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What does the "R" mean in "R-alpha-Lipoic Acid"?

The "R" in "R-alpha-Lipoic Acid" stands for the Latin prefix "Recto" (meaning "right"). The "S" in "S-alpha-Lipoic Acid" stands for "Sinistro" (meaning "left"). Alpha-Lipoic Acid is one of many chemical compounds that has two different molecular structures with analogous chemical properties. The two structures do not differ from each other in the number or types of atoms that make up the molecules, or in the order in which these atoms are connected together with chemical bonds; they differ in the way these atoms and bonds are configured in three dimensions. The R- and S-configurations have the same connectivity within their molecular frameworks, but they cannot be rotated or moved so as to coincide.

As an analogy, hold out both your hands with the fingers in the same relative configuration -- that is, whatever pattern your right-hand fingers are making, hold the fingers of the left hand in the same relative pattern. One hand is now like a mirror image of the other. But no matter how you try to move or rotate one hand, you can't make it coincide with the other -- you can't make a right hand look exactly like a left hand, even though they have the same relative configuration.

When a product is labeled simply "alpha-Lipoic Acid", this means that it is a 1:1 mixture of R- and S-alpha-Lipoic Acid. This mixed form is easiest and cheapest to make synthetically. If the product label says "R-alpha-Lipoic Acid", then the product is, of course, just the R-form of alpha-Lipoic Acid. This is the form normally found in the body.

The S-form of alpha-Lipoic Acid has, generally speaking, less biological activity than the R-form. The currently accepted view is that the S-form is not harmful, but that it may to some extent counteract the benefits of the R-form. Therefore, if one can afford the R-form, one should buy it instead of the mixed form. The mixed form, however, has been shown in many clinical studies to be beneficial, and certainly worth using if the R-form is too expensive.

March 11, 2003
written by Russell Mills
San Francisco, CA