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Low Dietary Magnesium Changes Cardiac Rhythm

Low intakes of dietary magnesium may increase supraventricular ectopy.

Thus, people who live in areas with soft water, who use diuretics, or who are predisposed to magnesium loss may need to take in more dietary magnesium than others, United States researchers suggest.

Investigators who studied low dietary magnesium concentration in 22 postmenopausal women found that women who took in less than half of the recommended dietary allowance (RDA) of 320 milligrams per day had a significant increase in supraventricular and supraventricular plus ventricular beats.

"Diet used in this study was both ordinary and adequate," say the researchers with the US Department of Agriculture in Grand Forks, North Dakota. "Its inadequacy in magnesium was shown by biochemical and physiologic changes."

They suggest the mean loss in depletion was slightly lower than the minimal daily loss assumed to be normal. "Intake of magnesium of less than one-half of the RDA changed cardiac rhythm.

"Daily dietary magnesium requirement of these middle-aged women exceeded 130 mg/d; 411 mg daily was sufficient. A criterion based on cardiac electrophysiology confirmed the correctness of the RDA.

"Whether extra dietary magnesium will decrease idiopathic ectopy is worth testing," investigators add.

Participants stayed in a metabolic unit for approximately six months and ate a diet of conventional foods with either less than one-half or more that the RDA for magnesium. Each diet was followed for 81 days. Assignment was random and double blind, in a crossover design.

The researchers measured magnesium concentrations by spectroscopy and ion-specific electrolyte analysis. They also used Holter electrocardiograms recorded over approximately 21 hours. "Magnesium concentrations in erythrocytes, serum and urine were significantly lower when dietary magnesium was lower," they report. "Holter monitors showed a significant increase in both supraventricular and supraventricular plus ventricular beats when the dietary magnesium concentration was low.

"Hypomagnesemia, hypocalcemia and hypokalemia were not found."

The investigators explain, "A magnesium-dependent ATPase controls sodium and potassium transport across cell membranes. Impairment of this pump as a result of a low intake of dietary magnesium may have altered membrane potential and myocardial irritability.

Magnesium is central to a variety of cellular mechanisms that control activity of muscle and nerve cells. Cardiac muscle seems to have been more sensitive to this intake than was skeletal muscle."

They point out some women may habitually eat a diet similar to the one in this study and for long periods of time.
American Journal of Clinical Nutrition, 2002; 75: 550-554.

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