Dosage
Trials commonly use 2 g to 6 g of L-ornithine per day, often split or taken before exercise. Take on an empty stomach for better absorption. Ask your healthcare provider before regular use if you have liver or kidney problems.
Ingredient guide
L-Ornithine: Ammonia, Exercise, and the Evidence
L-ornithine is a non-essential amino acid involved in the urea cycle, the body's ammonia-clearing pathway. Small trials show it can buffer ammonia and reduce subjective fatigue during exercise, but performance effects are mixed.
Limited evidenceBenefits
- May buffer ammonia rises during and after exercise.
- Reduced subjective fatigue in short trials of cycling exercise.
- A precursor for arginine and proline, supporting nitrogen handling.
Evidence summary
What L-ornithine is
L-ornithine is a non-essential amino acid that the body makes from arginine. It plays a central role in the urea cycle, the pathway that converts toxic ammonia (from protein metabolism) into urea for excretion. Supplements are taken before exercise to support ammonia clearance and reduce fatigue.
What the human research shows
A small trial of L-ornithine HCl before exercise found that it raised the body's ability to buffer ammonia, with higher plasma ammonia after exhaustion in the ornithine group than placebo, but reduced subjective fatigue during cycling. Performance effects across trials have been inconsistent. We grade the evidence as limited, supportive of ammonia buffering but mixed for performance.
Dosage & safety
Side effects
- Generally well tolerated at typical doses.
- Most common are mild stomach upset and loose stools at higher doses.
Interactions
- L-ornithine may add to the effect of arginine and some other amino acid supplements.
Warnings
- Speak with a doctor before regular L-ornithine use if you have liver or kidney problems.
- Tell your provider you take it before any planned surgery.
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Citations
- L-ornithine HCl on performance and ammonia metabolism during exercise pubmed.ncbi.nlm.nih.gov
- L-ornithine HCl on anaerobic cycling performance and fatigue recovery pubmed.ncbi.nlm.nih.gov
Frequently asked questions
Does L-ornithine help endurance?
Small trials show it can reduce subjective fatigue during exercise, but actual performance changes have been inconsistent.
How does L-ornithine work?
It is part of the urea cycle that clears ammonia, a fatigue-causing waste product. Extra ornithine appears to support that clearance during hard exercise.