Vitamin B-2; Riboflavin

 Riboflavin

Vitamin B-2

Riboflavin; also known as vitamin B-2, was once called "yellow enzyme" because it has a distinctive yellow-green fluorescence. In fact, its name comes from its color (flavin means "yellow" in Latin). Riboflavin contains 3 linked 6-membered rings, with a sugar alcohol attached to the middle ring.

Riboflavin in Foods:

Almost one-quarter of the riboflavin in our diets comes from milk products. The rest typically is supplied by enriched white bread, rolls, and crackers, as well as eggs and meat. Foods rich in riboflavin are liver, mushrooms, spinach and other green vegetables, broccoli, asparagus, milk, and cottage cheese. Exposure to light causes riboflavin to break down rapidly. To prevent this light-induced breakdown, paper and plastic containers __not glass __should be used as packaging for riboflavin rich-foods, such as milk, milk products, and cereals.

Riboflavin Needs and Upper Level:

The RDAs for riboflavin for adult men and women are 1.3 and 1.1 mg/day, respectively. The Daily Value on food and supplement labels is 1.7 mg. In the U.S., the average intake from food for riboflavin is approximately 2.49 mg/day for men and 1.85 mg/day for women. These  appear to be no adverse effects from consuming large amounts of riboflavin because of its limited absorption and rapid excretion via the urine, so no Upper Level has been set.

Absorption, Transport, Storage & Excretion of Riboflavin:

In the stomach, hydrochloric acid releases riboflavin from its bound forms. Above 60 to 65% of the free riboflavin is absorbed, primarily via active transport or facilitated diffusion in the small intestine. In the blood, riboflavin is transported by protein carriers. Riboflavin is converted to its coenzyme forms in most tissues, but this occur mainly in the small intestine, liver, heart, and kidneys. Any excess intake is excreted in the urine. For people who take excessive amounts in supplement form, riboflavin imparts a bright yellow color to the urine, which glows under a black light.

Functions of Riboflavin

Riboflavin is a component of 2 coenzymes that play key roles in energy metabolism: flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). These coenzymes, also referred to as flavins, have oxidation and reduction functions.

The riboflavin coenzymes are involved in many reactions in various metabolic ways. They are critical for energy metabolism and are involved in the formation of other compounds, including other B-vitamins and antioxidants.

Energy Metabolism:

➤ In the citric acid cycle, the oxidation of succinate to fumarate requires the FAD containing enzyme succinate dehydrogenase. The FADH2 formed in this reaction donates hydrogen to the electron transport chain.

➤ In fatty acid breakdown to acetyl-CoA,   the enzyme fatty acyl dehydrogenase requires FAD.

➤ FMN shuttles hydrogen atoms into the electron transport chain.

Activation of Other B-vitamins:

➤ The formation of niacin from the amino acid tryptophan requires FAD.

➤ The formation of the active vitamin B-6 coenzyme (pyridoxal phosphate) requires FMN.

➤ FAD is required for the synthesis of the folate metabolite 5-methyl-tetrahydrofolate in this way, riboflavin participates indirectly in homocysteine metabolism.

Antioxidant Function:

The synthesis of the antioxidant compound glutathione depends on the FAD-containing enzyme glutathione reductase. Since glutathione is an important part of the cell's antioxidant defense network.

Riboflavin Deficiency:

Riboflavin deficiency; Ariboflavinosis

Riboflavin deficiency, called ariboflavinosis, primarily affects the mouth, skin, and red blood cells. The symptoms include inflammation of the throat, mouth (stomatitis), and tongue (glossitis); cracking of the tissue around the corners of the mouth (angular cheilitis); and moist, red and scaly skin (seborrheic dermatitis). Anemia, fatigue, confusion, and headaches also may occur. Some of the symptoms of ariboflavinosis may result from deficiencies of other B-vitamins because they work in the same metabolic pathways as riboflavin are often supplied by same foods.

Ariboflavinosis develops after 2 months on a riboflavin-deficient diet and is rare in otherwise healthy people. Biochemical evidence of deficiency (low riboflavin levels in red blood cells or reduced activity of the enzyme glutathione reductase) is most often seen in adolescent girls and elderly people. Correcting moderate riboflavin deficiency with supplementation improves hematologic status. Diseases such as cancer, certain form of cardiovascular disease, and diabetes can lea  to or worsen a riboflavin deficiency. People

 with alcoholism, malabsorption disorders, or very poor diets may be at risk of riboflavin deficiency. The long-term use of phenobarbital also may adversely affect riboflavin status because this drug increases the breakdown of riboflavin and other nutrients in the liver. Marginal riboflavin intake may occur in those who do not consume milk or milk products. Presently, little is known about the effects of a marginal riboflavin deficiency.