Magnesium Balance

INTRODUCTION

Magnesium (Mg) is an essential cofactor for more than 300 separate enzymes within the human body. Thus know to be “indispensable to the metabolism of ATP, which means that it is essential in a great many metabolic processes such as glucose utilization; synthesis of fat, protein, and nucleic acids; muscle contraction; and some membrane transport systems.”(1) Those desirous of learning more about Mg should read the excellent short review by Elin.(2)

All physicians should learn about magnesium (Mg) balance. If the serum Mg or red cell Mg is low, the patient is in a state of negative Mg balance. Even if they are both normal, Mg deficiency or negative Mg balance may be present. In such cases, a low-dose Mg load test needs to be done. Childbearing females and premature neonates are at risk for Mg deficiency. Oral repletion can be accomplished with Mg gluconate, and a technique is suggested.

A survey of Mg content of food supplies and diet intakes was very revealing in that only 25% of the U.S. population in 1978 had a dietary Mg intake equal to or greater than the Recommended Dietary Allowance (RDA). Thirty-nine percent had less than 70% of the RDA.(3) Much of this has been changing through the years because of significant changes in our diet. For example, refinement of our foods has led to reductions in the amount of Mg as follows: wheat flour 82%, polished rice 83%, corn starch 97%, and white sugar 99%.(3)

A 1985 study of the U.S. population showed in a 3-day analysis of food intake that adolescent females (12-18 years), adult females (19-34 years), and adult females (35-50 years) had a daily average intake of Mg equal to 85%, 80%, and 85%, respectively of, the 300 mg/day RDA.(4) Besides the RDA, it has been recommended that females should get at least 6 mg/kg/day.(5)

MAGNESIUM BALANCE

An important part of the consideration of Mg homeostasis is ascertaining the body’s Mg balance. It is essential that all humans remain in positive Mg balance. This means that Mg intake must always exceed Mg redistribution and elimination. Negative Mg balance results whenever there is a change in one or more of the stages of metabolism, such as reduced intake, reduced absorption, redistribution, and increased excretion.(2) Human and animal bodies have unique mechanisms to maintain serum Mg levels. Normally 30% to 40% is absorbed from the gastrointestinal tract, but in severe Mg depletion, significantly more can be absorbed. Excess oral intake leads to diarrhea. Since the extracellular Mg represents about 1% of the total body Mg, a low serum Mg level is helpful in determining the Mg balance, yet the serum level can be normal in the presence of significant tissue depletion.

Bone contains about 50% of the body Mg, and part of this is the principal source of available Mg to maintain a normal serum level. Determination of red cell Mg is another way to assess Mg but is useful only if it is lower than normal. In the presence of normal renal function, ascertained by a normal serum creatinine, the kidney will not reabsorb Mg if the serum level exceeds 3.0 mg/dl.(6) Serum Mg levels are maintained by renal method of assessing Mg balance, which is by the use of low-dose Mg tolerance test.(6) There are multiple situations in all of these stages, which lead to Mg deficiency. Occasional or transient negative Mg balance is well established, especially with regard to the examples listed in Table 1.

There is an example of redistribution of Mg that could effect the asthmatic. Administration of intravenous epinephine in humans (0.1 µg/kg/min) for 2 hours resulted in a fall in serum Mg from 1.86± 0.04 mg/dl to 1.63± 0.05 mg/dl (p<0.01). Red blood cell Mg2+, mononuclear Mg content, and urine Mg excretion did not change. These researchers suggested that endogenous catecholamine release during stress or acute illness may contribute to negative Mg balance.(6)

VALUE OF ROUTINE MAGNESIUM SERUM ANALYSES

In a study of physician-initiated Mg serum requests, analysis of Mg had been requested in only a small percent of those with abnormal Mg values had the analysis of Mg been requested. Yet hypomagnesemia (less percent of those with abnormal Mg values had the analysis of Mg been requested. Yet hypomagnesemia (less than 0.74 mmol/liter) was present in 47,2% of sera

TABLE 1. CAUSES OF NEGATIVE MAGNESIUM BALANCE

Causes

Examples

A. Reduced intake

  1. Mg poor foods
  2. Dieting; starvation
  3. Infusion of intravenous fluids
  4. Vomiting

B. Reduces absorption

  1. Chronic diarrhea
  2. Starvation diarrhea
  3. Gastrointestinal problem
  4. Intake of substances that bind Mg Phosphates; soda drinks, Oxalates

C. Redistribution

  1. Increase serum free fatty acid
  2. Hypoalbuminemia
  3. Increase catacholamines

D. Increased excretion

  1. Multiple medications: diuretics, furosemide, the chlorothiazides, sodium lactate, sodium bicarbonate, theophylline
  2. During aciduria or glycosuria
  3. Following ingestion of 2 ounces of alcohol

Modified from ref. 2.

studied. In only 9.9% of those with hypomagnesemia and 13.5% of those with hypermagnesemia were tests requested.(8) This was despite the fact that this same group of investigators from the same veterans’ and university hospital had reported 6 years previously that of patients’ sera with hypokalemia, 42% had hypomagnesemia, and of sera with hypophosphatemia, 29% had hypomagnesemia, of sera with hyponatremia 23% had hypomagnesemia, and of sera with hypocalcemia 22% had hypomagnesemia. Cases have been reported where hypokalemia, hypophosphatemia, and hypocalcemia have been corrected only after the hypomagnesemia has been corrected.(9) Remember, however, that if the serum Mg value is normal, negative Mg balance may still be present.

The critically ill patient is at significant risk of being in negative Mg balance. At Los Angeles County Hospital, 65% of patients in the medical intensive care unit were found to have hypomagnesemia (<1.5 mEq/liter or 1.8 mg/dl). Low blood mononuclear cell Mg content was seen in 53% of patients with a normal serum Mg. A Mg load test undoubtedly would have demonstrated more patients in negative Mg balance in spite of normal magnesemia.(10)

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