Pick up the nearest bottle of magnesium and look at it. If it says «500 mg» in large type on the front, chances are you are not swallowing 500 mg of magnesium, but 500 mg of a salt in which magnesium is only a fraction.
This is not deception, it is chemistry. But almost nobody explains it, and the result is that many people have been supplementing for months at a dose that bears no relation to the one they believe they are taking.
Elemental magnesium, the only line that counts
Magnesium is never sold pure. It is bound to a carrier that makes it stable and absorbable: oxygen for oxide, citric acid for citrate, glycine for bisglycinate. The advertised weight is often that of the whole compound. The share of actual magnesium is computed from the salt's molar mass.
| Form | Elemental magnesium | In a 500 mg capsule |
|---|---|---|
| Oxide | ≈ 60% | ≈ 300 mg |
| Citrate | ≈ 16% | ≈ 80 mg |
| Malate | ≈ 15% | ≈ 77 mg |
| Bisglycinate | ≈ 14% | ≈ 70 mg |
| L-threonate | ≈ 8% | ≈ 40 mg |
Weight for weight, an oxide capsule therefore holds more than four times the magnesium of a bisglycinate one. You could stop there and conclude that oxide wins. That would miss the second half of the problem.
The most concentrated is the worst absorbed
A randomised trial published in 2019 compared citrate and oxide in 14 healthy men, pre-saturated with magnesium for five days to remove the effect of empty stores. After a single 400 mg dose, citrate significantly increased 24-hour urinary magnesium excretion. Oxide did not. Plasma concentration followed the same logic, higher with citrate at 4 and 8 hours.
Urinary excretion is a fair judge: whatever comes out through the kidneys necessarily got into the blood first. When it does not move, the magnesium never crossed the intestinal wall.
Which form for which use
| Form | Relevant for | Drawback |
|---|---|---|
| Bisglycinate | Daily use, evening, sensitive digestion | Low elemental content |
| Citrate | General use, good absorption per euro | Osmotic effect at higher doses |
| Malate | Fatigue, muscle recovery | Less human data |
| L-threonate | Brain target, the only one studied for it | Expensive, very low content |
| Oxide | Occasional laxative effect | Poor absorption, wrong choice for a deficit |
Sleep: 17 minutes, and a lot of caution
This is the claim that sells the most magnesium. The reference meta-analysis pools 3 randomised trials and 151 older adults with insomnia. Result: time to fall asleep drops by 17.4 minutes versus placebo, with a confidence interval that does not cross zero. That is a real effect.
The rest calls for restraint. Total sleep time gains 16 minutes, without reaching statistical significance. The authors rate all trials at moderate to high risk of bias, and the quality of evidence from low to very low. They conclude that the literature remains too weak for a physician to make a well-informed recommendation.
Three trials, 151 people, all elderly and insomniac. If you are thirty and sleeping decently, these data are not about you.
Your blood panel will not see your deficit
This is the most useful part of this article, and the least known. Serum magnesium is held in a narrow range by the body, which draws on bone and cells to maintain that value. A 2015 review notes that blood markers do not reflect intracellular status, and can stay perfectly normal while other indicators of magnesium status are degraded.
In other words: a serum magnesium within range proves nothing. Red blood cell magnesium comes closer to the real store. That is the marker worth tracking, and the one Helix uses.
How much, and how
The European authority sets the limit at 250 mg per day of magnesium from supplements, on the single criterion of osmotic diarrhoea. That limit does not apply to magnesium from food, which does not affect the gut the same way. It is contested: a US team reviewed trials published since 1997 and concluded that higher doses are consumed without adverse events. Worth knowing: those authors work at a research centre dedicated to magnesium, which does not disqualify their work but belongs in the picture.
- 1
Read the nutrition table, not the marketing
The only line that counts is elemental magnesium. If it appears nowhere, change brand.
- 2
Split it into two intakes
The osmotic effect depends on the concentration reached in the gut. Two doses of 150 mg go down far better than one of 300 mg.
- 3
Take it with a meal
Food dilutes magnesium in the intestinal contents and buffers the laxative effect.
- 4
Evening if you are after falling asleep faster
It is the only documented sleep benefit, so put it at the right time.
- 5
Space your intake from tetracyclines and fluoroquinolones
These antibiotics form complexes with magnesium, which lowers their absorption. Prescribing information states the gap to respect, and it is measured in hours.
Magnesium is the perfect example of a spend that looks rational and is not. You buy a product, you take it daily, you observe that nothing happens, and you conclude the molecule does not work. When in fact you were taking 70 mg where you believed you took 500, in a poorly absorbed form, while your blood panel reassured you because it cannot see your body in any precision.
Three errors, none visible on its own. That is exactly the kind of thing you only catch by cross-checking stack, dosages and biomarkers in the same place.
Know what you are actually taking, and how your body works.
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