Friday December 10 2010
Wholegrains are an important part of a healthy, balanced diet
“Eating three portions of wholegrain foods such as porridge every day protects your heart by slashing blood pressure levels,” reported the Daily Express. It said this could be as effective as taking medication.
The study behind this story demonstrates that wholegrain foods give a slight reduction in blood pressure in middle-aged individuals without cardiovascular disease. The study did not assess the effect of porridge itself on health, but compared a diet containing both wholewheat food and oats with a diet high in wholegrain wheat and a diet high in refined grains.
This was a well-conducted randomised controlled trial. Overall, the results support health advice that a diet containing recommended amounts of wholegrains can have a positive effect on blood pressure, which is an important marker of cardiovascular health. This positive effect on blood pressure can be expected to translate into benefits in terms of risk of coronary artery disease and strokes, although this study did not measure such health outcomes.
Where did the story come from?
The study was carried out by researchers from the University of Aberdeen. Funding was provided by the Food Standards Agency and the Scottish Government. Any oatcakes used in the study were provided by Paterson Arran Ltd. The study was published in The American Journal of Clinical Nutrition.
What kind of research was this?
This research assessed how three daily servings of wholegrain foods (either wheat or a mixture of wheat and oats) affected certain markers of cardiovascular disease risk in relatively high-risk individuals.
Evidence has built up over time that eating a diet high in wholegrain foods is associated with a reduced risk of chronic diseases such as diabetes, high blood pressure and coronary artery disease. The evidence is mainly from cohort studies, with one large meta-analysis of cohort studies concluding that three servings of wholegrains a day may protect the heart.
The researchers noted that evidence from a large intervention study was needed, which is why they carried out this randomised controlled trial.
What did the research involve?
Men and women aged between 40 and 65 years from Aberdeen volunteered to take part between September 2005 and December 2008. The participants’ BMIs ranged from 18.5 to 35 (from healthy weight to obese). Of these, the researchers included only people who were either sedentary or moderately active, and who had signs of metabolic syndrome or moderately high blood cholesterol. Those with diagnosed cardiovascular disease, diabetes, high blood pressure or thyroid conditions were excluded. People who habitually ate a lot of wholegrain fibres or took supplements were also excluded. This process resulted in 233 participants for the study.
The participants were randomly allocated to one of three treatment groups for 12 weeks: refined diet, wheat, or oats plus wheat. Apart from these dietary restrictions, the participants were allowed to eat normally. Before they began taking the treatment foods, they were asked to eat only refined cereals and food containing cereals that had been refined and white bread for four weeks) so that everyone was eating the same initially.
The main difference between the three groups was the type of refined cereals and bread being eaten. Those in the refined food group ate only refined cereals and white bread. The wheat group ate only wholemeal bread and wholegrain cereals, while those in the oat and wheat group had a mix of wholewheat foods and oats. Each group ate three servings of the treatment food a day. In total, the recommended servings for the day equalled the amount recommended by the Food Standards Agency for levels of nonstarch polysaccharides (18g/day). The researchers report that the participants were provided with refined, wheat- or oat-based wholegrain foods that are widely available in UK shops. They do not specify further which types. Aside from the food that the researchers supplied, the volunteers were able to select their own foods to eat and advice on what to substitute was tailored to individual participants' diets.
Several measurements, including weight, health, level of exercise, use of medications and anthropometric measures such as blood pressure, arterial stiffness and blood lipids were taken four times during the trial (before the run-in period, and at the start, end and during the trial). The participants kept seven-day food diaries before the study started as well as during the trial. At the end, the authors assessed whether there was an association between the experimental diet and the markers of health that they had measured.
What were the basic results?
In total, 206 participants completed the study and were available for analyses. As expected, at the end of the study, those in the wheat and the wheat plus oats groups were eating more nonstarch polysaccharides. Intakes of vitamin B6 and vitamin D were lower in the wheat plus oats group compared with the refined group, while zinc and magnesium were greater in both wholegrain groups.
After six weeks of their assigned diet, people in the wheat plus oats group had significantly reduced systolic blood pressure compared with the refined group (a reduction of 5mmHg versus 1.3mmHg) and by 12 weeks this improvement was seen in both wholegrain groups. Diastolic blood pressure did not change. There was also significantly reduced pulse pressure in both wholegrain groups (pulse pressure is the numerical difference between systolic and diastolic readings, so if systolic readings are going down pulse pressure would be expected to go down too). There were no changes in blood fats except that in the refined food group, ‘bad’ cholesterol (LDL cholesterol) reduced significantly as did total cholesterol. There was no apparent effect of the diet on other markers of cardiovascular health including C-reactive protein, interleukin-6 and markers of insulin problems, including glucose levels.
How did the researchers interpret the results?
The authors conclude that daily consumption of three portions of wholegrain fibre significantly reduces systolic blood pressure and pulse pressure in middle-aged, healthy, overweight men and women. They note that the observed reduction “was similar to” that seen in drug trials and that such levels could reduce the incidence of coronary heart disease and stroke by more than 15% and 25%, respectively.
This was a well-conducted randomised controlled trial, the results of which appear to show that eating wholegrains influences certain markers of cardiovascular health. The findings raise several issues:
- The researchers say this study is the first of its kind to demonstrate a positive effect of wholegrains on blood pressure. They discuss a similar study, the WHOLEheart study, which found beneficial effects in terms of blood lipids and insulin, but which did not find any effect of wholegrains on blood pressure. The researchers discuss the possible reasons for this, suggesting that their study is more robust when it comes to measuring the effects of wholegrain on blood pressure.
- The study measured only indirect outcomes (proxy outcomes) of cardiovascular health. This means that while the researchers claim that the reductions they have seen here equate to the level of effect that would reduce the risk of stroke and cardiovascular disease, this study cannot prove that wholegrains would have such an effect.
- The effect on blood pressure was only on systolic blood pressure, not diastolic. Blood pressure is a measure of the force of your blood in the blood vessels as the heart beats. A blood pressure reading constitutes two measurements: systolic when the heart pumps and the pressure is at its highest, and diastolic when the heart relaxes and the pressure is lowest. Both are recorded during a single heartbeat. When interpreting blood pressure both systolic and diastolic levels need to be taken into consideration together, as both are indicators of cardiovascular health. This study noted a small (about 4-5mmHg) improvement in systolic blood pressure with wheat, but the medical benefits of this are difficult to generalise and they are likely to depend on a person’s blood pressure at the start. People in this study did not have high blood pressure. Generally, above 140mmHg systolic and above 90mmHg diastolic is considered to be high. The average systolic blood pressure of participants in this study was around 130mmHg.
While on its own the study cannot prove that wholegrains reduce the risk of stroke and cardiovascular disease, overall, the findings support existing evidence that wholegrains in the diet are important for cardiovascular health. They are recommended as part of a healthy, balanced diet and, along with recommended levels of physical activity, may help to prevent cardiovascular disease. The study did not compare the effects of wholegrains with drugs for lowering blood pressure so any claims that this change to diet achieves the same effect as drugs do is premature.