Another Fiber Benefit - Lower Blood Pressure

Summarized by Robert W. Griffith, MD
May 4, 2005

Introduction

Fiber in the diet is good for you, in many ways. One of its benefits has not been fully established - its action on the blood pressure. Scientists from the Netherlands have conducted an analysis of a number of pooled relevant studies, and published their findings in the Archives of Internal Medicine. Here's a summary.

What was done

Many studies of dietary fiber and blood pressure have been done in the past, but they weren't larger enough detect small but meaningful effects. The Dutch scientists limited their search to randomized placebo-controlled studies, i.e. those where subjects were allocated at random to receive either a fiber-supplemented diet or their normal diet, without supplements. Only studies which had blood pressure as a major measurable outcome were included.

One study had two different forms of fiber supplementation. This meant there were 25 study results available, out of a total of 24 studies analyzed.

The information analyzed comprised the dose of fiber, type of fiber, changes in blood pressure, design of the study, and the sort of subjects studied (e.g. normal or raised blood pressure).

What the analyses showed

There were 1404 subjects in the 24 studies, which lasted an average of 9 weeks. Their average age was 42. The studies were double-blind in 16 instances i.e. neither the subject or the healthcare professionals concerned knew if the subject was getting a supplement or a non-fiber supplement. In those 12 studies where it was reported, the body mass index (BMI) demonstrated that 92% of the participants were overweight.

The average 'dose' of fiber supplementation was 11.5 grams daily, but it ranged widely, between 3.5 and 42.6 grams daily. In 4 of the 24 studies, the additional fiber was achieved by dietary manipulation.

The average reduction in systolic blood pressure (the top number) was 1.13 mm Hg. This change was not statistically significant - that is to say, the result could have occurred 'by chance alone'. The reduction in diastolic pressure (the lower number) was 1.26 mm Hg, which was a statistically significant result.

When just those participants over 40 years of age were considered, the average reduction in systolic blood pressure was 3.04 mm Hg, and in diastolic pressure it was 1.41 mm Hg. In a similar way, the benefits of fiber on blood pressure were greater in those participants who had hypertension (raised blood pressure); however, when the effect in hypertensive patients was adjusted to allow for age differences, the apparent benefit disappeared.

What these findings mean

The reductions in blood pressure over a 9-week period were small in this analysis; however, assuming they were extended over some years, they might indeed be sufficient to account for a reduction in the overall rate of hypertension in the community.

If this doesn't persuade you to take in more fiber, think of the other known benefits: decreased heart attack risk, decreased colon cancer risk, lessened diverticular disease, lessened constipation, help in controlling diabetes, lower cholesterol levels, and help with weight control.

Daily intake of fiber in the US and other Western countries averages about 15 grams. However, the American Heart Association recommends an intake of 25 to 30 grams daily. There seems to be plenty of room for improvement in the average diet!

Source

• Dietary fiber and blood pressure. A meta-analysis of randomized placebo-controlled trials. MT. Streppel, LR. Arends, P. van't Veer,  et al., Arch Intern Med, 2005, vol. 165, pp. 150--156

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