Magnesium is extensively distributed in soil, water and plants. It is essential for ehzymatic reactions requiring adenosine triphosphate, and the recommended dietary allowance in man is 5 - 10 mg/kg/d. About 50% of magnesium in man is stored in bone, where it is regulated by parathyroid hormone'and 1,25(OH)2-D3. Most of the remaining magnesium pool is intracellular, plasma magnesium accounting for less than 0,5% of the total

Leary, W.P.

Olhaberry, J.V.

Reyes, A.J.

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4. International Federation of Clinical Chemistry. Provisional recommendation(1979) on a scheme for a rwo year poStgraduate course in clinical chemistry. ClinChnn 1980; 26: 1748-1753.5. Sanz MC, Lous P. A definition of clinical chemistry. IFCC Newsletter 1971;6: l.6. Mitchell FL. The trend towards devolurion in-elinical chemistry.] AueomatChnn 1979; I: 179-181.7. Varley H. Developments in clinical chemistry since 1900. Proc Assoc ClinBiochem 1966; 5: 93-95.8. Whitby LG. Clinical chemistry - time for im·estmenr. Lancet 1963; ii: 1239-1243.9. Broughron PMG. Instrumentation - past and presem. Ann Clin Biochem1974; 11: 146-149.SA MEDICAL JOURNAL VOLUME 63 26 FEBRUARY 1983 31910. Murray JF. The dendritic pathologist. S Afr Med] 1969; 43: 528-529.11. Baron DN, Bold AM, F1ynn FV, Neill DW, Rinsler MG, Zilva JF. Chemicalpathology in the foreseeable future. Bull R Coli Pathologists 1981; No. 36, 6-10.12. Royal College of Pathologists. Staffing and Warkload. London: Royal Collegeof Pathologists, 1972.13. Joubert SM. Chemical pathology: a perspective. Newsleccer S Afr Assoc ClinBiochemists 1979; 5: 34-36.14. Baker TG. The graduate pathway: the specialist route. Gaz lnst MedLab Sci 1981; 25: 421-422.15. Marsden J. The graduate pathway: the general [Oute. Gaz lmt Med Lab Sci1981; 25: 421-422.16. McLmosh DB. Scientists in medicine. Newsleteer S Afr Assoc Clin Biochemists1979; 5: SQ.-53.Magnesium - distribution and basicmetabolismJ. V. OLHABERRY, W. P. LEARY, A. J. REYESSummaryMagnesium is extensively distributed in soil, waterand plants. It is essential for ehzymatic reactionsrequiring adenosine triphosphate, and the recom-mended dietary allowance in man is 5 - 10 mg/kg/d.About 50% of magnesium in man is stored ~n bone,where it is regulated by parathyroid hormone'and1,25(OHb-D3. Most of the remaining magnesiumpool is intracellular, plasma magnesium accountingfor less than 0,5% of the total.S Afr Med J 1983: 63: 319-320.The importance of electrolyte and fluid balance in man is gene-rally understood by practising clinicians, particularly wherelosses of water, sodium, chloride and potassium are concerned.However, it has become apparent of late that changes in thehomeostasis of elements such as magnesium may also be ofconsiderable practical importance. Magnesium should be ofparticular interest to clinicians in view ofthe numerous enzymaticreactions which require this cation as a co-factor.Magnesium (Mg2') is found within the earth's cortex, account-ing for 2% of its total mass.' It is widely distributed, principallyas silicates, magnesite (magnesium carbonate) and dolomite'(calcium and magnesium carbonate), although only 10% of theelement in soil is soluble and can be utilized by plants. InFundacion Procardias and Universidad de la Republica,Montevideo, UruguayJ. V. OLHABERRY, QF., c.P. BIOMAT. BIOEST.A. J. REYES, DR. MED., C.E. CARDIOL., c.P. BIOMAT. BIOEST.Department of Experimental ~md Clinical Pharmacology,University of Natal, DurbanW. P. LEARY, M.SC., M.B. B.CH., D.PHIL., F.C.P. (S.A.), F.R.C.P.Da(~ received: 13 October 1982.animals, magnesium is essential for enzymatic reactions requiringadenosine triphosphate (ATP) and is therefore of importance inDNA transcription, RNA aggregation, protein synthesis andvarious cell membrane functions. 