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Magnesium: What is it?
Magnesium is the fourth most abundant mineral in
the body and is essential to good health.
Approximately 50% of total body magnesium is found
in bone. The other half is found predominantly
inside cells of body tissues and organs. Only 1% of
magnesium is found in blood, but the body works very
hard to keep blood levels of magnesium constant [1].
Magnesium is needed for more than 300 biochemical
reactions in the body. It helps maintain normal
muscle and nerve function, keeps heart rhythm
steady, supports a healthy immune system, and keeps
bones strong. Magnesium also helps regulate blood
sugar levels, promotes normal blood pressure, and is
known to be involved in energy metabolism and
protein synthesis [2-3].
There is an increased interest in the role of
magnesium in preventing and managing disorders such
as hypertension, cardiovascular disease, and
diabetes. Dietary magnesium is absorbed in the small
intestines. Magnesium is excreted through the
kidneys [1-3,4].
What foods provide magnesium?
Green vegetables such as spinach are good
sources of magnesium because the center of the
chlorophyll molecule (which gives green vegetables
their color) contains magnesium. Some legumes (beans
and peas), nuts and seeds, and whole, unrefined
grains are also good sources of magnesium [5].
Refined grains are generally low in magnesium [4-5].
When white flour is refined and processed, the
magnesium-rich germ and bran are removed. Bread made
from whole grain wheat flour provides more magnesium
than bread made from white refined flour. Tap water
can be a source of magnesium, but the amount varies
according to the water supply. Water that naturally
contains more minerals is described as "hard".
"Hard" water contains more magnesium than "soft"
water.
Eating a wide variety of legumes, nuts, whole
grains, and vegetables will help you meet your daily
dietary need for magnesium. Selected food sources of
magnesium are listed in Table 1.
Table 1: Selected food sources of magnesium
[5]
| FOOD |
Milligrams (mg) |
%DV* |
| Halibut, cooked, 3 ounces |
90 |
20 |
| Almonds, dry roasted, 1 ounce |
80 |
20 |
| Cashews, dry roasted, 1 ounce |
75 |
20 |
| Soybeans, mature, cooked, ½ cup |
75 |
20 |
| Spinach, frozen, cooked, ½ cup |
75 |
20 |
| Nuts, mixed, dry roasted, 1 ounce |
65 |
15 |
| Cereal, shredded wheat, 2 rectangular
biscuits |
55 |
15 |
| Oatmeal, instant, fortified, prepared w/
water, 1 cup |
55 |
15 |
| Potato, baked w/ skin, 1 medium |
50 |
15 |
| Peanuts, dry roasted, 1 ounce |
50 |
15 |
| Peanut butter, smooth, 2 Tablespoons |
50 |
15 |
| Wheat Bran, crude, 2 Tablespoons |
45 |
10 |
| Blackeyed Peas, cooked, ½ cup |
45 |
10 |
| Yogurt, plain, skim milk, 8 fluid ounces |
45 |
10 |
| Bran Flakes, ¾ cup |
40 |
10 |
| Vegetarian Baked Beans, ½ cup |
40 |
10 |
| Rice, brown, long-grained, cooked, ½ cup |
40 |
10 |
| Lentils, mature seeds, cooked, ½ cup
|
35 |
8 |
| Avocado, California, ½ cup pureed |
35 |
8 |
| Kidney Beans, canned, ½ cup |
35 |
8 |
| Pinto Beans, cooked, ½ cup |
35 |
8 |
| Wheat Germ, crude, 2 Tablespoons |
35 |
8 |
| Chocolate milk, 1 cup |
33 |
8 |
| Banana, raw, 1 medium |
30 |
8 |
| Milk Chocolate candy bar, 1.5 ounce bar |
28 |
8 |
| Milk, reduced fat (2%) or fat free, 1
cup |
27 |
8 |
| Bread, whole wheat, commercially
prepared, 1 slice |
25 |
6 |
| Raisins, seedless, ¼ cup packed |
25 |
6 |
| Whole Milk, 1 cup |
24 |
6 |
| Chocolate Pudding, 4 ounce ready-to-eat
portion |
24 |
6 |
*DV = Daily Value. DVs are reference numbers
developed by the Food and Drug Administration (FDA)
to help consumers determine if a food contains a lot
or a little of a specific nutrient. The DV for
magnesium is 400 milligrams (mg). Most food labels
do not list a food's magnesium content. The percent
DV (%DV) listed on the table above indicates the
percentage of the DV provided in one serving. A food
providing 5% of the DV or less per serving is a low
source while a food that provides 10-19% of the DV
is a good source. A food that provides 20% or more
of the DV is high in that nutrient. It is important
to remember that foods that provide lower
percentages of the DV also contribute to a healthful
diet. For foods not listed in this table, please
refer to the U.S. Department of Agriculture's
Nutrient Database Web site:
http://www.nal.usda.gov/fnic/cgi-bin/nut_search.pl.
