Effects of Dietary Nitrate on Blood Pressure

To the Editor: Larsen and colleagues (Dec. 28 issue)1 report that oral nitrate (NO3) can lower mean arterial blood pressure and is associated with an elevation in blood levels of nitrite (NO2). They point out that nitrate is reduced by oral bacteria to nitrite, which can be converted into nitric oxide or absorbed, and they suggest that the mechanism involved in the blood-pressure–lowering effect needs to be clarified. In studies conducted over the past 5 years, we have shown that nitrite is an intrinsic vasodilator.2,3 Within the vasculature, nitrite is reduced to nitric oxide by reaction with deoxygenating hemoglobin and possibly other heme proteins.4,5 These studies show that nitrite levels are higher in the arteries than in the veins in humans, indicating metabolism across the peripheral circulation, and that when nitrite is infused into the brachial artery, it causes vasodilatation. We recently completed a study evaluating the effect of low-dose nitrite infusion in 15 healthy volunteers and found that levels as low as 350 nM resulted in measurable vasodilatation in the forearm; mean (±SE) values of blood flow ranged from 3.0±0.3 to 3.4±0.3 ml per minute per 100 ml of forearm tissue (P = 0.04). We are therefore not surprised that increases in nitrite levels from 138 nM to 219 nM caused by increased dietary intake of nitrate are associated with decreases in blood pressure.