Wide variation in niacin requirements of animals has been reported. This generally occurs because niacin is synthesized from tryptophan and much of dietary niacin is in a bound form unavailable to humans and animals. It is easier to establish the niacin requirement for the cat (also mink and most fish), since they lack the ability to convert tryptophan to niacin. For species like the dog that have the capacity to synthesize niacin from tryptophan, it is impossible to set the niacin requirement unless the tryptophan level is specified and it is known that the diet is adequate in vitamin B6 and riboflavin, since these vitamins are needed in synthesis of niacin from tryptophan. For the dog, niacin conversion from tryptophan is inefficient. It was estimated that 1 g of L-tryptophan is equivalent to about 7.6 mg of niacin in dogs (Singal et al., 1948). Adequate iron is also required by two enzymes for conversion of tryptophan to niacin (Oduho and Baker, 1993).
Other factors that may influence niacin requirements for dogs and cats include 1) genetic differences, such as selection for a leaner animal; 2) increased stress and subclinical disease level, as companion animals are in closer or more frequent contact; 3) newer methods of handling and processing feeds that may affect niacin and tryptophan level and availability; 4) various nutrient interrelationships, including amino acid imbalances; and 5) molds and antimetabolites in feeds that can increase certain nutrient needs.
The type of carbohydrate consumed may influence niacin requirements. For maximal growth the fish tilapia fingerlings required 26 ppm niacin when fed a glucose diet compared to 121 ppm for those fish fed a dextrin (from corn) diet (Shiau and Suen, 1992).
For dogs and cats, there are no experimental data available to give a requirement for niacin during pregnancy and lactation. However, for humans the rate of conversion of tryptophan to niacin appears to be enhanced in pregnancy due to higher levels of circulating estrogen (Rose and Braidman, 1971; Horwitt et al., 1975). This more efficient conversion may have application to dog niacin requirements.