Vitamin B9 (Folic Acid) - Nutrition

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What is Vitamin B9 (Folic Acid)?

Vitamin B9, also called folic acid or folate, is an essential water-soluble vitamin. Dietary folic acid is hydrolysed by intestinal brush border enzymes and absorbed into the enterocyte by both carrier-mediated transport and passive diffusion. Folic acid is further metabolised within the enterocyte to 5-methyl-tetrahydrofolate and released into the portal circulation. Circulating 5-methyl-tetrahydrofolate is then taken up by the liver where it is either retained or released back into circulation. Folic acid derivatives are filtered by the renal tubules, with active reabsorption during periods of low intake.

Why is it Important?

Folic acid is a cofactor used in one-carbon transfer in amino acid and nucleotide metabolism.

Roles in the Body

  1. Amino Acid Metabolism: Folic acid cofactors are used in the inter-conversion of serine and glycine, histidine catabolism, and the regeneration of methionine via donation of a methyl group to homocysteine.[1]
  2. Nucleotide Synthesis: Folic acid derivatives are required for purine synthesis and also act as cofactors for thymidylate synthase and dihydrofolate reductase used in synthesis of thymine.[1] Thymidylate synthase and dihydrofolate reductase have high expression in replicating tissue.
  3. Therapeutic Target: The folate antagonist methotrexate, which is used to treat certain types of cancers is a selective inhibitor of dihydrofolate reductase.

Consequences of Folic Acid Deficiency


Folic acid deficiency during gestation can result in cleft palate in puppies.[2][3] Puppies fed folic acid deficient diets experience poor growth and chronic folic acid deficiency in adult can result in low haemoglobin and haematocrit concentrations.[4]


Kittens fed folic acid deficient diets experience decreased growth rate and megablastic anaemia.[4] There are no published reports of folic acid deficiency in adult cats.


There are no published reports of folic acid toxicity in dogs and cats.

Dietary Sources

Naturally occurring folic acid is found in all foods, though in highest concentration in dark green leafy vegetables and animal proteins (e.g. muscle and organ meats, dairy, and eggs). Folic acid is also sensitive to degradation with heating and additional supplementation is required during commercial pet food manufacturing.

Diagnosing Folic Acid Deficiency

Diagnosis of folic acid deficiency is based on measurement of low serum folic acid concentration. Megaloblastic anaemia may be seen on haematology profiles.

A deficiency can also be suspected based on the presence of clinical signs consistent with deficiency and evaluation of diet.


  1. 1.0 1.1 Shane B. (2000) Folic Acid, Vitamin B12, and Vitamin B6. In Biochemical and physiological aspects of human nutrition. 2000 Philadelphia, PA: WB Saunders Company p. 484-500.
  2. Elwood JM and Calquhoun TA. (1997) Observations on the prevention of cleft palate in dogs by folic acid and potential relevance to humans. New Zeal Vet J 1997;45:254-256.
  3. Domoslawska A, et al. (2013) Oral folic acid supplementation decreases palate and/or lip cleft occurrence in Pug and Chihuahua puppies and elevates folic acid blood levels in pregnant bitches. Pol J Vet Sci 2013;16:33-37.
  4. 4.0 4.1 National Research Council (NRC) (2006) Vitamins. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p.227-230.

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