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NUTRITIONAL VALUE OF MAGNESIUM

NUTRITIONAL VALUE OF MAGNESIUM
Physiological functions

Magnesium has roles in energy metabolism, muscle contraction, and nerve impulse transmission, and bone mineralization. It is a required cofactor for an estimated 300 enzymes. Among the reactions catalyzed by these enzymes are fatty acid synthesis, protein synthesis, and glucose metabolism. Magnesium status is important for regulation of calcium balance through parathyroid hormone-mediated reactions. Secretion of parathyroid hormone and end-organ responsiveness to the hormone are dependent on availability of the mineral.

As a positively charged divalent cation, magnesium acts as a calcium antagonist at the cell membrane level which is necessary to maintain normal electrical potentials and to coordinate muscle contraction-relaxation responses. In muscle, increased intracellular calcium concentration triggers contraction while increased intracellular magnesium concentration counteracts this effect resulting in relaxation. Magnesium is also involved in metabolism of ATP which enables release of energy stored in the high-energy phosphate ester bond. It is also a potent vasodilator. Magnesium and calcium coordinate the constriction/relaxation of capillary blood vessels which contributes to regulation of blood pressure.

Factors affecting availability

Milling of whole grains reduces the magnesium content of the processed flour to 20% of the amount initially present. High intakes of calcium can inhibit magnesium absorption.

Deficiency

Magnesium deficiencies are usually observed as secondary to other clinical conditions rather than due to inadequate dietary intake. Use of diuretics, alcohol abuse, renal disease, and acidosis can contribute to magnesium deficiency through increasing urinary losses. Symptoms of magnesium deficiency include tremors, muscular weakness, confusion, nervousness, hallucinations, and swallowing difficulties.

Toxicity

Exposure to magnesium from dietary sources is unlikely to result in toxicity. Individuals who take magnesium-containing laxatives may experience diarrhea without any other adverse effects.

Since magnesium balance is regulated by renal mechanisms, patients with renal disease are prone to toxicity. Initial symptoms of magnesium toxicity include diarrhea, nausea, vomiting, and low blood pressure. Advanced stages of toxicity are characterized by cardiovascular and muscular irregularities.

The upper limit of safety for magnesium established by the Food and Nutrition Board of the Institute of Medicine is approximately 350 mg daily for adults. See table below for more age- and gender specific guidelines.

Magnesium tolerable upper intake levels

Children
   1-3 years 65 mg/day
   4-8 years 110 mg/day
Adults 350 mg/day

Requirements

The Daily Reference Intakes (DRI) for magnesium are shown in the table below.

Magnesium requirements Daily Reference Intakes

Infants
   0-6 months 30 mg
   7-12 months 75 mg
Children
   1-3 years 80 mg
   4-8 years 130 mg
Males
   9-13 years 240 mg
   14-18 years 410 mg
   19-30 years 400 mg
   >31 years 420 mg
Females
   9-13 years 240 mg
   14-18 years 360 mg
   19-30 years 310 mg
   >31 years 320 mg
Pregnancy
   >18 years 400 mg
   19-30 years 350 mg
   31-50 years 360 mg
Lactation
   <18 years 360
   19-30 years 310
   31-50 years 320

Dietary sources

Rich sources of magnesium include nuts and seeds, soybeans, tofu, chocolate, dark-green vegetables, legumes, yogurt, wheat germ and dairy products. See table below for detailed listing.

Magnesium content of food

Pumpkin seeds, 1/4 cup roasted 303 mg
Almonds, 1/2 cup 238 mg
Soy nuts, 1/2 cup 196 mg
Cashews, 1/2 cup 157 mg
Tofu, firm, 1/2 cup 128 mg
Peanuts, 1/2 cup 125 mg
Chili with beans, 1 cup 115 mg
Molasses, 2 Tbl 100 mg
Wheat germ, toasted, 2 Tbl 90 mg
Unsweetened chocolate, 50 g 155 mg
Sunflower seeds, 1/4 cup 82 mg
Halibut, baked, 100 g 92 mg
Swiss chard, cooked, 1/2 cup 75 mg
Spinach, 1/2 cup cooked 66 mg
Black beans, 1/2 cup 60 mg
Oatmeal, 1 cup cooked 56 mg
Peanut butter, 2 Tbl 51 mg
Baked potato with skin 55 mg
Cereal, raisin bran, 50 g 85 mg
Low fat yogurt, 1 cup 43 mg
Milk, nonfat, 1 cup 28 mg
Chicken, breast, 100 g. 30 mg
Green peas, cooked, 1/2 cup 23 mg
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