A diet rich in fiber can help lower blood pressure. But should you eat more soluble or insoluble fiber? Recent research provides the answer.

The shift in Western societies from a diet based on whole grains, vegetables, fruits, and legumes to a diet based on meats, refined grains, and processed foods has been associated with an increase in the incidence of heart disease and type 2 diabetes -- and several studies pointed to a lack of dietary fiber as a primary cause. Debate followed as to whether fiber has a protective effect or is simply a hallmark of a healthy diet. But recent studies are finding that fiber is an independent factor in the development of these diseases.

There are two types of fiber: soluble fiber (which dissolves in water) and insoluble fiber (which absorbs large amounts of water). Both types are important for disease prevention. Most plant foods contain some of each type, but usually one type predominates. Soluble fiber is found in legumes, barley, oats, and dried and fresh fruits. Wheat and other whole grains and some vegetables contain mostly insoluble fiber.

Diets rich in whole grains can protect against high blood pressure. But researchers wanted to know whether soluble or insoluble fiber was responsible for the effect.

To find out, seven men and 18 women with blood pressures less than 140/90 mm Hg were put on the American Heart Association’s Step I diet. After two weeks, refined carbohydrates in the diet were replaced by whole wheat/ brown rice (insoluble fiber), barley (soluble fiber), or half wheat rice/ half barley (a combination of soluble and insoluble fiber) for five weeks each. Blood pressure was taken weekly and weight measured daily.

Systolic blood pressure (the upper number) was reduced by the wheat-rice and the combination diets. Diastolic blood pressure (the lower number) was reduced by all three diets. There were no differences between the three groups in urinary excretion of minerals (such as sodium and potassium) that might affect blood pressure.

The researchers conclude that, in a healthful diet, replacing refined carbohydrates with any type of whole grains -- from brown rice and barley to whole-wheat bread and cereals -- can reduce blood pressure. Doing so may also help to control your weight. The participants lost about 2 lbs during the study. This study was reported in the Journal of the American Dietetic Association I (Volume 106, page 1445).