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Ask Dr. K: Adequate magnesium comes from healthy eating

DEAR DOCTOR K: What does magnesium contribute to good health? Should I have my magnesium level checked?

DEAR READER: Magnesium is important for good health. You need adequate magnesium for bone health, and it’s essential for proper nerve, muscle, heart rhythm and immune function. It helps regulate blood sugar levels and promotes healthy blood pressure.

We get most of the magnesium we need in food. About half of the magnesium we absorb is stored in bone; the other half is stored in cells throughout the body. Excess magnesium is excreted through the kidneys.

If you eat whole-grain bread and your tap water is “hard” — meaning it contains relatively high levels of minerals — you probably consume more magnesium than a person who favors white bread and drinks “soft” water. Why? The refining process used to make white flour strips away the magnesium-rich germ and bran layer of the wheat. And hard water contains more magnesium than soft water.

Most people don’t have to worry about the level of magnesium in their blood. If you’re otherwise healthy, your magnesium level is probably normal and you don’t need to check it regularly. Magnesium deficiency is rare in this country. When it does occur, it can cause muscle weakness, cramping or cardiac arrhythmias.

Conditions that impair magnesium absorption include chronic vomiting, Crohn’s disease (an inflammatory condition of the intestine), celiac disease and gastric bypass surgery. In all of these conditions, magnesium is lost in what is vomited or passed in bowel movements. As a result, people with these conditions often have magnesium deficiency. If you suffer from any of them, talk to your doctor about having your magnesium level checked periodically.

Commonly used diuretic medicines also can cause the kidneys to eliminate magnesium in the urine, lowering the level of magnesium in the blood and throughout the body. The thiazide diuretics (such as hydrochlorothiazide) or “loop diuretics” (such as furosemide) are both culprits. People with diabetes whose blood sugar is not well controlled also lose magnesium in the urine. The same is true for people who abuse alcohol. Finally, there are a group of rare inherited diseases that cause an excessive loss of magnesium.

How much magnesium should you get? For women ages 19-30, 310 milligrams (mg) daily; ages 31 and older, 320 mg. For men ages 19-30, 400 mg; ages 31 and older, 420 mg.

Popular multivitamin brands contain between 10 percent to 30 percent of the recommended daily allowance (RDA) for magnesium. Studies have shown that magnesium supplements may modestly lower blood pressure, but don’t take high-dose supplements without a doctor’s guidance.

You can get magnesium from a variety of healthy foods, such as nuts, fish, certain fruits, many vegetables and whole grains. I’ve put a table listing good food sources of magnesium on my website, AskDoctorK.com.

Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.

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