The Doctor's Corner

Q: Last issue, MUSCLE INSIDER columnist Dr. Ann de Wees Allen said you can only absorb 30 grams of protein at one meal! Is there any truth to that statement in your opinion?

A. The oft-quoted dictum that the human digestive system can only absorb 30 grams of protein at a single consumption is nonsense. In fact, I’m unaware of any human data documenting any upper limit to absorption of dietary protein consumed in a single sitting. Various human in-vivo research techniques have documented these facts about dietary protein absorption: Most ingested “whole-food” proteins are absorbed by the time they reach the initial segments of the small intestine (jejunum). This can be affected by many variables known to influence gastric emptying and intestinal transit times as well as the composition and form of the protein source. The rate of absorption of the amino acids contained in the protein meal is proportionate to their concentration in the protein source. Secreted proteins (from the gut) are always additive to the ingested protein source meal. The amount of this added protein load varies but can be as high as 50 to 60 percent of the ingested protein dose. Thus, a 30-gram ingestion may induce as much as an additional 15-gram protein delivery to the systemic circulation, producing a net 45-gram absorption. Although the major site of assimilation is the proximal small intestine, ingested protein is continuously absorbed along the length of the small bowel, and some in-vivo studies document continued absorption of protein derived peptides (especially from endogenously secreted proteins) in the large bowel. A review of existing literature suggests that for cooked whole-food protein sources (e.g., meat, poultry, fish) consumed as mixed meals, the dietary protein load (with no currently identified per-meal upper limit) will be completely absorbed at an average rate of 10 grams per hour. The efficiency (i.e., percentage of total ingested protein absorbed) can vary between dietary sources and presentation forms (e.g., hydrolyzed vs. complete proteins), but in general, commodity protein sources consumed as a part of a typical Western diet are associated with very high absorption efficiencies. The absolute metrics of nitrogen retention and whole-body protein metabolism are profoundly affected by the post-ingestion kinetics of the ingested protein source. Hence, complete proteins (as are typical in the diet) are associated with better whole-body nitrogen retention effects than rapidly digested protein sources such as whey and soy. So, persons consuming disproportionate amounts of their daily protein intake from either elemental sources (e.g., free-form amino acids) or rapidly assimilated protein products (e.g., soy, whey, hydrolyzed dairy proteins) would require frequent feedings throughout the day (every three hours in the case of whey proteins) to address the issue of suboptimal nitrogen retention associated with such products. There appear to be no essential differences between young and elderly humans with respect to the above considerations.