Aims: Dietary nitrate from sources such as beetroot juice lowers blood pressure (BP) via the nitrate–nitrite–nitric oxide (NO) pathway. However, NO and nitrite are inactivated via reoxidation to nitrate, potentially limiting their activity. Cytochrome P450-3A4 inhibition with troleandomycin prevents nitrite re-oxidation to nitrate in rodent liver. Grapefruit juice contains the CYP3A4 inhibitor furanocoumarin. We therefore hypothesized that grapefruit juice would enhance BP-lowering with beetroot juice by maintaining circulating [nitrite]. Methods: We performed a randomized, placebo-controlled, 7-hour crossover study in 11 healthy volunteers, attending on 3 occasions, receiving: a 70-mL shot of active beetroot juice (Beet-It) and either (i) 250 mL grapefruit juice (Active Beet+GFJ), or (ii) 250 mL water (Buxton, Active Beet+H 2O); or (iii) Placebo Beet+GFJ. Results: The addition of grapefruit juice to active beetroot juice lowered systolic BP (SBP): Active Beet+GFJ vs Active Beet+H 2O (P =.02), and pulse pressure, PP (P =.0003). Peak mean differences in SBP and PP were seen at T = 5 hours: −3.3 mmHg (95% confidence interval [CI] −6.43 to −0.15) and at T = 2.5 hours: −4.2 mmHg (95% CI −0.3 to −8.2), respectively. Contrary to the hypothesis, plasma [nitrite] was lower with Active Beet+GFJ vs Active Beet+H 2O (P =.006), as was salivary nitrite production (P =.002) and saliva volume (−0.34 mL/min [95% CI −0.05 to −0.68]). The taste score of Beet+GFJ was 1.4/10 points higher than Beet+H 2O (P =.03). Conclusion: Grapefruit juice enhanced beetroot juice's effect on lowering SBP and PP despite decreasing plasma [nitrite]. Besides suggesting more complex mechanisms, there is potential for maximising the clinical benefit of dietary nitrate and targeting isolated systolic hypertension.

Original languageEnglish
Number of pages11
JournalBritish Journal of Clinical Pharmacology
Issue number2
Publication statusE-pub ahead of print - 10 Jun 2020


  • blood pressure
  • cytochrome p450
  • nitric oxide


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