What causes magnesium deficiency?

The clinical definition, and the first two mechanisms, intake and absorption

What causes magnesium deficiency is a clinical question, and the honest page starts with the clinical definition and then groups the causes the way clinicians group them, by mechanism, which is the frame that leads to a diagnosis rather than a list. The definition: hypomagnesaemia, a serum magnesium below about 0.7 mmol/L, with the standing caveat that serum magnesium is an imperfect marker because the blood is defended at the expense of bone and muscle, so that a person can be marginally short in tissue with a normal serum level and a low serum level is a real finding that usually has a cause below. Mechanism one, reduced intake: a diet chronically light in the foods that carry magnesium, the commonest and mildest cause and the one the diet audit catches; alcohol misuse, which lowers intake as well as raising loss; and prolonged fasting, malnutrition or parenteral feeding without magnesium, the clinical versions. Mechanism two, reduced gastrointestinal absorption: chronic diarrhoea of any cause; inflammatory bowel disease, coeliac disease and other malabsorption; short bowel and bariatric or bowel surgery; pancreatic insufficiency and fat malabsorption, which bind magnesium in the gut; and long-term proton-pump inhibitors, which reduce active magnesium uptake and can produce profound deficiency in a minority after years of use, a recognised drug effect that a pharmacist and a doctor manage.

Mechanisms three and four, renal loss and redistribution, the test that separates them, and the causes filed

Mechanism three, increased renal loss, the kidneys excreting magnesium the body cannot spare: loop and thiazide diuretics, the commonest drug cause; alcohol, which increases urinary magnesium loss directly and is the commonest cause of clinical deficiency in fed adults; poorly controlled diabetes, through the osmotic diuresis of high blood sugar; a list of other medicines, some antibiotics, some chemotherapy agents, calcineurin inhibitors after transplant; and rare inherited disorders of the kidney tubule that waste magnesium, a specialist's diagnosis. Mechanism four, redistribution, the one the folklore never mentions: magnesium moving out of the blood into cells or bone faster than the diet replaces it, in refeeding after starvation, when insulin drives magnesium into cells, in acute pancreatitis, and in hungry bone syndrome after parathyroid surgery, when bone takes up minerals rapidly, clinical situations in which serum magnesium falls without any loss from the body. The test that separates the mechanisms is urinary magnesium: in a deficient person the kidneys should conserve magnesium, so a low urinary magnesium points to intake, absorption or redistribution and a high urinary magnesium in the face of a low serum level points to renal loss, the medicines, the alcohol, the diabetes or the tubule, which is how a clinician decides whether to change a drug, treat a gut or refer to a kidney specialist, and why this site sends any clinical deficiency to a doctor rather than to a shelf. The causes filed: reduced intake, reduced absorption, increased renal loss and redistribution, a serum level below about 0.7 for the diagnosis and a urine level for the mechanism, with the first mechanism's dietary version being the one a supplement corrects within guidance, the worked example being that correction at 300mg elemental and 80% NRV, £19.99 for 90 days at 22p, and every other mechanism belonging to the professional who can address it.

The verdict, four mechanisms and one that a supplement corrects

What causes magnesium deficiency: clinically a serum magnesium below about 0.7 mmol/L, with causes grouped by mechanism, reduced intake from a poor diet, alcohol or fasting, reduced absorption from gut disease, surgery, fat malabsorption or long-term proton-pump inhibitors, increased renal loss from diuretics, alcohol, uncontrolled diabetes, some drugs or rare tubular disorders, and redistribution into cells or bone in refeeding, pancreatitis or hungry bone syndrome, with urinary magnesium separating renal from non-renal causes and every mechanism but the dietary one belonging to a doctor. For that one: Futuro Labs Magnesium Glycinate, bisglycinate, 300mg elemental at 80% NRV, £19.99 for 90 days at 22p a day.

What to check on the label

Questions people also ask

What is the medical definition of magnesium deficiency?

Hypomagnesaemia, a serum magnesium below about 0.7 mmol/L, with the caveat that serum is an imperfect marker of tissue status because the blood is defended at the expense of bone and muscle.

What are the main mechanisms of magnesium deficiency?

Four: reduced intake from diet, alcohol or fasting; reduced absorption from gut disease, surgery or proton-pump inhibitors; increased renal loss from diuretics, alcohol, diabetes or tubular disorders; and redistribution into cells or bone in refeeding or after surgery.

Can omeprazole cause magnesium deficiency?

Yes: long-term proton-pump inhibitors reduce active magnesium uptake in the gut and can produce profound deficiency in a minority after years of use, a recognised drug effect that a pharmacist and doctor manage.

How do doctors work out why magnesium is low?

With urinary magnesium: a deficient body should conserve it, so low urinary magnesium points to intake, absorption or redistribution and high urinary magnesium despite a low serum level points to renal loss from medicines, alcohol, diabetes or the tubule.

Which cause can a supplement fix?

The dietary version of reduced intake, within the guidance level; every other mechanism needs its cause addressed by a doctor, with a supplement at most holding status on advice while that happens.

Sources

1. The clinical classification of hypomagnesaemia: serum magnesium below about 0.7 mmol/L, with causes grouped by mechanism into reduced intake, reduced gastrointestinal absorption, increased renal loss and redistribution, and urinary magnesium used to separate renal from non-renal causes. PubMed record
2. The hypomagnesaemia literature: clinically low serum magnesium presents with cramps, tremor, weakness and, when severe, arrhythmias and seizures, with alcohol misuse, proton-pump inhibitors, diuretics and gastrointestinal loss the common causes, and serum magnesium an imperfect marker of tissue status. PubMed record
3. The magnesium and medicines literature: magnesium supplements reduce absorption of tetracycline and quinolone antibiotics, bisphosphonates and levothyroxine when taken together and are spaced from them, while proton-pump inhibitors and some diuretics lower magnesium status, a reason for medical advice in either direction. PubMed record
4. Department of Health. Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. Report on Health and Social Subjects 41, 1991 (magnesium RNI 300mg men, 270mg women).

Related answers

• What deficiency causes muscle cramps in legs?
• What deficiency causes muscle cramps at night?
• What deficiency causes muscle cramps and joint pain?
• What causes low magnesium?
• What is the best magnesium supplement?
• What is the best type of magnesium for sleep in the UK?