7 Signs You're Magnesium Deficient — And How to Fix It

Why Magnesium Deficiency Is Surprisingly Common in the UK

Magnesium is the fourth most abundant mineral in the human body and is required as a cofactor for over 300 enzymatic reactions — yet surveys consistently show that a significant proportion of UK adults consume less than the Reference Nutrient Intake of 300 mg per day for women and 350 mg per day for men. The reasons are structural, physiological, and dietary.

Soil Depletion and the Modern Diet

Intensive farming practices over the last century have progressively reduced the magnesium content of UK agricultural soil. A landmark comparison of historical food composition data found that the magnesium content of vegetables fell by between 24% and 35% between the 1930s and the 1990s. When you eat less-nutritious food, you absorb less magnesium regardless of how varied your diet appears to be.

Processed and refined foods compound this problem. Refining wheat into white flour removes the magnesium-rich bran and germ, stripping out approximately 80% of the mineral. A diet heavy in refined carbohydrates, convenience foods, and sugar — the dietary pattern most common in the UK — is inherently low in magnesium.

Stress, Exercise, and Medication as Depleting Factors

Physiological stress directly accelerates magnesium excretion via the kidneys. Elevated cortisol, the primary stress hormone, increases urinary magnesium loss. This creates a compounding problem: low magnesium impairs the regulation of the hypothalamic-pituitary-adrenal (HPA) axis, making stress responses harder to dampen, which in turn depletes magnesium further.

Vigorous physical exercise also increases magnesium requirements substantially. Sweat losses during prolonged endurance exercise can reach 10–15% of daily intake, and muscle contraction itself relies on adequate intracellular magnesium. Athletes and regular exercisers who do not deliberately replenish magnesium are frequently in negative balance.

Several common medications further reduce magnesium status. Proton pump inhibitors (PPIs) such as omeprazole, widely prescribed across the UK, are a well-documented cause of hypomagnesaemia with long-term use. Diuretics, commonly used for hypertension and heart failure, increase renal magnesium excretion. Alcohol consumption — prevalent in the UK adult population — similarly increases urinary losses.

Recognising the Symptoms

Magnesium deficiency rarely presents dramatically. The symptoms are non-specific and overlap with other conditions, which is part of why it is so frequently missed.

Muscle cramps and spasms are among the most commonly reported symptoms. Magnesium acts as a natural calcium antagonist: when intracellular magnesium falls, calcium influx into muscle cells becomes dysregulated, leading to involuntary contractions. Nocturnal leg cramps — a phenomenon that affects roughly one third of UK adults over 50 — are strongly associated with low magnesium status.

Sleep disturbances represent another common presentation. Magnesium activates the parasympathetic nervous system and binds to GABA receptors in the brain, promoting the neurological quieting necessary for sleep onset. Low magnesium is associated with lighter, more fragmented sleep and reduced slow-wave sleep.

Anxiety and heightened stress reactivity are frequently reported. The NMDA receptor, which plays a central role in modulating excitatory neurotransmission, is normally blocked by magnesium ions at rest. When magnesium is deficient, this block weakens, and the nervous system becomes more easily over-stimulated.

Other recognised symptoms include fatigue, headaches (magnesium is used intravenously in hospitals for acute migraine), palpitations, and low mood.

Why Blood Tests Are Often Misleading

Standard NHS serum magnesium tests are notoriously insensitive for detecting suboptimal magnesium status. Less than 1% of total body magnesium is in the bloodstream; the vast majority is intracellular or in bone. The body actively maintains serum magnesium within narrow limits by drawing on intracellular reserves, meaning that serum levels appear normal until intracellular depletion is advanced.

A red blood cell (RBC) magnesium test, which measures magnesium inside red blood cells, is a better proxy for tissue magnesium status, though it is not routinely available on the NHS. In practice, clinical response to supplementation often provides more actionable information than any available blood test.

Choosing the Right Form

Not all magnesium supplements are equivalent. Magnesium oxide, the cheapest and most widely sold form, has extremely poor bioavailability — approximately 4% in some studies. It functions primarily as a laxative rather than a meaningful mineral supplement.

Magnesium glycinate (or bisglycinate) is consistently rated as one of the most bioavailable and best-tolerated forms. It is chelated to glycine, an amino acid with its own calming neurological properties that may complement magnesium's effect on sleep and anxiety. It is the form most commonly used in published clinical trials on magnesium and sleep.

Magnesium malate is another well-absorbed form, often favoured for energy metabolism and muscle recovery. Magnesium L-threonate is a newer form that has been shown in animal studies to cross the blood-brain barrier more efficiently, though human clinical data remains limited.

Dosing

For adults seeking to address potential deficiency, a dose of 300–400 mg of elemental magnesium per day is generally appropriate. This refers to elemental magnesium, not the weight of the compound — always check the label for elemental content. Taking magnesium in the evening, with or after food, tends to maximise absorption and supports the sleep benefits. Loose stools are the main dose-limiting side effect, particularly with oxide or poorly chelated forms; glycinate is significantly less likely to cause this.