Magnesium - Nutritional Aspects

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You can't take a breath, move a muscle, or think a thought if you don't have enough magnesium in your cells. But according to dietary surveys, many Americans may not be getting enough from their food. And research suggests that a shortage of magnesium may put you at increased risk of diabetes, high blood pressure, heart disease, osteoporosis, and maybe migraines.

Many Americans don't take in enough magnesium to replace what they lose every day. One reason: it's most plentiful in foods like green leafy vegetables, whole grains, beans, and nuts--not exactly staples in most households.

The National Academy of Sciences (NAS) recommends that women over 30 consume 320 mg of magnesium a day. Yet half of all women aged 31 to 70 get 230 mg or less. Half of all women over 70 get 205 mg or less.

(Those are probably underestimates, because people typically don't report everything they eat.)

The story's the same with men. Most should consume 420 mg a day. But half of all men aged 31 to 50 get less than 330 mg. Half of all men over 70 get less than 275 mg a day.

What's wrong with losing a little magnesium each day?

"Many of us in magnesium research feel that there are harmful consequences to getting less than the requirement--like diabetes, high blood pressure, osteoporosis, and atherosclerosis," says Robert Rude of the University of Southern California in Los Angeles. "But we haven't been able to prove any of that yet."

Rude served as the magnesium expert on the NAS panel that recommended higher magnesium intakes last year.

Others take a more skeptical view. "We don't know which is the cart and which is the horse," says Stephanie Atkinson of McMaster University in Hamilton, Canada, who chaired the panel.

"There is mounting evidence that people with a number of diseases have lower blood magnesium levels, as measured by the only readily available method. But we don't know if that is a result of the disease or a possible cause."

Here's some of that evidence.

Diabetes

Magnesium and insulin need each other. Without magnesium, your pancreas won't secrete enough insulin--or the insulin it secretes won't be efficient enough--to control your blood sugar. And without insulin, magnesium doesn't get transported from your blood into your cells, where it does most of its work.

When Jerry Nadler of the Gonda Diabetes Center at the City of Hope Medical Center in Duarte, California, and his colleagues placed 16 healthy people on magnesium-deficient diets, their insulin became less effective at getting sugar from their blood into their cells, where it's burned or stored as fuel.(1) In other words, they became less insulin sensitive. And that's a step on the road to diabetes.

Two large population studies also found that people who eat less magnesium have a higher risk of type 2, or adult-onset, diabetes. In a study of 65,000 nurses, those who consumed about 220 mg a day were about one-third more likely to develop diabetes over the next six years than those who consumed about 340 mg a day.(2) A study of 43,000 male health professionals turned up similar results.(3)

And last year, the Atherosclerosis Risk in Communities Study (ARIC) weighed in. It has been following about 14,000 middle-aged people for up to seven years.

"We have found that those men and women with the lowest levels of magnesium in their blood at the start of ARIC were twice as likely to be diagnosed later with diabetes as those with the highest levels of magnesium," says Frederick Brancati, an epidemiologist at the Johns Hopkins School of Medicine in Baltimore.(4)

What about people who already have diabetes?

"A high percentage of type 2 diabetics have a deficiency of magnesium inside their cells," says Nadler. But no one knows which came first--the diabetes or the magnesium deficiency.

And it's not clear whether magnesium supplements can help diabetics. Nine small studies have tested 360 mg to 390 mg a day for one to five months. Six found that it didn't lower blood sugar, while three found that it improved insulin sensitivity.

That's what you would expect, says Nadler. "Magnesium is not likely to change blood sugar very much because there are so many factors that influence glucose levels. But it may improve insulin sensitivity," he adds, and that would improve your long-term prospects of avoiding a heart attack or stroke.

High Blood Pressure

Magnesium helps signal muscles to contract and relax. And when the muscles that line the major blood vessels contract, your blood pressure rises.

When researchers studied the diets of 40,000 nurses and 30,000 male health professionals, they found lower blood pressures in people who ate more magnesium.(5,6)

And in the ARIC study, "women with lower levels of magnesium in their blood were more likely to develop high blood pressure over the next few years than women with higher levels," says Aaron Folsom of the University of Minnesota in Minneapolis. That was not true for men, though researchers don't know why.

What about people whose blood pressure is already high? Could magnesium supplements help them?

Since 1985, at least 16 trials have given magnesium to people with high or high-normal blood pressure. The two largest compared 360 mg a day to a (look-alike but magnesium-free) placebo in 285 people for six months.(7,8)

"Magnesium had no effect in either," says Frank Sacks of the Harvard School of Public Health in Boston, who headed one of the two studies.

Still, some smaller studies have found that magnesium lowered blood pressure modestly. In the latest, from Japan, 360 mg a day for eight weeks lowered blood pressure by an average of 3 points over 1 point in 60 men and women who started with high-normal blood pressures (they averaged 134/81).(9)

"Extra magnesium is not a definitive treatment for high blood pressure," says Marvin Moser, a hypertension expert at Yale University. "Eating magnesium-rich foods or taking magnesium supplements might result in some decrease in pressure. But the vast majority of people with high blood pressure will still need medication."

Heart Disease & Stroke

Since the 1960s, researchers have known that people who live in areas where the water is "hard" often have lower rates of heart disease and stroke. ("Hard" water contains more calcium, magnesium, and other minerals than "soft" water.)

But the amount of magnesium in hard water is typically no more than 3 mg to 20 mg per liter--less than ten percent of what people get from a day's food.

"It's hard to imagine how such a small amount could have such a big effect on preventing heart disease," says epidemiologist Raymond Neutra of the California Department of Health Services.

"Still, there's enough suggestive evidence out there that it needs to be seriously looked at."

In fact, there's some evidence that magnesium may be harmful. In what Neutra calls a "sobering" Dutch study from the early 1990s, more than 450 men and women with heart disease who were given 360 mg a day of magnesium for a year were more likely to suffer a second heart attack, need a bypass, or die a sudden death than heart disease victims who were given a placebo.(10)

"That could be due to chance," says Neutra. Another possibility, according to USC's Robert Rude: People with more severe heart disease may have inadvertently ended up in the group given magnesium.

Other Diseases

* Migraines. "Some people who suffer from migraine headaches may be deficient in magnesium," says Alexander Mauskop of the New York Headache Clinic.

In one of two small studies, 40 migraine patients who took 600 mg of magnesium a day for 12 weeks went from three attacks per month to two. Patients taking a placebo had no fewer attacks. There was no difference in the severity of the headaches, though.(11)

* Osteoporosis. "There are hints out there in human and animal research that magnesium is important for good quality bones," says Purdue University's Connie Weaver. "But the evidence that magnesium supplements can prevent osteoporosis in humans is pretty skimpy."

(1) Hypertension 21 (6 Pt 2): 1024, 1993.
(2) J. Amer. Med. Assoc. 277: 472, 1997.
(3) Diabetes Care 20(4): 545, 1997.
(4) Diabetes 46 (Suppl 1): 20A.
(5) Hypertension 27: 1065, 1996.
(6) Circulation 86: 1475, 1992.
(7) Annals of Epidemiology 5: 96, 1995.
(8) Hypertension 31 (Part 1): 131, 1998.
(9) Hypertension 32: 260, 1998.
(10) British Medical Journal 307: 585, 1993.
(11) Cephalalgia 16: 257, 1996.