In feedstuffs niacin occurs predominantly in bound forms. In plants niacin is found mostly as protein-bound nicotinic acid and in animal tissues mostly as nicotinamide. In grains, niacin is present in covalently bound complexes that are not normally available (Combs, 1998). Ng et al. (1998) evaluated the availability of niacin in common feed ingredients for channel catfish. Niacin availability ranged from approximately 100% in menhaden fish meal and meat and bone meal; 60% in cottonseed meal, wheat middlings, and soybean meal; 44% in cooked corn; and 28% in uncooked corn. Nicotinamide absorption in channel catfish and rainbow trout is by simple diffusion (Casirola et al., 1995; Casirola and Ferraris, 1997). Conversion of tryptophan to niacin is very limited in salmonids (Poston and DiLorenzo, 1973; Poston and Combs, 1980) and channel catfish (Ng et al., 1997); therefore, niacin must be supplied in the diet.
Dietary carbohydrate type has been found to influence niacin requirements in tilapia (Shiau and Suen, 1992). Tilapia fed diets containing 38% carbohydrate, supplied as dextrin, required 121 mg niacin per kg of diet for optimal growth, compared with a niacin requirement of 26 mg per kg for fish fed glucose. Part of the difference can be explained by the poorer growth of the fish fed glucose than that of those fed dextrin. Another reason is thought to be the poor metabolic utilization of a high dietary level of glucose.
