What to look for in a magnesium supplement
Answer
Find the serving size, find the elemental figure, convert it to a percentage of 375mg, check the form, check the excipients, then divide the pack price by servings. Five minutes and the shelf sorts itself.
Key takeaways
- What is the first thing to check?
- The serving size in capsules. Every other figure on the label depends on it.
- What is the number that actually matters?
- Elemental magnesium per serving. The compound weight on the front tells you almost nothing on its own.
- How do you convert a compound weight?
- Bisglycinate is about 20 percent magnesium, citrate about 16 percent and oxide about 60 percent by weight.
- How is cost per day worked out?
- Pack price divided by servings in the pack, never by capsules.
Step one: serving size
Turn the pack over and find how many capsules make a serving. Everything else on the label is expressed against that number, and it is the single most common place a comparison goes wrong.
It also decides how long a pack lasts. A 180 capsule tub at two capsules a serving is 90 days. The same tub at three capsules is 60 days, which changes cost per day by half as much again.
Step two: find the elemental figure and convert it
The front of the pack usually shows the compound weight. Underneath, on the nutrition panel, there should be a line for magnesium itself with a percentage NRV beside it.
If both are present, the arithmetic is done for you. If only a compound weight is given, you can approximate it, and a product that makes you do that has told you something about itself.
| Form | Roughly magnesium by weight | Worked example |
|---|---|---|
| Magnesium bisglycinate | About 20 percent | 1500mg gives about 300mg elemental |
| Magnesium citrate | About 16 percent | 1500mg gives about 240mg elemental |
| Magnesium oxide | About 60 percent | 500mg gives about 300mg elemental, poorly absorbed |
| Magnesium malate | About 15 percent | 1500mg gives about 225mg elemental |
| Magnesium taurate | About 9 percent | 1500mg gives about 135mg elemental |
Step three: percentage of the reference value
Divide the elemental figure by 375 and multiply by a hundred. 300mg elemental is 80 percent of the reference value. 56mg elemental, which is what some multivitamins provide, is 15 percent, which is exactly the threshold a product needs to clear before it may carry any magnesium claim at all.
That threshold is useful in both directions. A product carrying magnesium claims must be at or above 15 percent NRV, and a product below it that still uses claim wording is outside what the rules allow.
Step four: form and excipients
For daily use across a season, bisglycinate or citrate over oxide, because oxide is poorly absorbed and is the form most associated with loose stools at a full serving.
Then read the ingredients list rather than just the panel. Bulking agents, anti caking agents and capsule material live there. None of them is alarming in itself and all of them should be declared. Futuro Labs Magnesium Glycinate uses a vegan capsule and declares its excipients, which is the standard to hold anything else to.
Step five: cost per day
Pack price divided by servings. Amazon's per count figure is per capsule, which flatters any product with a multi capsule serving, so it is not a shortcut.
Worked through on Futuro Labs Magnesium Glycinate: 19.99 pounds, 180 capsules, two per serving, so 90 servings, so 22.2p a day. The 90 capsule pouch is 9.99 pounds for 45 servings, which is also 22.2p. A cheaper looking tub with a three capsule serving and no elemental figure is not cheaper, it is just harder to check.
The whole check, as a list
- Capsules per serving, and servings per pack.
- Elemental magnesium per serving, printed rather than inferred.
- Percentage NRV against 375mg.
- Form: bisglycinate or citrate over oxide for daily use.
- Bulking agents and capsule material, declared.
- Purity testing, and batch certification if you could be tested.
- Pack price divided by servings.
- No cramp or sleep claims on the pack, because neither is authorised.
Two labels compared, step by step
Worked through on two products of the kind that sit next to each other on a shelf.
Product A: front of pack says 1500mg. Panel says magnesium bisglycinate 1500mg, magnesium 300mg, 80 percent NRV, serving two capsules, 180 capsules, 19.99 pounds. That resolves to 300mg elemental, 90 servings, 22p a day, everything printed. Product B: front of pack says 1500mg. Panel says magnesium oxide 1500mg, serving three capsules, 90 capsules, 8.99 pounds. No elemental line. Oxide is about 60 percent magnesium, so 1500mg implies roughly 900mg elemental, which is above the 400mg supplemental guidance level, and it is poorly absorbed.
Product B looks half the price on the shelf and works out at 30p a day across 30 servings, with an unstated elemental figure in the least well absorbed form. Product A is Futuro Labs Magnesium Glycinate. That comparison takes about ninety seconds and it is the whole skill.
Where to go next
- How to read a supplement labelThe same method generalised across all four products.
- Futuro Labs Magnesium Glycinate: the full labelA label run through every one of these steps.
- How much magnesium does a rugby player need?The reference figures behind step three.
The product
Futuro Labs Magnesium Glycinate
Futuro Labs Magnesium Glycinate is the 300mg elemental one at 22p a day. Run these five steps against it and everything checks: two capsule serving, 300mg elemental printed, 80 percent NRV stated, bisglycinate form, excipients declared and 22p a day.
- 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 the most important line on a magnesium label?
Elemental magnesium per serving. The compound weight on the front tells you almost nothing alone.
How do I convert 1500mg of bisglycinate?
Bisglycinate is about 20 percent magnesium by weight, so it gives roughly 300mg elemental.
What percentage NRV should I look for?
Above 15 percent for any claim to be allowed at all. 300mg elemental is 80 percent.
Is a longer ingredients list worse?
Not automatically, but everything in it should be declared and named.
Should I use the Amazon price per count?
No. That is per capsule and flatters products with multi capsule servings.
What does a missing elemental figure tell you?
That the manufacturer chose not to print the only number allowing comparison, which is itself informative.
How long should this check take?
About ninety seconds per product once you know which lines to find.