Both nicotinic acid and nicotinamide are white, odorless, crystalline solids soluble in water and alcohol. They are very resistant to heat, air, light and alkali and thus are stable in feeds. Niacin is also stable in the presence of the usual oxidizing agents. However, it will undergo decarboxylation at a high temperature, when in an alkaline medium.
An important source of niacin for ruminants is ruminal synthesis. Synthesis of niacin in the rumen has been demonstrated in sheep (Rérat et al., 1959), cattle (Hungate, 1966) and goats (Porter, 1961). This synthesis also has been suggested to be under metabolic control; e.g., more is synthesized when small amounts are provided in the ration and vice versa (Porter, 1961; Abdouli and Schaefer, 1986).
Nicotinic acid and its amide are readily and very efficiently absorbed by diffusion at either physiological or pharmacologic doses. The mechanism by which nicotinamide nucleotides present in animal feeds are absorbed, however, is unknown. Whether the coenzyme is hydrolyzed in the lumen or after it is taken up by the mucosa is unclear.
In animals, absorption from the small intestine seems to be the major route by which niacin is made available to the host. Although direct absorption from the rumen is possible, normally it is thought to be limited because only a small portion (3% to 7%) of the vitamin is in the supernatant fraction of rumen fluid; most is bound within the microbes themselves (Rérat et al., 1959).
Niacin in foods occurs mostly in its coenzyme forms, which are hydrolyzed during digestion, yielding nicotinamide, which seems to be absorbed as such without further hydrolysis in the gastrointestinal tract. The intestinal mucosa is rich in niacin conversion enzymes such as NAD glycohydrolase (Henderson and Gross, 1979). Nicotinamide is released from NAD in the liver and intestines by glycohydrolases for transport to tissues that synthesize NAD as needed. In the gut mucosa, nicotinic acid is converted to nicotinamide (Stein et al., 1994). Nicotinamide is the primary circulating form of the vitamin and is converted into its coenzyme forms in the tissues. Blood transport of niacin is associated mainly with red blood cells. Niacin rapidly leaves the blood stream and enters kidney, liver and adipose tissues.
The tissue content of niacin and its analogs, NAD and NADP, is a variable factor, dependent on the diet and a number of other factors, such as strain, sex, age and treatment of animals (Hankes, 1984). Although niacin coenzymes are widely distributed in the body, no true storage occurs. The liver is the site of greatest niacin concentration in the body, but the amount stored is minimal.
Urine is the primary pathway of excretion of absorbed niacin and its metabolites. The principal excretory products in humans, dogs, rats, and pigs is the methylated metabolite N1-methylnicotinamide or as two oxidation products of this compound, 4-pyridone or 6-pyridone of N1-methylnicotinamide. On the other hand, in herbivores, niacin does not seem to be metabolized by methylation, but large amounts are excreted unchanged. In the chicken, however, nicotinic acid is conjugated with ornithine as either alpha- or delta-nicotinyl ornithine or dinicotinyl ornithine. The measurement of the excretion of these metabolites is carried out in studies of niacin requirements and niacin metabolism.
The amino acid tryptophan is a precursor for the synthesis of niacin in the body. There is considerable evidence that synthesis can occur in the intestine. There is also evidence that synthesis can take place elsewhere within the body. The extent to which the metabolic requirement for niacin can be met from tryptophan will depend first on the amount of tryptophan in the diet and second on the efficiency of conversion of tryptophan to niacin. The pathway of tryptophan conversion to nicotinic acid mononucleotide in the body is shown in Figure 1. Protein, energy, vitamin B6 and riboflavin nutritional status and hormones affect one or more steps in the conversion sequence shown in Figure 1, and hence can influence the yield of niacin from tryptophan. Iron is required by two enzymes for the conversion of tryptophan to niacin with a deficiency-reducing tryptophan utilization. At low levels of tryptophan intake, conversion efficiency is high. It decreases when niacin and tryptophan levels in the diet are increased.