Magnesium

What it is

Total body magnesium in an adult is about 25 g, of which 50 to 60 percent is in bone; only about 1 percent is outside cells.^[1] Magnesium is needed by more than 300 enzyme systems and takes part in both anaerobic and aerobic energy production, often bound to ATP, the cell's energy carrier.^[1] The kidney is the main organ that keeps magnesium in balance, filtering it and reabsorbing what the body needs.^[1]

Green leafy vegetables are rich sources, because chlorophyll is itself a magnesium compound.^[1] From a typical diet, about 50 percent of magnesium is absorbed.^[1] High alcohol intake and several medicines, including diuretics, increase how much the kidneys lose.^[1]

Supplements use many salts: oxide, citrate, chloride, glycinate and others. Replenish uses magnesium citrate, the magnesium salt of citric acid, alongside sodium, potassium and calcium.

How much adults need

Source: Institute of Medicine, Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride (1997), Chapter 6.^[1] *The UL applies to magnesium from supplements and medicines only, not food. RDA = Recommended Dietary Allowance; UL = Tolerable Upper Intake Level.

Amount in Replenish: confirmed at launch

The RDA is higher from age 31 for both sexes, partly because kidney function, which governs magnesium balance, declines with age.^[1] Intakes often fall short of it: in the 1994 survey data the committee used, the median intake of women aged 51 to 70 was 230 mg a day, below even their average requirement of 265 mg.^[1] Those figures are old, and current intake data would come from newer surveys.

The UL is unusual. Magnesium from food has not been shown to cause adverse effects, so there is no limit on food. The 350 mg limit applies only to magnesium from supplements and medicines, and it is set to avoid diarrhea, the first sign of too much.^[1] That means it is possible to exceed the RDA from food and stay well within the UL, or stay below the RDA in total and still pass the UL with a high-dose supplement.

What the evidence says

Blood pressure. A 2016 meta-analysis pooled 34 randomized, double-blind, placebo-controlled trials with 2,028 adults. At a median dose of 368 mg a day for a median of 3 months, magnesium lowered systolic blood pressure by 2.00 mm Hg and diastolic by 1.78 mm Hg compared with placebo, and raised serum magnesium by 0.05 mmol/L.^[2] The authors noted that residual heterogeneity may remain between trials and called for better-designed trials.^[2] The effect is small, and the median dose was above the UL for supplemental magnesium.

Bowel habits. Some magnesium salts draw water into the bowel; this osmotic effect is why diarrhea is the limiting effect at high supplemental doses.^[1] Magnesium oxide is used as a laxative in Japan, and two small placebo-controlled trials there tested it at 1.5 g of magnesium oxide a day for 28 days. In 34 women with mild to moderate chronic constipation, 70.6% on magnesium oxide met the primary endpoint of overall improvement against 25.0% on placebo.^[3] In a 90-patient trial, overall improvement was 68.3% on magnesium oxide, 69.2% on senna and 11.7% on placebo.^[4] These are laxative doses of a different salt, in people with constipation, and they say nothing about the much smaller amounts of magnesium in a daily drink mix. How fiber, fluid and movement fit together is in fiber and regularity on a GLP-1.

Low magnesium. Magnesium depletion is seen with diabetes, malabsorption, kidney disorders that waste magnesium, and alcoholism.^[1] Serum magnesium is the most commonly used test, but it may not reflect the magnesium available inside cells, and it does not accurately reflect muscle magnesium content.^[1] None of these studies was in people taking GLP-1 medications; the wider picture of minerals while eating less is in electrolytes on a GLP-1.

Safety and interactions

The UL for adults is 350 mg a day from nonfood sources. It rests on a lowest-observed-adverse-effect level of 360 mg a day, at which 6 of 21 patients given magnesium chloride developed gastrointestinal symptoms including diarrhea; because osmotic diarrhea is mild, reversible and obvious, an uncertainty factor close to 1 was used.^[1] Many people take more without problems.^[1]

The serious risk is high blood magnesium, which occurs mainly when impaired kidney function is combined with large nonfood intakes such as antacids. Deaths have been reported after very large exposures, especially in people with kidney failure.^[1] People with kidney disease should not take magnesium supplements without medical advice.

Tetracycline antibiotics. The prescribing information for doxycycline states that antacids containing aluminum, calcium or magnesium impair tetracycline absorption.^[5]

Quinolone antibiotics. The ciprofloxacin label says to take it at least 2 hours before or 6 hours after magnesium or aluminum antacids, multivitamins and other products containing multivalent cations such as calcium, iron or zinc.^[6]

Bisphosphonates. For alendronate, calcium supplements, antacids and medicines containing multivalent cations, a group that includes magnesium, interfere with absorption; the label says to wait at least 30 minutes after a dose before taking supplements or vitamins.^[7]

Levothyroxine. The label lists antacids, including magnesium hydroxide, among products that reduce absorption, and says not to take it within 4 hours of calcium supplements and antacids.^[8]

Proton pump inhibitors. A 2019 meta-analysis of 16 observational studies with 131,507 patients found hypomagnesemia in 19.4% of users against 13.5% of non-users, an adjusted odds ratio of 1.71.^[9] Another 2019 meta-analysis found the link uncertain because the studies were so heterogeneous.^[10] If you take a PPI long term, ask your prescriber whether your magnesium should be checked.

Diuretics. Diuretics used for high blood pressure and heart failure can increase magnesium loss in urine.^[1]

If you take prescription medicine, ask your pharmacist or prescriber before adding a supplement.

In Replenish

Replenish provides magnesium as magnesium citrate, one of its 4 electrolyte minerals plus zinc. Magnesium is one of the minerals in a formula that provides key vitamins and minerals.^† The amount per serving is published with the Supplement Facts panel, which lets you add it to any other supplemental magnesium and compare the total with the 350 mg UL. See Replenish.