What are the Dietary Reference
Intakes for magnesium?
Recommendations for magnesium are provided in
the Dietary Reference Intakes (DRIs) developed by
the Institute of Medicine of the National Academy of
Sciences [4].
Dietary Reference Intakes is the general
term for a set of reference values used for planning
and assessing nutrient intake for healthy people.
Three important types of reference values included
in the DRIs are Recommended Dietary Allowances
(RDA), Adequate Intakes (AI), and
Tolerable Upper Intake Levels (UL). The RDA
recommends the average daily intake that is
sufficient to meet the nutrient requirements of
nearly all (97-98%) healthy individuals in each age
and gender group. An AI is set when there is
insufficient scientific data available to establish
a RDA for specific age/gender groups. AIs meet or
exceed the amount needed to maintain a nutritional
state of adequacy in nearly all members of a
specific age and gender group. The UL, on the other
hand, is the maximum daily intake unlikely
to result in adverse health effects. Table 2 lists
the RDAs for magnesium, in milligrams, for children
and adults [4].
Table 2: Recommended Dietary Allowances for
magnesium for children and adults [4]
Age
(years) |
Male
(mg/day) |
Female
(mg/day) |
Pregnancy
(mg/day) |
Lactation
(mg/day) |
| 1-3 |
80 |
80 |
N/A |
N/A |
| 4-8 |
130 |
130 |
N/A |
N/A |
| 9-13 |
240 |
240 |
N/A |
N/A |
| 14-18 |
410 |
360 |
400 |
360 |
| 19-30 |
400 |
310 |
350 |
310 |
| 31+ |
420 |
320 |
360 |
320 |
There is insufficient information on magnesium to
establish a RDA for infants. For infants 0 to 12
months, the DRI is in the form of an Adequate Intake
(AI), which is the mean intake of magnesium in
healthy, breastfed infants. Table 3 lists the AIs
for infants in milligrams (mg) [4].
Table 3: Recommended Adequate Intake for
magnesium for infants [4]
Age
(months) |
Males and Females
(mg/day) |
| 0 to 6 |
30 |
| 7 to 12 |
75 |
Data from the 1999-2000 National Health and
Nutrition Examination Survey suggest that
substantial numbers of adults in the United States
(US) fail to consume recommended amounts of
magnesium. Among adult men and women, Caucasians
consume significantly more magnesium than
African-Americans. Magnesium intake is lower among
older adults in every racial and ethnic group.
African-American men and Caucasian men and women who
take dietary supplements consume significantly more
magnesium than those who do not [6].
When can magnesium deficiency
occur?
Even though dietary surveys suggest that many
Americans do not consume recommended amounts of
magnesium, symptoms of magnesium deficiency are
rarely seen in the US. However, there is concern
about the prevalence of sub-optimal magnesium stores
in the body. For many people, dietary intake may not
be high enough to promote an optimal magnesium
status, which may be protective against disorders
such as cardiovascular disease and immune
dysfunction [7-8].
The health status of the digestive system and the
kidneys significantly influence magnesium status.
Magnesium is absorbed in the intestines and then
transported through the blood to cells and tissues.
