Vitamin B2 (Riboflavin) - Nutrition

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What is Vitamin B2 (Riboflavin)?

Vitamin B2, also called riboflavin, is an essential water-soluble vitamin. Riboflavin is absorbed across the intestinal mucosa primarily via active transport, with minimal passive diffusion. Intracellular or plasma riboflavin is typically bound to protein. Riboflavin is not stored in the body and is freely filtered by the renal tubules.

Why is it Important?

Riboflavin is a component of the co-enzymes flavin mononucleotide (FMN) and flavin adenine di-nucleotide (FAD)[1]. It is also an essential component of the enzyme glutathione reductase.

Roles in the Body

The coenzymes FMN and FAD are play a catalytic role in redox reactions, such as the conversion of retinol to the active metabolite retinoic acid, tryptophan to niacin, and pyruvate to α-ketogluterate, and are cofactors in the electron transport chain. Riboflavin is also a component of the antioxidant enzyme, glutathione reductase.

Consequences of Riboflavin Deficiency


Riboflavin deficiency impairs growth rate in puppies[2][3], and can result in anorexia, weight loss, weakness, ataxia, collapse, and death[4]. Bilateral corneal opacities have also been described in adult dogs fed riboflavin deficient diets[3].


Riboflavin deficiency in cats can resulted in anorexia, weight loss, periauricular alopecia, bilateral cataracts, testicular hypoplasia, fatty accumulation in the liver, and death[5]. Conditions associated with diuresis (e.g. chronic disease, such as renal disease or diabetes, or therapeutic intervention, such as intravenous fluids or increased water intake with management of lower urinary diseases) can result in increased loss of riboflavin and may increase daily requirements. Patients on chronic haemodialysis are also at an increased risk for developing a deficiency. Riboflavin deficiencies can also occur due to low dietary intake and vitamin degradation during cooking especially under alkaline conditions.


There are no published reports of riboflavin toxicity in dogs and cats[6]. Excess intake is freely filtered through the renal tubules.

Dietary Sources

Riboflavin is naturally found in many different types of foods such as muscle, organ meats, eggs, dairy, and vegetables. Most of the riboflavin in foods occurs in the coenzyme form of FMN, FAD or flavins covalently bound to proteins. Milk is an exception where most of the riboflavin is free and not bound. Storage conditions and thermal processing can result in riboflavin degradation, although to a lesser extent than thiamin.

Diagnosing Riboflavin Deficiency

Diagnosis of riboflavin deficiency is based on a finding of low erythrocyte glutathione reductase activity, although this is not routinely tested for in veterinary reference laboratories. A clinical suspicion arises when there are clinical signs consistent with a riboflavin deficiency and dietary analysis indicates a deficiency.


  1. McCormick DB. Niacin, Riboflavin, and Thiamin. In Biochemical and physiological aspects of human nutrition. 2000 Philadelphia, PA: WB Saunders Company p.469-475.
  2. Axelrod AE, et al. The production of uncomplicated riboflavin deficiency in the dogs. Am J Physiol 1940;128:703-708.
  3. 3.0 3.1 Noel PR, et al. Riboflavin supplementation in the dog. Res Vet Sci 1972;13:443-450.
  4. Street HR and Cowgill GR. Acute riboflavin deficiency in the dog. Am J Physiol 1939;125:323-334.
  5. Gershoff SN, et al. The effect of the carbohydrate and fat content of the diet upon the riboflavin requirement of the cat. J Nutr 1959;68:75-88.
  6. National Research Council (NRC). Vitamins. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p.216-218.

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