L-ornithine is a non-essential, non-protein amino acid. L-ornithine is part of the urea cycle, along with L-arginine and L-citrulline. Together these amino acids help buffer ammonia in the blood. Ammonia activates the enzyme phosphofructokinase, which increases the production of lactate. This causes a decrease in muscle cell pH, a decrease in calcium release, and therefore a decrease in muscle contractility. Increased ornithine may reverse these effects—increasing the length of muscle contractility before fatigue, allowing for a longer, more productive workout.
Ornithine and arginine promote the synthesis of growth hormone, which is expected to increase protein synthesis and muscle mass. However, the potential effect of ornithine on growth hormone release is not fully understood.
Other studies show that ornithine may improve mood and sleep quality by affecting the hypothalamus-pituitary-adrenal axis. In a study, it reduced cortisol levels, and study participants reported they felt less stressed and had a more restful sleep.