“Understanding the relationship between the concentration of an analyte in the compartment being measured (e.g., blood, urine, and epithelial samples) and the status of that analyte in the body, or its relevance in the measured compartment is a fundamental principal that will render an analytical test useful or not. Due to the way in which magnesium is compartmentalized, the typical compartment (blood and urine) analytics may not provide an accurate proxy of magnesium status and will mislead the practitioner.”
3.1 blood levels
“The current “normal” range interval of serum magnesium is 0.7–1 mmol/L and was established based on serum magnesium levels gathered by a U.S. study between 1971 and 1974 of presumably healthy individuals aged 1–74 years [
120]. Serum changes can be influenced by dietary magnesium intake and albumin levels, but can also be affected by short term changes like day to day and hour to hour variability of the amount of magnesium absorbed and excreted through the kidneys [
121]. Blood levels have been shown to increase in response to magnesium supplementation, but this does not signal that complete equilibrium has been established between blood and the nearly 100-fold larger body reservoir of magnesium.
In fact, the much larger exchangeable pool of magnesium is more often called upon to augment blood levels to maintain a narrow range preferentially, which is a key reason why blood measurements can easily mask deficiency [
122,
123].
The tight control of magnesium serum levels, representing only 0.8% of total body stores (see
Section 2.4), therefore serves as a poor proxy for the 99.2% of magnesium in other tissues that constitutes the body’s true magnesium status. Furthermore, this narrow serum range feeds the common perception of clinicians that magnesium levels rarely fluctuate, and therefore, are not indicative of the condition for which the blood tests are ordered. Therefore, practitioners are apt to order blood tests for magnesium infrequently, if at all, and if a magnesium level is in the patient chart, it is more often as part of a blood test panel and not purposely ordered to determine the magnesium status [
89,
124,
125,
126]. This contributes significantly to magnesium deficiency not being recognized as a modifiable nutritional intervention, and magnesium in general, being the neglected mineral that it is.
Red blood cells’ (RBC; erythrocyte and monocyte) magnesium levels are often cited as preferable to serum or plasma levels due to their higher magnesium content (0.5% vs. 0.3%, respectively). Some RBC studies report correlation to magnesium status particularly when subjects are placed on long-term (~3 months) magnesium replete or deplete diets. However, most studies using RBC magnesium endpoints do not satisfy this long-term design and have not been performed in nearly enough randomized clinical studies to be considered sufficiently robust or reliable (
Table 1) [
127,
128,
129]. In addition, the majority of RBC studies do not validate the method through inter-compartmental sampling (e.g., urine and muscle), challenging the claim that this test is a reliable representation of the large magnesium pool.”
Magnesium is a critical mineral in the human body and is involved in ~80% of known metabolic functions. It is currently estimated that 60% of adults do not achieve the average dietary intake (ADI) and 45% of Americans are magnesium deficient, a ...
www.ncbi.nlm.nih.gov
probably worth a read