Water soluble vitamins, which are the b-vitamins such a niacin, thiamine, folic acid, and many others, are excreted from the body on a daily basis in the urine.  B-complex vitamins play an important part in allowing horses to metabolize the nutrients contained in feedstuffs, by facilitating carbohydrate, fat and protein utilization within body cells, providing the energy needed for growth, performance and reproduction. All of the B-complex vitamins are essential to horses, but they are synthesized by bacteria in the cecum and colon. After microbes form the vitamins, they are absorbed through the intestinal wall and are available for use by the horse’s body cells. Bacteria in healthy adult horses generally produce adequate levels of the B-complex vitamins.

Since bacteria in the horse’s digestive tract produce B vitamins, several factors influence the B vitamin status of horses. Changes that affect the bacterial population may change the synthesis and availability of B vitamins, and they include, but are not limited to:

• Drugs, which selectively kill certain species

• Horses going “off feed” thus reducing the availability of food for the bacteria

• Dietary changes which change the levels of fiber, carbohydrate and protein passing to the bacteria in the hindgut

In addition, situations that decrease the horse’s ability to absorb B vitamins have an effect on overall vitamin status include:

• Chronic or clinical diseases that interfere with efficient metabolism by the horse

• Parasitism, through ulceration of the mucosa and direct competition for vitamins in the feed, causes reduce availability

• Diarrhea bouts reduce B vitamins for the horse

• Moldy feed, especially hay contaminated by Streptomyces, makes biotin unavailable for horses

• Stress tends to decrease the horse’s ability to absorb B vitamins

• Anti-metabolites present in plants such as bracken fern, yellow star thistle or horsetail, interfere with thiamin utilization and transport

Finally, information on the B vitamin requirements and synthesis in young, growing horses and performance horses is limited. While there is much anecdotal evidence available about the effects of supplementing b-vitamins to horses, few actual research studies have been completed to determine the actual results of such supplementation, positive or negative.