In 30 seconds
- In healthy volunteers, magnesium oxide had a fractional absorption of 4%. Chloride, lactate and aspartate were absorbed better.
- In a 60-day trial with 46 adults, citrate and amino acid chelated magnesium were absorbed better than oxide, which did not differ from placebo.
- A systematic review of 14 studies concludes that inorganic forms appear to be absorbed less well than organic ones, and that the percentage absorbed depends on the dose.
- With 250 mg of magnesium a day as bisglycinate, 93% of participants in a 4-week trial had no side effects at all.
Not all the magnesium in a supplement is absorbed equally, or agrees with you equally. Studies comparing forms suggest that those that dissolve well, such as citrate, chloride or amino acid chelates, are absorbed better than oxide. If you take it every day, whether you train or not, how it agrees with you matters too. This is what they found, study by study.
How it is absorbed
When and how much magnesium is absorbed
How magnesium is absorbed
Magnesium is absorbed mainly in the small intestine, and in smaller amounts in the colon. Absorption starts about 1 hour after you take it and is 80% complete after 6 hours 6.
With an intake of about 370 mg a day, 30-50% is absorbed. In studies in people, the figures range from 10 to 75%, depending on the dose and the starting level 6.
To compare forms, researchers measure how much magnesium rises in urine or blood after it is taken, or label the magnesium with isotopes to track it 1 2 3. If you want to see how bioavailability works for other nutrients, you will find it in bioavailability: what it is and how to get more from your supplements.
Level of evidence by outcome
| Outcome | Evidence | What the data show |
|---|---|---|
| Forms that dissolve well compared with oxide | Moderate | Better absorption of citrate, chloride and amino acid chelates in several small studies in people. |
| Organic compared with inorganic forms | Moderate | Systematic review of 14 studies: inorganic forms appear to be absorbed less well, with exceptions such as chloride. |
| Differences between citrate and bisglycinate | Preliminary | No conclusive direct comparisons: the reviews do not point to a winning form. |
| Digestive tolerance of bisglycinate | Moderate | Better tolerated than oxide in a crossover trial; 93% with no side effects in another 4-week trial. |
| Absorption of L-threonate | Preliminary | EFSA considers it a bioavailable source, based on two studies in rats and one trial in people. |
| Dose and splitting the doses | Moderate | With a smaller amount, a higher percentage is absorbed; split and with food, a little more. |
The rating is ours, based on the number of studies, their quality and the consistency of the results.
The key studies
| Study | Type | Sample | Outcome |
|---|---|---|---|
| Firoz and Graber, 2001 | Bioavailability study | Healthy volunteers | Fractional absorption of oxide of 4%. Chloride, lactate and aspartate were absorbed better, and similarly to one another. 1 |
| Walker et al., 2003 | Placebo-controlled trial | 46 healthy adults | 300 mg a day for 60 days: citrate and the amino acid chelate gave more magnesium in urine than oxide; citrate, more in blood. Oxide did not differ from placebo. 2 |
| Schuette et al., 1994 | Crossover trial with isotopes | 12 people with part of the intestine removed | 100 mg of magnesium: similar absorption with diglycinate and oxide (23.5% and 22.8%). In the 4 who absorbed least well, twice as much with diglycinate, which all of them tolerated better. 3 |
| Blancquaert et al., 2019 | Laboratory and 2 trials | 15 products; 10 and 30 adults | Dissolution predicted absorption. A soluble tablet with 196 mg raised blood magnesium more than a poorly soluble one with 450 mg. 4 |
| Pardo et al., 2021 | Systematic review | 14 studies | Inorganic forms appear to be absorbed less well than organic ones, and the percentage absorbed depends on the dose. 5 |
| Schuster et al., 2025 | Placebo-controlled trial | 155 adults aged 18 to 65 | 250 mg a day of magnesium as bisglycinate for 4 weeks: 93% of participants had no side effects, and none were serious. 7 |
Oxide, the reference form
Almost all studies compare against oxide, because it is very common and very concentrated: 60% of its weight is magnesium 4. The problem is that it dissolves poorly.
- In healthy volunteers, its fractional absorption was 4% 1.
- In the 60-day trial, oxide did not differ from placebo 2.
- In a laboratory gut model, several oxide products were among the least well absorbed 4.
The way it is presented also matters: effervescent oxide tablets gave about twice as much magnesium in urine as capsules 6.
Citrate and chelated forms
In the Walker trial, 46 healthy adults took 300 mg a day of magnesium for 60 days. Citrate and amino acid chelated magnesium were absorbed better than oxide, measured in urine. Citrate also gave the highest levels in blood and saliva 2.
Bisglycinate, or diglycinate, is magnesium chelated with glycine. In 12 people who had had part of their intestine removed, its absorption was similar to that of oxide: 23.5% compared with 22.8%. In the 4 who absorbed oxide least well, diglycinate was absorbed twice as well, and they all tolerated it better 3.
The authors suggest that some of the diglycinate is absorbed whole, through the route the gut uses for small fragments of protein 3.
Oxide versus soluble forms
The key seems to lie in dissolution
Organic better than inorganic?
A systematic review brought together 14 studies. Its conclusion: inorganic forms appear to be absorbed less well than organic ones, and the percentage absorbed depends on the dose 5.
There are exceptions. Chloride is an inorganic salt, but it dissolves well: it was absorbed better than oxide and as well as lactate and aspartate, which are organic 1.
