Thiamin requirements in some species are difficult to establish due to endogenous vitamin synthesis by intestinal microflora. Diets containing starch rather than sucrose favor intestinal synthesis, and the practice of coprophagy (dogs) can markedly influence dietary requirements for this vitamin. For dogs and cats, however, it is doubtful whether the amount of thiamin produced by intestinal synthesis and absorbed is large enough to make a significant contribution to body needs.
Diet composition can dramatically influence thiamin requirements. Since thiamin is specifically involved in carbohydrate metabolism, level of dietary carbohydrate relative to other energy-supplying components influences thiamin requirement. The need for thiamin increases as consumption of carbohydrate increases (Benevenga et al., 1966). When thiamin is deficient, body reserves become depleted more rapidly when animals are maintained on a feed rich in carbohydrates than when they receive a diet rich in fat and protein. The "thiamin-sparing" effect of fats and protein has long been known. Dogs receiving diets with high fat levels required lower levels of thiamin to meet their thiamin needs than did dogs receiving low-fat diets (Arnold and Elvehjem, 1939). Excess intake of carbohydrate foods such as potatoes, bread and possibly certain grains can increase the thiamin requirement.
Size, genetic factors and metabolic status affect thiamin requirements. Periods of increased metabolism (e.g., fever, elevated muscular activity, pregnancy and lactation) can also increase thiamin requirements (Marks, 1975). Infectious and parasitic diseases also result in increased thiamin requirements. When dietary thiamin is marginal, typical deficiency signs of thiamin are more likely to develop in infected animals than in normal animals. Endoparasites such as strongylids and coccidia compete with the host for thiamin contained in food. It has been shown experimentally with poultry that infection with these coccidia results in considerable reduction in thiamin blood levels. Thiamin blood levels were found to be directly correlated to infection severity (McManus and Judith, 1972). Likewise, conditions such as diarrhea and malabsorption increase the requirement.
Thiamin requirements are higher if feeds contain raw ingredients (e.g., fish) or additives with anti-thiamin activity. Spoiled and moldy feeds may contain such antagonists or thiaminases. Chicks kept on a feed infected with Fusarium moniliforme developed polyneuritis that could be cured with thiamin injections (Fritz et al., 1973). High dietary intakes of sulfur as well as substances in tall fescue (Festuca arundinacea Schreb.) are antagonistic to thiamin, resulting in higher requirements (Edwin et al., 1968; Lauriault et al., 1990).