Approximately one-third to one-half of dietary
magnesium is absorbed into the body [9-10].
Gastrointestinal disorders that impair absorption
such as Crohn's disease can limit the body's ability
to absorb magnesium. These disorders can deplete the
body's stores of magnesium and in extreme cases may
result in magnesium deficiency. Chronic or excessive
vomiting and diarrhea may also result in magnesium
depletion [1,10].
Healthy kidneys are able to limit urinary excretion
of magnesium to compensate for low dietary intake.
However, excessive loss of magnesium in urine can be
a side effect of some medications and can also occur
in cases of poorly-controlled diabetes and alcohol
abuse [11-18].
Early signs of magnesium deficiency
include loss of appetite, nausea, vomiting, fatigue,
and weakness. As magnesium deficiency worsens,
numbness, tingling, muscle contractions and cramps,
seizures, personality changes, abnormal heart
rhythms, and coronary spasms can occur [1,3-4].
Severe magnesium deficiency can result in low levels
of calcium in the blood (hypocalcemia). Magnesium
deficiency is also associated with low levels of
potassium in the blood (hypokalemia) [1,19-20].
Many of these symptoms are general and can result
from a variety of medical conditions other than
magnesium deficiency. It is important to have a
physician evaluate health complaints and problems so
that appropriate care can be given.
Who may need extra magnesium?
Magnesium supplementation may be indicated when
a specific health problem or condition causes an
excessive loss of magnesium or limits magnesium
absorption [2,7,9-11].
- Some medicines may result in magnesium
deficiency, including certain diuretics,
antibiotics, and medications used to treat
cancer (anti-neoplastic medication) [12,14,19].
Examples of these medications are:
- Diuretics: Lasix, Bumex, Edecrin, and
hydrochlorothiazide
- Antibiotics: Gentamicin, and
Amphotericin
- Anti-neoplastic medication: Cisplatin
- Individuals with poorly-controlled diabetes
may benefit from magnesium supplements because
of increased magnesium loss in urine associated
with hyperglycemia [21].
- Magnesium supplementation may be indicated
for persons with alcoholism. Low blood levels of
magnesium occur in 30% to 60% of alcoholics, and
in nearly 90% of patients experiencing alcohol
withdrawal [17-18].
Anyone who substitutes alcohol for food will
usually have significantly lower magnesium
intakes.
- Individuals with chronic malabsorptive
problems such as Crohn's disease, gluten
sensitive enteropathy, regional enteritis, and
intestinal surgery may lose magnesium through
diarrhea and fat malabsorption [22].
Individuals with these conditions may need
supplemental magnesium.
- Individuals with chronically low blood
levels of potassium and calcium may have an
underlying problem with magnesium deficiency.
Magnesium supplements may help correct the
potassium and calcium deficiencies [19].
- Older adults are at increased risk for
magnesium deficiency. The 1999-2000 and 1998-94
National Health and Nutrition Examination
Surveys suggest that older adults have lower
dietary intakes of magnesium than younger adults
[6,23].
In addition, magnesium absorption decreases and
renal excretion of magnesium increases in older
adults [4].
Seniors are also more likely to be taking drugs
that interact with magnesium. This combination
of factors places older adults at risk for
magnesium deficiency [4].
It is very important for older adults to consume
recommended amounts of dietary magnesium.
Doctors can evaluate magnesium status when
above-mentioned medical problems occur, and
determine the need for magnesium supplementation.
Table 4 describes some important interactions
between certain drugs and magnesium. These
interactions may result in higher or lower levels of
magnesium, or may influence absorption of the
medication.
