Riboflavin (Vitamin B2): Dietary Source, RDA, and Functions

Vitamin B2, riboflavin, is an essential water-soluble vitamin found in lean meat, fish, milk and dairy products, whole grains, nuts, and some green vegetables. Riboflavin is important in carbohydrate, protein, and fat metabolism. 

Alternate name	Riboflavin
Year of Discovery	1922
Discovered by	Richard Kuhn and Theodor Wagner-Jauregg

It acts as a coenzyme in redox reactions. The main coenzyme forms of riboflavin are flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). 

Riboflavin was the first B complex component isolated in a pure state. Green plants and microorganisms synthesize this vitamin. Warburg (Nobel prize 1931) isolated the “yellow enzyme” of cellular respiration. Later Axel Theorell (Nobel prize, 1955) isolated riboflavin. In 1935, Paul Karrer (Nobel prize, 1937) determined the structure.

Structure of Riboflavin

Lactoflavin (milk), hepatoflavin (liver), and ovoflavin (eggs) are chemically identical to riboflavin.

Riboflavin is heat stable. Riboflavin exists in tissues tightly bound (but not covalently) with enzymes. Enzymes containing riboflavin are called flavoproteins. The two co-enzymes are FMN (flavin mononucleotide) and FAD (flavin adenine dinucleotide).

Dietary Sources

Riboflavin is present in various food sources, mainly meats and dairy. Several foods fortified with riboflavin, including bread, cereals, and milk, are also available. 

Rich sources of Riboflavins are; 

• Organ meat, especially kidney liver
• Meat (lean beef and pork)
• Poultry (chicken breast) 
• Dried yeast
• Almonds 
• Egg
• Whole milk  and milk products such as yogurt, cheese
• Fish (salmon) 
• Whole cereals
• Legumes
• Green leafy vegetables (spinach).[1,2]

Dietary Reference Intakes (DRI)

The RDA of riboflavin for adults is 1.1 mg/day for women and 1.3 mg/day for men.

Riboflavin is concerned mainly with the metabolism of carbohydrates, and its requirement is related to calorie intake. Adults on sedentary work require about 1.5 mg per day. An additional 0.2 to 0.4 mg/day is required during pregnancy, lactation, and old age [3].

Life Stage Group	Riboflavin
Infants
0-6 months	0.3 mg/day
7-12 months	0.4 mg/day
Children
1-3 years	0.5 mg/day
4-8 years	0.6 mg/day
Males
9-13 years	0.9 mg/day
14-18 years	1.3 mg/day
>19 years	1.3 mg/day
Females
9-13 years	0.9 mg/day
14-18 years	1.0 mg/day
>19 years	1.1 mg/day
Pregnancy
≤ 18 years	1.4 mg/day
19-50 years	1.4 mg/day
Lactation
≤ 18 years	1.6 mg/day
19-50 years	1.6 mg/day

Absorptions

Thiamin is mainly consumed as FMN and FAD bound to a food protein. In the stomach, the acidic environment releases the coenzymes FMN and FAD from the protein.

Most riboflavin absorption occurs in the small intestine. Riboflavin is transported in the plasma mainly bound to albumin and is excreted in the urine.

Functions

Free riboflavin has little biologic activity, but riboflavin functions as a coenzyme in numerous oxidation-reduction reactions. 

Riboflavin is a component of the metabolically essential coenzymes FMN and FAD. As coenzymes, FAD and FMN are often bound to enzymes that are oxidases and dehydrogenases.

Deficiency

Natural deficiency of riboflavin is rare because riboflavin is synthesized by the intestinal flora. Riboflavin deficiency, ariboflavinosis, usually occurs with severe malnutrition and chronic alcoholism and accompanies other deficiency diseases such as beriberi, pellagra, and Kwashiorkor.

The symptoms of riboflavin deficiency include sore throat, weakness, angular stomatitis, glossitis, mucositis, dermatitis, and cheilosis.