Supplements touted to increase nitric oxide seem to be popular among bodybuilders, judging from the number of companies selling them. The active ingredient in nearly all of them is the amino acid arginine, which is the immediate nutrient precursor of nitric oxide. Several other nutrients are also capable of boosting NO, however, including garlic, apple antioxidants and the isoflavone genistein, found in soy. In fact, most antioxidant nutrients will boost NO because increased oxidation blunts NO release.
A recent study disclosed another supplement capable of boosting NO release, glycine-propionyl-L-carnitine. It’s known as an “amino” carnitine, since it’s a bonded form of L-carnitine and the amino acid glycine. While L-carnitine has several functions in the body, it’s best known for shuttling fat into the portion of cells known as mitochondria. Fat is burned, or oxidized, in mitochondria in a process called beta-oxidation. Carnitine is essential for this process.
Glycine-propionyl-L-carnitine is a specialized form of carnitine, and it’s particularly beneficial for heart function. The heart preferably uses fat as an energy source, and GPLC is used therapeutically to treat heart failure and poor blood circulation. In Europe it’s used as a drug for that purpose. Other studies show that GPLC has more of an affinity for muscle than other forms of L-carnitine. These studies suggest that using GPLC may boost exercise efficiency by lowering lactic acid and using fat more efficiently as an energy source.
In the new study, 15 experienced weight-trained men were provided 4.5 grams a day of either GPLC or a placebo. The subjects were also given carbohydrates to promote insulin release, since insulin fuels carnitine uptake into tissues. They were then exposed to an ischemia-reperfusion routine by means of a blood pressure cuff. Inflating the cuff stops blood flow; releasing it produces a higher-than-usual blood flow, or reperfusion. It’s difficult to measure NO because it’s an ephemeral gas that rapidly disappears. Products of NO metabolism, however, such as nitrates and nitrites, can be measured in blood to determine NO activity, and that’s what the researchers did here.
Under normal circumstances, NO release occurs with turbulent blood flow, such as that induced by exercise. It is synthesized and released in the endothelium, or lining of blood vessels. In that manner, NO inhibits the clumping of blood platelets and expands blood vessel diameter. The net effect is increased blood flow and, in the case of bodybuilding exercise, increased muscle pump and delivery of nutrients and oxygen to working muscle.
The study showed that NO release was significantly higher in those who took the GPLC supplement than it was in those who took the placebo. The authors noted that there are always “nonresponders,” who get no apparent effect from any supplement. For example, 30 percent of creatine users get no apparent benefits, usually those who eat a lot of red meat, a rich natural source of creatine.
As to how GPLC works in relation to NO, the authors weren’t sure. One theory is that GPLC inhibits an oxidative enzyme that rapidly degrades NO because of an increase in free radicals. Another theory is that GPLC augments the activity of endothelial nitric oxide synthase, the enzyme in the blood vessel walls that produces NO from arginine.
GPLC may be a useful adjunct to NO supplements, since it independently augments NO release and has no known side effects. It also provides other benefits, such as increased muscle and heart efficiency, along with lower lactic acid production during intense exercise. In that sense, it would be complementary with beta-alanine, mainly used to control higher acidity levels produced during exercise. Anecdotal reports suggest that GLPC may blunt some of the negative side effects linked to ketosis, experienced by many who follow low-carbohydrate diets. GLPC also helps maintain androgen cell receptors, which increases the anabolic efficiency of testosterone. In fact, some preliminary studies suggest that GLPC may be of use in treating male impotence. IM
Bloomer, R.J., et al. (2007). Glycine propionyl-L-Carnitine increases plasma nitrate/nitrite in resistance-trained men. J Int Soc Sports Nutr. 4:22.