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Riboflavin, or B2, is a constituent of enzymes called flavoproteins. Flavin mononucleotide (FMN) and flavin-adenine dinucleotide (FAD) are vital in the respiratory chain of cellular energy metabolism. FMN is used in deamination, which is the process of removing the amino group from amino acids. FAD is used in the deamination of glycine, an amino acid. FAD is also involved in the oxidation of some fatty acids.

Absorption and excretion

The coenzymes FAD and FMN are released from proteins by the acid in the upper gut. Riboflavin is absorbed in the proximal small intestine. Bile salts facilitate the uptake of riboflavin. In human blood, riboflavin is primarily bound to proteins. The immunoglobulin IgG binds readily to the free form of riboflavin. Conversion of riboflavin to coenzymes occurs in cellular cytoplasm of tissues, primarily in the small intestine, liver, heart, and kidney. Lactoflavin is contained in the milk of lactating women. The synthesis of the coenzymes is tightly regulated and dependent on riboflavin status. Little riboflavin is actually stored in the body; it is excreted through the urine.

Clinical conditions

Riboflavin deficiency, also known as ariboflavinosis, occurs in areas with long periods of low intake. Deficiency is usually accompanied by deficiency of other B vitamins. Symptoms include the inflammation and breakdown of tissue, swollen and cra cked lips, swollen tongue, and red, itchy eyes. Newborn infants with jaundice that are treated with phototherapy have shown signs of riboflavin deficiency. Excess excretion occurs in catabolic patients undergoing nitrogen loss.

Toxicity is non-existent. Excess riboflavin is readily excreted in the urine.

Recommended intake

Riboflavin requirements are related to total energy and protein intakes. The DRIs are: 0.3-0.4 mg for infants; 0.5-0.6 mg for children; 0.9-1.3 mg for adolescents; 1.3 mg for men; 1.1 mg for women; 1.4 mg for pregnant women; and 1.6 mg for lactating wome n.

Food Sources

Milk is the most abundant source of riboflavin. Other sources include organ meats, whole grains, enriched grains, and broccoli. Riboflavin is easily lost in cooking due to its water solubility.