Abstract
Effects of gradually increasing levels of nicotinamide (Nam) in a niacin-free and tryptophan-limited diet on the blood NAD levels and the urinary excretion of Nam metabolites in rats were investigated. When the concentrations of Nam in diets were raised to 1 mg per 100 g of diet, the blood NAD levels increased in direct proportion to the intake of niacin-equivalent, but the urinary excretory metabolites of Nam such as N1-methylnicotinamide (MNA), N1-methyl-2-pyridone-5-carboxamide (2-py), and N1-methyl-4-pyridone-3-carboxamide (4-py) were almost constant regardless of the Nam levels. On the contrary, when the concentrations of Nam in diets were raised to 3 mg and 5 mg per 100 g of diet, the blood NAD levels were almost constant regardless of the Nam levels and it reached nearly normal levels, but the urinary excretion of MNA, 2-py, and 4-py increased in direct proportion to the intake of niacin-equivalent. These results mean that Nam is first used for the synthesis of blood NAD and then, when it has reached nearly normal levels, Nam is used for the increase in the urinary excretion of Nam metabolites. The ratio of 2-py plus 4-py to MNA excretion remained constant regardless of the Nam levels under the conditions of the experiment.
