Oral amino acid tracer delivery detects feeding and exercise changes in myofibrillar protein synthesis rates in male adults

The efficacy of oral administration of leucine and phenylalanine tracers to measure MyoPS (LEUMyoPS and PheMyoPS, respectively) in response to varying anabolic stimuli was investigated. Participants were randomized to a rested-fasted (FAST), rested-fed (FED), or exercise-fed (EXFED) condition. FED and EXFED consumed a mixed carbohydrate and AA beverage enriched with L-[1-¹³C]leucine (25%) and L-[ring-²H₅]phenylalanine (30%) at rest or after a bout of resistance exercise, while FAST consumed only the equivalent tracer dose. Blood samples were obtained every 30 min for 300 min to determine tracer precursor enrichment with muscle biopsies obtained before and at 120 and 300 min after tracer ingestion to determine MyoPS by the precursor-product method. LEUMyoPS over 300 min was greater (p < 0.01) in EXFED (0.090 ± 0.024%/h; mean ± SD) and FED (0.067 ± 0.028%/h) compared to FAST (0.024 ± 0.015%/h). PHEMyoPS over 300 min was greater (p < 0.01) in EXFED (0.128 ± 0.034%/h) and FED (0.098 ± 0.020%/h) compared to FAST (0.056 ± 0.012%/h). There was a positive correlation (r = 0.81, p < 0.0001) between LEUMyoPS and. Oral essential amino acid tracer ingestion can be used as an alternative method to detect changes in MyoPS in response to mixed macronutrient feeding and feeding plus exercise and may be an option for the study of human muscle protein synthesis where intravenous isotope administration is logistically difficult or prohibitively expensive.