Q. What is the difference between L-glutamine and glutamine, and the same for the other amino acids? Is one more effective as a supplement than the other?
A. Unfortunately the answer to this question will involve some chemistry, though it may not be necessary to completely understand all the chemistry to understand the gist of the answer.
All the amino acids in our bodies have a common structure that enables them to link up to one another by what is known as a peptide bond, producing the long strands of amino acids that we define as proteins. Without these long protein chains we could not survive. Amino acids are constructed so that two mirror images can exist, like your right and left hands. If you have ever tried to put a left-handed glove on your right hand you can see how difficult it would be for a "right-handed" amino acid to connect to a "left-handed" amino acid.
Louis Pasteur observed the "handedness" of amino acids and many other molecules in our bodies. He found that when you shine a beam of polarized light through a solution of amino acids, the light beam would be rotated counterclockwise. The amino acids are thus designated "levo" (left) or " " amino acids. A minor problem was that not every amino acid rotates light counterclockwise even though they all had the common chemical structure that allows them to link up. Pasteur solved the problem by calling all amino acids "L" amino acids (the capital letter is important) whether they actually rotated light clockwise or counterclockwise.
Whether the "L" prefix is present or not, all the glutamine that is found in our food or food supplements is L-glutamine because the mirror image of L-glutamine (called D-glutamine for "dextro" or right) has no nutritional value and is useful only for chemical lab research.
D-Amino acids occur in nature in small amounts. All the amino acids that are made into proteins are L-amino acids, but some bacteria use D-amino acids in their cell walls to help them resist the natural antibiotics that are produced by other bacteria. Mostly D-amino acids in foods are the product of bacterial fermentation and so are found in products such as yogurt and cheese. D-Amino acids in the amounts found in food are generally thought to be harmless to humans.
One D-amino acid has been proposed to have benefits in the human body, not for protein synthesis but as a pain reliever. D-Phenylalanine, the mirror image of L-phenylalanine, is not normally found in the body but may have a pharmacological effect on the production of enkephalins, the natural pain relievers our bodies produce in high-stress situations. The third World Congress on Pain recommended trials of D-Phenylalanine for pain relief, but unfortunately there have as yet been no "double-blind-placebo-controlled" studies, which are necessary to establish the credibility of a treatment for most clinicians. A mixture, DL-phenylalanine, is much more readily available than D-phenylalanine because it is cheaper and easier to make. DL-phenylalanine may also be an effective antidepressant drug on a par with imipramine, one of the first and, for a long time, the standard antidepressant in psychiatry. In clinical studies, DL-Phenylphenyl worked faster and with fewer side effects and was about as effective as imipramine. You should always check with your health care provider before trying DL-phenylalanine.