25: Phosphorus (Chapter 23b)

Abstract

Magnesium is a cofactor for over 600 enzymatic reactions vital for life and is a controlling factor in nerve transmission, skeletal and smooth muscle contraction, cardiac excitability, vasomotor tone, blood pressure, and bone turnover. Thus, magne­sium deficiency can have detrimental consequences that include impaired physical and mental well-being and risk for chronic disease. Before 1990, a nutritional magne­sium deficiency was considered rare and to be present mainly in clinical conditions associated with gastrointestinal or renal loss of magne­sium. However, numerous reports of an association between a low magne­sium intake and chronic diseases, especially those associated with chronic inflam­matory stress, has led to the recognition that mild or subclinical magne­sium deficiency, also known as chronic latent magne­sium (CLMD) deficiency, may be quite prevalent. As a result, magne­sium has become a nutrient of public health concern and a simple, rapid, and reliable clinical measure is needed to assess for magne­sium deficiency. Numerous methods of magne­sium status assessment have been developed, but all have shortcomings that impact their use in the clinical setting to accurately assess magne­sium status.

The predominant method for assessing magne­sium status at present is the determination of total serum or plasma magne­sium. However, this method is not a sensitive indicator of body stores of magne­sium. In the reference range of serum or plasma magne­sium values, there is an interval in which individuals may have either CLMD or an adequate status. Combining measurement of total serum or plasma magne­sium with both the determination of magne­sium from dietary intake and urinary excre­tion appears to enhance the usefulness of this method to assess magne­sium status.

Recent models of ion-selective electrodes have enabled the measurement of plasma ionized magne­sium, the physiologically active form, to be acceptably accurate and precise using whole blood. As a result, this method is becoming more prevalent in the clinical setting because the procedure is simpler and requires less blood than the measurement of total magne­sium in plasma or serum magne­sium. However, plasma ionized magne­sium has shortcomings similar to those for total plasma and serum magne­sium regarding the sensitivity of the reference range to deter­mine CLMD accurately.

Measurements of magne­sium in erythro­cytes, erythro­cyte membranes, and mono­nuclear cells give a reasonable assessment of physiologically active body stores of magne­sium. However, all these methods are laborious and prone to error, and hence only suitable for use in a research setting and not for routine clinical use.

Urinary magne­sium excre­tion is an excellent indicator of magne­sium intake. However, a single urinary magne­sium determination may not reflect magne­sium stores because it responds so rapidly to changes in dietary intake. Hence, it is mainly appropriate for use in population studies, or in combination with other measures of status.

The magne­sium load test provides the most valid assessment of magne­sium status. It has been used to identify magne­sium deficiency in elderly individuals, in chronic alcoholism, and in several chronic diseases in which hypomagnesemia was not present. The test deter­mines the per­cent­age of urinary magne­sium retained over a given period of time after parenteral admin­istration of a magne­sium load and yields an abbreviated balance determination. The test is invasive, time-consuming, and cumbersome; it requires close supervision for at least 24 hours after a magne­sium load. Because of these drawbacks, it has been used mostly as a research tool.

Several other methods used to assess magne­sium status also have drawbacks that preclude their use as routine clinical methods. Muscle magne­sium determination is extremely invasive and requires skill to obtain suitable samples for analysis. Buccal cell magne­sium determinations also require skill in obtaining cells, and special expensive equipment for analysis. Magnesium balance requires controlled and consistent dietary intakes and careful collection of urine and stool over a lengthy study period. Fractional excre­tion of magne­sium requires the determination of three variables and has no apparent advantage over other well-estab­lished methods. The magne­sium depletion score needs further evaluation and validation, although seems to be most useful for establishing status in individuals with a chronic disease. However, all these methods do not have an estab­lished validated reference range that indicates magne­sium adequacy or deficiency.

At present, there is no simple, rapid, and reliable single clinical method to deter­mine the presence of chronic, latent magne­sium deficiency (CLMD). Determination of serum total or ionized magne­sium remains the most acceptable choice in the clinical setting. The reliability of these measures of status could be improved if their values for the reference ranges indicative of deficient or adequate magne­sium status are confirmed by other measures such as the determination of magne­sium in both dietary intakes and urinary excre­tion.