Also known as: Ornithine, L-ornithine HCl, OKG, Ornithine alpha-ketoglutarate
Muscle & RecoveryEmerging evidence
A non-protein amino acid central to the urea cycle, where it helps clear ammonia. Marketed for growth hormone and recovery, though the better evidence concerns fatigue and sleep quality.
Typical dose
400.0–2000.0 mg per day
Evening for fatigue and sleep; pre-exercise for ammonia clearance
What it is
L-ornithine is a non-protein amino acid, meaning it is not incorporated into proteins. It is produced in the body from arginine and functions primarily within the urea cycle, the pathway that converts ammonia into urea for excretion.
Dietary intake is minimal, since it is not a protein constituent. It appears in small amounts in some foods but is essentially obtained through internal synthesis from arginine.
It is often paired with arginine in supplements on the basis of their metabolic relationship, and ornithine alpha-ketoglutarate, or OKG, is a separate compound combining ornithine with a citric acid cycle intermediate, studied mainly in clinical nutrition rather than sport.
How it works
Within the urea cycle, ornithine combines with carbamoyl phosphate to begin the sequence that converts ammonia into urea. Supplying additional ornithine is proposed to support the cycle's capacity to clear ammonia.
This matters for exercise because ammonia accumulates during prolonged or intense effort and contributes to both peripheral and central fatigue. Improved clearance would in principle delay that.
The growth hormone rationale comes from studies using intravenous administration at high doses, which does produce a growth hormone spike. Oral doses at supplemental levels produce a far smaller and transient response, and there is no good evidence that this translates into muscle or body composition change. This is a case where an intravenous finding was carried into oral marketing without justification.
What the evidence shows
Evidence for muscle growth, strength or body composition is weak, and trials in trained individuals have generally found no meaningful benefit. The growth hormone story does not hold up at oral doses.
More interesting findings concern fatigue and stress. A Japanese trial reported reduced subjective fatigue and improved sleep quality alongside favourable changes in stress hormone markers, and other small trials have reported reduced ammonia accumulation during endurance exercise.
There is a larger body of supervised research using the related compound OKG, but that concerns a different compound in a different setting. Overall ornithine is a modest ingredient whose supplement positioning does not match where its evidence actually sits.
Dosing
Common supplemental range is 400 mg to 2 g daily. The fatigue and sleep research used around 400 mg daily, which is at the lower end and reassuringly modest.
Higher doses of 2-6 g have been used in ammonia and endurance research, taken before exercise, but gastrointestinal tolerance becomes limiting above roughly 3 g.
Often taken in the evening given the sleep and fatigue findings, or before endurance work if the goal is ammonia clearance. If muscle growth is the goal, the evidence does not support taking it at all.
At a glance
Common forms
L-ornithine HCl, Ornithine alpha-ketoglutarate (OKG), Combined with arginine, Capsules and powder
Half-life / duration
Plasma ornithine peaks within 1-2 hours
Timing
Evening for fatigue and sleep; pre-exercise for ammonia clearance
Competition status
Permitted — not on the WADA Prohibited List
Category
Muscle & Recovery
Safety and side effects
Generally well tolerated at typical doses. The main adverse effect is gastrointestinal, with cramping and diarrhoea reported at higher intakes, particularly above 3 g in a single serving.
Because ornithine and arginine share transport mechanisms, taking large doses of both together may reduce absorption of each, which is worth knowing given how often they are combined in products.
Long-term safety data at supplemental doses is limited. Anyone under medical care should check before use.
Who should avoid it
Speak to your doctor before use if you take regular medication or are under medical care. Take care alongside high-dose arginine, given shared transport. Not established for use in pregnancy or breastfeeding at supplemental doses. Reduce the dose if digestive symptoms occur.
This entry is provided for information only. Food supplements should not be used as a substitute for a varied and balanced diet and a healthy lifestyle. Consult a qualified healthcare professional before use if you are pregnant, breastfeeding, taking medication or under medical supervision.
