Liposoluble vitamins (present and soluble in fats), whose needs seem to be more or less well defined, must be discriminated from hydro soluble vitamins (soluble in water), [which] requirements remain hypothetic.
Most vitamins must be given by feeding, as the horse is not able to synthesi[s]ze, to develop the vitamins in its organism (except vitamin B, C and D).
Vitamins are indispensable for the maintenance of the organism, growth and reproduction.
All vitamins are brought through feeding, as far as it is fresh, good-quality and in required quantity.
Ageing, stress, preparation for competition, daily work, illness, antibiotics, inflammation, and food shortage are all factors which can lead to a lack of vitamins (e.g. B and K) produced by the intestinal flora. This is what is called avitaminosis.
However, hypervitaminosis can also be dangerous and lead to metabolism troubles. But it is of little effect to administer vitamins without adding the nutritive elements (i.e. minerals and amino acids) they control and which make them efficient. As an example, vitamin D works with calcium and phosphorus, vitamin H works with zinc.
It is therefore highly recommended to carefully manage the daily allowance in order to make it complete and well-balanced.