Introduction

Magnesium (Mg) 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.

Magnesium is needed for more than 300 biochemical reactions in the body. It helps to maintain normal muscle and nerve function, keeps heart rhythm steady, supports a healthy immune system, and keeps bones strong. Magnesium also helps to regulate blood sugar levels, promotes normal blood pressure, and is known to be involved in energy metabolism and protein synthesis. 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.

Food Sources

Minerals from plant sources may vary from place to place because soil mineral content varies geographically.

Green vegetables such as spinach are good sources of magnesium because the centre of the chlorophyll molecule (which gives green vegetables their colour) contains magnesium. Some legumes (beans and peas), nuts and seeds, and whole, unrefined grains are also good sources of magnesium. Refined grains are generally low in magnesium. 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 meet daily dietary needs for magnesium.

Some important food sources of Magnesium:

Cocoa powder	Sunflower seeds	Pumpkin seeds	Bran	Nuts
Peanut butter	Wheat flakes	Popcorn	Spinach	Wholegrain bread

Recommended Dietary Allowance (RDA)

The European Union RDA for the general population is set at 300 mg/day.

In general, the intake of magnesium depends on the age of the person. Pregnant women need a higher intake for the developing baby.

Inhibitors/stimulators:

The following food components have been found to stimulate the absorption of magnesium.

Vitamin D and calcium – The active form of vitamin D (1,25-dihydroxycholecalciferol, calcitriol) may increase the intestinal absorption of magnesium to a small extent. Inadequate blood magnesium levels are known to result in low blood levels, resistance to parathyroid hormone (PTH), and resistance to some of the effects of vitamin D.

The following food components have been found to inhibit the absorption of magnesium.

Zinc – High doses of zinc in supplement form appear to interfere with the absorption of magnesium.

Functions in the Body

Magnesium is involved in over 300 essential metabolic reactions.

Energy production

The metabolism of carbohydrates and fats to produce energy require numerous magnesium-dependent chemical reactions. Magnesium is required by the adenosine triphosphate (ATP) synthesizing protein in mitochondria. ATP, the molecule that provides energy for almost all metabolic processes, exists primarily as a complex with magnesium (MgATP).

Synthesis of essential molecules

Magnesium is required at a number of steps during the synthesis of nucleic acids (DNA and RNA) and proteins. A number of enzymes participating in the synthesis of carbohydrates and lipids require magnesium for their activity. Glutathione, an important antioxidant, requires magnesium for its synthesis.

Structural roles

Magnesium plays a structural role in bone, cell membranes, and chromosomes.

Ion transport across cell membranes

Magnesium is required for the active transport of ions like potassium and calcium across cell membranes. Through its role in ion transport systems, magnesium affects the conduction of nerve impulses, muscle contraction, and the normal rhythm of the heart.

Cell signalling

Cell signalling requires MgATP for the phosphorylation of proteins and the formation of the cell signalling molecule, cyclic adenosine monophosphate (cAMP). cAMP is involved in many processes, including the secretion of parathyroid hormone (PTH) from the parathyroid glands.

Cell migration

Calcium and magnesium levels in the fluid surrounding cells affect the migration of a number of different cell types. Such effects on cell migration may be important in wound healing.

Deficiency

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.

The health status of the digestive system and the kidneys significantly influences 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. 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.

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.

Toxicity

Adverse effects have not been identified from magnesium occurring naturally in food. However, adverse effects from excess magnesium have been observed with intakes of various magnesium salts (supplemental magnesium). The initial symptom of excess magnesium supplementation is diarrhoea--a well-known side effect of magnesium that is used therapeutically as a laxative.

Individuals with impaired kidney function are at higher risk for adverse effects from magnesium supplementation, and symptoms of magnesium toxicity have occurred in people with impaired kidney function taking moderate doses of magnesium-containing laxatives or antacids. Elevated serum levels of magnesium (hypermagnesemia) may result in a fall in blood pressure (hypotension). Some of the later effects of magnesium toxicity, such as lethargy, confusion, disturbances in normal cardiac rhythm, and deterioration of kidney function, are related to severe hypotension. As hypermagnesemia progresses, muscle weakness and difficulty breathing may occur. Severe hypermagnesemia may result in cardiac arrest

Regulation

Magnesium homeostasis is regulated by a fine balance between gastrointestinal absorption and renal excretion.

Magnesium is absorbed homogeneously throughout the small intestine, which is vitamin D dependent. The percentage of magnesium absorbed is inversely proportional to the amount of this cation in the diet.

The other major site of regulation of magnesium homeostasis is the kidney. The excretion of magnesium varies between 3% to 25% of the amount filtered in the glomeruli, depending on the serum level of magnesium. Magnesium restricted diet causes an increase in magnesium reabsorption in the loop of Henle before the occurrence of a detectable change in serum concentration. The most important mechanism of regulation of magnesium excretion in the kidney is changes in the reabsorption of the filtered magnesium in the loop of Henle and the proximal tubule. Magnesium reabsorption occurs mainly in the loop of Henle (65%), and in the proximal tubule (20%-30%) and a smaller percentage of magnesium is reabsorbed in the distal tubule (2%-5%). The endocrine system might also have a role in the regulation of magnesium homeostasis. Different studies have shown that aldosterone might either increase or have an effect on urinary magnesium excretion