Relationship between ascorbic acid dose and N-nitrosoproline excretion in humans on controlled diets

Area of Science:

• Nutritional Biochemistry
• Human Physiology
• Toxicology

Background:

• Dietary nitrate can be converted to N-nitrosamines, potential carcinogens.
• Ascorbic acid (vitamin C) is known to inhibit nitrosamine formation in vitro.
• The in vivo dose-response relationship between ascorbic acid and N-nitrosoproline (NPRO) formation in humans requires further elucidation.

Purpose of the Study:

• To establish the dose-response relationship between ascorbic acid intake and endogenous N-nitrosoproline (NPRO) excretion in humans.
• To determine the efficacy of ascorbic acid in inhibiting NPRO formation from dietary precursors under controlled conditions.

Main Methods:

• Seven healthy males consumed a controlled low-nitrate, low-ascorbic acid diet.
• Nitrate and L-proline were administered daily for a period.
• Increasing doses of ascorbic acid were given with L-proline from day 5-10, with urine analysis for nitrate, NPRO, and ascorbic acid.

Main Results:

• A logarithmic dose-response relationship was observed between ascorbic acid dose and NPRO excretion.
• Even low doses of ascorbic acid (0.05 mmol) significantly reduced NPRO excretion.
• High doses (5.68 mmol) of ascorbic acid did not completely inhibit NPRO formation to baseline levels.

Conclusions:

• Complete inhibition of endogenous NPRO formation requires a higher ascorbic acid to nitrite molar ratio than previously shown in vitro.
• Dietary recommendations regarding nitrate exposure and vitamin C intake may benefit from these findings.
• The study provides valuable data for interpreting epidemiological studies on nitrate exposure and health.