If there is one supplement I'd nominate as a reasonable default for a significant proportion of the UK population, magnesium would be it. Widespread dietary insufficiency, a staggering number of biological roles, and a safety profile that is genuinely hard to harm yourself with at standard doses. But the conversation usually stops there — and it shouldn't, because there are eight common clinical forms, each with a different mechanism and application, and there are real contraindications that don't get discussed often enough.
This is the first in a monthly series. The format is always the same: three robust human studies, the genuine pros, the genuine cons, the clinical forms ranked by application, the contraindications, and a testing recommendation. No supplement gets featured here on rodent data alone.
What Magnesium Actually Does
Magnesium is a cofactor in over 300 enzyme systems. That number gets cited so often it has become almost meaningless, so let me make it concrete. The processes that require magnesium include: ATP production (the currency of cellular energy), DNA synthesis and repair, protein synthesis, neuromuscular transmission, blood pressure regulation via smooth muscle relaxation, blood glucose regulation via insulin signalling, and the synthesis of glutathione — the body's master antioxidant.
Magnesium also modulates the NMDA receptor — the primary excitatory glutamate receptor in the central nervous system. In a magnesium-replete state, Mg²⁺ ions physically block the NMDA receptor channel at rest, preventing excessive neuronal excitation. This is one of the mechanisms by which adequate magnesium status is associated with lower anxiety, better sleep quality, and reduced migraine frequency. It's also the mechanism most directly relevant to the emerging evidence on depression.
In the UK National Diet and Nutrition Survey, average magnesium intake from food was about 91% of the reference intake for men and 80% for women, and about 14% of men and 12% of women aged 20–59 ate less than the lower reference intake (190 mg a day for men, 150 mg for women), the level below which deficiency becomes likely (Derbyshire 2018, NDNS years 1–6; the analysis was funded by the Health & Food Supplements Information Service). That's intake, not clinical deficiency. Clinical deficiency (hypomagnesaemia) is less common but significantly more serious, and standard serum magnesium testing misses a large proportion of functional deficiency because serum magnesium is tightly regulated even when intracellular and tissue stores are depleted.
Evidence Base — Three Key Human Studies
✓ Where Magnesium Genuinely Helps
- Sleep initiation and quality — particularly where racing mind, muscle tension, and difficulty switching off are the pattern. The NMDA receptor block mechanism is relevant here.
- Muscle cramps and spasm — including restless legs syndrome, exercise-related cramping, and menstrual cramps. Smooth and skeletal muscle relaxation is a direct physiological effect.
- Migraine prevention: mixed trial evidence. In one placebo-controlled trial, 600 mg a day of magnesium as trimagnesium dicitrate for 12 weeks cut attacks by 42%, against 16% on placebo, though about 1 in 5 people got diarrhoea (Peikert 1996). A second placebo-controlled trial found no benefit and was stopped early (Pfaffenrath 1996). The 2012 American Academy of Neurology and American Headache Society guideline graded magnesium "probably effective" (Level B). Glycinate and malate, the forms I use most for sleep, have not been tested for migraine. If you're trying magnesium for migraine, the tested form is dicitrate, and the tested dose is above the 250 mg supplemental upper level, so agree it with your GP and check kidney function first.
- Insulin sensitivity and glucose regulation — magnesium is required for insulin receptor function. Low magnesium is independently associated with insulin resistance and type 2 diabetes risk in prospective studies.
- Stress resilience and anxiety — magnesium depletion raises baseline cortisol and exacerbates the HPA axis stress response. Repletion does not fix anxiety caused by psychological factors but meaningfully reduces the physiological substrate of it.
- Constipation — magnesium oxide and citrate specifically draw water into the colon via osmotic effect, softening stool and stimulating peristalsis. A legitimate first-line intervention for functional constipation.
- Cardiovascular risk markers — blood pressure, endothelial function, and inflammatory markers all improve with magnesium repletion in deficient individuals.
✗ Limitations and What Magnesium Won't Fix
- It won't overcome a fundamentally poor diet. Magnesium is in green leafy vegetables, legumes, nuts, seeds, and whole grains. If the dietary pattern isn't changing, supplementation is a compensatory intervention, not a solution.
- Poor gut absorption is a genuine barrier. Active gut inflammation, low stomach acid, or significant intestinal dysbiosis will impair magnesium absorption regardless of what form you're taking. Fix the gut first — or at minimum, address gut function alongside supplementation.
- It won't fix the sleep problems that aren't about nervous system excitation. Sleep apnoea, circadian disruption, chronic pain, or psychological hyperarousal from trauma are not primarily magnesium problems. The supplement helps with a specific mechanism; it doesn't address everything.
- High-dose protocols can cause diarrhoea. Magnesium oxide and citrate especially. Some people use this therapeutically (constipation); most people don't want it as a side effect. The solution is usually a different form at a sensible dose.
- Serum testing is a poor diagnostic tool. A normal serum magnesium result does not rule out functional deficiency. If you want to know your actual tissue status, red blood cell (RBC) magnesium is more informative. Better still, clinical response to supplementation in a trial period tells you most of what you need to know.
