Magnesium:
Why the form
matters as much
as the dose

The majority of magnesium supplements sold in UK pharmacies contain magnesium oxide, primarily because it is cheap to manufacture and contains a high percentage of elemental magnesium by weight. However, magnesium oxide has a bioavailability of approximately 4%, the vast majority of the dose passes through the gut without being absorbed. This is why people taking standard magnesium supplements often notice minimal benefit and why magnesium oxide has a reputation as a laxative: the unabsorbed magnesium draws water into the colon. The form of magnesium determines how much actually enters circulation and reaches target tissues. This is not a minor detail, it is the difference between a supplement that does something and one that does not.

Magnesium glycinate, magnesium bound to the amino acid glycine, combines two sleep-supporting compounds. Magnesium acts as an NMDA receptor antagonist, reducing neuronal excitability, and as a GABA agonist, promoting the inhibitory neurotransmission associated with sleep onset and maintenance. Glycine independently promotes sleep: a 2012 RCT found that 3g of glycine before bed significantly improved sleep quality, reduced daytime sleepiness, and shortened sleep onset latency. The glycinate form also has substantially higher bioavailability than oxide, is generally well-tolerated, and produces minimal gastrointestinal side effects. A 2025 RCT specifically investigating magnesium bisglycinate in adults with self-reported poor sleep found significant improvement in insomnia severity scores versus placebo.

Muscle contraction requires calcium; muscle relaxation requires magnesium. Magnesium regulates calcium channels in muscle cells, enabling the relaxation phase of the contraction cycle. Insufficient magnesium availability is associated with muscle cramps, impaired recovery, and reduced exercise performance, effects most consistently observed in people with below-adequate intake. Magnesium is also lost in sweat, making athletes and people who train regularly more susceptible to functional insufficiency even with otherwise adequate dietary intake. The exercise physiology literature consistently shows that low magnesium status impairs aerobic and anaerobic performance, and that repletion in deficient athletes improves strength, power output, and recovery metrics.

The hypothalamic-pituitary-adrenal (HPA) axis regulates the body's stress response. Magnesium inhibits the release of ACTH from the pituitary and modulates downstream cortisol output, acting as a physiological buffer to acute stress. Deficiency is associated with HPA hyperreactivity, a heightened cortisol response to stressors that perpetuates a cycle of elevated stress hormones, disrupted sleep, and further magnesium depletion (cortisol promotes magnesium excretion). A 2024 systematic review found that magnesium supplementation produced modest but consistent improvements in self-reported anxiety in adults with inadequate dietary intake. The effect was most pronounced in those with identified deficiency, less clear in those with adequate baseline status. This mirrors the broader pattern seen across magnesium research: the benefit is primarily from correcting insufficiency, not from supraoptimal dosing.

Magnesium is necessary for the enzymatic conversion of vitamin D to its active hormonal form (1,25-dihydroxyvitamin D3). This means that vitamin D supplementation in a state of magnesium insufficiency may produce limited benefit because the conversion step is impaired. The interaction is bidirectional: vitamin D increases intestinal magnesium absorption, and magnesium is required for vitamin D to function. Magnesium is also directly incorporated into the bone mineral matrix and is a cofactor for alkaline phosphatase, the enzyme responsible for bone mineralisation. Deficiency is associated with reduced bone mineral density independently of calcium and vitamin D status. For anyone focused on bone health, or supplementing vitamin D, adequate magnesium intake is a meaningful consideration.

The richest dietary sources of magnesium are dark leafy vegetables (spinach, kale), nuts (almonds, cashews, Brazil nuts), seeds (pumpkin seeds are particularly high), whole grains, legumes, and dark chocolate. A diet consistently including these foods can meet the recommended intake of 300–375mg daily without supplementation. The problem in typical Western diets is that these foods are systematically under-consumed. If your diet is low in vegetables, nuts, and whole grains, or if you train frequently and lose meaningful amounts through sweat, dietary intake alone may not maintain adequate status. In this case, supplementation at a highly bioavailable form (glycinate, citrate, or malate) is a practical and inexpensive intervention.

The recommended dietary allowance for magnesium in adults is approximately 300–375mg/day. Supplemental doses in research studies typically range from 200–500mg of elemental magnesium daily. The tolerable upper intake level for supplemental magnesium is 350mg elemental magnesium daily in adults, above this, gastrointestinal symptoms (particularly loose stools) become more likely, especially with poorly absorbed forms like oxide. At therapeutic doses from bioavailable forms (glycinate, citrate), gastrointestinal tolerance is good in most people. The key practical rule is to look at the elemental magnesium content on the label, not the total compound weight. 500mg of magnesium glycinate contains roughly 50–75mg of elemental magnesium, so multiple capsules are typically required to reach 200–400mg of elemental magnesium daily.