Racemic mixture versus R-lipoic acid only
R-lipoic acid is the isomer that is synthesized by plants and animals and functions as a cofactor for mitochondrial enzymes in its protein-bound form (see Biological Activities). Direct comparisons of the bioavailability of the oral racemic mixture and R-lipoic acid supplements have not been published.
Following the ingestion of R,S-lipoic acid, peak plasma concentrations of R-lipoic acid were found to be 40%-50% higher than S-lipoic acid, suggesting better absorption of R-lipoic acid. Both isomers were nonetheless rapidly metabolized and eliminated (6, 8, 11). In rats, R-lipoic acid was more effective than S-lipoic acid in enhancing insulin-stimulated glucose transport and metabolism in skeletal muscle (110), and R-lipoic acid was more effective than R,S-lipoic acid and S-lipoic acid in preventing cataracts (111).
However, all of the published human studies have used R,S-lipoic acid (racemic mixture). It has been suggested that the presence of S-lipoic acid in the racemic mixture may limit the polymerization of R-lipoic acid and enhance its bioavailability (52). At present, it remains unclear which supplemental form is best to use in clinical trials.
While R-lipoic acid is the more beneficial component of the racemic form, the S-lipoic acid does lend enhanced bioavailability to the R, by keeping it in it's smaller molecular form.
Alpha-lipoic acid is one of the most powerful antioxidants ever discovered. It is a vitamin-like sulfur-containing compound that is synthesized naturally in the human body. Sometimes it is referred to as alpha-lipoate, thioctic acid, or just lipoic acid. One of its most important characteristics is that it is both fat-soluble and water-soluble. This enables it to provide antioxidant protection in a much wider range of physiological environments throughout the body, which has resulted in some scientists referring to alpha-lipoic acid as the "universal" antioxidant.
In the body, alpha-lipoic acid is converted to dihydrolipoic acid (DHLA), which also functions as a strong antioxidant. Lipoic acid is part of two enzyme systems: PDH (pyruvate dehydrogenase) and alphaketoglutarate dehydrogenase. These enzymes are part of the Krebs cycle and are essential in the production of energy.
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