These eight vitamins-all but the last one from the B family--are supplied in the proper amounts through the average hay-and-grain diet. No natural excesses or deficiencies have ever been reported in horses.

Choline is a B vitamin that is produced in the liver, rather than in the intestine. It is necessary for transmitting nerve impulses between neurons, and it is a component of cell membranes. Scientists once speculated that choline could be used to treat heaves, but studies showed no benefits.

Cyanocobalamin, more commonly known as B12, is produced in the horse's intestines in a quantity sufficient to meet his needs. Nonetheless, it is a popular, but unproven, practice to inject racehorses with B12 to improve performance. There are no natural sources of this vitamin in the typical horse's diet.

Folacin is the B vitamin more commonly known as folic acid. It is used to form red blood cells and has been studied extensively in human nutrition because of its role in preventing birth defects. Grass is one of the richest sources of folacin, so a deficiency in horses is highly unlikely. Nevertheless, it is usually included in supplements for pregnant mares.

Niacin, another of the B vitamins, occurs naturally in all living tissue. It is produced by the equine gastrointestinal tract and by the amino acid tryptophan. Niacin plays a role in your horse's metabolism.

Panthothenic acid is used in general metabolic function. This B vitamin is produced by intestinal bacteria and is present in forage and grain. Experts have not established any dietary requirement for pathothenic acid in horses.

Pyridoxine (B6) is necessary for the formation of red blood cells and the synthesis of the hormone epinephrine. Pyridoxin is found in plants and produced by the body. It is converted into one of three active forms only in the presence of compounds that contain niacin and riboflavin.

Riboflavin (B2) is used in a number of processes that convert feed to energy. It once was thought that the eye disorder uveitis was caused by a B2 deficiency, but this has since been disproved. Experts estimate that a horse needs B2 in amounts less than 2 mg/kg of dry diet matter daily, which can readily be supplied by good-quality hay and through synthesis in the gut by bacteria.

Vitamin C (ascorbic acid) plays a very small role in a horse's diet. It is created from glucose in the liver and used to form bone and collagen and to absorb iron. Vitamin C is poorly absorbed by the intestines, so the benefits of supplementing vitamin C to improve cartilage and bone health are questionable.

Excerpted from an article by Christine Barakat in EQUUS Magazine, Issue 263 (September 1999), copyright 1999 by PRIMEDIA Enthusiast Publications, Inc. All rights reserved.

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