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Department of Physiology and Pharmacology, Karolinska Institutet, S-171 77 Stockholm, Sweden, E-mail: jon.lundberg{at}ki.se
Dear Sir:
I read with interest the article by Loke et al (1) published in the October issue of the Journal. In their study they show that the dietary flavonoids quercetin and (–)-epicatechin increase plasma and urinary concentrations of nitric oxide (NO) reaction products. More specifically, the concentrations of nitrite and S-nitrosothiols increased in plasma, whereas nitrate concentrations increased in urine. These stable NO reaction products are commonly used as markers of endogenous NO formation. Consequently, the authors suggest that stimulation of endothelial NO synthase by the flavonoids is the underlying mechanism. Although I have little doubt that flavonoids can increase endogenous NO generation and bioavailability, I was concerned about the actual concentrations of NO reaction products reported in the study. Loke et al reported plasma nitrite concentrations of 
4 µM, and these increased to >5 µM after ingestion of flavonoids. It seems clear from a great number of recent studies (some of which are cited by Loke et al) that fasting circulating nitrite concentrations are in the 50–500 nM range, ie, 10–100-fold lower (2–7). I wonder if the authors have confirmed these high concentrations by using another method, eg, the chemiluminescence technique that they used for S-nitrosothiols in the present study. This highly sensitive method is also commonly used for nitrite measurements (8). It was also surprising that the observed increases in urinary nitrate were not paralleled by increases in circulating nitrate. Moreover, in the Discussion, the authors state that normal plasma concentrations of S-nitrosothiols are 7 µM, but at the same time they now report concenterations of 8 nM, ie, 1000-fold lower. Finally, although the authors attempted to control basal intake of flavonoids during the study periods, I could find no information on restriction in dietary intakes of nitrate and nitrite—the actual markers measured in this study. Diet has a profound impact on circulating nitrate and nitrite (4) and vegetables are particularly rich in nitrate (9, 10). For example, a plate of lettuce or spinach contains more nitrate than what is produced during 1 d by all 3 NO synthase isoforms combined (5).
ACKNOWLEDGMENTS
The author declared no financial or personal conflict of interest.
REFERENCES
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