Safety and tolerability of L‑Proline intake in healthy adult men: a randomized, double‑blind, placebo‑controlled crossover study

The aim of this study was to evaluate the safety of L-proline supplementation over a four-week period by determining the highest tested dose without an identified safety signal under the conditions of this study. In a randomized, double-blind, placebo-controlled crossover trial, 30 healthy adult men received graded doses of L-proline (0, 3, 6, 9, or 12 g/day). Each participant was assigned to a randomized sequence comprising four of the five doses, with each intervention period separated by a two-week washout period. Primary endpoints included clinical laboratory parameters (hematological, biochemical, and electrolyte indices), while the secondary endpoint was the incidence of adverse events. Anthropometric measures and dietary intake were also monitored. Plasma amino acid concentrations following supplementation were assessed as an exploratory outcome. L-proline supplementation at all tested doses was well tolerated. No clinically meaningful or statistically significant abnormalities were observed in hematological, biochemical, or electrolyte parameters. Anthropometric and dietary variables remained unchanged throughout the study. No serious adverse events were reported, and the frequency of mild to moderate adverse events did not differ from placebo. Under fasting conditions at the end of supplementation, plasma L-proline concentrations increased at higher doses (9 and 12 g/day), whereas other amino acids were unaffected. Overall, daily L-proline supplementation at doses up to 12 g for four weeks is safe and well tolerated in healthy adult men, with no evidence of adverse clinical or biochemical effects. Our results support 12 g/day as the highest tested dose without an identified safety signal for L-proline under the conditions of this study. The findings of the present study provide safety data regarding supplementation with L-proline, thus allowing us to propose a tentative highest experimental intake without detected adverse effects that might be useful for future clinical and nutritional studies.