Vitamin B1 (thiamine)

Deficiency

Thiamine deficiency is rare, but plants containing thiaminase can cause acute deficiency ^[1].

Supplements

Up to double the maintenance intake may be recommended for strenuous exercise, and oral thiamine is preferred because intravenous thiamine can cause adverse reactions ^[1].

A calming effect of thiamine on nervous horses has been both shown and disproved in studies, but oral thiamine has not been shown to have a calming effect ^[2].

B vitamins in general

Studies since the mid-20th century have shown that thiamine, riboflavin, niacin, pantothenic acid, pyridoxine, folic acid, biotin and cobalamin are made in the horse's colon. Apart from experiments with deliberately deficient diets, clinical B-vitamin deficiency has never been reported in horses ^[2].

Claims that B-vitamin supplements improve hindgut function or help with coat changes have not been tested in studies, so they can't be validated ^[2].