People with hypertension who replace a portion of the carbohydrates in their diet with soy protein or low-fat dairy may see a small yet meaningful decrease in their blood pressure, a new study suggests.
The findings aren't entirely surprising: Diets high in refined carbohydrates are generally believed to be unhealthy for the heart, while soy foods such as tofu have been shown to improve cholesterol levels and decrease the risk of heart disease, especially when eaten in place of meat. Relatively little is known about how these foods affect blood pressure, however.
To answer this question, researchers put more than 350 people with mild hypertension or higher-than-normal blood pressure on three separate regimens of daily supplements containing soy protein, milk protein, and complex carbohydrates.
Without knowing which was which, the study participants took each supplement exclusively for a two-month period. The carbohydrate supplement -- which contained largely refined carbohydrates -- had no measurable effect on blood pressure, but when the participants took the soy and milk protein supplements, their systolic blood pressure (the top number in a blood pressure reading) dropped by about two points, on average.
"Our message really encourages people to eat more beans -- especially soybeans -- and to eat more low-fat dairy," says the lead author of the study, Jiang He, M.D., a professor of epidemiology at the Tulane University School of Public Health and Tropical Medicine, in New Orleans.
The decreases in blood pressure seen in the study were modest on an individual level, but they would be "very, very important at a population level," He adds. If the average systolic blood pressure in the U.S. dropped by about two points, he and his coauthors note in the study, annual deaths from heart disease and stroke could be expected to drop by 6 percent and 4 percent, respectively.
And even for individuals, a two-point drop is not insignificant, says Penny Kris-Etherton, Ph.D., a distinguished professor of nutrition at Penn State University, in University Park. "Every little bit is good, and helpful in terms of reducing [the] risk of cardiovascular disease," she says.
But people with high blood pressure shouldn't start gulping soy milk or popping milk-protein supplements, Kris-Etherton says. Nor should they confuse heart-healthy whole grains with the refined carbohydrates used in the study.
"People shouldn't just tip the balance and say, 'Those carbohydrates are so bad and I'm just gong to load up on protein,'" she says. "Be sensible about it, and just don't go overboard."
Some previous research has suggested that consuming soy protein and low-fat dairy products may help lower blood pressure, but He and his colleagues are the first to explore the impact of protein and carbohydrate intake on blood pressure in a randomized controlled trial. Their findings appear in the August 2 issue of Circulation, a journal published by the American Heart Association.
The study participants all had mild hypertension (known as stage 1 hypertension) or moderately elevated blood pressure (known as prehypertension), and none were taking drugs to lower blood pressure. They were instructed to slightly reduce their usual daily calorie intake to allow for the calories in the supplements, but they were told to otherwise maintain their usual diet and exercise habits for the duration of the study.
The supplements included 40 grams (about 1.5 ounces) of protein or carbohydrates. On average, the researchers found, the participants' systolic blood pressure was 2.0 and 2.3 millimeters of mercury (mmHg) lower when they took the soy and milk protein, respectively, than when they took the carbohydrate supplement.
Kris-Etherton says it's not clear from the results whether replacing carbs with protein from lean meats, beans, and other sources not addressed in the study would have the same effect on blood pressure as soy and milk protein.
He and his colleagues are planning future studies to investigate that question.