⚠ Clinical Contraindications and Cautions
- Kidney disease or renal impairment. The kidneys are responsible for regulating magnesium excretion. In renal impairment, magnesium can accumulate to toxic levels. This is the most important contraindication — anyone with reduced kidney function should not supplement magnesium without medical supervision.
- Certain cardiac medications. Magnesium has cardiac effects including slowing of atrioventricular conduction. In patients on digoxin, calcium channel blockers, or with pre-existing cardiac conduction disorders, magnesium supplementation requires physician oversight.
- Myasthenia gravis. Magnesium can worsen neuromuscular blockade in myasthenia gravis. Not common, but absolute — avoid without specialist clearance.
- Drug interactions. Magnesium can reduce absorption of certain antibiotics (fluoroquinolones, tetracyclines), bisphosphonates, and levothyroxine. Take at least two to four hours away from these medications.
- High-dose supplementation during pregnancy. Standard dietary magnesium is safe and often recommended in pregnancy (leg cramps, blood pressure). High-dose supplementation warrants midwife or physician awareness.
Clinical Forms — Which to Use and When
| Form | Best For | Bioavailability | Notes |
|---|---|---|---|
| Glycinate Top Pick | Sleep, anxiety, stress, general repletion | High | Chelated to glycine — calming amino acid. Minimal GI side effects. My most commonly used form. |
| Malate | Energy, muscle pain, fibromyalgia | High | Malic acid is a Krebs cycle intermediate. Mitochondrial support. Good for fatigue and myalgia. |
| L-Threonate | Cognitive function, brain health | High (CNS) | Crosses the blood-brain barrier more effectively than other forms. Premium cost. Specific application. |
| Citrate | Constipation, general use | Good | Osmotic laxative effect at higher doses. Effective and affordable. Can cause loose stools — dose carefully. |
| Taurate | Cardiovascular health, blood pressure | Good | Chelated to taurine — additional CV benefits. Worth considering alongside a cardiac risk context. |
| Oxide | Constipation only | Poor | Low bioavailability — much of it stays in the gut and acts as an osmotic laxative. Still used in the Abbasi sleep trial above. |
| Sulphate (Epsom salt) | Topical, baths | Transdermal (variable) | Transdermal absorption is contested in literature. Relaxing regardless — worth doing. Don't rely on it as primary repletion route. |
Practical Dosing
The UK reference nutrient intake (RNI) for magnesium is 300 mg a day for men and 270 mg for women. It's set to cover the needs of about 97.5% of people, so many need less. The level below which deficiency becomes likely is the lower reference nutrient intake: 190 mg a day for men and 150 mg for women (Derbyshire 2018). Most therapeutic protocols for sleep, anxiety, and muscle function use 200–400mg elemental magnesium in the evening. From supplements, I keep to 250 mg of elemental magnesium a day by default. That's the EU upper level for supplemental magnesium, set on diarrhoea risk; the UK's Expert Group on Vitamins and Minerals set a guidance level of 400 mg. Food doesn't count towards either. I go higher only for a specific reason and a set period, and not with reduced kidney function.
Take at night, away from meals if possible, and at least two to four hours away from any medications with potential interaction (see contraindications above). Glycinate is my default recommendation for most people because of its absorption profile and the added benefit of the glycine component for nervous system calming.
"Magnesium won't fix a broken lifestyle. But in a population as systematically depleted as the UK adult population, getting the basics right — including adequate magnesium — is a meaningful foundation."
Should You Test Before Supplementing?
For magnesium specifically, I generally support a trial-first approach for most healthy adults — the safety profile at standard doses is very good, and clinical response within two to four weeks tells you a lot. If you've been supplementing without noticing improvement, or you want an accurate picture of your status, RBC magnesium is more informative than serum. Ask your GP or request it through a private lab.
If you're considering the TDG programme, magnesium status is assessed as part of the comprehensive blood chemistry panel — the Blood Chemistry Interpretation Guide covers this in detail.
Stephen Duncan FDN-P MSc. This is educational content, not personalised medical advice. Key references: Abbasi B et al. (2012), J Res Med Sci — magnesium and insomnia; Tarleton EK et al. (2017), PLOS ONE — magnesium and depression; Zhang X et al. (2016), Hypertension — magnesium and blood pressure meta-analysis; Barbagallo M & Dominguez LJ (2015), World J Diabetes — magnesium and type 2 diabetes; Kass L et al. (2012), European Journal of Clinical Nutrition — magnesium and blood pressure; Peikert A et al. (1996), Cephalalgia 16:257–63 (doi:10.1046/j.1468-2982.1996.1604257.x) and Pfaffenrath V et al. (1996), Cephalalgia 16:436–40 (doi:10.1046/j.1468-2982.1996.1606436.x) — magnesium and migraine; Holland S et al. (2012), Neurology 78:1346–53 (doi:10.1212/WNL.0b013e3182535d0c) — AAN/AHS guideline. Abbasi 2012: PMID 23853635. Tarleton 2017: doi:10.1371/journal.pone.0180067. UK dietary data: National Diet and Nutrition Survey Rolling Programme; Derbyshire E (2018), Front Nutr 5:55 (doi:10.3389/fnut.2018.00055).