Supplement analysis backed by scientific evidence · PT

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Limited evidenceStress & mood

L-Ornithine

L-ornithine has a coherent biochemical story around ammonia clearance and a couple of small positive Japanese trials for fatigue and sleep, but the total human evidence amounts to a few dozen participants.

What the science says

  • Reduced subjective fatigue in a small crossover trial Limited evidence

    A randomized crossover study in 17 healthy volunteers found 2 g/day of L-ornithine for one week reduced subjective fatigue after a cycling test and shifted markers of ammonia detoxification and lipid metabolism. With fewer than 20 participants and no independent replication, this is preliminary.

  • Modest improvements in stress markers and sleep in office workers Limited evidence

    A randomized controlled trial in 52 healthy workers found 400 mg/day of L-ornithine for 8 weeks lowered the serum cortisol to DHEA-S ratio and improved several sleep quality subscales versus placebo. It was a single-site trial with self-reported sleep outcomes and no objective sleep measurement.

  • Do not confuse it with L-ornithine L-aspartate for liver disease Moderate evidence

    The strong ammonia-lowering evidence in hepatic encephalopathy relates to L-ornithine L-aspartate, usually given intravenously in hospital, not to over-the-counter L-ornithine capsules. That clinical literature does not transfer to healthy people taking a gram or two orally.

  • No convincing performance or growth hormone effect Limited evidence

    Ornithine's popularity in bodybuilding rests on studies where very large oral doses transiently raised growth hormone, an effect that has never been shown to translate into strength or muscle gains. Doses high enough to move GH also reliably cause diarrhea.

Dosage & safety

Studied doseFatigue trials used about 2 g/day for 7 days; the stress and sleep trial used 400 mg/day for 8 weeks. Ammonia-focused exercise protocols have used 6-12 g/day, with poor GI tolerance.
SafetyDoses above roughly 10 g/day commonly cause osmotic diarrhea, cramping and nausea; below 3 g/day it is generally well tolerated. Because ornithine feeds the urea cycle, people with urea cycle disorders, cirrhosis or advanced kidney disease should not supplement without medical advice. No significant drug interactions are established, but data are sparse. There is no meaningful safety information for pregnancy, breastfeeding or use beyond a few months.

Scientific references

Where to buy

Nutricost, L-Ornithine, 500 mg, 180 Capsules

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