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Magnesium is one of the key minerals in the body. It plays an essential role in hundreds of enzymatic reactions — in other words, processes that regulate energy production, muscle and nerve function, heart rhythm stability and bone health.
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At the same time, insufficient magnesium intake is relatively common. This is largely because many modern diets are low in foods naturally rich in magnesium.
What is magnesium, and where is it found in the body?
Magnesium deficiency can be difficult to identify because magnesium is not only present in the bloodstream, but also cannot be reliably assessed through standard blood tests alone.
How much magnesium is in the body, and where is it stored?
In an adult body, there is approximately ~25g of magnesium. Roughly 50–60% is stored in the bones, and the rest is in soft tissues (muscles, organs).
Only about 1% of total body magnesium is present in the blood (plasma and blood cells combined). This means the vast majority is stored outside the bloodstream.
So even if you have your serum magnesium measured, you’re only assessing a small fraction of your total magnesium reserves.
Why serum magnesium does not always reveal a problem
Total serum magnesium (tMg) is the most commonly measured marker. However, it reflects less than 1% of total body magnesium and can be influenced by factors such as binding to albumin. It also poorly reflects intracellular magnesium levels. As a result, there is currently no single universally reliable biomarker for assessing overall magnesium status.
“Low normal” can still be insufficient
Some studies also point out that values in the low‑normal range (e.g. <0.85 mmol/l) may be associated with higher cardiometabolic risk. This has led to discussions that commonly used thresholds may underestimate the true extent of the problem.
Why magnesium is important
Magnesium is involved in so many physiological processes that without it, many fundamental systems in the body would function poorly — or not at all.
Energy and fatigue
One of magnesium’s best‑known roles is its involvement with ATP (adenosine triphosphate), often described as the “energy currency” of the cell. In simple terms, ATP in the body typically functions as a complex with magnesium (Mg–ATP). This is one reason why long‑term low magnesium levels may be associated with fatigue and reduced performance — the body is less well equipped for efficient energy metabolism.
Nerves and muscles
Magnesium plays an important role in neuromuscular stability — in other words, it helps prevent the nervous and muscular systems from becoming overly excitable.
Heart rhythm
The heart is extremely sensitive to the balance of ions (Mg, K, Ca). In hypomagnesemia, EKG changes and increased risk of arrhythmias are described, from extrasystoles to serious ventricular arrhythmias.
Interaction with potassium and calcium
Magnesium is often a “hidden” factor in other electrolyte imbalances:
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Hypokalemia (low potassium):This can be difficult to correct if magnesium levels are also low. Clinical reviews note that hypomagnesaemia is frequently associated with hypokalaemia.
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Hypocalcemia (low calcium):Magnesium plays a role in calcium regulation, and deficiency may contribute to reduced calcium levels.
Blood sugar regulation and insulin
Low magnesium status has also been linked to increased cardiometabolic risk and is associated with metabolic conditions such as type 2 diabetes.
Muscle health and performance
In a review focusing on muscle and neuromuscular conditions, magnesium is discussed in relation to muscle contraction, mitochondrial function and oxidative stress — mechanisms that may influence recovery and overall muscle health.
Oxidative stress and mitochondria
Within muscle cells (and cells more broadly), magnesium is involved in mitochondrial energy processes and may help reduce the formation of reactive oxygen species — commonly referred to as free radicals.
How magnesium deficiency develops: low intake vs. increased losses
When people hear “magnesium deficiency”, they often assume it simply means not eating enough magnesium. That’s frequently true — but it’s not the only cause. In practice, magnesium deficiency usually develops due to a combination of three factors:
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Low dietary intake
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Reduced intestinal absorption
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Increased losses (primarily via the kidneys)
Low magnesium intake is the most common cause
A review examining the impact of magnesium deficiency notes that insufficient intake is common and is linked to broader changes in modern dietary patterns.
In particular, many people consume fewer magnesium‑rich foods — such as nuts, seeds, pulses, wholegrains and leafy green vegetables — while eating more highly processed foods, which typically contain less magnesium.
In practice, this means that average magnesium intake is often below recommendations — approximately by a third in women and by a quarter in men.
Absorption: when intake alone is not enough
Magnesium is absorbed in the intestine, and its bioavailability varies depending on the chemical form and what it is consumed alongside.
