Journal Title (Medline/Pubmed accepted abbreviation): Am J Clin Nutr
Summary of background and research design
Elderly individuals have a reduced skeletal muscle protein synthetic response following a meal. Among all individuals, this response is dependent on the quantity and quality of ingested protein. Research suggests that consumption of whey protein results in greater protein retention than consumption of casein protein. This difference is attributed to the faster digestion kinetics of whey (“fast protein”), which result in greater increase in plasma amino acids following ingestion. Additionally, these two protein sources have different amino acid compositions, which may also contribute to theses differences. It has been shown that hydrolysis of intact casein alters the digestion and absorption characteristics to resemble a “fast protein”.
Forty-eight healthy older men (mean age: 74) were randomly assigned to consume a single 20 gram bolus of whey, casein, or hydrolyzed casein following an overnight fast. The proteins were intrinsically labeled with a stable isotope of phenylalanine, an essential amino acid. Serial blood and muscle samples were collected for 2 hours prior to ingestion of the protein and for 6 hours after.
Summary of research findings
- Peak insulin concentrations in plasma were higher and peak essential amino acid concentrations after whey and hydrolyzed casein compared to native casein.
- Peak plasma leucine from whey was greater than from the other two sources.
- Whey and hydrolyzed casein increased plasma essential amino acid availability to a greater extent than native casein during the first 2 hours following ingestion.
- The fractional synthesis of new protein in skeletal muscle following whey consumption was significantly higher than both native casein and hydrolyzed casein.
- Of all measured variables, the post-prandial increase in leucine had a strong correlation with the rise in protein synthesis.
Interpretation of findings/Key practice applications
Essential amino acids that are crucial for stimulating new muscle protein synthesis appeared more rapidly after consumption of both whey and hydrolyzed casein. However, whey produced greater increases in plasma leucine, and correspondingly the highest increase in protein synthesis. Hydrolyzed casein was digested at the same rate as whey, while native casein was digested more slowly. Despite similar digestion rates, the higher enrichment of leucine in whey relative to hydrolyzed casein (12.5% vs. 8.5%) likely led to the higher spike in plasma leucine. It has been established that elevations in post-prandial leucine are highly correlated with increases in muscle protein synthesis, and it is likely that the total amount of ingested leucine in large part determines the response in synthesis of new protein. In this elderly population, whey protein produced the most desirable acute outcome on muscle protein homeostasis, likely due to its rapid digestion and favorable essential amino acid profile (enriched in leucine). It has been suggested that elderly individuals have a reduced sensitivity to leucine, so this response would likely be reflected to a higher magnitude among younger individuals.
One additional comment is that the use of the term “accretion” in the title of the article might not be the best choice of words. Accretion might imply accumulation or build-up of protein. However, in this study, the main measurement was of fractional synthetic rate for protein, not net muscle protein balance. Net muscle protein balance measures would incorporate measures of both protein synthesis and degradation, with accretion meaning that synthesis was greater than degradation. The authors did not present data on protein degradation or net muscle protein balance in this paper.