N-Acetyl-L-Cysteine and Acetyl-L-Carnitine

Acetyl-l-carnitine and N-acetyl-l-cysteine are nutritional supplements derived from naturally occurring amino acids. Although they share somewhat similar names, these antioxidant supplements have markedly different properties and effects.

Acetyl-l-carnitine, or ALCAR, is synthesized from carnitine, while N-acetyl-l-cysteine, or NAC, is derived from the amino acid cysteine. ALCAR and NAC are used to treat a wide variety of medical conditions.

Both are also used by weightlifters and athletes who believe these supplements help enhance performance and build muscle. Consult your doctor before taking ALCAR or NAC.

L-carnitine, synthesized in your liver from the amino acids lysine and methionine, works to convert fat to energy. Acetyl-l-carnitine is simply l-carnitine to which acetyl groups have been added; the Linus Pauling Institute at Oregon State University notes that this may increase bioavailability.

The Office of Dietary Supplements reports that carnitine can reduce age-related oxidative stress, decrease mortality in people with heart conditions and improve exercise performance in individuals with peripheral arterial disease.

According to Chris Aceto, author of "Championship Bodybuilding," ALCAR helps athletes maintain testosterone levels following heavy workout schedules, and may help improve uptake of glucose and promote loss of body fat.

Dr. Eric P. Brass, Ph.D, a professor of medicine at University of California Los Angeles, reports that 20 years of research have yielded no real evidence that carnitine enhances athletic performance in healthy people. Brass notes, however, that a negative finding hasn't been proved either.

Noting that l-carnitine appears to be generally safe in recommended amounts, University of Maryland Medical Center recommends dosages of between 1,000 and 3,000 milligrams a day. Side effects -- including increased appetite, rashes, diarrhea and a fishy body odor -- have been reported with carnitine; dosages in excess of 3,000 milligrams make side effects more likely.

The Linus Pauling Institute at Oregon State University cautions that you should avoid taking a D-carnitine formulation; this can compete with l-carnitine and can cause muscle weakness. Supplemental carnitine, including ALCAR, can interact with prescription medications. Consult your physician before taking ALCAR.

L-cysteine, from which NAC is derived, is a nonessential amino acid synthesized by your body. Cysteine helps to produce the antioxidants glutathione and taurine; it also plays a role in digestive and immune health. Supplementary NAC is converted to cysteine in your body, where it helps to fight free-radical damage and possibly slow the onset of aging and reduce disease risk.

NAC, which helps break down toxic chemicals in the body, is used to treat acetaminophen overdose and lung conditions such as bronchitis and chronic obstructive pulmonary disease. According to bodybuilding expert Clayton South, NAC increases glutathione levels and helps to preserve lean muscle mass. In a clinical study published in 2000 in "European Journal of Clinical Investigation," researchers found that oral NAC replenished glutathione in patients with HIV.

According to UMMC, the usual dose of NAC for general health and antioxidant protection is 500 milligrams a day. Follow your physician's dosage directions for NAC; very large doses of over 7,000 milligrams can be toxic and even fatal. Avoid D-cysteine, D-cystine and 5-methyl cysteine; UMMC reports that these are toxic.

NAC should be taken with a multivitamin to ensure proper levels of B-complex vitamins. Serious allergic reactions have been reported with cysteine, and it can interact with prescription medications. Consult your doctor before taking NAC.