Table 4: Common and important magnesium/drug
interactions
| Drug |
Potential Interaction |
|
Loop and thiazide diuretics (e.g. lasix,
bumex, edecrin, and hydrochlorthiazide
Anti-neoplastic drugs (e.g. cisplatin)
Antibiotics (e.g. gentamicin and
amphotericin)
|
These drugs may increase
the loss of magnesium in urine. Thus, taking
these medications for long periods of time
may contribute to magnesium depletion [9-10,12].
|
|
Tetracycline antibiotics
|
Magnesium binds
tetracycline in the gut and decreases the
absorption of tetracycline [24].
|
|
Magnesium-containing antacids and
laxatives
|
Many antacids and laxatives contain
magnesium. When frequently taken in large
doses, these drugs can inadvertently lead to
excessive magnesium consumption [25-26]
and hypermagnesemia, which refers to
elevated levels of magnesium in blood.
|
What is the best way to get
extra magnesium?
Eating a variety of whole grains, legumes, and
vegetables (especially dark-green, leafy vegetables)
every day will help provide recommended intakes of
magnesium and maintain normal storage levels of this
mineral. Increasing dietary intake of magnesium can
often restore mildly depleted magnesium levels.
However, increasing dietary intake of magnesium may
not be enough to restore very low magnesium levels
to normal.
When blood levels of magnesium are very low,
intravenous (i.e. by IV) magnesium replacement is
usually recommended. Magnesium tablets also may be
prescribed, although some forms can cause diarrhea [27].
It is important to have the cause, severity, and
consequences of low blood levels of magnesium
evaluated by a physician, who can recommend the best
way to restore magnesium levels to normal. Because
people with kidney disease may not be able to
excrete excess amounts of magnesium, they should not
consume magnesium supplements unless prescribed by a
physician.
Oral magnesium supplements combine magnesium with
another substance such as a salt. Examples of
magnesium supplements include magnesium oxide,
magnesium sulfate, and magnesium carbonate.
Elemental magnesium refers to the amount of
magnesium in each compound. Figure 1 compares the
amount of elemental magnesium in different types of
magnesium supplements [28].
The amount of elemental magnesium in a
compound and its bioavailability influence the
effectiveness of the magnesium supplement.
Bioavailability refers to the amount of magnesium in
food, medications, and supplements that is absorbed
in the intestines and ultimately available for
biological activity in your cells and tissues.
Enteric coating of a magnesium compound can decrease
bioavailability [29].
In a study that compared four forms of magnesium
preparations, results suggested lower
bioavailability of magnesium oxide, with
significantly higher and equal absorption and
bioavailability of magnesium chloride and magnesium
lactate [30].
This supports the belief that both the magnesium
content of a dietary supplement and its
bioavailability contribute to its ability to replete
deficient levels of magnesium.
The information in Figure 1 is provided to
demonstrate the variable amount of magnesium in
magnesium supplements.
What are some current issues
and controversies about magnesium?
Magnesium and blood pressure
"Epidemiologic evidence suggests that magnesium may
play an important role in regulating blood pressure
[4]."
Diets that provide plenty of fruits and vegetables,
which are good sources of potassium and magnesium,
are consistently associated with lower blood
pressure [31-33].
The DASH study (Dietary Approaches to Stop
Hypertension), a human clinical trial, suggested
that high blood pressure could be significantly
lowered by a diet that emphasizes fruits,
vegetables, and low fat dairy foods. Such a diet
will be high in magnesium, potassium, and calcium,
and low in sodium and fat [34-36].
An observational study examined the effect of
various nutritional factors on incidence of high
blood pressure in over 30,000 US male health
professionals. After four years of follow-up, it was
found that a lower risk of hypertension was
associated with dietary patterns that provided more
magnesium, potassium, and dietary fiber [37].
For 6 years, the Atherosclerosis Risk in Communities
(ARIC) Study followed approximately 8,000 men and
women who were initially free of hypertension. In
this study, the risk of developing hypertension
decreased as dietary magnesium intake increased in
women, but not in men [38].
Foods high in magnesium are frequently high in
potassium and dietary fiber. This makes it difficult
to evaluate the independent effect of magnesium on
blood pressure. However, newer scientific evidence
from DASH clinical trials is strong enough that the
Joint National Committee on Prevention, Detection,
Evaluation, and Treatment of High Blood Pressure
states that diets that provide plenty of magnesium
are positive lifestyle modifications for individuals
with hypertension. This group recommends the DASH
diet as a beneficial eating plan for people with
hypertension and for those with "prehypertension"
who desire to prevent high blood pressure
http://www.nhlbi.nih.gov/health/public/heart/hbp/dash/
[39-41].
