Magnesium Oxide

Magnesium oxide is the form our naturopaths spend the most time explaining in store, because the label reads impressively and the experience often doesn't match. These are the questions they ask, in the same order.

A good fit if

• You already take an oxide-based tablet and it's never changed your bowel habits
• You want the most magnesium per tablet and you're happy to adjust the dose to suit you
• You want to understand why one product lists 440 mg and another 265 mg from the same tablet
• You're comparing labels and want to know which number is the compound and which is the mineral
• Your kidneys are healthy and you don't take any medicine that needs to be spaced away from minerals

Check with us first if

• Loose bowels are why magnesium hasn't worked for you before. The oxide part of a blend is usually the cause, and a chelate is usually the answer.
• The product says high strength. Check whether oxide is carrying the magnesium figure, because that's where the gap between the label and what you absorb opens up.
• You already take a laxative or stool softener. Adding more water-drawing magnesium on top is worth a chat rather than an experiment.
• You take bisphosphonates, tetracycline or quinolone antibiotics, diuretics (water tablets) or long-term reflux medicine (a proton pump inhibitor). The spacing and monitoring apply to every form of magnesium and are set out on our magnesium page.
• The product is for a child aged 1 to 8. The Australian thresholds for the label warning are much lower for them than for adults.
• You take any prescription medicine. A quick check with your doctor, pharmacist or our naturopaths first is the sensible step.

• Magnesium oxide (Schedule 1 item 3168). Best known for: The reference entry, and the easiest way to put a large magnesium number on a label. Absorption: Poor. It's slow to dissolve, and a substantial share passes through unabsorbed. Typical use: High-strength tablets, where it supplies most of the magnesium on the label.
• Heavy magnesium oxide (item 2497). Best known for: A denser powder grade that presses into a smaller tablet that's easier to swallow. Absorption: The same chemistry once dissolved; the grade is about manufacturing, not quality. Typical use: The wording you'll see on most Australian high-strength magnesium labels.
• Light magnesium oxide (item 3019). Best known for: The bulkier, lower-density grade of the same compound. Absorption: No different from the heavy grade once dissolved. Typical use: Chosen when powder flow or volume matters to the manufacturer.
• Magnesium hydroxide (item 3162). Best known for: The other poorly soluble inorganic salt in common use. Absorption: Also low, and counted in the same total that triggers the advisory statement. Typical use: Carries the same laxative statement and the same restriction for babies.
• Magnesium citrate. Best known for: The best-studied organic salt, and the one absorption studies usually compare against. Absorption: Higher than oxide, measured by magnesium in the urine, in a randomised crossover trial. Typical use: Everyday tablets and powders, where the bowel effect is acceptable or wanted.
• Magnesium glycinate. Best known for: Gentle on the bowel, because it's a chelate rather than an inorganic salt. Absorption: It doesn't count toward the inorganic magnesium total at all, so no statement applies. Typical use: Larger single doses; the form people often move to after oxide disagreed with them.

Reference values. The NHMRC sets its recommended amounts for actual (elemental) magnesium, not for any particular form, and the full list by age and sex is on our magnesium page. What matters here is the conversion. Magnesium makes up roughly 60 per cent of magnesium oxide by weight, so a 440 mg dose of the compound gives you close to 265 mg of actual magnesium, before you even ask how much is absorbed.

Typical supplemental range. Australian tablets usually list heavy magnesium oxide at somewhere between 250 mg and 440 mg per tablet, giving roughly 150 mg to 265 mg of actual magnesium, usually alongside a second or third form. The trial that tested magnesium oxide against a placebo for long-term constipation used a much bigger 1.5 g a day, split into three 0.5 g doses, over four weeks.

Upper level. The NHMRC upper limit for magnesium from supplements is on our magnesium page, and it applies to the actual magnesium. Oxide reaches that limit with less compound than any other form. That's exactly why the Australian label warning is based on the total magnesium from mineral (inorganic) forms, rather than the dose of the salt itself.

Worth knowing. Two numbers on an oxide label do different jobs and are easy to mix up. The compound weight tells you how much powder is in the tablet. The elemental figure underneath tells you how much magnesium that is. Neither tells you how much you'll absorb, which is the one number no label can give you. What you need depends on your age, diet and health, so talk to one of our naturopaths.

Food first, where the diet allows it.

• Milk: about 24 to 27 mg of magnesium per cup
• Halibut, cooked: about 24 mg per 85 g
• Raisins: about 23 mg per half cup
• Beef mince, 90% lean, cooked: about 20 mg per 85 g
• Broccoli, chopped and cooked: about 12 mg per half cup
• Apple: about 9 mg per medium apple

If any of these sound like you, talk to one of our naturopaths or your doctor before you start.

• Reduced kidney function or kidney failure. Your body can't clear excess magnesium, so it can build up; don't supplement without medical supervision.
• Babies under 12 months. Schedule 1 says the medicine must not be directed for use in babies younger than 12 months when taken by mouth.
• Children aged 1 to 8 years. The Australian advisory thresholds are 65 mg of total inorganic magnesium for ages 1 to 3 and 110 mg for ages 4 to 8, far below the adult figure.
• If you already take a laxative or stool softener. Magnesium oxide draws water into the bowel, which adds to what you're already taking.
• Bisphosphonates, tetracycline and quinolone antibiotics, diuretics and long-term proton pump inhibitors. See our magnesium page.
• If you decided magnesium disagrees with you after an oxide-based product. The salt, not the mineral, is the usual explanation.
• Taking more than the label dose. Extremely high intakes can lead to an irregular heartbeat and cardiac arrest.
• A lasting change in bowel habit. That's a reason to see a doctor, not to change products.
• If you take prescription medicines, check with your doctor, pharmacist or our naturopaths before you start.