The purpose of this randomized, double-blind, controlled trial is to examine the effect of soybean protein supplementation on blood pressure in persons with prehypertension or stage-1 hypertension.

Background: Epidemiologic studies suggest that vegetable protein intake is inversely related to blood pressure.

Objectives: To examine the effect of soybean protein supplementation on blood pressure in persons with prehypertension or stage-1 hypertension

Design: Randomized, double-blind, controlled trial

Study Participants: 350 participants, aged 35-64 years, with an initial untreated systolic blood pressure of 130-159 mmHg and/or diastolic blood pressure of 80-99 mmHg

Intervention: Study participants will be randomly assigned to take 40-grams of isolated soybean protein supplements per day or complex carbohydrate control for 12 weeks.

Outcomes: Blood pressure measurements will be obtained using random-zero sphygmomanometers at baseline and during the trial at weeks 6 and 12. Net changes in systolic and diastolic blood pressure before and after the intervention is the primary outcome.

Inclusion Criteria:

1. Systolic BP of 120 to 159 mm Hg and diastolic BP of 80 to 95 mm Hg (average of 6 measurements at 2 screening visits)
2. Willing to participate in all aspects of the study

Exclusion Criteria:

1. Consumption of dietary protein greater than or equal to 1.63 grams/kg per day, as determined by two 24-hour dietary recalls
2. Stage-2 or higher severe hypertension (systolic BP greater than or equal to 160 mm Hg and/or diastolic BP greater than or equal to 95 mm Hg)
3. Use of antihypertensive medications or medications that affect BP
4. History of clinical heart disease (e.g., angina/heart attack, coronary revascularization, heart failure, stroke/transient ischemic attack, peripheral arterial disease)
5. Kidney disease (serum creatinine level greater than or equal to 1.7 mg/dL for men and greater than or equal to 1.5 mg/dL for women)
6. Current hypercholesterolemia (fasting serum total cholesterol greater than or equal to 240 mg/dL) or use of cholesterol-lowering medications
7. Current diabetes (fasting serum glucose greater than or equal to 126 mg/dL) or use of insulin or oral hypoglycemic agents
8. Severe obesity (body mass index greater than or equal to 40 kg/m²)
9. Current use of prescription weight loss medications, underwent weight loss surgery, and/or experienced weight loss greater than 15 pounds within the 6 months prior to study entry
10. Cancer (except non-melanoma skin cancer) that required treatment during the year prior to study entry
11. Consumption of more than 14 alcoholic beverages per week
12. Current participation in another medical study
13. Consumption of milk or soy protein greater than or equal to the 90th percentile of intake in the U.S. general population
14. Has another member of the household participating in the study
15. Study employees or living with study employees
16. Allergy or intolerance to soy protein or milk protein products
17. Allergy to aspartame
18. Plans to move out of the study area (greater than or equal to 50 miles from the study site) and has difficulty coming to the study site
19. Inability or unwillingness to cooperate during the screening visits
20. Poor compliance during the screening period (intake of less than 85% of supplements)
21. Pregnant or plans to become pregnant during the study

Participants randomized to this three-phase cross-over trial.

Through study completion, an average of 24 weeks

This is a three-phase cross-over trial: 284 participants completed soy protein supplementation, 286 completed milk protein supplementation, and 287 completed carbohydrate supplementation.

The randomization assignment list was generated by a computer program that could only be accessed by the study data coordinator. Apart from the data coordinator, all research personnel, including investigators, study coordinators, and BP technicians, and the study participants were unaware of treatment assignment.