Metabolic fate of an oral dose of 15N-labeled nitrate in humans: effect of diet supplementation with ascorbic acid.

The metabolic fate of a p.o. dose of 3.5 mmol 15N-labeled nitrate has been investigated in 12 healthy young adults. Samples of urine, saliva, plasma, and feces were collected over a period of 48 hr following administration of the dose. Subjects received either 60 mg of ascorbic acid, 2 g of ascorbic acid, or 2 g of sodium ascorbate per day. An average of 60% of the 15NO3- dose appeared in the urine as nitrate within 48 hr. Less than 0.1% appeared in the feces. The 15N label of nitrate was also found in the urine (3%) and feces (0.2%) in the form of ammonia or urea. The fate of the remaining 35% of the 15NO3- dose administered is unknown. No effect of ascorbic acid or sodium ascorbate on the nitrate and nitrite levels of plasma, saliva, urine, or feces was observed. A one-compartment pharmacokinetic model was used to describe the relationships between intake, plasma concentration, and urinary excretion of nitrate. The half-life of nitrate in the body was found to be approximately 5 hr, and its volume of distribution was about 30% of body weight. Daily endogenous biosynthesis of nitrate was estimated to be about 1 mmol/day.