Magnesium

Learning Objectives Dietary sources Daily Requirements Metabolism Important functions and Deficiency diseases

MAGNESIUM Mg At. No. 12 Atomic Mass: 24

Magnesium

MAGNESIUM Magnesium is the 4th most abundant and important cation in humans. It is extremely essential for life and is present as intracellular ion in all living cells and tissues

DIETARY SOURCES Magnesium is widely distributed in vegetables Found in porphyrin group of chlorophyll of vegetable cells Found in almost all animal tissues. Other important sources are cereals, beans, potatoes, almonds and dairy products

Recommended Dietary Allowances Infants 0 – 0.5 50 mg 0.5 – 1.0 70 mg Children 1 – 3 150 mg 4 – 6 200 mg 7 – 10 250 mg

Recommended Dietary Allowances Adults Male Female 11 – 14 350 300 15 – 18 400 300 19 – 22 350 300 23 – 50 350 300 51+ 350 300 Pregnancy and Lactation + 150 mg

ABSORPTION Average daily intake in humans is 250-300 mg, much of which is obtained from green vegetables. Roughly 1/3 of dietary Mg is absorbed Remainder is passively excreted in feces.

ABSORPTION Absorption takes place primarily in small intestine beginning within hour after ingestion Continues at a steady rate for 2 to 8 hours By that time 80% of total absorption has taken place .

FACTORS AFFECTING ABSORPTION Size of Mg load: Absorption is doubled when normal dietary Mg requirement is doubled and vice versa. Dietary calcium: Increased absorption in calcium deficient diets. Decreased absorption occurs in presence of excess of Ca. A common transport mechanism from intestinal tract for both Ca and Mg suggested.

FACTORS AFFECTING ABSORPTION Motility and mucosal state: In hurried bowel, absorption is decreased. Absorption decreases in damaged mucosal state. Vit-D: helps in increased absorption. Parathormone: increases absorption. Growth hormone: increases absorption

OTHER FACTORS: High protein intake and Neomycin therapy increases absorption. Fatty acids, phytates and phosphates decrease absorption.

Excretion Magnesium is lost from the body in feces, sweat and urine. 2/3rd of orally taken Mg is lost through these routes Sweat loss: Currently it is drawing attention; 0.75 mEq of Mg is lost daily in perspiration in normal health with normal diet. Loss is much increased with visible frank sweating. Urine: Regulation of Mg balance is principally dependant on renal handling of the ion. In a normal healthy adult with normal diet 3 to 17 mEq are excreted daily.

Factors Affecting Renal Excretion Calcium intake: Increased dietary calcium increases excretion of Mg. Parathormone (PTH): diminishes excretion. Antidiuretic hormone (ADH): increases Mg excretion Growth hormone (G.H): also increases excretion of Mg. Aldosterone: increases excretion

Factors Affecting Renal Excretion Thyroid hormones: 80% greater excretion in hyperthyroidism. Alcohol ingestion: oral ingestion of as little as 1.0 ml of 95% alcohol per kg, increases urinary excretion 2 to 3-fold. The increased excretion partially accounts for Mg-deficiency in chronic alcoholics with Delirium tremens. Administration of acidifying substances (NH4Cl) is followed by increased urinary elimination of Mg.

FUNCTIONS Role in Enzyme Action: Mg is involved as a cofactor and as an activator to wide spectrum of enzyme actions. It is essential for: Peptidases, Hexokinases,Fructokinase and PF kinase Ribonucleases, Adenyl Cyclase, cAMP and ATP requiring enzymes, Glycolytic enzymes and Co-carboxylation reactions.

FUNCTIONS Neuromuscular Irritability: Mg exerts an effect on neuromuscular irritability similar to that of Ca, High levels depress nerve conduction and Low levels may produce tetany (hypomagnasemic tetany). Thus it helps maintain the electrical potential in nerves and muscle

FUNCTIONS Constituent of Bones and Teeth: About 70% of body magnesium is present as apatites in bones, dental enamel and dentin. Is involved in active transport across cell membrane

DISTRIBUTION Total body Magnesium is about 20 grams. Mainly in intracellular Fluid. 75% complexed with Calcium in bones

Distribution Total body magnesium is approx 20 grams Approximately 2/3rd occurs in bones 1/3rd is in ECF and soft tissues. Plasma level: 1.5 to 1.8 mEq/L, which is rigorously maintained within normal limits. 15% is exchangeable

Blood Magnesium exists in blood partly bound to proteins. Under conditions of physiological pH roughly 1/3 is “protein-bound” The remainder 2/3rd is ionic. C.S. Fluid: Concentration of Mg is high than in plasma.

Plasma Mg in Diseases: Hypermagnaesemia: Raised values have been reported in: Uncontrolled Diabetes mellitus Adreno-cortical insufficiency Hypothyroidism Advanced renal failure, and Acute renal failure.

Plasma Mg in Diseases: Hypomagnaesemia: Low values are observed in: Malabsorption syndrome and kwashiorkor, Prolonged gastric suction Hyperthyroidism Portal cirrhosis

Plasma Mg in Diseases: Hypomagnaesemia: Prolonged use of diuretics Chronic alcoholism Delirium tremors Renal diseases Primary aldosteronism