Riboflavin plays a critical role in protein metabolism and is a key component of the oxidative phosphorylation enzyme system that is intimately involved in the production of cellular energy. Its chemical structure is shown in Figure 7.2.
Riboflavin is primarily absorbed in the proximal small intestine, and uptake is facilitated by bile salts. Transport in the blood is accomplished via attachment to protein complexes (i.e., albumin). Very little riboflavin is stored in the body, and therefore, urinary excretion of metabolites (7- and 8-hydroxymethylflavins; i.e., 7-a-hydroxyriboflavin) reflects dietary intake.
FIGURE 7.2 Vitamin B2 (riboflavin).
The coenzyme forms of riboflavin are flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). Riboflavin is converted to its coenzyme forms within the cellular cytoplasm of most tissues, but particularly in the small intestine, liver, heart, and kidney. In this process, FMN forms of riboflavin are complexed with specific active enzymes to form several functional flavoproteins, but most of it is further converted to FAD. Thus, the biosynthesis of flavo-coenzymes is tightly regulated and dependent on riboflavin status. Thyroxine and triiodothyroxine stimulate FMN and FAD synthesis in mammalian systems. Riboflavin has been shown to participate in oxidation-reduction reactions in numerous metabolic pathways and in energy production via the respiratory chain. A variety of chemical reactions are catalyzed by flavoproteins.4
When a riboflavin deficiency occurs, its symptoms include sore throat, cheilosis, angular stomatitis, glossitis, seborrheic dermatitis, and normocytic, normochromic anemia. Riboflavin deficiencies have rarely been reported. Riboflavin toxicities are almost never reported; however, electroencephalographic abnormalities have been mentioned as a rare side effect of excessive riboflavin intake.4
Riboflavin is present in a variety of foods. Some of the best sources are eggs, lean meats, milk, broccoli, and enriched breads and cereals. The RDA for riboflavin in adolescent and adult males is 1.3 mg/d, whereas in adolescent females, it is 1.0 mg/d, and in adult females it is 1.1 mg/d.8
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