Supplements
Unlike lipoic acid in foods, lipoic acid in
supplements is not bound to
protein. Moreover, the amounts of lipoic acid available in dietary supplements (50-600 mg) are likely as much as 1,000 times greater than the amounts that could be obtained from the diet. In Germany, lipoic acid is approved for the treatment of diabetic neuropathies and is available by prescription
(108). Lipoic acid is available as a dietary supplement without a prescription in the US. Most lipoic acid supplements contain a
racemic mixture of
R-lipoic acid and
S-lipoic acid (sometimes noted
d,l-lipoic acid). Supplements that claim to contain only
R-lipoic acid are usually more expensive, and information regarding their purity is not publicly available
(109). Since taking lipoic acid with a meal decreases its
bioavailability, it is generally recommended that lipoic acid be taken 30 min prior to a meal (see also
Metabolism and Bioavailability)
(8).
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.