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Nutrition: vitamins

ISSN 2398-2977

Podcast: Nutrition: vitamins


  • Vitamins are organic substances required in very small quantities for the maintenance of tissues and metabolic systems.


  • Diet.
  • Tissue synthesis (vitamin C (ascorbic acid), vitamin A from ß-carotene).
  • Skin synthesis as a reaction to ultraviolet light (vitamin D3).
  • Alternative metabolic pathways (substitution of methionine and other sources of methyl groups for choline).
  • Large intestinal micro-organisms (K and some B complex vitamins).


  • Amounts reported as 'requirements' generally represent the level below which clinical signs of deficiency may occur.
  • In some cases, eg vitamins E and B complex, increased supplementation may be beneficial.
  • Depends on:
    • Age: foals' rapid growth increases the requirement for vitamins A   Vitamin A  , D   Vitamin D3  , B   Vitamin B1   complex and K. In very young foals the poorly developed large intestine is not a reliable source.
    • Parasitism: intestinal parasites may increase vitamin requirements.
    • Reproductive status: requirements are greater for lactating mares   Nutrition: brood mare  but tend to be eating more anyway, so vitamins per unit of feed are not affected.
    • Work levels: decreased appetite of horses in heavy work   Nutrition: work    →   increased requirement for certain B vitamins   Vitamin B1  .
    • Latitude: horses with dark skin, heavy coats, those kept indoors or in northern latitudes all have increased requirements for vitamin D   Vitamin D3  .
    • Disease: horses with rhabdomyolysis syndrome   Muscle: myopathy - exertional rhabdomyolysis  (azoturia/tying-up) may benefit from increased levels of vitamin E   Vitamin E   to protect muscle tissues.

Fat soluble vitamins

  • Fat soluble vitamins are poorly excreted from the body. Excessive intake   →   toxicosis.

Vitamin A

  • Vitamin A   Vitamin A  is required for:
    • Healthy epithelial tissues plus the visual process, endochondral ossification, hematopoiesis in foals and disease resistance.
    • Produced from carotenoid pigments, almost entirely ß-carotene, present in green herbage - hay more than 6 months old contains very little whether or not it is green.
    • ß-carotene serves as an anti-oxidant in the ovary.
    • Conversion to vitamin A in the liver and the wall of the small intestine.
    • Requires adequate protein for its effective utilization, especially by pregnant mares.
    • Absorbed ß-carotene and other carotene pigments are responsible for the yellow coloration of milk, fat and skin.
    • ß-carotene also involved in control of progesterone secretion and therefore ovulation regulation and pregnancy maintenance.
    • Liver storage should provide adequacy for 4-5 months.
  • Signs of deficiency (uncommon):
    • Susceptibility to infections, eg respiratory, gut, genito-urinary tract.
    • Decreased feed intake.
    • Infertility in older mares.
    • Slow growth.
    • Night blindness.
    • Weight loss   Weight loss: overview  .
    • Keratinization of skin and cornea.
    • Abnormal bone growth.

Many of the same signs will be seen with vitamin A toxicity so the dietary intake must be established before supplementing.

  • Requirement of 30-60 iu/BW/day.
  • ß-carotene - has separate functions to vitamin A   Vitamin A  , but evidence in horses is slight.
  • Extra vitamin A will not necessarily counteract ß-carotene deficiency.

Vitamin D

  • Vitamin D   Vitamin D3   is required for:
    • Metabolism of calcium and phosphorus.
    • Absorption of calcium from the small intestine.
  • Signs of deficiency:
    • Stiffness.
    • Reduced feed intake.
    • Enlarged joints and growth plates.
    • Bone demineralization.

Many of the same signs will be seen with vitamin D toxicity which can be fatal. Vitamin D functions as a hormone - some plants, not found in the UK, produce this hormone causing vitamin D toxicity. 

