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.
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.
* 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.
RELATED ARTICLE: GETTING ENOUGH?
Last year, the National Academy of Sciences announced new recommended intakes for magnesium. Women need 310 mg a day (ages 19 through 30) or 320 mg (over 30). Men need 400 mg (19 through 30) or 420 mg (over 30).
How much magnesium are you getting? And is it enough to keep you from running short?
"Unfortunately, there isn't a reliable test of magnesium deficiency that's widely available," says the University of Southern California's Robert Rude. "About all a primary care physician can do is measure the level of magnesium in the blood. But that doesn't tell you if the level of magnesium is adequate within the cells, which is where it's critically important."
"I think that magnesium should be obtained from the diet, if possible," says Jerry Nadler of the City of Hope Medical Center in Duarte, California. "That's because many magnesium-rich foods are the healthier foods to eat." But if someone can't get enough from food (see "Magnesium Counts," p. II), "then taking a magnesium supplement clearly is better than nothing."
Multivitamin and mineral supplements seldom contain a day's recommended dose of magnesium, simply because the 300 or 400 mg necessary won't fit into a pill small enough for most people to swallow. So if you want more than 25 percent or so of a day's supply from a supplement, you'll probably need to buy it separately.
Don't worry about how the magnesium is bundled--whether it comes in an oxide, chloride, or any other form. "There's no convincing evidence that one is better-absorbed than another," says Connie Weaver of Purdue University. And don't be afraid to get your calcium and your magnesium in a single supplement. "Calcium doesn't interfere with magnesium absorption, as some people believe," says Weaver.
But not all magnesium supplements are equal. Dolomite (a naturally occurring calcium-magnesium combination) is more likely to contain lead than other kinds of magnesium.
Can you get too much magnesium? "Taking too much from magnesium-confining antacids or drugs causes diarrhea," says magnesium expert Mildred Seelig. "So most people find out quickly when they've exceeded a safe dose."
No cases of magnesium toxicity from food have ever been reported, says the National Academy of Sciences. As for supplements, the NAS recommends a ceiling of 350 mg a day as a Tolerable Upper Intake Level (UL).
RELATED ARTICLE: MAGNESIUM COUNTS
Food Magnesium (mg) All-Bran (1/2 cup) 120 Black or navy beans (1 cup cooked) 105-120 Great Northern or pinto beans (1 cup cooked) 90-95 Brown rice (1 cup cooked) 85 Almonds or cashews (3 Tbs.) 75-85 Garbanzos (chickpeas) or lima beans (1 cup cooked) 80 Kellogg's Raisin Bran (1 cup) 80 Spinach (1/2 cup cooked) 75 Lentils (1 cup cooked) 70 Grape-Nuts (1/2 cup) 60 Multi-Bran Chex or Wheat Chex (1 cup) 60 Baked potato, with skin (1-7 oz.) 55 Seafood (4 oz. cooked) 35-55 Peanuts (1/5 cup) or peanut butter (2 Tbs.) 50 Whole wheat bread (2 slices--2 oz.) 50 Acorn squash (1/2 cup cooked) 45 Yogurt (1 cup) 35-45 Banana (1) 35 Peas (1/2 cup cooked) 35 Watermelon (2 cups) 35 Chocolate, semisweet or sweet (1 oz.) 30-35 Milk (1 cup) 30-35 Grapefruit, orange, or prune juice (1 cup) 25-35 Beef, pork, or poultry (4 oz. cooked) 20-35 Butternut squash (1/2 cup cooked) 30 Cheerios or Wheaties (1 cup) 30 Broccoli (1/2 cup cooked) 20 White rice (1 cup cooked) 20 White bread (2 slices--2 oz.) 15 Kellogg's Corn Flakes (1 cup) or Frosted Flakes (3/4 cup) 0
Chart compiled by Ingrid VanTuinen.
Sources: USDA and manufacturers.
RELATED ARTICLE: THE BOTTOM LINE
* The average American gets too little magnesium from food. The richest sources are legumes, nuts, whole grains and whole-grain breads and cereals, and some vegetables.
* Too little magnesium in the diet could increase the risk of diabetes, high blood pressure, heart disease, and stroke, but the evidence isn't conclusive.
* With the possible exception of treating migraines, there's no good evidence that getting more than the recommended levels of magnesium (see "Getting Enough?")--from food or supplements--provides any additional benefits.