L-ornithine is a non-proteinogenic amino acid (not incorporated into proteins) that plays a central role in the urea cycle — the body's primary system for removing ammonia. Our research shows the primary supplemental benefit is ammonia detoxification during prolonged exercise and in liver disease. A 2010 RCT found L-ornithine supplementation reduced fatigue by 52% in a cycling test and lowered blood ammonia by 28%. L-ornithine also stimulates growth hormone (GH) release during sleep, but the magnitude is too small to produce meaningful body composition changes. The most evidence-backed application is hepatic encephalopathy treatment (L-ornithine-L-aspartate, or LOLA).
L-ornithine is a key intermediate in the urea cycle — the metabolic pathway that converts toxic ammonia (NH₃) to urea for excretion. During intense or prolonged exercise, ammonia accumulates from amino acid catabolism and AMP deamination, contributing to central and peripheral fatigue. Supplemental ornithine provides additional substrate for the urea cycle, accelerating ammonia clearance. For GH release, ornithine stimulates the anterior pituitary via inhibition of somatostatin. The sleep improvement mechanism involves cortisol reduction — ornithine accelerates ammonia clearance, reducing hypothalamic stress signaling.
No critical interactions identified.
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Not medical advice. Based on published clinical research and systematic reviews.