Ask the Doctors

Dear Doctor: Much is said about calcium and vitamin D, but what about magnesium? As I understand it, that's a crucial part of the bone-building equation. Should people be taking magnesium supplements?

Dear Reader: The element magnesium is necessary for bone health, yes, but also for normal heart function and nerve conduction and to maintain a proper calcium balance in the body. Notably, although it's found in leafy vegetables, nuts, fish and shellfish, soy and cereals, magnesium levels in our foods appear to be slowly decreasing. One reason may be the overutilization of fields, leading to depletion of magnesium in the soil. A survey conducted in 1999 and 2000 found that 79 percent of American adults do not meet the recommended daily allowance for magnesium.

Magnesium, which is stored in the bone, is very similar in atomic structure to calcium, and deficiencies can lower the body's calcium levels. This happens in several ways. First, low levels of magnesium impair the release of parathyroid hormone (PTH), which raises calcium levels when they fall too low. Second, low magnesium levels actually block the function of PTH. And third, low levels decrease the formation of the active form of vitamin D, also needed for healthy bones. Further, low magnesium causes an inflammatory process in the bone, which leads to bone breakdown.

Animal studies show that rats fed magnesium-deficient diets, even in the short term, have more brittle and fragile bones than animals fed a normal diet. This happens because of decreased bone formation and increased bone breakdown. In humans, deficiencies can develop among people who don't absorb the element due to gastrointestinal disease, chemotherapy or alcoholism.

Accordingly, data from the well-known Framingham Heart Study found that people with higher dietary intake of magnesium had greater bone density. As for whether a magnesium supplement will help decrease the risk of osteoporosis, however, the data are definitely not conclusive.

Although a 2010 study found that postmenopausal women who took 1,830 milligrams of extra magnesium daily for 30 days had fewer markers for bone breakdown than those who didn't take extra magnesium, most studies assess the impact of only dietary magnesium.

A notable 2016 study combined data from 12 different studies (involving 109,564 people), assessing the risk of bone density and fracture in relation to magnesium intake. The authors found a slight increase in hip bone density among those with higher magnesium intake, but no increase in spinal bone density.

If you have a good dietary intake of magnesium and if you don't drink heavily or have intestinal or kidney disease, you're unlikely to benefit from taking a magnesium supplement. The best thing you can do for your bones is to eat a diet high in magnesium and calcium; get outside so your body can produce vitamin D; and exercise.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

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