Some plant compounds can bind magnesium and reduce its absorption — typically phytates (e.g., in whole grains and legumes) and oxalates (e.g., in spinach).
Excessive magnesium losses via the kidneys
The kidneys are the main organ that determines how much magnesium the body retains. And some factors can increase magnesium excretion.
Higher sodium intake (typically a very salty diet) and very high calcium intake can increase magnesium excretion.
Who is at greater risk of magnesium deficiency?
Older people — with age, the efficiency of intestinal absorption tends to decline, and urinary magnesium excretion may increase. As a result, the risk of deficiency is higher in older individuals.
Pregnant women (and often postmenopausal women) — pregnancy increases magnesium requirements. In some women, this can make low magnesium levels more likely.
People with chronic diseases (e.g., diabetes, hypertension, chronic kidney disease, intestinal diseases) may increase urinary or intestinal magnesium losses or alter its distribution within the body.
People with chronic diarrhoea / malabsorption (e.g., Crohn’s disease, ulcerative colitis, celiac disease) can impair magnesium absorption and increase losses in the stool.
People taking certain medications — typically diuretics and proton pump inhibitors (PPIs), but also some antibiotics or other drugs. They can reduce magnesium reabsorption in the kidneys or its intestinal absorption.
People with higher alcohol consumption — alcohol is associated with increased urinary magnesium losses and poorer overall nutritional status. This often results in lower serum and intracellular magnesium levels.
Highly physically active people (especially with heavy sweating) — magnesium can be lost more through sweat and urine. If intake is not increased at the same time, magnesium status may slightly decrease.
Socially disadvantaged groups — the risk increases mainly due to poorer access to quality care and the presence of other risks (co‑morbidities, medication), with data showing a link between lower socioeconomic status and lower magnesium intake.
Many athletes may have a magnesium deficiency
During exercise, magnesium is lost not only in urine but also through sweat. Prolonged or intense activity can increase urinary excretion further, potentially raising magnesium requirements by around 10–20%. Interestingly, athletes sometimes show lower blood magnesium levels, even though their intake from food is higher than in the general population.
Symptoms of magnesium deficiency: acute vs. chronic deficiency
Symptoms of magnesium deficiency can be difficult to recognise because they often begin with non‑specific signs (fatigue, weakness, “odd sensations”) that are easily attributed to stress, poor sleep or overwork.
More characteristic symptoms affecting muscles, nerves and the heart tend to appear only with more significant deficiency or in higher‑risk situations.
Acute (rapid‑onset) deficiency
Acute magnesium deficiency means that magnesium levels drop suddenly — over hours to days — usually due to increased losses or an acute medical issue. The onset is often subtle: nausea, vomiting, loss of appetite, fatigue and weakness appear.
If the deficit continues, “irritability” of nerves and muscles increases, and tremors, muscle cramps/spasms, tetany and, in more severe cases, seizures or delirium may occur.
Chronic (long‑term) deficiency: when it lasts months to years
With long‑term low intake and gradual depletion of stores, symptoms often persist and are not dramatic. Chronic deficiency can lead to milder, nonspecific symptoms such as:
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Persistent fatigue
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Weakness
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Reduced physical performance
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More frequent muscle cramps
Chronic deficiency is often associated with disturbances in other minerals. Hypomagnesaemia frequently coexists with hypokalaemia (low potassium) and hypocalcaemia (low calcium), and may partly explain why these imbalances can be difficult to correct.
Is there a test for magnesium deficiency?
There is no single perfect test for magnesium deficiency. The most commonly measured marker is serum (total) magnesium, but this represents only a small part of the body’s magnesium and may not reliably reflect tissue stores. Therefore, it is recommended to use a comprehensive assessment — combine symptoms and risk factors with laboratory results, and depending on the situation, add urine testing (24‑hour collection) to distinguish low intake/absorption from increased kidney losses.
How to replenish magnesium quickly
The most effective approach is usually a combination of dietary improvements and appropriate food preparation methods, with supplementation added if necessary. According to research, magnesium status in the body is most influenced by diet, but in real‑world data, many adults do not reach recommendations from food alone — and in such cases, supplementation is a reasonable addition.
