Magnesium lowers blood pressure by about 2 points. The DASH diet lowered it 11 in people with hypertension.

Magnesium lowers blood pressure, by about 2 points. A 2016 meta-analysis by Zhang and colleagues in Hypertension pooled 34 randomized, double-blind, placebo-controlled trials with 2,028 participants. At a median dose of 368 mg a day for a median of 3 months, magnesium lowered systolic blood pressure by 2.00 mmHg and diastolic by 1.78 mmHg, and raised serum magnesium by 0.05 mmol/L. The authors note that residual heterogeneity may remain and call for further well-designed trials.

Two points is small next to a whole-diet change. In the DASH trial (Appel, NEJM 1997), the combination diet, rich in fruits, vegetables and low-fat dairy, lowered systolic pressure by 11.4 mmHg and diastolic by 5.5 mmHg more than the control diet among the 133 participants with hypertension. The groups are different (DASH looked at one subgroup with hypertension, the magnesium meta-analysis pooled people with and without it), so the two figures are a comparison of scale, not a head-to-head. DASH foods are also naturally high in magnesium.

People with insulin resistance saw a little more. A 2017 meta-analysis by Dibaba and colleagues in the American Journal of Clinical Nutrition pooled 11 randomized trials with 543 participants who had insulin resistance, prediabetes or another chronic disease. At 365 to 450 mg a day for 1 to 6 months, systolic pressure fell by 4.18 mmHg and diastolic by 2.27 mmHg. The trials were small, so treat the larger drop as a lead, not a promise.

What your blood test can and cannot show. The NIH Office of Dietary Supplements notes that less than 1% of the body's magnesium is in blood serum, and the body keeps that level under tight control. A normal serum result therefore says little about total stores. FixFirst reads serum magnesium from an uploaded report against the 1.7 to 2.2 mg/dL range and ranks it with your potassium, calcium and kidney markers. Blood pressure itself needs a cuff.

Before you start. The NIH Office of Dietary Supplements sets the upper limit for supplemental magnesium at 350 mg a day, and notes that high supplemental doses often cause diarrhea. The trials above used a median of 368 mg, just over that limit. The same fact sheet says the kidneys eliminate excess magnesium, so if you have kidney disease, talk to your doctor before taking it.