That is why the key seems to lie in dissolution. In a study with 15 products, dissolution in the laboratory predicted absorption, and a soluble tablet with 196 mg raised blood magnesium more than a poorly soluble one with 450 mg 4.
Among the organic forms, such as citrate and bisglycinate, there is no clear winner: a review on absorption concludes that it is not clear which form achieves the highest bioavailability 6.
The dose matters as much as the form
The same review concludes that the type of salt matters less than is often thought: the dose and your starting magnesium level count for more 6.
- Smaller amount, higher percentage: in one study, 65% of 36 mg a day was absorbed, compared with 11% of 973 mg a day. Even so, the total amount absorbed went up with each increase 6.
- Split doses: the same amount taken in seven doses was absorbed better than in two 6.
- With food: 52.3% was absorbed with a meal, compared with 45.7% with water alone 6.
In healthy people with no prior deficiency, all the forms studied can maintain normal levels. In older people or people with illnesses, this is not guaranteed 5.
Digestive tolerance
With high doses from supplements, the most common effects are diarrhoea, nausea and stomach cramps. The forms most often linked to diarrhoea are carbonate, chloride, gluconate and oxide 8.
- Bisglycinate: better tolerated than oxide in the crossover trial 3. In another 4-week trial with 250 mg a day of magnesium, 93% of participants had no side effects at all, none were serious and there were fewer side effects than with placebo 7.
- Limit: for magnesium from supplements, the upper limit is 250 mg a day for adults, set to avoid mild diarrhoea. It does not include magnesium from food 10.
L-threonate: a new form
EFSA assessed magnesium L-threonate in 2024. About 3,000 mg provide about 250 mg of magnesium. It considers it safe under the proposed conditions and a bioavailable source of magnesium, based on a dissolution study, two studies in rats and one trial in people 9.
What it means for you
- Look at the elemental magnesium, not the weight of the compound: it is what the label declares 8.
- Choose a form that dissolves well and agrees with you, such as citrate or bisglycinate.
- Do not go over 250 mg a day of magnesium from supplements 10.
You will find each type explained, with which to choose depending on what you are looking for, in the guide to types of magnesium. And what the studies say about sleep, muscle and tiredness, in magnesium: what the studies say.
Bisglycinate
Magnesium bound to two glycine molecules
And our magnesium?
Our Magnesium Bisglycinate + B6 is from NG Advanced, our other supplement range. Two vegetable capsules provide 200 mg of elemental magnesium (53% NRV), from 1,000 mg of bisglycinate, and 6 mg of vitamin B6 (429% NRV). Ideally taken in the evening; 60 days for €26.90.
At NG Advanced we also explain what magnesium bisglycinate is and how to take it.
Sleep Recovery also contains magnesium: if you want to take both, check with your pharmacist or doctor first. If you take medication, have kidney disease, or are pregnant or breastfeeding, check with your doctor.
Frequently asked questions
Which form of magnesium is absorbed best?
The forms that dissolve well. In studies, citrate, chloride, lactate, aspartate and amino acid chelated magnesium were absorbed better than oxide. Among the organic forms there is no clear winner.
Is organic magnesium absorbed better than inorganic?
In general, yes: a systematic review of 14 studies concludes that inorganic forms appear to be absorbed less well. There are exceptions, such as chloride, an inorganic salt that dissolves well and was absorbed better than oxide.
Which form of magnesium is gentlest on the stomach?
The forms most often linked to diarrhoea are carbonate, chloride, gluconate and oxide. Bisglycinate was better tolerated than oxide in a crossover trial, and in another 4-week trial 93% of participants had no side effects.
How much magnesium is absorbed from a supplement?
It depends on the dose and your starting level. With about 370 mg a day, 30-50% is absorbed, and the percentage rises when the amount is smaller.
Sources
- Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res. 2001;14(4):257-262. View publication
- Walker AF, Marakis G, Christie S, Byng M. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnes Res. 2003;16(3):183-191. View publication
- Schuette SA, Lashner BA, Janghorbani M. Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection. JPEN J Parenter Enteral Nutr. 1994;18(5):430-435. View publication
- Blancquaert L, Vervaet C, Derave W. Predicting and testing bioavailability of magnesium supplements. Nutrients. 2019;11(7):1663. View publication
- Pardo MR, Garicano Vilar E, San Mauro Martín I, Camina Martín MA. Bioavailability of magnesium food supplements: a systematic review. Nutrition. 2021;89:111294. View publication
- Schuchardt JP, Hahn A. Intestinal absorption and factors influencing bioavailability of magnesium: an update. Curr Nutr Food Sci. 2017;13(4):260-278. View publication
- Schuster J, Cycelskij I, Lopresti A, Hahn A. Magnesium bisglycinate supplementation in healthy adults reporting poor sleep: a randomized, placebo-controlled trial. Nat Sci Sleep. 2025;17:2027-2040. View publication
- Office of Dietary Supplements, National Institutes of Health. Magnesium: fact sheet for health professionals. View publication
- EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA). Safety of magnesium L-threonate as a novel food pursuant to Regulation (EU) 2015/2283 and bioavailability of magnesium from this source in the context of Directive 2002/46/EC. EFSA J. 2024;22:e8656. View publication
- Nordic Council of Ministers. Nordic Nutrition Recommendations 2023: magnesium. Nord 2023:003. View publication
Information based on published studies. It does not replace the advice of a doctor or registered dietitian. The results describe what was measured in each study, not the properties of any specific product. A varied and balanced diet and a healthy lifestyle are important.
