Magnesium in the Blood: What the Test Shows and Why It Matters for an Athlete

Magnesium is one of the most popular minerals in sports nutrition, but the blood test for it is often interpreted incorrectly. The editorial team explains what role magnesium plays for an athlete, what the laboratory actually measures and why a "normal" result does not always mean sufficient reserves.

Magnesium: why it matters for an athlete

Magnesium is the fourth most abundant cation in the body and the second most abundant intracellular one after potassium. It is a cofactor for hundreds of enzymatic reactions. It is especially important that the cell's energy "currency", ATP, is biologically active predominantly in the form of a complex with magnesium.

For an athlete the functions of magnesium have a direct practical dimension: energy metabolism in the muscles, the contraction and relaxation of muscle fibers, the conduction of nerve impulses, heart rhythm, protein synthesis, maintenance of bone tissue. Magnesium also affects insulin sensitivity and glucose metabolism.

The review by Nielsen and Lukaski (2006) describes how physical activity redistributes magnesium between tissues and increases its losses through urine and sweat. Athletes with a restricted diet, in particular those who control weight, may receive less of it than recommended.

The recommended daily allowance for adults, according to the US Institute of Medicine, is 400–420 mg for men and 310–320 mg for women. The sources are whole-grain products, nuts, seeds, legumes, leafy greens and cocoa.

What the blood test actually shows

The standard test determines total magnesium in the serum. The problem is that the serum contains less than 1% of all the magnesium in the body; the rest is concentrated in the bones, muscles and other tissues. The body actively keeps the blood concentration stable by using bone reserves and regulating excretion through the kidneys.

Because of this, serum magnesium can remain normal even when tissue reserves are already reduced. The review by Elin (2010) and the perspective article by Costello and co-authors (2016) directly indicate: a normal serum level does not rule out a chronic latent deficiency.

In the serum magnesium exists in three forms: ionized (biologically active, about two-thirds), bound to proteins (mainly albumin) and in complexes with anions. Most laboratories measure total magnesium; the ionized form is determined less often, with special electrodes.

Other methods — magnesium in red blood cells, daily urinary excretion, the magnesium loading test — are used in research and in individual clinical situations, but they are less standardized and are not routine.

Reference values and nuances of interpretation

Most laboratories use an interval of about 0.70–1.0 mmol/L for total serum magnesium (see the report for the specific limits). Magnesium may also be reported in mg/dL or mEq/L, so it is important not to mix up the units.

Costello and co-authors (2016) argued that it would be reasonable to raise the lower limit to approximately 0.85 mmol/L, since values between 0.75 and 0.85 mmol/L may already reflect an insufficient status. This is an expert position, not a generally accepted standard, but it is useful for understanding "borderline" results.

The result is affected by the albumin level: with low albumin, total magnesium is also reduced, although the active fraction may be normal. In addition, magnesium is closely linked to potassium and calcium: persistent hypokalemia and hypocalcemia often cannot be corrected without correcting magnesium.

A substantial pre-analytical factor is hemolysis. There is more magnesium in red blood cells than in plasma, so the destruction of cells in the tube falsely raises the result. Prolonged application of the tourniquet and vigorous fist clenching are also undesirable.

When an athlete should take the test

Routinely determining magnesium in healthy people without symptoms is not necessary. However, in sports medicine there are situations in which the test is appropriate.

• frequent muscle cramps, twitching, weakness without an obvious cause;
• heart rhythm disturbances or a sensation of skipped beats;
• long-term use of diuretics, proton pump inhibitors or some antibiotics;
• chronic diarrhea, intestinal diseases;
• significant weight loss and strict diets, work in conditions of heavy sweating;
• detected low potassium or calcium;
• long-term use of high doses of magnesium supplements, especially in kidney diseases.

The test is taken in the morning fasting. For a day beforehand it is advisable not to take magnesium supplements, in order to assess the baseline level, and to avoid exhausting workouts the day before, which temporarily redistribute electrolytes.

It is important to understand the limits of the method. If symptoms are present but serum magnesium is normal, the doctor may assess the diet, losses and other electrolytes and decide whether additional methods are needed.

The systematic review by Heffernan and co-authors (2019) emphasizes that magnesium supplements bring the most benefit to people with a low status. That is precisely why assessing status makes practical sense.

Editorial conclusions

Magnesium is critically important for energy metabolism, muscle contraction and heart function, so its status matters for an athlete.

The serum magnesium test reflects only a small, tightly regulated fraction of the reserves. A normal result does not rule out a latent deficiency, and hemolysis can falsely inflate the indicator.

Assess the result together with albumin, potassium, calcium, diet and symptoms — and discuss it with your doctor.

We also recommend reading our materials on the causes of magnesium deviations from the norm, on the vitamin D test and on electrolytes for athletes.

References

1. de Baaij JH, Hoenderop JG, Bindels RJ. Magnesium in man: implications for health and disease. Physiol Rev. 2015;95(1):1–46.
2. Costello RB, Elin RJ, Rosanoff A, et al. Perspective: the case for an evidence-based reference interval for serum magnesium: the time has come. Adv Nutr. 2016;7(6):977–993.
3. Elin RJ. Assessment of magnesium status for diagnosis and therapy. Magnes Res. 2010;23(4):S194–S198.
4. Nielsen FH, Lukaski HC. Update on the relationship between magnesium and exercise. Magnes Res. 2006;19(3):180–189.
5. Institute of Medicine. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. Washington, DC: National Academy Press; 1997.
6. Heffernan SM, Horner K, De Vito G, Conway GE. The role of mineral and trace element supplementation in exercise and athletic performance: a systematic review. Nutrients. 2019;11(3):696.