FAD

Flavin adenine dinucleotide, abbreviated FAD, is a flavin nucleotide and one of the two principal coenzyme forms of riboflavin, the water-soluble B vitamin also known as vitamin B2 ^[1][2]. Chemically it is built from a flavin group joined through a sugar and a diphosphate bridge to an adenine nucleotide, and this arrangement lets it accept and donate electrons during metabolism ^[1]. Because humans cannot synthesize riboflavin, the vitamin must be obtained from the diet and is then converted inside cells into FAD ^[2].

The body assembles FAD in two enzymatic steps: riboflavin is first phosphorylated by riboflavin kinase to give flavin mononucleotide (FMN), and FAD synthetase then attaches an adenine nucleotide to produce FAD, with both reactions consuming ATP ^[1]. FAD is the more common of the two flavin coenzymes; surveys of the human flavoproteome indicate that the large majority of flavin-dependent enzymes use FAD, a smaller share use FMN, and a few require both ^[1].

Enzymes that carry FAD take part in many core pathways, including the citric acid cycle, the breakdown of fatty acids, the metabolism of amino acids and the mitochondrial electron transport chain ^[1]. A familiar example is succinate dehydrogenase, which uses FAD as it links the citric acid cycle to respiration; other flavoenzymes, such as dihydropyrimidine dehydrogenase, employ FAD together with FMN and iron-sulfur centers to move electrons between reactions ^[1][3]. Through these roles FAD supports the production of cellular energy and helps maintain the cell's antioxidant defenses ^[2].

FAD itself is not a drug but a naturally occurring biomolecule, and dietary riboflavin is its ultimate source. Riboflavin and its phosphate derivatives are recognized nutrients, and when riboflavin intake is inadequate the resulting shortage of FAD and FMN impairs flavoenzyme activity and contributes to the signs of riboflavin deficiency ^[2].

Riboflavin and its flavin forms are water soluble and considered very safe, with excess mostly excreted in urine (which turns bright yellow). No meaningful toxicity is established at normal supplement doses. As always, high doses aren't studied much for the isolated FAD form specifically.