Water soluble B vitamin critical for the ability of cells to generate energy from carbohydrates. Requirement rises with carbohydrate intake and workload. Microbial synthesis in the hind gut is sufficient for spelling horses but working horses require additional dietary supply – grains and seeds are good sources of Thiamin and brewers yeast is rich in this vitamin.
Deficiency relates to impaired energy metabolism. In humans this is classical Beriberi. In horses on typical diets deficiency does not occur but experimentally symptoms are fatigue, poor appetite, depression, gait disturbances, wandering, a crouching stance and cardiovascular effects. Poisoning by bracken fern which contains Thiaminase I enzyme which destroys the vitamin, causes deficiency disease as will the coccidiostat Amprolium used mainly for poultry.
There is some justification for supplementing this vitamin in hard working horses, e.g. racehorses where normal supply may not keep pace with the increased demand.