2 The recommended dietaryallowance of magnesium varies from 5 mg/kg/d in healthy adultmales to 6 mg/kg/d in children or during pregnancy or lactation,although an intake as high as 7 - 10 mg/kg/d has beenrecommended in the latter circumstances.3,4 Somatic depletionof the ion and clinical manifestations of magnesium deficiency,including neuromuscular and cardiac symptoms and signs, maydevelop when inadequate intake is coupled with changes in theabsorption or excretion of or physiopathological demand for. 145magnesIUm...Magnesium in waterMagnesium is present in both fresh and salt water. Water forhuman consumption varies in its solute content and is describedas 'hard', when calcium and magnesium levels are high, or 'soft',when the concentrations of these ions are low. In most regions alitre of drinking water contains 5 - 30 mg magnesium, andhealthy adults usually obtain 3 - 60 mg from this source daily.Magnesium in foodThe magnesium concentration in common foods is low. Fruit,flour, vegetables, dairy products, meat and sugar usually containless than 30 mg/ 100 g. Refinement of products such as sugar andflour always leads to a major loss of magnesium content.'However, bananas are an excellent source of the ion, 3-4containing sufficient magnesium to provide an adult with therecommended daily allowance of the cation.AbsorptionIn conditions of magnesium balance, 50-60% of the ingestedmineral passes through the gastro-intestinal tract unabsorbed.The percentage absorbed increases in conditions of relativemagnesium deficiency. Magnesium absorption from the gut isaffected by nonspecific factors such as vomiting, diarrhoea,320 SA MEDIESE TYDSKRIF DEEL 63 26 FEBRUARIE 1983alterations in the total gut absorptive surface, interactions withother substances within the intestinal lumen, and systemicpathological conditions.6 Intestinal absorption of the ion isfavoured by the presence of protein and decreased by intra-luminal fat.Glucose depresses magnesium absorption whereas lactose andvitamin D increase it. None of the known hormones appears toaffect magnesium absorption to a significant extent.Plasma transportThe measurement ofmagnesium in biological fluids is compli-cated by technical problems,7 although experimental error canbe minimized by the use of atomic absorption spectrophoto-metry.B-1l Normal plasma magnesium levels range from 0,75 to1,05 mmol/I (1,5 - 2,1 mEq/l) in the adult but are normallymaintained within narrow limits in the individual. It is conven-tionally accepted that a plasma magnesium level below 0,75mmol/I is diagnostic of magnesium deficiency, although somaticdepletion of magnesium may exist in the presence of normalplasma levels.DistributionMagnesium and potassium are the most important intracellularcations and share a similar distribution within the body. In an'adult magnesium accounts for approximately 0,36 g of everykilogram body weight. Only about 0,04% of this is in the plasma,whereas 50-55% is in bone, approximately 46% intracellular and1,0% transcellular; 1,0% is contained in lymph and interstitialtissues and 0,04% in dense connective tissue and cartilage.Magnesium turnover is slow in comparison with that of potas-sium9 and is altered by parathormone and 1,25(OH)2-D,.Bone magnesium is either incorporated in the thin lamella orforms part of the superficial mineral sheet. These superficialstores account for 30% of bone magnesium, correlate well withplasma magnesium levels and serve as a source of magnesiuminterchange for the maintenance of normal plasma magnesiumconcentrationsY No correlation exists between striated musclemagnesium and magnesium in plasma, bone or erythrocytes; thecontent of magnesium in erythrocytes largely depends upon theconcentration in extracellular fluid during haematopoiesis. As aresult erythrocyte magnesium levels do not correlate with plasmamagnesium levels and fall slowly when magnesium deficiencydevelops. In cells of electrically excitable tissues such as themyocardium, the concentration of magnesium reaches 6 - 9mmol/kg wet weight and most of the ion is found in themicrosomes and mitochondria. Intracellular magnesium reachesa concentration of I mmol/I, but its cytosolic concentration is notuniform, since it accumulates at the inner surface of the poly-ionic macromolecules, anracted by their electronegativity.Renal excretionRenal excretion of magnesium usually ranges between 4 and 8mmol/d (100-400 mg/d). Between 80% and 95% of filteredmagnesium is reabsorbed, 20-30% in the proximal convolutedtubule, 50-60% in the thick limb of Henle's loop and 1-5% in thedistal convoluted tubule. Parathyroid hormone positively affectsreabsorption of magnesium in the thick limb of Henle's loop andpossibly also in the distal convoluted tubule. 13 No other factorseems to be of importance in the regulation of renal magnesiumexcretion. 