the tracking of where the food-derived amino acids ended up after study participants ingested them.
The yolks were removed from one subset of leucine-supplemented eggs, while the whites and eggs were mixed prior to storage. “The macronutrient composition and energy content were 18 g protein (1.57 g leucine), 17 g fat, and 226 kcal for the whole eggs and 18 g protein (1.60 g leucine), 0 g fat, and 73 kcal for the egg white treatments,” as reported by S Van Vliet et al. In crossover trials, 10 young men, approximately 21 years of age, received infusions of primed continuous l-[ring-2H5] phenylalanine and l-[1-13C]leucine infusions, while performing resistance exercise. Following exercise, the men consumed either the supplemented whole eggs or the egg whites.
The results showed that removal of the nutrient-dense egg yolk may ultimately affect muscle protein synthesis rates, as whole eggs stimulate myofibrillar muscle protein at a greater rate than that of egg whites. Other regulators of post-exercise muscle protein synthesis rates, such as total postprandial plasma leucine availability, whole-body leucine oxidation rates, skeletal muscle amino acid transport protein content, or molecular readouts associated with metabolic and anabolic protein signaling, showed no differences between whole egg and egg white consumption, notes study authors.
(Originally published October 4, 2017 and posted August 30, 2018)