Dietary Nitrate – an Unrecognised Nutrient?

Abstract

Background: Hypertension (high blood pressure) is a major risk factor for cardiovascular disease such as stroke, NICE [1]. Dietary nitrate supplements, including beetroot juice, may have positive effects on health: for example by reducing blood pressure (BP) Siervo et al. [2] and improving exercise tolerance in peripheral arterial disease Allen et al. [3]. However, nitrate is a strictly controlled, environmental contaminant and not regarded as a nutrient essential for health. Plasma [nitrate] and [nitrite] increase following ingestion of nitrate containing supplements, but few studies have used whole, nitrate-rich vegetables to supplement a normal diet. The purpose of this study was to assess the effects of supplementation with high- and low-nitrate vegetables on plasma [nitrate], [nitrite], and BP.  Method: Following ethical approval, fifteen non-smoking, physically active males of 18-40 years of age were recruited between January 2011 and March 2012. In a randomised, balanced, cross-over design, participants received high- or low-nitrate vegetables for a 2-week period and, after a 2-week wash-out, participants received the remaining diet (low- or high-nitrate) for a 2-week period. Data were analysed using repeated-measures ANOVAs. Results: There were significant main and interaction effects by diet on plasma [nitrate] (P<0.05) and plasma [nitrite] (P<0.05). Post hoc tests revealed that high-nitrate diet significantly increased plasma [nitrate] (pre: 29.5±20.0 µM; post: 129.4±87.1 µM, P<0.05) and plasma [nitrite] (pre: 118.9±35.2 nM; post: 226.5 ± 89.3 nM, P<0.05). There were significant inverse correlations between changes in plasma [nitrate] and systolic BP (r =–0.49, P<0.05), plasma [nitrate] and the mean arterial pressure (r =–0.44, P=0.05) and plasma [nitrite] and diastolic BP (r =–0.56, P<0.05). No significant changes were observed in these variables after the low-nitrate diet. Discussion: This is the first study to compare whole, fresh, high-nitrate and low-nitrate vegetables as a dietary intervention with potential to reduce BP. It was shown that increases in plasma concentrations of nitrate and nitrite were associated with a reduction in BP. These findings are comparable to previous studies using high-nitrate supplements, such as natural or concentrated beetroot juice or sodium nitrate (Siervo et al. [2]. Conclusion: The present findings support the hypothesis that increasing dietary nitrate intake in the form of nitrate-rich vegetables reduces BP, with major public health implications for dietary interventions to reduce hypertension. A population wide promotion of normal BP could substantially reduce the risk of stroke (Seshadri et al. [4].