Magnesium, an abundant mineral in the body, is naturally present in many foods, added to other food products, available as a dietary supplement, and present in some medicines (such as antacids and laxatives). Magnesium is a cofactor in more than 300 enzyme systems that regulate diverse biochemical reactions in the body, including protein synthesis, muscle and nerve function, blood glucose control, and blood pressure regulation [1-3]. Magnesium is required for energy production, oxidative phosphorylation, and glycolysis. It contributes to the structural development of bone and is required for the synthesis of DNA, RNA, and the antioxidant glutathione. Magnesium also plays a role in the active transport of calcium and potassium ions across cell membranes, a process that is important to nerve impulse conduction, muscle contraction, and normal heart rhythm .
An adult body contains approximately 25 g magnesium, with 50% to 60% present in the bones and most of the rest in soft tissues . Less than 1% of total magnesium is in blood serum, and these levels are kept under tight control. Normal serum magnesium concentrations range between 0.75 and 0.95 millimoles (mmol)/L [1,5]. Hypomagnesemia is defined as a serum magnesium level less than 0.75 mmol/L . Magnesium homeostasis is largely controlled by the kidney, which typically excretes about 120 mg magnesium into the urine each day . Urinary excretion is reduced when magnesium status is low .
Assessing magnesium status is difficult because most magnesium is inside cells or in bone . The most commonly used and readily available method for assessing magnesium status is measurement of serum magnesium concentration, even though serum levels have little correlation with total body magnesium levels or concentrations in specific tissues . Other methods for assessing magnesium status include measuring magnesium concentrations in erythrocytes, saliva, and urine; measuring ionized magnesium concentrations in blood, plasma, or serum; and conducting a magnesium-loading (or “tolerance”) test. No single method is considered satisfactory . Some experts  but not others  consider the tolerance test (in which urinary magnesium is measured after parenteral infusion of a dose of magnesium) to be the best method to assess magnesium status in adults. To comprehensively evaluate magnesium status, both laboratory tests and a clinical assessment might be required .
Sources of Magnesium
Magnesium is widely distributed in plant and animal foods and in beverages. Green leafy vegetables, such as spinach, legumes, nuts, seeds, and whole grains, are good sources [1,3]. In general, foods containing dietary fiber provide magnesium. Magnesium is also added to some breakfast cereals and other fortified foods. Some types of food processing, such as refining grains in ways that remove the nutrient-rich germ and bran, lower magnesium content substantially . Selected food sources of magnesium are listed in Table 2.
Tap, mineral, and bottled waters can also be sources of magnesium, but the amount of magnesium in water varies by source and brand (ranging from 1 mg/L to more than 120 mg/L) .
|Table 2: Selected Food Sources of Magnesium |
|Almonds, dry roasted, 1 ounce||80||20|
|Spinach, boiled, ½ cup||78||20|
|Cashews, dry roasted, 1 ounce||74||19|
|Peanuts, oil roasted, ¼ cup||63||16|
|Cereal, shredded wheat, 2 large biscuits||61||15|
|Soymilk, plain or vanilla, 1 cup||61||15|
|Black beans, cooked, ½ cup||60||15|
|Edamame, shelled, cooked, ½ cup||50||13|
|Peanut butter, smooth, 2 tablespoons||49||12|
|Bread, whole wheat, 2 slices||46||12|
|Avocado, cubed, 1 cup||44||11|
|Potato, baked with skin, 3.5 ounces||43||11|
|Rice, brown, cooked, ½ cup||42||11|
|Yogurt, plain, low fat, 8 ounces||42||11|
|Breakfast cereals, fortified with 10% of the DV for magnesium||40||10|
|Oatmeal, instant, 1 packet||36||9|
|Kidney beans, canned, ½ cup||35||9|
|Banana, 1 medium||32||8|
|Salmon, Atlantic, farmed, cooked, 3 ounces||26||7|
|Milk, 1 cup||24–27||6–7|
|Halibut, cooked, 3 ounces||24||6|
|Raisins, ½ cup||23||6|
|Chicken breast, roasted, 3 ounces||22||6|
|Beef, ground, 90% lean, pan broiled, 3 ounces||20||5|
|Broccoli, chopped and cooked, ½ cup||12||3|
|Rice, white, cooked, ½ cup||10||3|
|Apple, 1 medium||9||2|
|Carrot, raw, 1 medium||7||2|
*DV = Daily Value. DVs were developed by the U.S. Food and Drug Administration (FDA) to help consumers compare the nutrient contents of products within the context of a total diet. The DV for magnesium is 400 mg for adults and children aged 4 and older. However, the FDA does not require food labels to list magnesium content unless a food has been fortified with this nutrient. Foods providing 20% or more of the DV are considered to be high sources of a nutrient.
The U.S. Department of Agriculture’s (USDA’s) Nutrient Database Web site  lists the nutrient content of many foods and provides comprehensive list of foods containing magnesium arranged by nutrient content and by food name.