Ingestion of Free Amino Acids Compared with an Equivalent Amount of Intact Protein Results in More Rapid Amino Acid Absorption and Greater Postprandial Plasma Amino Acid Availability Without Affecting Muscle Protein Synthesis Rates in Young Adults in a Double-Blind Randomized Trial

Abstract

Background The rate of protein digestion and amino acid absorption determines the postprandial rise in circulating amino acids and modulates postprandial muscle protein synthesis rates. Objective We sought to compare protein digestion, amino acid absorption kinetics, and the postprandial muscle protein synthetic response following ingestion of intact milk protein or an equivalent amount of free amino acids. Methods Twenty-four healthy, young participants (mean ± SD age: 22 ± 3 y and BMI 23 ± 2 kg/m2; sex: 12 male and 12 female participants) received a primed continuous infusion of l-[ring-2H5]-phenylalanine and l-[ring-3,5–2H2]-tyrosine, after which they ingested either 30 g intrinsically l-[1–13C]-phenylalanine–labeled milk protein or an equivalent amount of free amino acids labeled with l-[1–13C]-phenylalanine. Blood samples and muscle biopsies were obtained to assess protein digestion and amino acid absorption kinetics (secondary outcome), whole-body protein net balance (secondary outcome), and mixed muscle protein synthesis rates (primary outcome) throughout the 6-h postprandial period. Results Postprandial plasma amino acid concentrations increased after ingestion of intact milk protein and free amino acids (both P < 0.001), with a greater increase following ingestion of the free amino acids than following ingestion of intact milk protein (P-time × treatment < 0.001). Exogenous phenylalanine release into plasma, assessed over the 6-h postprandial period, was greater with free amino acid ingestion (76 ± 9%) than with milk protein treatment (59 ± 10%; P < 0.001). Ingestion of free amino acids and intact milk protein increased mixed muscle protein synthesis rates (P-time < 0.001), with no differences between treatments (from 0.037 ± 0.015%/h to 0.053 ± 0.014%/h and 0.039 ± 0.016%/h to 0.051 ± 0.010%/h, respectively; P-time × treatment = 0.629). Conclusions Ingestion of a bolus of free amino acids leads to more rapid amino acid absorption and greater postprandial plasma amino acid availability than ingestion of an equivalent amount of intact milk protein. Ingestion of free amino acids may be preferred over ingestion of intact protein in conditions where protein digestion and amino acid absorption are compromised.

Mycoprotein ingestion within or without its wholefood matrix results in equivalent stimulation of myofibrillar protein synthesis rates in resting and exercised muscle of young men

“…Though the timing of the biopsies in the present study prohibited us from being able to detect a potential earlier (e.g. 2 rather than 4 h) stimulation of MyoPS and mTOR phosphorylaion in the PCM condition, our data contribute towards a growing number of studies that imply the plasma leucinaemic response is more, but not consistently so (30,31) , predictive of postprandial MPS rates when comparing consumption of isolated rather than whole-food protein sources (12,13,26,29,32) . However, taking the pivotal role of leucine at facevalue, it is interesting to speculate as to the role that the wholefood matrix may still have played in the stimulation of MyoPS in the present study.…”

Differential effect of two dietary protein sources on time course response of muscle anabolic signaling pathways in normal and insulin dysregulated horses

“…With non-structural carbohydrate (NSC) levels kept consistent between protein sources, a greater increase in plasma AA concentrations was expected to have a more potent stimulatory effect on pancreatic insulin release. This has been documented in human studies, where consumption of “faster” digested proteins or free amino acids resulted in greater elevation of blood insulin concentrations compared to “slower” digested proteins ( 7 , 37 ). To the best of our knowledge no studies exist to date comparing insulin responses to different isolated protein sources in horses, however, higher insulin responses were observed after nasogastric intubation with a glucose + leucine bolus vs. a glucose + whey protein bolus in thoroughbred horses ( 38 ).…”

Protein metabolism in critical illness

“…Improving protein digestibility by providing dietary protein as free amino acids or oligo peptides that do not require digestion increases amino acid availability in the circulation for peripheral tissues in health [40]. This could be a potential strategy in critically ill patients where protein is delivered into the stomach and delayed gastric emptying may compromise the systemic release of dietary amino acids [41].…”