This double-blind, placebo-controlled randomized clinical trial will test whether a magnesium glycinate supplement (480 mg/day) taken for 12 weeks lowers blood pressure.

This double-blind, placebo-controlled randomized clinical trial will test whether a magnesium glycinate supplement (480 mg/day) taken for 12 weeks lowers blood pressure in 60 adults aged 30-74 who have elevated blood pressure or stage 1 hypertension and are not taking anti-hypertensive medication. Animal studies, epidemiologic studies, and small randomized trials suggest that supplemental Mg may reduce blood pressure, but the evidence is not definitive.

Eligibility to participate in the trial will be determined by a 2-stage screening process -- completion of a pre-screening form online followed by an in-person screening clinic visit. Eligible participants will assigned by chance (like a coin toss) to daily magnesium or to placebo and will receive a supply of study capsules via U.S. mail. Participants will take 4 study capsules per day (2 capsules in the morning and 2 capsules in the evening).

Assessments at the screening visit include seated blood pressure; 24-hour ambulatory blood pressure (monitor will be worn by the participant for 24 hours and then returned via mail); pulse; weight, height, waist and hip circumference measurements; fasting blood and urine collection; and health and diet questionnaires. Participants will return for clinic visits at 6 weeks and at 12 weeks to assess these measures, including 24-hour ambulatory blood pressure.

Inclusion Criteria:

• Self-report of systolic blood pressure 120-139 mmHg or diastolic blood pressure 80-89 mmHg at prescreening
• Measured seated systolic blood pressure 120-149 mmHg or measured seated diastolic blood pressure 80-94 mmHg at screening visit
• Body mass index less than 40 kg/m2
• Total Mg intake from supplements of no more than 100 mg/day
• Willing to maintain current diet and supplement use patterns during the intervention period

Exclusion Criteria:

• History of antihypertensive use
• History of cardiovascular disease (myocardial infarction, stroke, revascularization [coronary artery bypass graft or percutaneous transluminal coronary angioplasty], or angina pectoris)
• History of invasive cancer diagnosed within the last 5 years (non-melanoma skin cancer permitted)
• History of type 1 or 2 diabetes
• History of renal disease
• History of kidney failure
• History of dialysis
• History of pancreatitis
• History of inflammatory bowel disease
• History of hypermagnesemia
• Women who are pregnant, nursing, or intend to become pregnant during the treatment period
• Plan to relocate out of Boston area within the next year
• Unwillingness and/or inability to swallow 4 pills per day
• Inability to provide written informed consent
• Excessive antacid or laxative use within the past 3 months