Magnesium — The Complete Guide

Magnesium is involved in more than 300 enzymatic reactions in the human body, including processes that affect muscle function, nerve signalling, blood sugar regulation, blood pressure, and the production of energy and protein. Despite being one of the most abundant minerals on earth, a substantial portion of UK and US adults consistently fall short of the recommended daily intake — partly because modern wheat-based diets contain less magnesium than they used to, partly because magnesium is one of the first things lost in food processing.

This guide covers what magnesium does, how much you need, the best food sources, and — for those who don’t get enough from diet — how to choose a supplement that actually works rather than one that wastes your money on a poorly absorbed form.

Quick disclaimer: Information here is for general guidance and is not medical advice. Magnesium supplementation interacts with several medications and conditions. If you have kidney disease, heart conditions, or take prescription medication, speak to your GP or pharmacist before starting a magnesium supplement above the dietary level.

What magnesium does in the body

The European Food Safety Authority has authorised the following health claims for magnesium:

• Magnesium contributes to a reduction of tiredness and fatigue
• Magnesium contributes to electrolyte balance
• Magnesium contributes to normal energy-yielding metabolism
• Magnesium contributes to normal functioning of the nervous system
• Magnesium contributes to normal muscle function
• Magnesium contributes to normal protein synthesis
• Magnesium contributes to normal psychological function
• Magnesium contributes to the maintenance of normal bones and teeth
• Magnesium has a role in the process of cell division

That’s an unusually broad list — magnesium genuinely is involved in a lot of basic biological machinery. Beyond the EFSA list, several other potential roles are being actively researched and we summarise the current evidence below.

Sleep quality (modest evidence, mostly in older adults)

Magnesium supplementation in older adults with sleep difficulties has shown modest improvements in sleep quality in several small randomised trials. The mechanism is plausible — magnesium activates the parasympathetic nervous system and regulates GABA receptors, the same neurotransmitter system that prescription sleep medications target. Whether magnesium helps sleep in younger adults without deficiency is less clear, but it’s a reasonable thing to try if you have sub-optimal sleep and your magnesium intake is on the low side.

Anxiety (limited but suggestive evidence)

Several reviews have found a modest association between low magnesium status and anxiety, and limited trial evidence suggests supplementation may help in some people. The effect is not large or consistent enough to recommend magnesium as a treatment for anxiety, but it’s worth getting your intake up to recommended levels if you have anxiety symptoms.

Migraine prevention (reasonable evidence)

Magnesium is one of the few nutrients with reasonable evidence for migraine prevention, and the American Academy of Neurology lists it among possible preventive treatments. Typical doses studied for migraine are 400-600 mg per day of supplementary magnesium — significantly higher than the dietary RDA — and should only be undertaken with medical advice because of side effects at these doses.

Blood pressure (modest evidence)

A meta-analysis of randomised trials found that magnesium supplementation produces small reductions in both systolic and diastolic blood pressure, particularly in people with hypertension or insulin resistance. The effect is modest (a few mmHg) and shouldn’t replace prescribed antihypertensive medication, but as part of overall lifestyle measures it has some support.

Type 2 diabetes risk (reasonable observational evidence)

Higher dietary magnesium intake is associated with lower risk of developing type 2 diabetes in several large cohort studies. Whether supplementation reduces risk in people already at high risk is less clear, but ensuring adequate intake is sensible.

Muscle cramps (mixed evidence, except in pregnancy)

The popular use of magnesium for muscle cramps is supported in pregnant women (some evidence of benefit) but not consistently in the general population. The Cochrane review found magnesium unlikely to help leg cramps in non-pregnant older adults. If you get severe cramps with exercise, hydration and overall electrolyte balance is usually more important than magnesium alone.

Constipation (well-established, depending on form)

Magnesium oxide and magnesium citrate at higher doses (hundreds of milligrams) reliably loosen stools — they’re commonly used as over-the-counter laxatives. This is a known property to be aware of when choosing a supplement: oxide is poorly absorbed and most of the dose ends up doing this. Glycinate, malate, and threonate are far gentler.

