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Magnesium

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, ? cup 75 20
Spinach, frozen, cooked, ? 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, ? cup 45 10
Yogurt, plain, skim milk, 8 fluid ounces 45 10
Bran Flakes, ? cup 40 10
Vegetarian Baked Beans, ? cup 40 10
Rice, brown, long-grained, cooked, ? cup 40 10
Lentils, mature seeds, cooked, ? cup 35 8
Avocado, California, ? cup pureed 35 8
Kidney Beans, canned, ? cup 35 8
Pinto Beans, cooked, ? 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, ? 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 people. 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 get recommended amounts of magnesium in their diets. Among adult men and women, the diets of Caucasians have significantly more magnesium than do those of African-Americans. Magnesium intake is lower among older adults in every racial and ethnic group. Among African-American men and Caucasian men and women who take dietary supplements, the intake of magnesium is significantly higher than in those who do not [6].

When can magnesium deficiency occur?

Even though dietary surveys suggest that many Americans do not get recommended amounts of magnesium, symptoms of magnesium deficiency are rarely seen in the US. However, there is concern that many people may not have enough body stores of magnesium because dietary intake may not be high enough. Having enough body stores of magnesium 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 make up 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 (sudden changes in behaviors caused by excessive electrical activity in the brain), 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 get 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 take 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 (the outer layer of a tablet or capsule that allows it to pass through the stomach and be dissolved in the small intestine) 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 restore deficient levels of magnesium.

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