Magnesium and diabetes
Diabetes is a disease resulting in insufficient
production and/or inefficient use of insulin.
Insulin is a hormone made by the pancreas. Insulin
helps convert sugar and starches in food into energy
to sustain life. There are two types of diabetes:
type 1 and type 2. Type 1 diabetes is most often
diagnosed in children and adolescents, and results
from the body's inability to make insulin. Type 2
diabetes, which is sometimes referred to as
adult-onset diabetes, is the most common form of
diabetes. It is usually seen in adults and is most
often associated with an inability to use the
insulin made by the pancreas. Obesity is a risk
factor for developing type 2 diabetes. In recent
years, rates of type 2 diabetes have increased along
with the rising rates of obesity.
Magnesium plays an important role in carbohydrate
metabolism. It may influence the release and
activity of insulin, the hormone that helps control
blood glucose (sugar) levels [13].
Low blood levels of magnesium (hypomagnesemia) are
frequently seen in individuals with type 2 diabetes.
Hypomagnesemia may worsen insulin resistance, a
condition that often precedes diabetes, or may be a
consequence of insulin resistance. Individuals with
insulin resistance do not use insulin efficiently
and require greater amounts of insulin to maintain
blood sugar within normal levels. The kidneys
possibly lose their ability to retain magnesium
during periods of severe hyperglycemia
(significantly elevated blood glucose). The
increased loss of magnesium in urine may then result
in lower blood levels of magnesium [4].
In older adults, correcting magnesium depletion may
improve insulin response and action [42].
The Nurses' Health Study (NHS) and the Health
Professionals' Follow-up Study (HFS) follow more
than 170,000 health professionals through biennial
questionnaires. Diet was first evaluated in 1980 in
the NHS and in 1986 in the HFS, and dietary
assessments have been completed every 2 to 4 years
since. Information on the use of dietary
supplements, including multivitamins, is also
collected. As part of these studies, over 127,000
research subjects (85,060 women and 42,872 men) with
no history of diabetes, cardiovascular disease, or
cancer at baseline were followed to examine risk
factors for developing type 2 diabetes. Women were
followed for 18 years; men were followed for 12
years. Over time, the risk for developing type 2
diabetes was greater in men and women with a lower
magnesium intake. This study supports the dietary
recommendation to increase consumption of major food
sources of magnesium, such as whole grains, nuts,
and green leafy vegetables [43].
The Iowa Women's Health Study has followed a group
of older women since 1986. Researchers from this
study examined the association between women's risk
of developing type 2 diabetes and intake of
carbohydrates, dietary fiber, and dietary magnesium.
Dietary intake was estimated by a food frequency
questionnaire, and incidence of diabetes throughout
6 years of follow-up was determined by asking
participants if they had been diagnosed by a doctor
as having diabetes. Based on baseline dietary intake
assessment only, researchers' findings suggested
that a greater intake of whole grains, dietary
fiber, and magnesium decreased the risk of
developing diabetes in older women [44].
The Women's Health Study was originally designed to
evaluate the benefits versus risks of low-dose
aspirin and vitamin E supplementation in the primary
prevention of cardiovascular disease and cancer in
women 45 years of age and older. In an examination
of almost 40,000 women participating in this study,
researchers also examined the association between
magnesium intake and incidence of type 2 diabetes
over an average of 6 years. Among women who were
overweight, the risk of developing type 2 diabetes
was significantly greater among those with lower
magnesium intake [45].
This study also supports the dietary recommendation
to increase consumption of major food sources of
magnesium, such as whole grains, nuts, and green
leafy vegetables.
On the other hand, the Atherosclerosis Risk in
Communities (ARIC) study did not find any
association between dietary magnesium intake and the
risk for type 2 diabetes. During 6 years of
follow-up, ARIC researchers examined the risk for
type 2 diabetes in over 12,000 middle-aged adults
without diabetes at baseline examination. In this
study, there was no statistical association between
dietary magnesium intake and incidence of type 2
diabetes in either black or white research subjects
[46].
