Oral essential amino acid tracer ingestion can be used as an alternative method to detect changes in myofibrillar protein synthesis rates in response to mixed macronutrient feeding and feeding plus exercise where intravenous isotope administration is logistically difficult or prohibitively expensive.
Key Findings
Results
LEUMyoPS over 300 min was significantly greater in exercise-fed and fed conditions compared to fasted.
EXFED LEUMyoPS: 0.090 ± 0.024%/h
FED LEUMyoPS: 0.067 ± 0.028%/h
FAST LEUMyoPS: 0.024 ± 0.015%/h
Differences were statistically significant (p < 0.01) for both EXFED and FED versus FAST
Results
PheMyoPS over 300 min was significantly greater in exercise-fed and fed conditions compared to fasted.
EXFED PheMyoPS: 0.128 ± 0.034%/h
FED PheMyoPS: 0.098 ± 0.020%/h
FAST PheMyoPS: 0.056 ± 0.012%/h
Differences were statistically significant (p < 0.01) for both EXFED and FED versus FAST
Results
There was a strong positive correlation between LEUMyoPS and PheMyoPS across conditions.
Pearson correlation coefficient r = 0.81
Correlation was statistically significant (p < 0.0001)
This suggests the two oral tracer methods provide concordant measures of myofibrillar protein synthesis
Methods
The study used a randomized design with three distinct anabolic conditions to test the oral tracer methodology.
Participants were randomized to rested-fasted (FAST), rested-fed (FED), or exercise-fed (EXFED) conditions
FED and EXFED consumed a mixed carbohydrate and amino acid beverage enriched with L-[1-13C]leucine (25%) and L-[ring-2H5]phenylalanine (30%)
FAST consumed only the equivalent tracer dose without macronutrients
FED received the beverage at rest while EXFED received it after a bout of resistance exercise
Blood samples were obtained every 30 min for 300 min and muscle biopsies were obtained before and at 120 and 300 min after tracer ingestion
Results
Oral tracer delivery using both leucine and phenylalanine was sensitive enough to detect differences in myofibrillar protein synthesis rates between fasting, feeding, and exercise plus feeding conditions.
Both LEUMyoPS and PheMyoPS detected significantly higher synthesis rates in FED versus FAST (p < 0.01)
Both tracers detected significantly higher synthesis rates in EXFED versus FAST (p < 0.01)
The precursor-product method was used to calculate MyoPS rates using blood tracer precursor enrichment and muscle biopsy product enrichment
Authors conclude oral tracer ingestion 'can be used as an alternative method to detect changes in MyoPS in response to mixed macronutrient feeding and feeding plus exercise'
Mazzulla M, Hodson N, Scaife P, Smith K, Atherton P, Burd N, et al.. (2026). Oral amino acid tracer delivery detects feeding and exercise changes in myofibrillar protein synthesis rates in male adults.. Physiological reports. https://doi.org/10.14814/phy2.70776