1) Foods rich in magnesium
Magnesium is widely distributed in foods, with plant sources contributing the most in the general population. The richest natural sources of magnesium include nuts, seeds, legumes, whole grains and green leafy vegetables.
Dairy products and fish can also contribute, as can fortified foods (such as some breakfast cereals and certain plant‑based milk alternatives).
2) How to increase magnesium absorption from food
It’s not just about how much magnesium you eat, but also how much you actually absorb. Some substances (especially phytates and oxalates) can bind magnesium and reduce its intestinal absorption.
In practice, however, there are simple methods that improve absorption. You can try fermentation, soaking or sprouting plant foods.
3) Mineral waters: an often overlooked source
Drinking water can also provide magnesium. The amount varies depending on the source and treatment of the water. In waters rich in magnesium (typically Magnesia), there is a link to improved magnesium levels, so they can serve as a useful supplement, especially for people with suboptimal intake.
4) Food supplements: when to use them, which forms to choose and what to consider
Diet is still the foundation. At the same time, it is true that in the real world a significant portion of adults do not reach recommended intakes from food alone — and in such situations, a supplement can be a reasonable “insurance” (especially for people with increased needs or limited dietary options).
When to consider supplements
Magnesium supplements usually make sense when you know long‑term that your diet lacks main sources of magnesium (nuts/seeds/legumes/whole grains/“greens”), and you can’t change that and/or you have higher needs or risk factors (e.g., certain medications, repeated diarrhoea/vomiting, older age, etc.— these connections were discussed earlier).
How to choose the right form
If your goal is to replenish magnesium stores in the body, the most important things are: how much elemental magnesium you actually take in and in what form (because forms differ in solubility, absorption and tolerability). Organic salts (e.g., citrate, glycinate, malate) usually have higher solubility and absorption than inorganic forms like oxide or sulphate.
Always check the elemental magnesium content
On the label, look for the amount of “elemental magnesium (Mg)”. This is the figure that counts towards your daily intake — not the total weight of the compound.
When compensating for a deficiency, in most cases, a dose of 300‑600mg of elemental magnesium per day is chosen.
To replenish magnesium stores, choose forms with higher bioavailability
Magnesium citrate
Magnesium citrate is often a suitable option for correcting a deficiency.Studies in humans repeatedly show that citrate increases both serum and urinary magnesium levels more effectively than magnesium oxide, and in some comparisons may even be better absorbed than certain chelated forms.
The downside for some people is that in higher doses it can cause looser stools. This can be prevented by splitting the dose into several smaller doses per day.
For those with sensitive digestion, magnesium bisglycinate may be a better option than citrate. Studies report good absorption and favourable gastrointestinal tolerance. This is thought to be due to its chelated structure, which may allow easier transport and fewer interactions with other minerals.
Magnesium malate is also considered a form with good bioavailability and is generally well tolerated. It is sometimes discussed in relation to energy production and muscle performance because of its malate component. However, the amount of malate provided in a typical magnesium supplement is usually too low to produce a significant standalone effect.
Magnesium threonate is formulated to more effectively raise magnesium levels within the central nervous system.
In a recent study involving healthy adults with subjectively poorer sleep, six weeks of magnesium threonate (Magtein®) supplementation led to improvements in some cognitive measures and certain subjective sleep parameters. However, objective sleep measures were less conclusive.
How long does it take to replenish magnesium?
In cases of mild deficiency, supplementation can raise serum magnesium levels relatively quickly — often within about a week. However, restoring intracellular magnesium levels takes longer. For this reason, supplementation may need to continue for several additional weeks to fully replenish tissue stores.
Bottom line
Magnesium is a crucial mineral for energy metabolism, nerve and muscle function, and heart health. Yet many people consume less than recommended amounts over the long term. The preferred approach is “food first” — prioritising nuts, seeds, pulses, wholegrains and "greens", and using appropriate preparation methods to support absorption.
Food supplements make sense mainly when intake cannot be increased or if you belong to a risk group (e.g., PPI/diuretics, diarrhoea, older age, pregnancy, athlete). If you supplement to replenish stores, more absorbable forms like citrate or bisglycinate are usually preferable, and the dose should be chosen with regard to tolerance (diarrhoea is the most common side effect).
