While this difference was modest, the researchers say it could mean a 6% drop in stroke deaths and a 4% drop in deaths from heart attack.
"Our study suggests that partially replacing a high-carbohydrate diet with soy or low-fat dairy may be beneficial for blood pressure," study researcher Jiang He, MD, PhD, of Tulane University tells WebMD.
Some earlier studies have suggested that eating a diet rich in low-fat dairy products can lower blood pressure.
The most well known of these studies was the Dietary Approaches to Stop Hypertension (DASH) diet study. It found that lowering sodium, limiting red meats and sweets, and eating two to three daily servings of low-fat dairy could reduce hypertension.
But other studies have not shown the association and some others have even suggested that a high-carbohydrate diet lowers blood pressure.
In an effort to better understand the impact, if any, of dietary protein and carbohydrates on blood pressure, Jiang He and colleagues conducted a study involving 352 patients with prehypertension or moderate hypertension (systolic blood pressure ranged from 120 to 159 and diastolic of 80 to 95).
Each study participant took powdered supplements of soy protein, milk protein, or refined carbohydrate in liquid twice a day for eight weeks. Milk and soy supplementation delivered 40 grams of protein daily, while the protein in the carbohydrate powder was negligible.
After a three-week "wash out" period the participants were put on another supplement for eight more weeks, followed by another three-week period without supplementation and another eight weeks on the third supplement.