L-carnitine is classified as an amino acid by-product because it can be synthesized in the body if certain nutrients are present. The primary aminos needed are lysine and methionine, although several other nutrients, including vitamin C and iron, are also required.
The primary function of carnitine is to help ferry long-chain fatty acids into the portion of the cell called mitochondria. Once in the mitochondria, fats are oxidized in a process known as beta-oxidation. Without carnitine, fats cannot enter the cell, and so no fat can be oxidized, or burned. That explains carnitine’s reputation as a fat-burner supplement.
The problem with advocating carnitine as a fat burner is that most people synthesize enough of it in their bodies to support fat oxidation, and obese people tend to produce even higher levels of carnitine than their leaner peers. That’s likely because of an obese person’s need to process more fat.
Carnitine was first discovered in meat extracts in 1905. It’s stored mainly in muscle, which has 50 to 200 times more carnitine than blood plasma has. The best dietary sources of carnitine are meat and dairy foods. Losses of carnitine in urine and bile average 60 milligrams a day and drop to less than 20 milligrams when people don’t eat any meat or dairy foods. Some evidence shows that choline, similar in chemical structure to carnitine, appears to help conserve it in the body.
Whether supplemental carnitine actually helps increase fat oxidation is a matter of debate, but the nutrient has other, lesser known, features that can improve training efficiency.1
Carnitine helps decrease lactate levels during intense exercise, which may lead to less fatigue and greater endurance. Several studies have shown that carnitine promotes recovery after intense training. Subjects who took three grams of carnitine daily for three weeks experienced less muscle soreness following training and lower levels of a muscle enzyme associated with muscle damage. The effect is thought to occur through increased cellular membrane stabilization promoted by carnitine in muscle cells. Carnitine also helps to decrease the effects of free radicals, by-products of oxygen metabolism that induce muscle inflammation and delay full muscular recovery.
Hard training tends to temporarily depress the immune system. When that occurs, you’re more vulnerable to infection. Carnitine appears to help stabilize and promote immune-system competence after training. It also helps promote the development of new red blood cells, which increases oxygen delivery to muscles.
The lethal dose of carnitine is about 630 grams a day, but the chances of anyone’s taking so much is remote. You can get diarrhea from taking a dose of six grams. A reasonable daily intake of carnitine is two to four grams in divided doses. Taking two grams after a workout will aid recovery and help prevent excessive muscle soreness. IM
1 Karlic, H., et al. (2004). Supplementation of L-carnitine in athletes: does it make sense? Nutrition. 20:709-15.