Harvard Heart Letter, August 1991, pp 6-7
Magnesium: Coming of Age
"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."
Tip of the iceberg
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.
Who loses magnesium?
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.
Replacement strategies
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.
Here to stay
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
http://www.mgwater.com/
Healthy Water
Association--USA