  • 2-4 h in sunlight daily provides adequate skin synthesis, except in northern latitude in winter; 500-700 iu/kg dry diet for adult horses. 800 iu/kg dry diet for growth, lactation and pregnancy.

Vitamin K

  • Vitamin K is required for:
    • Blood clotting   Blood: overview  .
    • Also for bone calcification, but no evidence in horses.
    • No evidence of a need for it in horse diets, except following extended treatment with sulfonamides   Therapeutics: sulfonamides  .
  • There are several forms of vitamin K   Phytomenadione  . Natural forms include:
    • K1 (phylloquianone) - found in green leafy plants.
    • K2 (menaquinones) - synthesized by gastrointestinal bacteria.
    • K3 (menadione) - a synthetic water-soluble form used in feed stuffs.
  • Signs of deficiency:
  • Low forage diet, extended treatment with sulfonamides and/or diarrhea may compromise vitamin K production.

Vitamin E

Water soluble vitamins

B complex vitamins
  • Water soluble vitamins are easily excreted from the body and so excessive intakes rarely cause problems.
  • Required for:
    • Metabolism of proteins   Nutrition: protein  , carbohydrates and fats - act as co-enzymes in reactions, or as a methyl donor in the case of choline.
  • Adequate levels of riboflavin (vitamin B2), pyridoxine (vitamin B6), pantothenic acid and nicotinic acid are usually provided by good quality feed plus intestinal synthesis.
  • Situations involving poor feed, hard exercise, traveling for long periods, weaning in foals or lactating mares may require dietary supplements.
  • Working in hot climates   →   diarrhea   →   prolonged antibiotic therapy.

Vitamin B1 (thiamin)

  • Signs of deficiency from vitamin B1   Vitamin B1  (from research conducted several decades ago) include:
    • Loss of appetite.
    • Weight loss   Weight loss: overview  .
    • Inco-ordination of the hind legs.
    • Cardiac hypertrophy.
  • Proprietary form available   Vitamin B1  .

Vitamin B12 (cyanocobalamin)

  • Adequate amounts of vitamin B12   Cyanocobalamin   can be produced by micro-organisms in the intestine of horses provided cobalt is present.
  • Deficiency may occur in foals.
  • Signs of deficiency:

Folic acid

  • Folic acid   Folic acid  is associated with formation of red blood cells. Present in good quality pasture.
  • Signs of deficiency:
    • Anemia   Anemia: overview  , macroscopic anemia.
    • Leukopenia.
    • Dysphagia.
    • Glossitis.
    • Neurological disorders.


  • Biotin   Biotin  is present in a bioavailable form in grass and clover.
  • Poor availability from cereal grains.
  • Signs of deficiency include:
    • Crumbling hooves   Hoof: biotin deficiency 02  .
  • Proprietary form available   Biotin  .

Vitamin C (ascorbic acid)

  • Cereals contain very little vitamin C.
  • Vitamin C is required for:
    • Membrane protection from free-radicals.
    • Collagen production.
    • Hormone production.
    • Synthesized from glucose but decreased serum concentrations may be associated with wound infection.
    • Aids utilization of B vitamins.
  • Signs of deficiency:
    • Questionable as to whether deficiencies occur in horses as able to synthesize sufficient to meet requirements.
    • Epistaxis and respiratory disease have been attributed to vitamin C deficiency, but no evidence has been presented.

    Print-off the Owner Factsheet onNutrition - keeping your horse on top form  Nutrition - keeping your horse on top form   to give to your client.

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Hintz H F (1992) Clinical nutrition. Vet Clin North Am 6, (2).
  • Roneus B O et al (1986) Vitamin E requirements of adult Standardbred horses evaluated by tissue depletion and repletion. Equine Vet J 18 (1), 50-58 PubMed.

Other sources of information

  • Frape D (2004) Equine Nutrition and Feeding. 3rd edn. Blackwell Publishing Ltd, UK. ISBN: 1405105984.

Further Reading