Hemoglobin synthesis rates in young trained and untrained males and females are not different

We employed deuterated water (²H₂O) to compare the fractional synthetic rate (FSR) of hemoglobin in trained (n = 10) and untrained humans (n = 10). Participants had a mean V̇O₂max of 49.8 [SD: 10.9] mL/kg/min, hemoglobin mass of 775 [180] g, and red blood cell volume of 2370 [550] mL. After a loading dose, participants ingested ²H₂O daily for 28 days to maintain a stable ²H body water enrichment (~0.5 atom percent excess (APE)), as measured in saliva samples. ²H-enriched alanine APE was measured in RBC protein using gas chromatography pyrolysis isotope ratio mass spectrometry. The increase in APE for Hb protein was nonlinear for the first 2 weeks but stabilized from day 14 to day 28, with mean APE reaching 0.033 [0.005]% on day 28. Hb FSR calculated over this 2-week period was 0.84 [0.15]%/day, which equated to an Hb absolute synthetic rate of 6.5 [2.2] g/day and a lifespan of 126 [30] days. Hb FSR was not different between trained (0.83 [0.19] %/day) and untrained (0.86 [0.24] %/day, p = 0.81) individuals or between males (0.80 [0.25] %/day) and females (0.88 [0.17] %/day, p = 0.24). Endurance training and sex did not affect Hb FSR. The ²H₂O method to measure Hb FSR is feasible in humans.