Niacin is a very stable vitamin when added to feed or premixes, being little affected by heat, oxygen, moisture, or light. In plant-source feed ingredients, much of the niacin activity, mostly nicotinamide nucleotides, is bound and therefore unavailable. Roughly 85 90% of the niacin activity in cereal grains and 40% in oilseeds is in a bound unavailable form. Alkaline hydrolysis is the only means by which niacin can be efficiently released from its bound state in these ingredients. Meat and milk products, on the other hand, contain no bound niacin, but instead contain free nicotinic acid, nicotinamide, and nico-tinamide nucleotides.
Because excess tryptophan is converted to nicotinic acid and because all common feed ingredients contain tryptophan as well as nicotinic acid, there is no good way to assess the bioavailability of niacin per se. Thus, 50 mg of tryptophan yields 1 mg of nicotinic acid.[6,7] Iron is required in two metabolic reactions in the pathway of tryptophan to nicotinate mononucleotide. Oduho et al. established that Fe deficiency in chicks will reduce the conversion efficiency of tryptophan to niacin [i.e., from 42:1 to 56:1 (wt:wt)].
Niacin activity can be purchased as either free nicotinic acid or free nicotinamide. Relative to nicotinic acid, nicotinamide has been observed to be roughly 120% bioavailable in delivering niacin bioactivity.[9'10]
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