How much you need

The UK NHS Reference Nutrient Intake (RNI) for adults:

Group	NHS RNI (UK)	US RDA
Men 19+	300 mg	400-420 mg
Women 19+	270 mg	310-320 mg
Pregnancy	270 mg	350-360 mg
Breastfeeding	320 mg	310-320 mg
Children 1-3	85 mg	80 mg
Children 4-6	120 mg	130 mg
Children 7-10	200 mg	240 mg
Children 11-14 (boys/girls)	280/280 mg	410/360 mg

Notice the US figures are notably higher than the UK ones — this is one of the larger international discrepancies in nutrient recommendations. The honest answer is that the “right” amount is debated, but if your diet is providing well below the UK RNI on most days, supplementing the difference is reasonable.

Tolerable Upper Intake Level (UL) for supplementary magnesium: 250 mg per day according to UK SACN, 350 mg per day according to US authorities. Above this, the main risk is digestive disturbance (diarrhoea), but very high doses can cause more serious effects. Magnesium from food doesn’t have an upper limit because food forms are absorbed differently from supplemental forms.

What blood tests can and can’t tell you

A standard serum magnesium test is widely available but only catches severe deficiency, because the body tightly regulates blood levels by pulling magnesium from bones if intake is low. A red blood cell magnesium test or an erythrocyte magnesium test gives a better picture of overall status but is not routinely available on the NHS. In practice, dietary intake is usually a more useful indicator than a blood test for mild-to-moderate insufficiency.

Where to get it from food

Magnesium is widely distributed across plant foods, particularly leafy greens, nuts, seeds, whole grains, and legumes. Approximate amounts per typical serving:

Food	Magnesium per serving
Pumpkin seeds — 30g (small handful)	165 mg
Almonds — 30g (small handful)	80 mg
Cashews — 30g	75 mg
Spinach (cooked) — 100g	80 mg
Black beans (cooked) — 100g	70 mg
Edamame (cooked) — 100g	60 mg
Dark chocolate (70%+) — 30g	65 mg
Brown rice (cooked) — 100g	40 mg
Wholemeal bread — 2 slices	45 mg
Avocado — 1 medium	60 mg
Banana — 1 medium	30 mg
Salmon (cooked) — 100g	30 mg
Yoghurt (plain) — 200g	40 mg
Tofu (firm) — 100g	35 mg

Hitting the UK RNI (270-300 mg) from food is achievable with a varied diet — for example, two slices of wholemeal toast + a banana for breakfast (75 mg), a handful of almonds as a snack (80 mg), spinach with dinner (80 mg), and a small piece of dark chocolate (60 mg) gets you to roughly 295 mg without supplementation.

Why intake has dropped

Several factors contribute to lower magnesium intake than in previous generations:

• Whole-grain bread and brown rice have largely been replaced by refined white versions in many diets — refining removes most of the magnesium
• Modern wheat varieties contain less magnesium than older varieties grown in less-depleted soils
• Processed foods are typically low in magnesium
• Soft water has less magnesium than hard water (a small but cumulative source of dietary magnesium in some regions)
• High-stress lifestyles can increase urinary magnesium loss

Who’s at higher risk of insufficiency

Some groups are more likely to fall short:

• People with type 2 diabetes — both because of higher urinary losses and because metformin can reduce absorption
• People with gastrointestinal conditions (Crohn’s, coeliac, after gastric bypass)
• People taking proton pump inhibitors (long-term PPI use is associated with low magnesium)
• People taking certain diuretics (especially loop and thiazide diuretics)
• Heavy alcohol users
• Older adults (lower intake + reduced absorption)
• People who restrict whole grains, nuts, or leafy greens for any reason
• Athletes with high sweat losses

How to choose a supplement

The single biggest decision in magnesium supplementation is choosing the form. This matters more than brand, dose, or price — get the form wrong and most of what you take won’t actually be absorbed.

