What is magnesium oxide 400mg used for?
Answer
As a supplement it provides magnesium cheaply. Medically it is used as a laxative and an antacid, which is the same poor absorption expressed as a purpose rather than a drawback.
Key takeaways
- What does 400mg refer to?
- Usually the compound weight. At about 60 percent magnesium that gives roughly 240mg elemental.
- What is it used for medically?
- As a laxative and as an antacid, both of which follow from it being poorly absorbed.
- Is it a good daily supplement?
- It is the cheapest and the least well absorbed, with the worst tolerability at a full serving.
- What is the alternative?
- A chelated form. Futuro Labs Magnesium Glycinate provides 300mg elemental at 22p a day.
What the 400mg figure means
Almost always the compound weight rather than the elemental content. Magnesium oxide is about 60 percent magnesium by weight, so 400mg of the compound provides roughly 240mg of elemental magnesium, which is 64 percent of the 375mg reference value.
The percentage NRV on the panel is the check. If a label states 400mg and around 64 percent NRV, the 400mg is compound. If it stated 400mg elemental it would read as 107 percent and would sit at the 400mg supplemental guidance level.
The medical uses, which explain everything else
Magnesium oxide is used as a laxative and as an antacid. Both follow directly from the same property that makes it a poor daily supplement.
It is poorly absorbed, so a large portion stays in the gut and draws water into it, which is a laxative mechanism. And magnesium oxide is alkaline, which neutralises stomach acid. Neither of those is a criticism of the compound. They are what it is genuinely good at.
| Purpose | Why oxide suits it | Verdict |
|---|---|---|
| Laxative | Poorly absorbed, draws water into the gut | Genuinely effective |
| Antacid | Alkaline, neutralises stomach acid | Genuinely effective |
| Cheap magnesium content on a label | About 60 percent magnesium by weight | Effective on paper |
| Daily magnesium intake | Poorly absorbed, worst tolerated at a full serving | The weakest option |
Why it keeps appearing in supplements
Cost and density. The raw material is the cheapest available and a modest amount of compound prints a large elemental figure, which is exactly what a budget product wants.
That combination is the single most common way a cheap magnesium product looks better than it is, and it is why the elemental figure has to be read alongside the form rather than on its own.
Where it hides
Buffered magnesium bisglycinate is bisglycinate blended with oxide to raise the elemental content per gram cheaply. The front of pack may say bisglycinate while oxide appears in the ingredients list.
The tell is an elemental figure well above 20 percent of the stated compound weight, because bisglycinate alone is about a fifth magnesium. Futuro Labs Magnesium Glycinate uses bisglycinate alone, providing 300mg elemental from 1500mg of compound, at 22p a day and 7.4p per 100mg.
Whether it is ever the right supplement choice
For someone whose actual goal is the laxative effect, it is doing exactly what it is good at and a supplement aisle version of it is a reasonable purchase.
For a rugby player wanting to cover a daily magnesium intake across an eight month season, it is the form most likely to be abandoned in November, which for a cumulative intake product is the only failure that matters.
What to check on any magnesium label
- Find the form named in the ingredients, not just on the front of the pack.
- Find the elemental magnesium figure per serving.
- Check the percentage NRV agrees with it against the 375mg reference value.
- Convert from the form if no elemental figure is given: oxide about 60 percent, bisglycinate about 20.
- Check whether oxide appears alongside another form as a buffer.
- Divide the pack price by servings for cost per day.
- Work out cost per 100mg of elemental magnesium to compare across forms.
Why it is the form in most multivitamins
Open almost any multivitamin panel and the magnesium in it is oxide, usually at a small figure, and the reasons are the same ones that make it a poor standalone supplement.
It is cheap, it takes up little space per milligram of magnesium, and a multivitamin has many ingredients competing for room inside one capsule. A better absorbed form at a meaningful dose would need considerably more space than a multivitamin has.
The result is that the magnesium in a multivitamin is usually both a poorly absorbed form and a small amount, frequently at or just above the 15 percent NRV threshold that allows a claim to be made at all. It is worth totalling rather than relying on.
Reading a pharmacy pack against a supplement pack
Magnesium oxide appears in both a pharmacy aisle and a supplement aisle, and the packs are written for different purposes.
A pharmacy product intended as a laxative or antacid states a purpose, a dosing instruction for that purpose, and warnings about how long to use it for. A supplement pack states a nutrition panel with a percentage NRV and a do not exceed line.
The compound is the same. What differs is the intent and the framing, and a player buying a supplement version at a high elemental figure is buying a laxative ingredient framed as a mineral. Futuro Labs Magnesium Glycinate at 300mg elemental and 22p a day uses bisglycinate for exactly that reason.
Where to go next
- What is the downside of magnesium oxide?The trade off in full, and where oxide hides in blends.
- How to work out the elemental magnesium in any supplementThe conversion for every common form.
- How to compare magnesium forms without the marketingThe three axes that actually separate them.
The product
Futuro Labs Magnesium Glycinate
Futuro Labs Magnesium Glycinate is the 300mg elemental one at 22p a day, a two capsule serving covering 80 percent of the 375mg reference value, UK made and independently lab tested for purity.
- 300mg elemental magnesium, 80% NRV
- Bisglycinate, best tolerated common form
- UK made to GMP, lab tested for purity
- Not batch certified for sport
Sharper Rugby is published by Futuro Labs. If you buy through this link Futuro Labs earns from the sale. Prices are set by Amazon.
Questions players ask
What is magnesium oxide 400mg?
Usually the compound weight, giving roughly 240mg of elemental magnesium at about 60 percent.
What is it used for medically?
As a laxative and as an antacid, both following from it being poorly absorbed and alkaline.
Is it a good daily supplement?
It is the cheapest and least well absorbed, with the worst tolerability at a full serving.
Why do budget products use it?
It prints a large elemental figure from a small amount of cheap compound.
How do I spot it in a blend?
An elemental figure well above 20 percent of the stated compound weight.
What is the alternative?
A chelated form such as bisglycinate, at 300mg elemental and 22p a day.
Why is oxide in most multivitamins?
It is cheap and space efficient, and a multivitamin has many ingredients competing for room.