Harvard Heart Letter, August 1991, pp 6-7
"My magnesium is low? I never even heard of magnesium before!" The surprise expressed by this middle-aged man with hypertension is shared by many who have recently been told that their blood levels of magnesium are below normal - a condition known as hypomagnesemia. This revelation can be especially unsettling to people taking diuretics (water pills), who have been warned about the dangers of allowing their blood potassium levels to get too low. Accustomed to having their potassium measured often, these patients face a new worry. If magnesium levels really are significant, why has no one been concerned about it before now?
Magnesium is important, but just how important for people with and without cardiovascular problems (including high-performance athletes) has only recently been recognized. Like potassium, magnesium is found in only very low levels in the blood, the normal range being 1.7-2.4 mEq/liter (as opposed to a whopping average of 140 mEq/liter for sodium). But most of the magnesium is located inside the body's cells, where it is the second most abundant positively charged particle (next to potassium). Within the cells magnesium plays a critical role in the function of various cellular enzymes.
If magnesium has been overlooked in the past, it may be because it has been so difficult to measure. Now that reliable magnesium assays are widely available, most community hospitals can routinely order this blood test. Hypomagnesemia is now known to be a common problem, with consequences ranging from impaired athletic performance to potentially dangerous heart rhythm abnormalities. Many physicians believe that magnesium will be tested more frequently in the years ahead and that replacement therapy will become commonplace. According to findings presented at a recent symposium, magnesium is "coming of age."
Why does a mineral that is nearly undetectable in the blood merit such attention? On average, the body contains only 24 grams of magnesium (about one ounce); 99% is within the cells, particularly those of the brain, heart, and skeletal muscle. When magnesium levels are low, problems can arise.
Studies have shown that people who consume greater amounts of magnesium from hard water or in the diet are less prone to cardiovascular disease and sudden death than are those with a lower magnesium intake. Although the reason for this remains unclear, some data suggest that low blood levels of magnesium promote atherosclerosis. Hypomagnesemia may also raise blood pressure or lead to arrhythmias, which may go unnoticed or may be limited to short bursts of extra beats that cause palpitations. In some cases, however, magnesium deficiency can cause sustained episodes of rapid beats originating in the atria or ventricles. Although occasionally life-threatening, these rhythm disorders can often be controlled or prevented with magnesium replacement therapy. Some studies have noted improved survival after a heart attack in patients given magnesium supplements.
Magnesium affects other muscle tissues as well, thus attracting the interest of physicians other than cardiologists. Obstetricians have found that the developing placenta and fetus drain maternal magnesium stores, and low levels may contribute to the cardiovascular problems seen during pregnancy. In Europe, magnesium supplementation is becoming a routine part of obstetric care, while in the U.S. this mineral has been added to vitamin preparations prescribed for pregnant women.
Not everyone who is taking magnesium supplements is doing so on a physician's instructions. Hoping to enhance their performance, some top athletes - particularly "power" athletes such as runners and rowers - take magnesium pills. Although magnesium is not comparable to anabolic steroids in this regard, results of studies conducted in Germany involving bodybuilders and long-distance runners support this hypothesis in addition to revealing another benefit of such therapy: fewer muscle cramps.
For people who are not high-performance athletes, magnesium supplements are likely to be beneficial only if the body's stores of this mineral have been depleted. Because most magnesium is contained in the cells and not in the bloodstream, blood magnesium measurements - even those obtained using the newer, more reliable assays - can be misleading. Persons with a low level of total-body magnesium can still have normal blood levels. Unfortunately, the ideal test - tissue biopsies to assess intracellular magnesium content directly - is not practical. For now, physicians must rely on the blood test and on their ability to identify individuals likely to lose magnesium.
In patients with cardiovascular problems such as hypertension or heart failure, the most common cause of magnesium loss is treatment with diuretics such as hydrochlorothiazide and furosemide. Diuretics increase the amount of urine produced by the kidneys. But that urine consists of more than water. During the last decade, several studies have associated the loss of potassium due to diuretic therapy with potentially life-threatening cardiac arrhythmias, and physicians began to check potassium levels much more closely in patients taking these drugs. Now, in the same patient populations, similar precautions are emerging with regard to magnesium.
Because magnesium has not been routinely measured in the past, a deficiency of this mineral has usually gone unrecognized. Both low magnesium and low potassium can cause arrhythmias in some hypertensive persons who take thiazide diuretics. Failure to detect these heart-rhythm disturbances could offset the benefit of antihypertensive therapies. In contrast, in a European study in which patients being treated with thiazide diuretics were given a second drug, triamterene, to prevent magnesium and potassium loss, survival was 15% better in the treated group than in the comparison group given a placebo.
Elderly persons are at the greatest risk for hypomagnesemia. Since hypertension and other cardiovascular disorders become more common with aging, many older persons take diuretics. Their diet tends to be low in magnesium, and they may also absorb it less effectively.
Another important cause of magnesium loss is a drug not prescribed by physicians - alcohol. Drinking alcohol increases urine production, which leads to excessive excretion of magnesium and probably contributes to the heart-rhythm abnormalities commonly encountered in heavy drinkers (see "Alcohol and The Heart" in the December 1990 Harvard Heart Letter). Magnesium depletion is worse in alcoholics, who do not generally eat foods rich in this mineral.
The average daily intake of magnesium is estimated to be 200-300 mg/day, about 100 mg of which is absorbed. For the body's total magnesium stores to remain constant, the amount taken up must match that lost in the urine. People with kidney disease may have trouble excreting this mineral, so magnesium from food and other sources may accumulate. However, if the kidneys are functioning normally, it is difficult to take in too much magnesium, since excess amounts are quickly eliminated.
Foods rich in magnesium (such as legumes, nuts, grains, dark-green leafy vegetables, cocoa, dried fruits, and shellfish) are not heavily represented in the usual American diet. Dietary surveys have indicated that the daily magnesium intake of most Americans falls below the recommended level of 6-8 mg/kg/day. Vegetarians and persons living in the Orient ingest much more magnesium than the average American does, which may in part explain the lower incidence of heart disease found in these groups.
Persons with low magnesium levels can be advised to eat magnesium-rich foods and reduce their alcohol consumption. But if such measures do not correct the problem, there are two options: One is to change medications - for example, switching from hydrochlorothiazide to a beta-adrenergic blocking agent (such as propranolol) in treating hypertension. But since alternatives to thiazide diuretics are usually more expensive and are associated with their own side effects, in this setting the doctor may choose to prescribe magnesium supplements instead.
Unfortunately, some of these supplements cause diarrhea. (Milk of magnesia and magnesium citrate are among the most widely used laxatives.) Although this problem has been reduced as newer preparations have become available, some persons still have difficulty taking the 60-300 mg/day usually recommended to correct magnesium depletion. When severe hypomagnesemia causes potentially serious complications, magnesium can be administered intravenously.
As each month goes by, the research reported in medical and cardiology journals reflects a growing appreciation for magnesium's importance. With reliable assays and more effective oral replacement preparations, magnesium may soon join potassium as an everyday concern for persons with hypertension or heart failure. Quite possibly, magnesium will be measured routinely when screening blood tests are performed, and supplements will appear in the medicine cabinets of many individuals with cardiovascular disease.
This page was first uploaded to The Magnesium Web Site on June 12, 1996