The FDA offers this definition of defines dietary supplements from it’s in its Guidance for Industry web sitehere: http://www.fda.gov/Food/GuidanceRegulation/GuidanceDocumentsRegulatoryInformation/DietarySupplements/ucm070519.htm#1-1
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Dietary supplements are defined, in part, as products (other than tobacco) intended to supplement the diet that bear or contain one or more of the following dietary ingredients:
A vitamin;
A A mineral;
An herb or other botanical;
An amino acid;
A dietary substance for use by man to supplement the diet by increasing the total dietary intake; or
A concentrate, metabolite, constituent, extract or a combination of any ingredient mentioned above.
Further, dietary supplements are products intended for ingestion, are not represented for use as a conventional food or as a sole item of a meal or the diet, and are labeled as dietary supplements. The complete statutory definition is found in section 201(ff) of the Federal Food, Drug, and Cosmetic Act (the act) (21 U.S.C. 321).