Intestinal Synthesis of Niacin and the Metabolic Interrelationship of Tryptophan and Niacin in the Rabbit

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Abstract
The feeding of a niacin deficient diet to rabbits resulted in a decreased gain in weight. One group of rabbits fed a purified diet supplemented with 60 mg of niacin per 100 gm of diet showed an average gain in weight of 1,637 gm, as compared to a gain of 1,057 gm made by a group that received a similar diet without the addition of niacin. Balance studies showed that considerable amounts of niacin were being synthesized by the rabbits fed the low niacin diet. The average daily ingestion of niacin for this group was 11.2 μg, with a corresponding niacin excretion of 178.7 μg. The average daily fecal excretion of niacin amounted to 38.8 μg, showing that synthesis was taking place in the digestive tract. Growth data secured with 6 groups of rabbits fed different levels of niacin and fed tryptophan as the free amino acid and as additional protein showed that niacin could be synthesized from its precursor tryptophan. One group of rabbits receiving the basal diet with 20% casein but no additional tryptophan or niacin lost an average of 34 gm in 10 weeks. The group receiving the basal diet supplemented with niacin gained an average of 680 gm in the same period. Rabbits receiving the basal ration plus tryptophan gained 864 gm, and the rabbits in the control group receiving the basal diet supplemented with both niacin and tryptophan gained 693 gm. Tryptophan fed as protein was not as effective as when fed as free amino acid, as evidenced by the fact that the rabbits in a group fed additional tryptophan in the form of casein gained only 247 gm as compared to the 864 gm gained by the group which was fed the same amount of tryptophan in the form of free amino acid.