Arginine behaviour after arginine or citrulline administration in older subjects.
Level 3 - non-randomized controlled study
Small crossover pharmacokinetic study with unstated randomization status
PubMed 26619904 · doi:10.1017/S0007114515004638
What was done
Eight fasting healthy elderly men participated in a crossover pharmacokinetic study comparing two separate isomolar oral loading tests: 10 g of citrulline versus 9.94 g of arginine. Serial blood samples were collected over an 8-hour period to measure plasma amino acid kinetics and assess tolerability.
What was found
Plasma concentration changes were limited to citrulline, ornithine, and arginine. Volume of distribution did not differ between interventions. Following arginine ingestion, arginine elimination was significantly higher compared to citrulline ingestion (P=0.041), and the admission period plus time to peak concentration was shorter (P=0.033). Combined, these kinetic differences produced a marked increase in systemic arginine availability after citrulline loading compared to arginine loading itself (P=0.033). Single-dose citrulline was well tolerated.
Why it matters
These findings show that oral citrulline serves as a more efficient precursor for elevating circulating arginine levels than arginine itself in older adults, circumventing typical first-pass splanchnic metabolism.
Limits
The study was conducted in a very small sample (n=8) of healthy older men, limiting generalizability to women or frailer populations. The abstract does not confirm randomization or blinding, and only single-dose 8-hour pharmacokinetics were evaluated without long-term efficacy or clinical outcomes.
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