It can be confusing to read about studies that
examine the same issue but have different results.
Before reaching a conclusion on a health issue,
scientists conduct and evaluate many studies. Over
time, they determine when results are consistent
enough to suggest a conclusion. They want to be sure
they are providing correct recommendations to the
public.
Several clinical studies have examined the potential
benefit of supplemental magnesium on metabolic
control of type 2 diabetes. In one such study, 63
subjects with below normal serum magnesium levels
received either 2.5 grams of oral magnesium chloride
daily "in liquid form" (providing 300 mg elemental
magnesium per day) or a placebo. At the end of the
16-week study period, those who received the
magnesium supplement had higher blood levels of
magnesium and improved metabolic control of
diabetes, as suggested by lower Hemoglobin A1C
levels, than those who received a placebo [47].
Hemoglobin A1C is a test that measures overall
control of blood glucose over the previous 2 to 3
months, and is considered by many doctors to be the
single most important blood test for diabetics.
In another study, 128 patients with poorly
controlled type 2 diabetes were randomized to
receive a placebo or a supplement with either 500 mg
or 1000 mg of magnesium oxide (MgO) for 30 days. All
patients were also treated with diet or diet plus
oral medication to control blood glucose levels.
Magnesium levels increased in the group receiving
1000 mg magnesium oxide per day (equal to 600 mg
elemental magnesium per day) but did not
significantly change in the placebo group or the
group receiving 500 mg of magnesium oxide per day
(equal to 300 mg elemental magnesium per day).
However, neither level of magnesium supplementation
significantly improved blood glucose control [48].
These studies provide intriguing results but also
suggest that additional research is needed to better
explain the association between blood magnesium
levels, dietary magnesium intake, and type 2
diabetes. In 1999, the American Diabetes Association
(ADA) issued nutrition recommendations for diabetics
stating that "…routine evaluation of blood magnesium
level is recommended only in patients at high risk
for magnesium deficiency. Levels of magnesium should
be repleted (replaced) only if hypomagnesemia can be
demonstrated" [21].
Magnesium and cardiovascular disease
Magnesium metabolism is very important to insulin
sensitivity and blood pressure regulation, and
magnesium deficiency is common in individuals with
diabetes. The observed associations between
magnesium metabolism, diabetes, and high blood
pressure increase the likelihood that magnesium
metabolism may influence cardiovascular disease [49].
Some observational surveys have associated higher
blood levels of magnesium with lower risk of
coronary heart disease [50-51].
In addition, some dietary surveys have suggested
that a higher magnesium intake may reduce the risk
of having a stroke [52].
There is also evidence that low body stores of
magnesium increase the risk of abnormal heart
rhythms, which may increase the risk of
complications after a heart attack [4].
These studies suggest that consuming recommended
amounts of magnesium may be beneficial to the
cardiovascular system. They have also prompted
interest in clinical trials to determine the effect
of magnesium supplements on cardiovascular disease.
Several small studies suggest that magnesium
supplementation may improve clinical outcomes in
individuals with coronary disease. In one of these
studies, the effect of magnesium supplementation on
exercise tolerance, exercise-induced chest pain, and
quality of life was examined in 187 patients.
Patients received either a placebo or a supplement
providing 365 milligrams of magnesium citrate twice
daily for 6 months. At the end of the study period
researchers found that magnesium therapy
significantly increased magnesium levels. Patients
receiving magnesium had a 14 percent improvement in
exercise duration as compared to no change in the
placebo group. Those receiving magnesium were also
less likely to experience exercise-induced chest
pain [53].
In another study, 50 men and women with stable
coronary disease were randomized to receive either a
placebo or a magnesium supplement that provided 342
mg magnesium oxide twice daily. After 6 months,
those who received the oral magnesium supplement
were found to have improved exercise tolerance [54].
In a third study, researchers exami | | |
