Magnesium

About half of adults in industrialised countries don't get enough magnesium from food — and because the body defends blood levels by quietly draining bone and tissue stores, most of that shortfall stays invisible on a standard blood test. Supplemental magnesium has one of the cleanest blood-pressure trials in the supplement aisle, a real-but-modest sleep signal, and a large mechanistic literature tying it to the biology of ageing. The form you pick matters more than the dose for what you'll actually feel: glycinate for sleep, citrate for daily use, threonate if you're chasing the (still-preliminary) cognitive angle.

Magnesium is a cofactor for over 300 enzymatic reactions, including ATP utilisation, DNA repair, and neuromuscular signalling. The dietary baseline has eroded over the last fifty years — modern soils are magnesium-depleted, refined grains lose most of the mineral during milling, and the "drink lots of water" message has accidentally pushed people toward reverse-osmosis-filtered water that strips minerals out. Replenishment via diet (leafy greens, nuts, legumes, whole grains) is the first move; supplementation closes the gap when diet can't.

The headline evidence

The strongest randomized signal is for blood pressure. Pooling 34 trials, a median ~368 mg/day of supplemental magnesium lowered blood pressure modestly — by about 3/2 mmHg in healthy adults, and noticeably more (about 8/3 mmHg) in people already being treated for hypertension.^[1] For a population-scale intervention that's a real effect; for an individual it's roughly the size of cutting back on salt or losing a few kilograms.

A second under-appreciated route: drinking water itself. In a 16-year follow-up of 26,733 postmenopausal Swedish women, those whose tap water carried about 10 mg/L of magnesium had 31% lower risk of ischemic stroke versus those at about 5 mg/L. The effect was driven entirely by what came out of the tap — reverse-osmosis filtration that strips minerals out (and isn't remineralised) works against this. See Water.

Other randomized and observational signals:

Sleep. Magnesium (especially glycinate, 200–400 mg before bed) modestly improves subjective sleep quality and reduces sleep latency. The best recent randomized trial — 155 adults with poor sleep on 250 mg of elemental magnesium as bisglycinate — found a statistically significant but small improvement in insomnia severity (effect size ~0.2).^[2] Magnesium is a physiological normaliser, not a sedative: the effect is real, small, and largest where dietary intake is low. See Sleep and anxiety supplements.
Inflammation. Long-term supplementation lowers C-reactive protein (CRP), a core marker of systemic inflammation, with the clearest effect in trials running 12–16 weeks.^[3] Effects on direct oxidative-stress markers are mixed, which suggests the benefit is mostly indirect — restoring normal physiology rather than scavenging free radicals.
Type 2 diabetes prevention. Higher dietary magnesium intake associates inversely with incident type 2 diabetes across multiple cohorts; randomized trials show modest improvements in insulin sensitivity in deficient or prediabetic adults.
Migraines. 400 mg/day has moderate evidence for prophylaxis (American Headache Society Level B).
Cognition (preliminary). Magnesium threonate uniquely raises brain magnesium in animal models;^[4] a 2025 randomized trial of 2 g/day in healthy adults reported gains in working and episodic memory and faster reaction time.^[5] It's a single trial of a branded form, and the headline "younger brain age" framing outpaces the evidence — but the working-memory signal is the most concrete human data the form has produced.

Why magnesium keeps appearing in longevity research

Most of the magnesium-and-ageing literature is mechanistic and preclinical, not outcome trials — worth understanding, but not the same evidentiary tier as the blood-pressure data above. The throughline is that magnesium sits upstream of several processes that go wrong with age, mapped comprehensively onto the hallmarks of aging in a 2024 review.^[6] A few concrete examples:

Energy. ATP is biologically active only as a magnesium complex (Mg-ATP), so every reaction that spends cellular energy depends on magnesium status.
DNA integrity. Magnesium is a required cofactor for the main DNA-repair pathways and stabilises the double helix directly.
Cellular senescence. Human fibroblasts grown in magnesium-poor media accumulate senescence markers and exhaust their replicative lifespan faster — an in-vitro result, but a striking one.^[7]
Anti-ageing signalling. Magnesium status tracks with circulating Klotho, a protein with cardiovascular- and kidney-protective effects that declines with age.^[8]

The one animal lifespan result worth flagging: adding soluble magnesium chloride to drinking water extended lifespan and reduced vascular calcification in a mouse model of accelerated ageing (progeria).^[9] That rhymes with the human hard-water/stroke association, but it's a long way from proving magnesium extends human life. Treat this section as why magnesium is plausible as a longevity-relevant nutrient, not as evidence that supplementing it lengthens lifespan.

