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Ornithine: Time for Another Look?

About 25 years ago ornithine was a popular supplement among bodybuilders. It’s a free amino acid but is not commonly found in high-protein foods.


About 25 years ago ornithine was a popular supplement among bodybuilders. It’s a free amino acid but is not commonly found in high-protein foods. The body produces ornithine from the amino acid arginine through the activity of the enzyme arginase. It’s a key player in the urea cycle, in which nitrogen waste products left over from protein metabolism are broken down. The urea cycle is vital in preventing an increase in ammonia, which occurs during exercise and is linked to training fatigue. The primary reason for ornithine’s popularity, however, was that it was considered an effective growth hormone trigger; in fact, it was said to be twice as effective as arginine.

You still see ornithine in many sports supplements, such as those touted to boost growth hormone, and in some products that aid in nitric oxide release. A recent double-blind, placebo-controlled study, however, suggests that some of ornithine’s most interesting effects may have been overlooked in past research. Nine women and eight men, average age 40, received either a placebo for eight days or 2,000 milligrams of ornithine a day for seven days and 6,000 milligrams for one day. Exercise testing involved fatigue on repetitive-cycling tests.

After a week those taking 2,000 milligrams of ornithine experienced a rise in serum triglyceride, which is fat; free fatty acids and ketones, which are by-products of fat metabolism. Such changes indicate an increase in growth hormone release. That’s interesting because prior to the exercise the subjects had taken in a sugar solution, and high blood sugar blunts the release of GH. In fact, one of the functions of GH is to offset extended hypoglycemia, or low blood sugar. The subjects in the ornithine group also had more urea and ornithine in their blood, suggesting that the ornithine did indeed help along the urea cycle. The implication is that taking ornithine prior to exercise not only boosts GH release but also decreases blood ammonia, which would help keep you going longer without fatiguing.

In addition, the subjects in the ornithine group had higher counts of branched-chain amino acids than those in the placebo group, which suggests that ornithine may suppress the drop in muscle BCAAs produced during hard training. Higher amounts of BCAAs compete with another amino acid, L-tryptophane, for entry into the brain. That’s significant because when tryptophane enters the brain, it rapidly converts into serotonin, a brain chemical linked to feelings of fatigue. The ornithine group also had more of another amino acid, alanine, which can be converted in the liver into glucose, then circulated back to the active muscle as a readily available energy source.

Based on all this, you’d expect that the subjects in the ornithine group would be less fatigued during the exercise testing portion of the study, and that’s precisely what occurred—but only in the women, whose bodies had higher counts of ornithine. The authors say that if the plasma ornithine had remained higher in the men, they would probably have been less fatigued as well. Because ornithine isn’t common in protein foods, the authors suggest that it may be an effective nutritional supplement for treating fatigue. IM

Sugino, T., et al. (2008). L-ornithine supplementation attenuates physical fatigue in healthy volunteers by modulating lipid and amino acid metabolism. Nut Res. 28:738-43.

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