The post-workout muscle-building response for people eating whole eggs is 40 percent greater than for those consuming an equivalent amount of protein from egg whites, a study of the University of Illinois (UI) found.
The finding has been newly published in the American Journal of Clinical Nutrition.
In the study, 10 young men engaged in a single bout of resistance exercise and then ate either whole eggs or egg whites containing 18 grams of protein. Researchers administered infusions of stable-isotope-labeled leucine and phenylalanine, two important amino acids, to participants. This allowed the researchers to maintain and precisely measure amino acid levels in participants' blood and muscles.
The researchers took repeated blood and muscle biopsy samples to assess how the egg-derived amino acids were appearing in the blood and in protein synthesis in muscles before and after the resistance exercise and eating.
"By using those labeled eggs, we saw that if you ate the whole egg or the egg whites, the same amount of dietary amino acids became available in your blood," said Nicholas Burd, a UI professor of kinesiology and community health who led the research. "In each case, about 60 to 70 percent of the amino acids were available in the blood to build new muscle protein. That would suggest that getting one' s protein from whole eggs or just from the whites makes no difference."
But when the researchers directly measured protein synthesis in the muscle, they found a very different response.
"We saw that the ingestion of whole eggs immediately after resistance exercise resulted in greater muscle-protein synthesis than the ingestion of egg whites," Burd said.
Previous studies suggest this difference has nothing to do with the difference in energy content of whole eggs and egg whites. Whole eggs containing 18 grams of protein also contain about 17 grams of fat, whereas egg whites have no fat.
The study suggests that the widespread practice of throwing away egg yolks to maximize one's dietary protein intake from eggs is counterproductive, said Nicholas Burd, a University of Illinois professor of kinesiology and community health who led the research.