Drinking a cup of nitrate-rich beetroot juice significantly lowered blood pressure (BP) in hypertensive individuals, a small, proof-of-principle study showed.
Three to 6 hours after a single dose of 250 mL of beetroot juice, systolic BP decreased by a mean 11.2 mm Hg in patients with hypertension compared with a decrease of 0.7 mm Hg in controls, according to Amrita Ahluwalia, PhD, of Queen Mary University of London, and colleagues.
At 24 hours, systolic BP remained significantly lower in the treatment group compared with controls, with a difference of 8.5 mm Hg (P<0.01), the researchers reported in Hypertension: Journal of the American Heart Association.
Recent studies have suggested that the health benefits of fruits and vegetables in the diet may derive from the presence of inorganic nitrate (NO3), which is found in particularly high levels in beetroot and leafy vegetables.
Following consumption, NO3 is bioactivated by conversion to inorganic nitrite (NO2), which in turn is reduced to nitric oxide (NO) through various pathways, resulting in vasodilation.
In experiments conducted in both animals and healthy volunteers, high doses of nitrite lowered BP, but neither the specific enzymatic process nor its precise location -- in the vessel wall or the erythrocyte -- has been identified.
To expand the understanding of the physiologic effects of nitrite, Ahluwalia's group first conducted a series of experiments comparing the effects of nitrite administration in hypertensive and normotensive rats. They determined the reductase pathway responsible for nitrite conversion involved xanthine oxidoreductase (XOR) and established the erythrocyte as the site of nitrite reduction in hypertension.
In addition, the administration of allopurinol blocked these effects in the hypertensive animals, but had no effect in the normotensive animals.
The subsequent proof-of-principle study included 15 participants with BPs above 140/90 who had not previously received antihypertensive therapy.
The group consisted of eight women and seven men whose mean age was 53 and who received a relatively low (13.2 × 10−3 mol/L) dose of nitrite in beetroot juice or an equivalent amount of water containing nitrite levels of 0.07 × 10−3 mol/L.
The dose of nitrite in the beetroot group had previously shown little effect on BP in healthy volunteers, the researchers noted.
Significant decreases in both systolic and diastolic BPs occurred in participants receiving beetroot juice compared with controls (P<0.001), with diastolic pressures showing a peak lowering of 9.6 mm Hg.
Pulse wave velocity also fell significantly (P<0.05) in those receiving the juice.
Changes in systolic BP correlated with change in plasma nitrite levels (r=0.280, P<0.001), and decreases correlated with baseline systolic BP (r=0.561, P<0.05).
As with the animal studies, the erythrocyte was the site of bioactivity, with high levels of XOR expression and inhibition by allopurinol being observed.
In discussing their findings, the researchers noted that circulating levels of nitrate and nitrite had returned to normal 24 hours after administration, yet BP reductions persisted.
"It is possible that while circulating nitrite levels have diminished, tissue levels remain elevated providing a continued supply of NO, although further mechanistic studies exploring this possibility are warranted," they wrote.
The study demonstrated that BP lowering occurs with nitrite administration most strongly in hypertensive animals and humans and could represent a therapeutic option, they concluded.
The study participants tolerated the regimen well. "This is a natural compound, and is quite innocuous," said William O'Neill, MD, of Henry Ford Hospital in Detroit, who was not involved in the study.
But O'Neill cautioned against the use of beetroot juice by patients with renal failure.
"I'm not sure what's going to happen to potassium levels in the bloodstream if they have renal insufficiency," he told MedPage Today.
"Given that approximately 50% of treated hypertensive subjects fail to achieve their target BP, an additional strategy, based on intake of nitrate-rich vegetables, may prove to be cost-effective, easily achievable, safe, and favorable for public health," they noted.