Get your blood flowing with L-arginine: popular in heart and sexual health formulas, L-arginine works by improving blood flow in the body. Here's advice on buying and using this truly cutting-edge supplement.(SHOP SMART: SUPPLEMENTS)

THE BASICS: Arginine, an amino acid, or building block of protein, is the only precursor to nitric oxide, one of the body's most important regulatory molecules. (Nitric oxide is not to be confused with nitrous oxide, or laughing gas.) Nitric oxide controls blood vessel tone and flexibility, so it has a profound effect on the cardiovascular system. Arginine's benefits are practically synonymous with nitric oxide, and all of nitric oxide's effects depend on the body's reserves of arginine.

ALIAS: Like many other amino acids, arginine is formally identified as L-arginine. Supplemental L-arginine boosts nitric oxide levels, improving the heart and blood vessels, immunity, neurotransmitter activity, erectile function in men and possibly gastric reflux. The conversion of arginine to nitric oxide requires the enzyme nitric oxide synthase, which requires vitamin C.

HOW THEY WORK: Arginine supplements rapidly boost levels of nitric oxide, which in turn increases blood vessel flexibility and improves blood flow. Arginine is also necessary for normal protein synthesis--important when you consider that after water, most of the body consists of protein. Some arginine is also converted to ornithine, an amino acid needed for the body's production of growth hormone. A small amount of arginine is converted to gamma-aminobutyric acid, a calming neurotransmitter.

HEALTH BENEFITS: Supplemental arginine has several important and varied health benefits.

* Blood vessel tone. Arginine, by way of nitric oxide, regulates the endothelium, a thin layer of cells in blood vessel walls. The endothelium gently expands and contracts, helping the heart move blood throughout the body. Dietary trans fats, especially when combined with refined carbohydrates in fast foods, interfere with normal endothelial activity, leading to "endothelial dysfunction," a risk factor for heart disease. Supplemental arginine can improve endothelial function in patients with heart disease.

* Blood pressure. Arginine supplements have been found to reduce blood pressure, likely as a result of nitric oxide's ability to relax and dilate blood vessels. In one study, supplements of a sustained-release form of arginine (1050mg, twice daily) led to substantial reductions in systolic blood pressure and modest reductions in diastolic blood pressure.

* Erectile dysfunction. The same vascular problems underlying endothelial dysfunction also impact a man's ability to maintain an erection. Supplemental arginine ...