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In page Lipoic acid:

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Lipoic acid is present in many foods in which it is bound to lysine in proteins,[1] but slightly more so in kidney, heart, liver, spinach, broccoli, and yeast extract.[2] Naturally occurring lipoic acid is always covalently bound and not readily available from dietary sources.[1] In addition, the amount of lipoic acid present in dietary sources is low. For instance, the purification of lipoic acid to determine its structure used an estimated 10 tons of liver residue, which yielded 30 mg of lipoic acid.[3] As a result, all lipoic acid available as a supplement is chemically synthesized.[citation needed]