Digestibility, Whey Protein, and Hypertrophy: Finding Balance

Research sounds good but then you have to validate the source’s credibility as a consultant to the dairy industry.

According to Dr. Stuart M. Phillips, Professor in Kinesiology at MacMaster University, protein quality plays a big role in determining resistance exercise-induced muscle hypertrophy. In a recent article in Nutrition and Metabolism1, Dr. Phillips looked into the impact of quality of supplemental protein on changes in muscle mass, strength and body composition when combined with strength training. The leucine content of a protein is the strongest determinant of the capacity of a protein to affect muscle protein synthesis (MPS) and likely hypertrophy.

While a strength training session may lower the threshold for protein/leucine required to stimulate MPS the importance of leucine content for MPS and likely subsequent hypertrophy needs to be appreciated. According to the article, it’s not just about getting your workout protein fix but, it’s about its digestibility. Leucine is the key amino acid that helps in the increase in MPS. Therefore, ingested proteins with a high leucine content are more likely to trigger a rise in MPS. Protein quality is determined by a combination of IAA in the formula and the efficacy of digestibility.

Protein quality has been measured by the protein digestibility-corrected amino acid score (PDCAAS); however, the digestible indispensable amino acid score (DIAAS) has been recommended as a better method for protein quality scoring. Under DIAAS there is the recognition that amino acids are individual nutrients and that protein quality is contingent on IAA content and ileal (as opposed to fecal) digestibility. In other words, protein absorption is a better indication of protein excretion when it comes to looking at measuring digestibility.

“My assessment of the data on protein supplementation and resistance exercise reveals that the amount of leucine in a protein supplement has the greatest impact on muscle protein synthesis,” said, Dr. Phillips. “Leucine is not only a building block for protein, but a trigger for working muscles to synthesize more protein. In essence, it turns on muscle protein synthesis like a light switch so that over time, there could be greater gains in lean body mass and strength, and subsequently, body composition improvements.”

Whey protein isolate or concentrate tend to have the greatest concentration of leucine. Whey protein is a milk protein that is considered high-quality due to its amino acid profile and high score for digestibility.

“The outcome of this review isn’t just applicable to strength trainers,” Dr. Phillips notes. “As we age, muscle loss becomes prevalent if we don’t thwart the decline. Leucine-rich whey protein supplementation, combined with resistance exercise, may be one way to help preserve muscle mass throughout the lifespan.”

Protein source PDCAAS DIAAS Limiting AA
Whey PI 1.00 1.09 His
Whey PC 1.00 0.97 His
Soy PI A 1.00 0.91 Met + Cys
Soy PI B 0.98 0.90 Met + Cys
Pea P 0.89 0.82 Met + Cys
Rice PC 0.42 0.37 Lys

PDCAAS and DIAAS scores, the limiting amino acid assessed by the amino acid reference ratio for selected proteins. (Source: British Journal of Nutrition)

Now, here is the proviso, the author of this review does admit to receiving funding, honoraria for speaking, and travel expenses from the following non-government agencies: The US National Dairy Council, The US National Cattlemen’s Beef Association, and Dairy Farmers of Canada. These groups love their Whey protein. We have no research on that here, but we are guessing.


1. Phillips, Stuart M. “The Impact of Protein Quality on the Promotion of Resistance Exercise-Induced Changes in Muscle Mass.” Nutrition & Metabolism 13.64 (2016):