The forms, ranked roughly by bioavailability and tolerability

Magnesium glycinate (also called bisglycinate) — magnesium bonded to two molecules of the amino acid glycine. Generally the best default form: well absorbed, very gentle on the digestive system, doesn’t cause diarrhoea at normal doses, and the glycine itself has a mild calming effect that some people notice. Good for evening use if you’re hoping for any sleep benefit. The main drawback is it’s more expensive per milligram of elemental magnesium than oxide.

Magnesium citrate — magnesium bonded to citric acid. Well absorbed, but at higher doses (above ~300 mg elemental) it has a laxative effect that’s useful if you want it, unpleasant if you don’t. A reasonable budget-friendly option at moderate doses.

Magnesium malate — bonded to malic acid. Well absorbed, generally well tolerated. Sometimes recommended for daytime use (malic acid is involved in energy metabolism), and some people with fibromyalgia find it helpful, though evidence for the specific fibromyalgia claim is limited.

Magnesium L-threonate — a newer form bonded to threonic acid, marketed as crossing the blood-brain barrier better than other forms and supporting cognitive function. The lab evidence is interesting but human trial evidence is limited and the form is significantly more expensive. Possibly worth trying if cognitive support is your specific goal; not necessary for general supplementation.

Magnesium taurate — bonded to taurine. Some evidence for cardiovascular benefits given taurine’s separate effects. Niche but reasonable.

Magnesium oxide — widely sold because it’s cheap and contains a high percentage of elemental magnesium by weight. Poorly absorbed (around 4% bioavailability in some studies), so most of the dose passes through and acts as a laxative. Useful if you want a laxative, not useful as a supplement.

Magnesium sulphate (Epsom salts) — generally used in baths rather than orally. Trans-dermal absorption of magnesium from Epsom salt baths is debated and probably minimal, but the warm water itself helps muscle relaxation.

Avoid: “Magnesium chelate” without specifying which chelate (usually a marketing term — could be anything), proprietary blends with multiple magnesium forms in undisclosed amounts, and aspartate (some safety concerns about excitotoxicity in high doses, though probably minor in practice).

Dosage

For general intake-topping: 100-200 mg of elemental magnesium per day from supplements is a sensible range for most adults. This brings most people up to or comfortably past the RNI without approaching the upper limit.

For specific therapeutic uses (migraine prevention, evening sleep support, etc.), higher doses are sometimes used but should be approached with medical advice.

Pay attention to “elemental magnesium” vs “total compound” on labels. A capsule containing 1,000 mg of magnesium glycinate provides only about 140 mg of elemental magnesium (the rest is glycine). The label should make this clear; if it doesn’t, calculate it or pick a different brand.

Quality markers

• Third-party tested (USP Verified, NSF Certified, or independent lab results published)
• Clear listing of elemental magnesium per serving
• Single form per product (not a “magnesium blend”)
• No proprietary blends
• Reasonable price — glycinate from a reputable brand costs around £0.10-£0.20 per 100 mg elemental dose

When and how to take it

• With food — improves tolerance, especially for forms that can cause digestive symptoms
• Time of day: glycinate in the evening if hoping for sleep effects; citrate in the morning if you want the mild laxative effect; otherwise whenever you’ll remember to take it consistently
• Split doses if taking more than 200 mg elemental — improves absorption and reduces digestive effects
• Daily, not all-at-once — magnesium doesn’t have a meaningful loading effect; consistent daily intake matters

Signs of insufficiency

Mild magnesium insufficiency often produces no obvious symptoms. More noticeable signs of low intake can include:

• Muscle twitches, cramps, or tics
• Fatigue and low energy
• Poor sleep quality
• Headaches
• Sensitivity to noise
• Low mood or anxiety
• Constipation (paradoxically — magnesium is needed for healthy bowel function even though too much causes the opposite)
• Numbness or tingling (severe deficiency)
• Abnormal heart rhythms (severe deficiency — get medical advice)

Note these are non-specific. If symptoms are significant or persistent, see your GP rather than self-diagnosing a magnesium deficiency.