14-16 Detailed accounts of the renal regulation ofmagnesium excretion have been published elsewhere.Il.17-19Intracellular functionsMagnesium acts as a co-factor for various enzymes, therebyregulating the excitability of heart20-24 and skeletal muscle,19neuromuscular junctions and, to a lesser extent, the centralnervous system25•26 and smooth muscle. Magnesium modulatesthe production ofenergy within the cells12,17 and also the cellularresponse to various hormones,z2,27,28REFERENCESI. Jokinen R. The magnesium status of Finnish mineral soils and the requirementof the magnesium supply. Magnesium Bull 1981; 3: 1-5.2. Rude RK, Singer FR. Magnesium deficiency and excess. Annu Rev Med 1981;32: 245-259.3. Seelig MS. Magnesium requirements in human nutrition. Magnesium Bull1981; 3: 26-47.4. Marier JR. Tutritional and myocardial aspects of magnesium in drinking-water. Magnesium Bull 1981; 3: 48-54.5. Huber HG, Disney GW, Mason RL. Urinary excretion and dietary intake ofmagnesium in girls. NUlr Rep Inr 1981; 23: 127-133.6. Tansy M F, Kendall FM. Magnesium and the gastrointestinal tract. MagnesiumBull 1981; 3: 55-66.7. Bergstrom J. Determination of electrolytes - methJ>dological problems. ActaMed Scand 1981; suppl 647: 39-46.8. Massry SG. Role ofhormonal and non-hormonal factors in the control of renalhandling of magnesium. Magnesium Bull 1981; 3: 277-281.9. Olesen KH. Exchangeable electrolytes in heart disease. ACla Med Scand 1981;suppl 647: 47-60.10. Reyes AJ, Leary WP. The magnesiuric effect of a single dose of furosemide inhealthy adults. Curr Ther Res 1982; 32: 406-416.11. Reyes AJ, Leary WP. Urinary magnesium and zinc excretions after monodosinghealthy volunteers with chlorthalidone. CUTT Ther Res 1982; 32: 128-137.12. Rude RK, Singer FR. Magnesium deficiency and excess. Annll Rev Med 1981;32: 245-259.13. Quamme GA, Dirks JH. Magnesium transport in the nephron. Am] Physiol1980; 239: F393-F401.14. Jiuuti M, Alhava EM, Lammi S. Excretion of urinary phosphate and magne-sium on three diets in patients with urolithiasis. Ann Clin Res 1981; 13: 1-5.15. Poujeol P, Touvay C, Roinel N, Rouffignac C. Stimulation of renal magnesiumreabsorption by calcitonin in the rat. Am] Physiol 1980; 239: F524-532.16. Rude RK, Bethune JE, Singer FR. Renal tubular maximum for magnesium innormal, hyperparathyroid and hypoparathyroid man.] Clin Endocnnol Metab1980; 51: 1425-1430.17. Leary WP, Reyes AJ. Diuretics, magnesium, potassium and sodium. S AfrMed] 1982; 61: 279-280.18. Leary WP, Reyes AJ. The magnesiuric effect of a single dose ofhydrochloro-thiazide in healthy adults, Curr Ther Res 1982; 61: 279-280.19. Leary WP, Reyes AJ. Urinary volume and solute flows after a single dose ofchlorexolone in healthy adults. CIITT Ther Res 1982; 32: 417-424.20. Fischer B, Fischer U.'Magnesium in der inneren Medizin, Pathophysiologieand Klinik. Magnesium Bull 1981; 3: 249-275.21. Davis WH, Ziady F. The effect oforal magnesium chloride therapy on the QTcand AUc intervals of the electrocardiogram. S Afr Med] 1978; 53: 591-593.22. Gunther T. Biochemistry and pathobiochemistry of magnesium. Arrery 1981;9: 167-181.23. Renker H, Muller A. Miiglichkeiten lOr Basistherapie bei der kardialen Insuf-fizienz mic verschiedenen Kalium-Magnesium-Kombinationen. MagnesiumBlIll 1980; 2: 93-96.24. Reyes AJ. Bases farmacol6gicas de la terapeutica cardiovascular con diureticos.Arch Inse Cardiol Mex 1981; 51: 291-303.25. Durlach J. Deficit magnesique, tetanie et dystonie neuro-vegetative: donneesnouvelles sur les formes neuromusculaires du deficit magnesique ch'l'onique.Magnesium Bu//1981; 3: 121-136.26. Chutkow JG. The neurophysiologic function of magnesium: an update. Mag-nesium BII//1981; 3: 115-120.27. Shils ME. Magnesium, calcium and parathyroid hormone interactions. AnnNY Acad Med 1980; 355: 165-180.28. Thode J, Borresen T, Beck K, Madsen SN. Effect offurosemide on parathy-roid hormone stimulated guinea pig renal adenylate cyclase and thyrotrophinand fluoride stimulated human thyroid adenylate cyclase. Acea PharmacolToxicol (Copenh) 1981; 49: 285-289.

Magnesium - distribution and basic metabolism

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Magnesium - distribution and basic metabolism

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