Calcium or Magnesium: what is the difference

Calcium and magnesium are often sold in a single tablet, but in the body they do very different work. The editorial team explains how these minerals differ in quantity, function and regulation, how the various supplement forms are absorbed, and which of the popular notions about them are confirmed by research.

Two minerals of different scale

Calcium and magnesium are "neighbors" in the periodic table and are often sold in one tablet, so they are frequently perceived as interchangeable minerals "for bones and muscles." In reality they differ in quantity in the body, in distribution, in function and in the way they are regulated. Understanding these differences helps you not to buy a supplement at random.

Calcium is the most abundant mineral in the human body: an adult has about a kilogram of it, and almost all of it is concentrated in the bones and teeth as part of hydroxyapatite. There is far less magnesium in the body - about 25 g. Roughly half or a little more is in the bones, the rest mainly in muscles and soft tissues, and in blood serum less than one percent (de Baaij et al., 2015).

Daily requirements differ accordingly. According to the norms of the US Institute of Medicine, adults under 50 are recommended about 1000 mg of calcium per day, and magnesium - 310-320 mg for women and 400-420 mg for men. That is, a person needs about 2.5-3 times more calcium.

For both minerals the consequences of deficiency develop differently. A lack of calcium mostly manifests over years - as a decrease in bone mineral density. A lack of magnesium can produce faster symptoms: muscle twitching, cramps, heart rhythm disturbances, although a moderate deficiency often proceeds unnoticed.

Functions of calcium

The main "visible" role of calcium is structural: it gives strength to bones and teeth. However, bone is not only a store but also a reservoir. When the level of calcium in the blood drops, the body mobilizes it from the bones to maintain the extremely stable plasma concentration needed for the work of nerves and muscles.

In muscle cells calcium is the trigger of contraction. A nerve impulse causes the release of calcium from the sarcoplasmic reticulum, calcium binds to troponin, and the muscle filaments begin to slide relative to one another. For relaxation the calcium must be pumped back, and this consumes ATP energy.

Calcium is also involved in blood clotting, the release of neurotransmitters, hormone secretion and signal transmission inside the cell. The concentration of calcium in the blood is tightly regulated by parathyroid hormone, the active form of vitamin D and calcitonin.

Because of such tight regulation, the level of calcium in a blood test hardly reflects its intake from food. Normal blood calcium does not mean the diet is adequate: the body can maintain it by gradually "borrowing" the mineral from the bones.

Functions of magnesium

Magnesium is a cofactor of hundreds of enzymatic reactions, including those related to energy metabolism. Most ATP in cells exists as a complex with magnesium, so without it protein synthesis, the work of ion pumps and muscle contraction are impossible. The review by de Baaij and colleagues (2015) describes this role in detail.

With respect to calcium, magnesium often acts as a physiological antagonist: it regulates calcium channels and promotes the relaxation of muscle fibers. That is exactly why magnesium is associated with the prevention of cramps, although, as we will show further, the evidence for this is less clear-cut than commonly thought.

Magnesium also affects the metabolism of calcium itself: it is needed for the secretion of parathyroid hormone and the sensitivity of tissues to it. In pronounced magnesium deficiency a persistent hypocalcemia can develop that cannot be corrected without correcting magnesium.

Unlike calcium, the balance of magnesium is regulated mainly by the kidneys. The level of magnesium in serum is an inaccurate indicator of stores: it can remain normal even with a significant intracellular deficiency. This complicates diagnosis.

Comparison of absorption and supplement forms

Calcium is absorbed better the smaller the single serving; therefore large daily amounts in supplements are usually divided into several intakes. Calcium carbonate contains the most elemental calcium but requires stomach acid, so it is taken with food. Calcium citrate is less dependent on acidity, which is convenient for the elderly and those taking proton pump inhibitors.

Magnesium in supplements is represented by oxide, citrate, glycinate (bisglycinate), lactate, malate and other salts. Oxide contains a lot of elemental magnesium but dissolves poorly and more often acts as a laxative. Organic salts are usually better tolerated, although the real difference in absorption between forms in studies is not always large.

Calcium and magnesium can compete for absorption when large doses are taken simultaneously, but in ordinary amounts the clinical significance of this competition is small. A more practical problem is that calcium reduces the absorption of iron, so these two supplements are better spaced out in time.

Both minerals are better obtained primarily from food. Calcium supplements, unlike dietary calcium, are associated with a possible increase in cardiovascular risk: the meta-analysis by Bolland and colleagues (2010) found an association of calcium supplements without vitamin D with myocardial infarction. This topic is still debated, but the argument in favor of food sources is weighty.

Calcium, magnesium and training

For athletes both minerals are important, but for different reasons. Calcium is critical for bone health, especially in female athletes with low energy availability, where the risk of stress fractures rises. Magnesium is linked to energy metabolism and is lost in sweat, so its status in athletes has been studied separately (Nielsen, Lukaski, 2006).

The common notion that magnesium "cures cramps" is confirmed worse than it seems. The Cochrane review by Garrison and colleagues (2020) concluded that magnesium is unlikely to be effective for the prevention of ordinary muscle cramps in the elderly. Cramps associated with physical exercise have a complex nature, and muscle fatigue plays no less a role in them than electrolytes.

As for direct improvement of athletic performance, the data are limited: benefit from magnesium supplements is usually observed in people with a low baseline status. For a person with a full diet, additional magnesium will probably give no gain in strength or endurance.

• Calcium:key for bones; special attention - to female athletes, vegans, people with lactose intolerance.
• Magnesium:important for energy metabolism; deficiency is more likely with a poor diet and large losses in sweat.
• Both:are not standalone means of improving performance.

Editorial conclusions

Calcium and magnesium are not interchangeable minerals. Calcium is primarily a structural element of the bones and a trigger of muscle contraction, which the body needs in large quantities. Magnesium is a universal cofactor of enzymes and ATP, necessary for muscle relaxation and the metabolism of calcium itself.

They differ in their depot in the body, in regulation, in food sources and in the side effects of supplements. For both, the best source remains the diet, and supplements make sense with insufficient intake or a confirmed deficiency.

The notion of magnesium as a universal remedy for cramps and of calcium supplements as unconditionally useful for everyone needs correction in light of modern research.

Practical advice on choosing can be found in the article "Calcium vs Magnesium: what to choose and for whom." We also recommend our materials on vitamin D and bone health and on electrolytes for athletes.

References

1. Institute of Medicine. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. Washington (DC): National Academy Press; 1997.
2. Ross AC, Taylor CL, Yaktine AL, Del Valle HB (eds); Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington (DC): National Academies Press; 2011.
3. de Baaij JHF, Hoenderop JGJ, Bindels RJM. Magnesium in man: implications for health and disease. Physiol Rev. 2015;95(1):1–46.
4. Weaver CM, Heaney RP (eds). Calcium in Human Health. Totowa (NJ): Humana Press; 2006.
5. Nielsen FH, Lukaski HC. Update on the relationship between magnesium and exercise. Magnes Res. 2006;19(3):180–189.
6. Bolland MJ, Avenell A, Baron JA, et al. Effect of calcium supplements on risk of myocardial infarction and cardiovascular events: meta-analysis. BMJ. 2010;341:c3691.
7. Garrison SR, Korownyk CS, Kolber MR, et al. Magnesium for skeletal muscle cramps. Cochrane Database Syst Rev. 2020;9:CD009402.