Form matters more for magnesium than for almost any other supplement

Form	Best use	Absorption	Notes
Glycinate (bisglycinate)	Sleep, anxiety, daily	High	Best tolerated; calming via the glycine; no laxative effect
Citrate	Daily, mild constipation	High	Well-absorbed; mildly laxative at higher doses
Threonate	Cognition (preliminary)	Moderate	Uniquely crosses the blood-brain barrier; expensive; low elemental yield
Malate	Fatigue, fibromyalgia	High	Reasonable alternative; some prefer for daytime use
Taurate	Cardiovascular nuance	High	Taurine-bound; some evidence for BP and arrhythmia
Chloride	Skin spray, topical	Low (oral)	Topical only; oral absorption inconsistent
Oxide	Constipation only	Very low	Almost entirely laxative effect; poor systemic absorption
Sulfate (Epsom salt)	Bath, occasional laxative	Low (oral)	Skin absorption is minimal; the bath is mostly relaxation

The practical short version:

Glycinate in the evening if you want the sleep or anxiety effect — it's the cleanest, best-tolerated form.
Citrate if you want a daily generalist that's cheap and well-absorbed.
Threonate if you're specifically targeting cognition and accept that the evidence is preliminary and the cost is 5–10× higher. Its low elemental yield (a 2 g dose delivers only ~140 mg of elemental magnesium) means it's a poor choice for general repletion — pair it with a cheaper form if you also want to close the dietary gap.
Skip oxide for general supplementation — it's cheap but barely absorbed.

Magnesium, vitamin D, and calcium

Magnesium doesn't act alone, and two interactions are worth getting right.

Vitamin D needs magnesium to work. The enzymes that convert vitamin D into its active hormone are magnesium-dependent, so low magnesium can blunt the response to vitamin D supplementation. A randomized trial found that magnesium status changed how people metabolised vitamin D — nudging low 25-hydroxyvitamin D up and very high levels down.^[10] If you're supplementing vitamin D and not getting the blood-level response you expect, magnesium status is one thing to check. See Vitamin D.

Calcium and magnesium compete. They share absorption pathways, and modern diets heavy in dairy and calcium supplements have pushed the dietary calcium-to-magnesium ratio well above the ~2:1 that observational data associate with the best outcomes; ratios above ~2.6 track with more inflammation and worse metabolic and bone markers.^[11] The practical reading: prioritise dietary calcium over high-dose calcium supplements (which carry their own cardiovascular signal — see Calcium), and don't let calcium crowd magnesium out.

If you're taking vitamin D for bone, the coherent stack is D3 + K2 + adequate magnesium: D raises calcium absorption, K2 directs that calcium into bone rather than arteries, and magnesium activates the D. See Bone density.

Practical dosing

200–400 mg/day of elemental magnesium covers the gap for most adults.
The EU upper limit for supplemental magnesium is 250 mg/day; the US upper limit is 350 mg/day. Food-based magnesium has no upper limit. (The reason: high-dose supplemental magnesium can cause osmotic diarrhea, which is the body's safety valve.)
Split the dose if you take more than 200 mg at once — absorption per dose declines as the amount rises.
Take glycinate or threonate in the evening if you're using it for sleep or relaxation. Citrate is anytime.

How to read the label

Supplement labels list the elemental magnesium content, not the total weight of the compound. A "magnesium glycinate 1000 mg" capsule typically delivers 100–200 mg of elemental magnesium, depending on how the manufacturer measures it. Check the supplement facts panel for the "Magnesium (elemental)" line.

Testing

Serum magnesium is insensitive for detecting body-store inadequacy — the body holds serum levels constant by pulling magnesium out of bone and intracellular stores. By the time serum magnesium drops, the deficit is severe. The international reference ranges are themselves contested: the Magnesium Global Network argues the common lab cutoffs understate deficiency, and proposes 0.85 mmol/L as the threshold for low magnesium.

RBC magnesium (red blood cell magnesium) is more sensitive and is the practical lab to ask for if you want a real read. The "optimal" high-normal targets promoted in longevity circles are expert opinion, not outcome-validated — a result inside the lab's normal range is reassurance, not a number to chase upward.
Magnesium Depletion Score — a simple clinical score built from kidney function, diuretic and proton-pump-inhibitor use, and alcohol intake — predicts low body magnesium better than a single blood draw, and tracks with cardiovascular and mortality risk in cohort data.^[12]
Symptoms of inadequacy are subtle: muscle cramps, low-grade fatigue, irritability, occasional palpitations. None are specific.

For most healthy adults with a normal diet who eat some nuts, leafy greens, and whole grains, magnesium status is adequate. The supplementation case is strongest in: heavy exercisers (sweat losses), older adults (declining absorption), people on diuretics or PPIs (depletion), and people drinking very-low-mineral filtered water.

Cautions and interactions

Chronic kidney disease. The kidneys excrete excess magnesium; impaired renal function risks hypermagnesemia. Discuss with a nephrologist.
Antibiotics (quinolones and tetracyclines). Magnesium chelates these drugs and reduces absorption. Separate doses by at least 2 hours.
Bisphosphonates (for osteoporosis): same problem. Separate by at least 2 hours.
Proton pump inhibitors (PPIs). Chronic use lowers magnesium absorption; supplementation often makes sense for long-term PPI users.
Diuretics (especially loop and thiazide): increase magnesium loss; supplementation often warranted.

What's overrated

Transdermal magnesium — sprays, lotions, and Epsom-salt baths. The skin is an effective barrier to magnesium ions, and a 2017 review found no good evidence that topical application raises body magnesium meaningfully.^[13] The Epsom-salt bath is relaxing, but it isn't a repletion route. Oral organic salts remain the only well-supported way to raise magnesium.
Magnesium oxide for repletion — cheap and high in elemental magnesium by weight, but absorbed so poorly that most of it stays in the gut as a laxative. Fine if a laxative is what you want; a poor choice for systemic repletion.