Australasian Science Magazine

By Various experts

Australian researchers have found that dark chocolate and cocoa powder have a small but statistically significant effect in lowering blood pressure in the short term. The authors say there is a need for long-term trials to determine whether or not blood pressure is reduced on a chronic basis by eating cocoa every day.

High blood pressure is an important risk factor for cardiovascular disease, contributing to about half the cardiovascular events, such as strokes and heart attacks, worldwide and around a third of cardiovascular-related deaths. Evidence from epidemiological studies has suggested that cocoa might reduce this risk, the possible explanation being that cocoa contains flavonols, which are responsible for the formation of nitric oxide in the body, and nitric oxide causes blood vessel walls to relax and open wider, thereby reducing blood pressure.

Our new Cochrane Review of 20 randomised trials updated previous reviews with fewer trials, and has shown dark chocolate and flavonol-rich cocoa products to effectively reduce blood pressure by 2–3 mm of mercury. The reviewed trials involved nearly 900 people consuming 3–100 g of dark chocolate or cocoa powder each day (containing 30–1000 mg of flavonols) usually for 2–8 weeks and, in one study, for 18 weeks. The reduction in blood pressure achieved with cocoa is somewhat comparable to other lifestyle modifications, such as diet and exercise (3–5 mm Hg reduction), and may serve as a complementary treatment option.

As studies in our review were of short duration, longer-term studies are needed to look into long-term effects. Available data did not allow any recommendations regarding optimal dosage. Smaller dosages may be as effective as larger dosages. Though larger daily intakes may not be as acceptable as smaller daily dosages.

While our research focused on the effect of blood pressure, research by others has linked flavonol-rich cocoa products to other cardiovascular benefits.

Dr Karin Ried is Research Director at the National Institute of Integrative Medicine, and Honorary Visiting Research Fellow in the Discipline of General Practice at The University of Adelaide.

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This meta-analysis showed that potentially cocoa may lower systolic blood pressure by 3 mm Hg, but there are many aspects of the studies included that means the conclusion cannot be any stronger than “may”.

Firstly, only studies of 2 weeks duration showed an effect while longer studies did not. Although the studies of 2-week duration mostly used a flavonol-free control, while the longer studies did not, I don’t believe a dose of 6–40 mg of flavonols used in the longer trials would have an effect on blood pressure. There was no relationship between dose and blood pressure-lowering effect despite there being over a 30-fold dose range.

As the authors themselves commented, the quality of the evidence is low so it is too early to encourage people to eat chocolate to lower their blood pressure.

Losing 3 kg would have an equivalent effect on blood pressure but would also lower your glucose and decrease your risk of Type 2 diabetes.

Professor Peter Clifton is Laboratory Head of the Nutritional Interventions at the Baker IDI Heart and Diabetes Institute, Adelaide.

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This is a well-conducted analysis of the current evidence of the effect of cocoa products on blood pressure in both hypertensive and non-hypertensive people. The analysis is consistent with previous meta-analyses in the literature that cocoa consumption (flavonoids) has an effect on both systolic and diastolic blood pressure lowering in the order of 2–3 mm Hg.

The studies included were relatively short-term (2–18 weeks), and whilst the trials included varied in design (control groups comparisons, flavonoid source etc.), the findings were relatively consistent across a number of subgroups (age, gender etc.). The absence of long-term trials was identified as a limitation to see whether the blood pressure reduction translated to impact on cardiovascular events.

The implications of the study relate to the provision of further evidence to support the inclusion of dietary flavonoids in a balanced dietary approach to cardiovascular risk reduction through the modification of risk factors that are known to impact on outcomes.

Professor Christopher Reid is Associate Director of Monash University’s Centre of Cardiovascular Research and Education in Therapeutics.