Magnesium supplementation

Magnesium is the fourth most abundant cation in the body, around 25 g in total, with 50 to 60 percent locked in bone and less than one percent circulating in serum. That last number is why a normal serum magnesium result means considerably less than patients assume: the body defends the serum compartment by pulling from bone, so you can be depleted with a perfectly reassuring blood test.

The real roles are not in dispute. The physiologically active form of ATP is Mg-ATP, so magnesium is an obligate cofactor for every kinase reaction in the cell. It is required by several hundred enzymes, including Na+/K+-ATPase. And it acts as a natural calcium antagonist, blocking NMDA receptors and modulating voltage-gated calcium channels. That last mechanism is where the entire relaxation narrative comes from, and it is not fictional. It is just extended far past its evidence.

Take the cramp claim specifically. Exercise-associated muscle cramping is now best explained by altered neuromuscular control: fatigue-driven increase in alpha motor neuron excitability with reduced inhibitory feedback from Golgi tendon organs. The tell is that cramp appears in one worked muscle group while the rest of the body is fine, which is difficult to explain with a whole-body electrolyte model. A systemic deficit does not produce a local spasm in the one muscle you overused.

Form is where the practical difference lives. Magnesium oxide is around 60 percent elemental magnesium by mass, which looks efficient on a label, but it is poorly soluble and poorly absorbed, which is precisely why it works as a laxative. Citrate, malate, and glycinate are more soluble and better absorbed. Fractional absorption also falls as dose rises, so three doses of 100 mg beat one dose of 300 mg.

The deficiency claim has a real basis and a dishonest presentation. National intake surveys do show a large share of the population consuming below the Estimated Average Requirement, because magnesium lives in green leafy vegetables, legumes, nuts, and whole grains. But intake below the EAR on a food frequency questionnaire is not clinical deficiency, and the 75 percent figure is an intake statistic wearing a diagnosis costume.

Performance: the review literature (Zhang et al. 2017 is a reasonable entry point) converges on a boring and sensible answer. Supplementation improves performance measures in subjects who were deficient or newly training, and does close to nothing in replete, trained athletes. This is the standard shape of a micronutrient effect, and it is the shape supplement marketing is designed to obscure.

Cramps: the Cochrane review (Garrison et al.) found no clinically meaningful benefit of magnesium for idiopathic cramp in adults, with the pregnancy literature mixed and separate. Updates have not changed the conclusion. This is a well-answered question with an unpopular answer.

Sleep: small trials, mostly in older adults or diagnosed insomnia, showing modest improvements in sleep latency and subjective quality. Almost nothing in healthy young athletes, and the magnesium glycinate story is partly a glycine story, since glycine itself has a small sleep literature of its own.