Interactions and cautions

Magnesium supplementation can interact with several conditions and medications:

• Kidney disease: people with reduced kidney function should not supplement magnesium without medical advice — risk of accumulation and toxicity
• Antibiotics: magnesium can reduce absorption of tetracycline and quinolone antibiotics. Take at least 2 hours apart.
• Bisphosphonates (osteoporosis medication): same — separate doses by 2+ hours
• Diuretics: some diuretics (loop, thiazide) increase magnesium loss; potassium-sparing diuretics can increase magnesium retention
• Proton pump inhibitors (omeprazole etc.): long-term use associated with low magnesium
• Diabetes medication: magnesium can affect blood sugar; monitor if you have diabetes
• Blood pressure medication: magnesium may have additive effects

If you take any prescription medication or have a chronic condition, ask your GP or pharmacist before starting a magnesium supplement above the dietary level.

Frequently asked questions

What’s the difference between magnesium glycinate and bisglycinate?
None — they’re the same thing. “Bisglycinate” specifies that two glycine molecules are bonded to one magnesium ion, which is the standard form. Brands use the names interchangeably.

Will magnesium really help me sleep?
Possibly, if your intake is on the low side. The evidence is strongest in older adults with sleep difficulties. In younger adults with adequate intake, the effect is more modest and may be largely a placebo or relaxation response. Magnesium glycinate taken about an hour before bed is the form most often used for this purpose. It’s worth trying for a few weeks; don’t expect a prescription-sleep-medication-strength effect.

Can I get enough magnesium from food alone?
For most people, yes, with a varied diet that includes leafy greens, nuts/seeds, whole grains, and legumes. If your diet is heavy in processed foods and refined grains, hitting the RNI from food alone is harder.

Is magnesium safe to take long-term?
For most healthy adults, supplementing within the recommended ranges (under the 250-350 mg upper limit) appears safe long-term. People with kidney disease or on certain medications should not supplement without medical advice.

Will taking magnesium give me diarrhoea?
Depends on the form and dose. Magnesium oxide and citrate at higher doses commonly do. Magnesium glycinate, malate, and threonate are far gentler. If you experience loose stools, switch form or reduce dose.

Can I take magnesium with calcium?
Yes — they’re sometimes paired in supplements. Some people prefer to take calcium and magnesium at different times of day because they compete for absorption at very high doses, but this matters less at typical supplement doses.

Does magnesium spray or oil work?
Trans-dermal magnesium products are popular but the evidence for meaningful absorption through skin is weak. The relaxation effect people report from Epsom salt baths is probably mostly from the warm water. If you want measurable magnesium intake, oral supplements are more reliable.

Should athletes take magnesium?
Athletes lose more magnesium in sweat and often benefit from being at the higher end of the intake range. Whether supplementation improves performance specifically is less clear unless the athlete is deficient.

Where to go from here

• For dietary improvement: aim to add a daily serving of pumpkin seeds, almonds, or leafy greens — small habit, meaningful magnesium boost
• For supplementation: start with 100-200 mg elemental magnesium glycinate per day, with food
• For sleep: try glycinate in the evening for 2-4 weeks
• For migraine prevention: discuss with your GP — therapeutic doses (400+ mg) need supervision

Related reading on Nutrition Encyclopedia

• Best foods for sleep — coming soon
• Spinach — nutrition facts — coming soon
• Almonds — nutrition facts — coming soon
• Vitamin D — the complete guide

Primary sources

• NHS — Magnesium guidance
• Scientific Advisory Committee on Nutrition (UK)
• NIH Office of Dietary Supplements — Magnesium fact sheet for health professionals
• European Food Safety Authority — authorised health claims register
• Cochrane reviews on magnesium supplementation for various indications
• USDA FoodData Central — for food magnesium content

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Information on this page is for general guidance and is not a substitute for medical advice. Please speak to your GP or pharmacist before starting magnesium supplementation if you have kidney disease, heart conditions, or take prescription medication.

Last updated: May 2026