10.4 Riboflavin (Vitamin B2)

Riboflavin (Vitamin B2)

A student once asked this question:

“I started taking this Super Athlete Multivitamin I bought from the health food store and about an hour or two after consumption, my pee is bright, practically neon yellow. What does that mean?”

The culprit is the vitamin riboflavin found in the Super Athlete Multivitamin. Indeed, flavin in the name riboflavin means yellow in Latin, and riboflavin in solution is bright yellow, as shown below.

Figure 10.41 Riboflavin in solution1

Riboflavin is also a water-soluble B vitamin, so the student was excreting large amounts of riboflavin in his urine, leading it to become “bright, practically neon yellow.”

Riboflavin is important for the production of two cofactors: flavin adenine dinucleotide (FAD) & flavin mononucleotide (FMN).

FAD has been introduced before (Chapter 6). FMN is similar to FAD, except that it only contains one phosphate group (versus 2) and doesn’t have ring structures attached to the phosphate groups that are found in FAD.

Riboflavin is photosensitive, meaning that it can be destroyed by light. This was a problem in the old days when the milkman delivered milk in clear glass bottles, resulting in the destruction of the riboflavin in the milk. These have now been replaced by cartons or opaque plastic containers to help protect the riboflavin content of the milk.

Figure 10.42 Milk is no longer packaged in clear glass bottles to help protect its riboflavin from light destruction

Foods that are particularly rich in riboflavin include eggs, organ meats (kidneys and liver), lean meats, and milk. Green vegetables also contain riboflavin. Grains and cereals are fortified with riboflavin in the United States and many other countries. The largest dietary contributors of total riboflavin intake in U.S. men and women are milk and milk drinks, bread and bread products, mixed foods whose main ingredient is meat, ready-to-eat cereals, and mixed foods whose main ingredient is grain. The riboflavin in most foods is in the form of FAD, although the main form in eggs and milk is free riboflavin4.

Riboflavin in foods is free, protein-bound, or in FAD or FMN. Only free riboflavin is absorbed so it must be cleaved, or converted before absorption2. Riboflavin is highly absorbed through a yet unknown process, though it is believed a carrier is involved3. As you would guess from the description above, riboflavin is primarily excreted in the urine.

For more information on thiamin, see the Required Web Link below.

Required Web Link:Riboflavin Fact Sheet for Health Professionals

Subsections:

• 10.41 Riboflavin Functions
• 10.42 Riboflavin Deficiency & Toxicity

References & Links

• http://en.wikipedia.org/wiki/File:Riboflavin_solution.jpg
• Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
• Said H, Mohammed Z. (2006) Intestinal absorption of water-soluble vitamins: An update. Curr Opin Gastroenterol 22(2): 140-146.
• Riboflavin Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Riboflavin- HealthProfessional/#h3

Riboflavin Functions

Riboflavin is required for the production of FAD and FMN. Below are some of the functions of FAD and FMN1:

• Citric Acid Cycle
• Electron Transport Chain

• Fatty Acid Oxidation
• Niacin Synthesis
• Vitamin B₆ Activation
• Neurotransmitter Catabolism
• Antioxidant Enzymes

• Citric Acid Cycle – FAD is reduced to FADH₂ in the citric acid cycle. FADH₂ will then move to the electron transport chain during aerobic conditions.

• Electron Transport Chain – Under aerobic conditions, the electron transport chain is where the FADH₂ is used to produce ATP.

• Fatty Acid oxidation – During fatty acid oxidation FAD is converted to FADH₂. FADH₂ can then be used to produce ATP by the electron transport chain.

• Niacin synthesis – As you will hear more about in the niacin section, niacin can be synthesized from tryptophan. An intermediate in this synthesis is kynurenine, and one of the multiple steps between kynurenine to niacin requires FAD.

• Vitamin B₆ Activation – The enzyme that creates the active form of vitamin B₆ (pyridoxal phosphate) requires FMN.

• Neurotransmitter Catabolism – The enzyme monoamine oxidase (MAO) requires FAD. This enzyme is important in the breakdown of neurotransmitters such as dopamine and serotonin.

• Antioxidant Enzymes – The antioxidant enzymes glutathione reductase and thioredoxin reductase both require FAD as a cofactor.

In addition to the functions listed above, FAD is also used in folate activation, choline catabolism, and purine metabolism1.

References & Links

1. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.

Riboflavin Deficiency & Toxicity

Ariboflavinosis, riboflavin deficiency, is a rare condition that often occurs with other nutrient deficiencies. The symptoms of this condition include:

• Fatigue
• Slowed growth
• Digestive problems
• Lesions at the corners of the mouth (angular stomatitis)
• Swollen magenta-colored tongue (glossitis)
• Eye fatigue
• Swelling and soreness of the throat
• Sensitivity to light

The most notable symptoms include angular stomatitis which is a lesion that forms at the corners of the mouth as shown in Figure 10.421.

Figure 10.421 Angular Cheilitis2

Glossitis is the inflammation of the tongue, which can be accompanied by redness or inflammation of the oral cavity. Dermatitis is also frequently a symptom3,4.

There has been no toxicity of riboflavin reported.

References & Links

• University of Maryland Medical Center. Vitamin B2 (Robiflavin). https://www.umm.edu/health/medical/altmed/supplement/vitamin-b2-riboflavin
• http://en.wikipedia.org/wiki/File:Angular_Cheilitis.JPG
• Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
• Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw’s perspectives in nutrition. New York, NY: McGraw-Hill.