Blueberries may deserve a place on the list of “heart-healthy foods,” according to this randomized controlled trial. Study participants showed remarkable decreases in blood pressure after the daily addition of freeze-dried blueberry powder to their diet.
Johnson SA, Figueroa A, Navaei N, et al. Daily blueberry consumption improves blood pressure and arterial stiffness in postmenopausal women with pre- and stage 1-hypertension: a randomized, double-blind, placebo-controlled clinical trial. J Acad Nutr Diet. 2015;115(3):369-377. Epub 2015 Jan 8.
This was an 8-week, randomized, double-blind, placebo-controlled clinical trial.
Forty-eight postmenopausal women with prehypertension and stage 1-hypertension were recruited from the greater‒Tallahassee, Florida area to participate in this study.
Study Medication and Dosage
Participants were randomly assigned to receive either 22 g freeze-dried blueberry powder or 22 g control powder. The participants were asked to consume half of their daily blueberry or placebo ration mixed with 1 c water in the morning and the remaining half in the evening, at least 6 hours apart.
Resting brachial systolic and diastolic blood pressures were evaluated and arterial stiffness was assessed using carotid-femoral pulse wave velocity and brachial-ankle pulse wave velocity. C-reactive protein, nitric oxide, and superoxide dismutase were measured at baseline, 4 weeks, and 8 weeks.
After 8 weeks of blueberry supplementation, systolic blood pressure and diastolic blood pressure and brachial-ankle pulse wave velocity decreased significantly from baseline levels. There were no changes in the group receiving the control powder. Nitric oxide levels were greater in the blueberry powder group at 8 weeks compared with baseline values whereas there were no changes in the control group (Table).
Table. Freeze-dried Blueberry Powder vs Placeboa
| || ||Blueberry|| || ||Placebo|| |
|Variable||0 wk||4 wk||8 wk||0 wk||4 wk||8 wk|
|Systolic blood pressure, mm Hg||138±14 ||136±15||131±17||138±15||136±15||139±15|
|Diastolic blood pressure, mm Hg||80±7||77±10||75±9||78±8||78±11||80±8|
|Mean arterial pressure, mm Hg||99±9||97±11||95±11||98±9||97±11||97±11|
|Heart rate, beats/minute||65±10||66±9||66±9||66±7||66±6||65±6|
aReported in mean standard deviation
We probably should add blueberries to our existing list of foods that are good for lowering risk of cardiovascular disease.
Consuming the blueberry product was associated with a 5.1% and 6.3% reduction in mean systolic and diastolic blood pressures, respectively, while no change blood pressure was seen in those who consumed the placebo powder. These results are close to those reported by Basu et al, who in 2010 reported 6% and 4% reduction in systolic and diastolic blood pressure in middle-aged obese men and women diagnosed with metabolic syndrome who consumed 50 g freeze dried blueberry powder per day for 8 weeks.1 It should be noted that in an earlier and somewhat similar trial by McNulty et al, no significant change in blood pressure was noted, though this study looked at chronic smokers who were provided with 250 g fresh blueberries a day for 3 weeks.2 This would not be the first clinical trial in which smokers respond differently to dietary interventions than nonsmokers do. The McNulty study may not have found significant associations because it was too short in duration, 3 weeks rather than 8 weeks, or perhaps the process of freeze-drying blueberries makes them more effective.
Let’s not confuse the strength of this evidence, though; even if blueberries are now on our 'good for the heart list,' there are still no clinical trials that suggest a change in mortality or even cardiovascular events in people consuming blueberries.
While the drop in blood pressure reported by Johnson et al in the current study may be statistically significant, the clinical significance might be questioned. The women participating in this study started out with blood pressure defined as prehypertensive (120-129 mm Hg systolic and 80-89 mm Hg diastolic3), and though their blood pressure dropped significantly over the course of the study, it did not drop enough to move these women out of the prehypertensive range. Greater quantities of blueberries taken over a longer period might have had greater impact on blood pressure.
Pulse wave velocity is a measure of arterial stiffness and is predictive of cardiovascular events.4 Rodriguez-Mateos et al reported dose-dependent improvements in pulse wave velocity in men given varying doses of blueberry polyphenol extracts.5 Earlier studies have already suggested that flavonoids and polyphenols from other plants, including cranberries,6 grape juice,7 turmeric,8 chocolate,9 and even watermelon,10 are associated with improved pulse wave velocity or reduced arterial stiffness. The authors of the current study speculate that the drop in blood pressure was in part due to an increase in endothelial nitric oxide synthase and not due to inducible nitric oxide synthase because there was no significant increase in inflammatory markers (C-reactive protein) during the course of the study.
The impact of a single food on blood pressure through nitric oxide production is most pronounced with beets. Beets contain more nitrate than almost any other food and as a result are associated with increased nitric acid production, which results in acute vasodilation and decreased blood pressure.11-13
Flaxseed meal also lowers blood pressure, apparently through another mechanism. Caliguri et al reported in July 2014 that 30 g ground flaxseed per day for 6 months was associated with a 10 mm Hg drop in systolic and 7 mm Hg drop in diastolic blood pressure. These authors hypothesized that the drop was due to the α-linolenic acid content of the flax altered plasma oxylipin levels.14 Of course, many foods act on blood pressure through multiple avenues; chocolate, for example, both decreases arterial stiffness and increases nitric oxide.
Let’s not confuse the strength of this evidence, though; even if blueberries are now on our “good for the heart list,” there are still no clinical trials that suggest a change in mortality or even cardiovascular events in people consuming blueberries. These studies that suggest blueberry consumption is associated with modest reductions in blood pressure hardly compare with data such as those from the Prevención con Dieta Mediterránea or PREDIMED trial in which consumption of nuts or extra virgin olive oil has been associated with significant changes in morbidity and mortality.