Blood magnesium: what it is, what it tells you and why we test it
Blood magnesium is the amount of magnesium in your blood. Magnesium keeps your muscles, nerves and heart working, but your blood holds only about one percent of your body’s total. That is why a normal result does not rule out a shortage.
Why we test it
Magnesium keeps your muscles, nerves and heart working, and helps your body use the energy in food.
Almost all of it sits in your bones and inside your cells. Your blood holds little, and keeps it steady by drawing on your stores.
That makes it a weak but useful signal: if it is low, your stores are usually low too. And a low magnesium can keep potassium and calcium low as well.
What it tells you, and what it does not
If it is low, a diet short on vegetables, legumes and whole grains, alcohol, lasting diarrhoea or some medicines are often behind it.
If it is normal, that does not have to mean you have enough: your blood stays in range even while your stores run down.
If it is high, supplements or laxatives with magnesium are usually behind it, especially when the kidneys filter less.
That is why it is read together with other tests:
- Potassium: When magnesium is short, a low potassium struggles to come back up
- Calcium: Without enough magnesium, calcium can drop too
- Vitamin D: Magnesium also helps activate it
- Creatinine: Your kidneys clear magnesium: if they filter less, it can rise
Normal magnesium levels
The lab decides what is normal. It is usually 1.7 to 2.4 mg/dL, the same for women and men. Your own report may print different limits.
Some labs measure it in mmol/L: the numbers are smaller, usually 0.7 to 1.0, but you read them the same way. Compare your result with the limits printed next to it.
A normal result does not rule out a shortage: your blood holds only a small part of your body’s magnesium.
In the Rosa report you also see your optimal range, with potassium and calcium in the same panel.
What your result means
Find the row for your result, in mg/dL. A result outside the limits needs explaining with your doctor, especially if your potassium is low too or your kidneys filter less.
- Below your report’s limit, Usually below 1.7: Low. Diet, alcohol, diarrhoea or some medicines are often behind it. Your stores are usually low too. Check your potassium and calcium too, and talk to your doctor before taking supplements.
- Within your report’s limits, Usually 1.7 to 2.4: Normal. Usually fine. But your blood holds little magnesium, so it does not rule out a shortage. In the Rosa report you also see whether you are in your optimal range.
- Above your report’s limit, Usually above 2.4: High. It is uncommon. Supplements or laxatives with magnesium, or kidneys that filter less, are usually behind it. Check your creatinine and eGFR and talk to your doctor. If you take products with magnesium, tell them.
If your magnesium is low or high
- Low magnesium: Diet, alcohol, diarrhoea or some medicines are often behind it. The causes and what to do are further down this page.
- High magnesium: It is uncommon: supplements or laxatives with magnesium, or the kidneys, are usually behind it. More on that below.
What causes low magnesium
There is often more than one, and many can be changed.
- Little magnesium on your plate: A diet with few leafy greens, legumes, nuts and whole grains.
- Alcohol: Drinking often makes you lose more magnesium in your urine.
- Lasting diarrhoea: Your gut loses magnesium, especially if diarrhoea goes on for a long time or keeps coming back.
- Some medicines: For example stomach acid medicines called proton pump inhibitors (such as omeprazole or pantoprazole), and some water tablets. Do not stop them on your own: tell your doctor.
- Diabetes: Low magnesium is more common with diabetes: the body loses more of it in the urine.
Symptoms of low or high magnesium
A magnesium just outside the limits usually causes no symptoms. Cramps and tiredness are very common and often have no clear cause. For night cramps in older adults, for example, magnesium supplements usually do not help.
When it is very low, tremors, muscle twitches or an irregular heartbeat can appear. A high magnesium is uncommon, and if it rises a lot it can cause weakness, nausea and drowsiness.
With a very irregular or very slow heartbeat, severe weakness or drowsiness, trouble breathing, fainting or a seizure, call 112 or go to the emergency department.
How to raise a low magnesium
It usually starts with food. Where to start:
- Eat leafy greens, legumes, nuts, seeds and whole grains every day: they are among the richest sources.
- Drink less alcohol.
- If you often have diarrhoea, talk to your doctor: the cause needs finding.
- If you take stomach acid medicines or water tablets, tell your doctor and do not stop them on your own.
- Take magnesium supplements only if your doctor advises them, especially if your kidneys filter less: they may not be able to clear it.
- If your potassium or calcium is low too, talk to your doctor: they are usually corrected together.
What can skew the result
A few things change the number for a while. It helps to know them before you read it.
- A supplement before the test: Magnesium taken in the hours before can raise the number for a while, without changing your stores. Tell your doctor.
- A haemolysed sample: If red blood cells break in the tube, for example when the sample waits too long, they release magnesium and the number rises by mistake. Your doctor can have it repeated.
- A different unit: If your report uses mmol/L, the numbers are smaller. Compare your result with the limits printed next to it.
When to worry
Talk to your doctor if:
- Your magnesium is below your report’s limit.
- It is low and your potassium or calcium is low too.
- It is high, especially if your kidneys filter less or you take supplements or laxatives with magnesium.
- Your magnesium is low and you have tremors or muscle twitches: talk to your doctor soon. If you notice your heart beating irregularly, see a doctor today.
- You have a very irregular or very slow heartbeat, severe weakness or drowsiness, trouble breathing, fainting or a seizure: call 112 or go to the emergency department.
Frequently asked questions
Does a normal magnesium mean I am not short of it? Not necessarily. Your blood holds about one percent of your body’s magnesium, and keeps it steady by drawing on your stores. So it can stay normal while your stores run down.
High magnesium: what does it mean? It is uncommon. Supplements, laxatives or antacids with magnesium are usually behind it, especially when the kidneys filter less. Talk to your doctor and check your creatinine and eGFR too.
What does too much magnesium from supplements do? Usually diarrhoea and an upset stomach. It rarely builds up in the blood, because the kidneys clear the extra, but it can when they filter less.
What do magnesemia and magnesiemia mean on my report? They are two Italian names for magnesium measured in the blood. Your report may also call it magnesio or Mg.
Do I need to fast for a magnesium test? Not for magnesium, usually. It is done with other tests that do need fasting, though, so it is best to arrive fasted. If you take magnesium supplements, tell your doctor.
How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it once a year. If it is low and you change what you eat, it is usually checked again after about three months, or when your doctor says.
Sources
- Jahnen-Dechent and Ketteler, magnesium basics: where it sits in the body, how it is measured and what the blood level shows. Clinical Kidney Journal, 2012. PMID 26069819
- de Baaij and colleagues, magnesium in health and disease, including the medicines that lower it. Physiological Reviews, 2015. PMID 25540137
- Huang and Kuo, why a low potassium is hard to correct when magnesium is low. Journal of the American Society of Nephrology, 2007. PMID 17804670
- Gommers and colleagues, low magnesium in diabetes. Diabetes, 2016. PMID 26696633
- Garrison and colleagues, Cochrane review of magnesium for muscle cramps. Cochrane Database of Systematic Reviews, 2020. PMID 32956536
