Where does magnesium go in the body?
An adult body holds about 24 g of magnesium: a little over half in the bones, most of the rest in the muscles and organs, less than 1% in the blood. The gut absorbs 30 to 40% of it and the kidneys set how much is kept. It helps muscles relax; for sleep, the evidence remains weak.
Where does magnesium go, from the plate to the bones?
Mg1 dot = a little circulating magnesiumShare of the body's magnesium
Organs and routes simplified; the number of small dots gives the order of magnitude, not a measurement. Tap an organ to go to its step.
The journey in text, and where these numbers come from
| Step | Number | What happens |
|---|---|---|
| 1. Magnesium arrives from the plate | 320 to 420 mg a day recommended for adults | Magnesium comes mostly from seeds, legumes, whole grains and green vegetables, where it sits at the heart of chlorophyll. Refining grains removes much of it. |
| 2. The small intestine lets part of it through | 30 to 40% of the magnesium eaten passes into the blood | It passes mainly in the small intestine: between the cells when it is plentiful, through channels (TRPM6, TRPM7) when it is scarce. What is not absorbed leaves in the stool. |
| 3. The blood keeps almost none | < 1% of the body's magnesium is in the blood | The blood is only a place of passage. 55 to 70% circulates free, the rest is mostly carried by proteins. So a normal blood test does not prove that stores are full. |
| 4. The bones hold the biggest reserve | 50 to 60% of the body's magnesium is in the bones | In bone, magnesium is stuck to the calcium crystals. About a third stays on the surface and can go back into the blood when it runs low: it is a reserve that can be lent. |
| 5. Muscles and organs put it to use | > 300 enzymes need magnesium | In cells, magnesium is almost always attached to ATP, the cell's fuel: without it, ATP does not work. It also helps make DNA and proteins, and controls the channels of nerves and muscles. |
| 6. The kidneys sort it out | ≈ 100 mg leave in the urine each day | Each day, about 2,400 mg of magnesium pass through the kidneys' filter. They send 95% back into the blood, mostly in the loop of Henle, and let only about 100 mg go in the urine. When you eat less of it, they let less of it go: they are the ones that keep the blood level steady. |
| 7. A day's balance | ≈ 24 g of magnesium in an adult's body | In an adult in balance, what the gut absorbs each day equals what the kidneys let go into the urine. The rest of the plate leaves in the stool. |
- humanTotal amount. About 24 g in adults; 50 to 60% in the bones (Jahnen-Dechent and Ketteler, 2012), about 25% in the muscles (ANSES, 2021), less than 1% in the blood (NIH). The other organs hold the rest, about 20% (our calculation).
- humanAbsorption. 30 to 40% of the magnesium eaten passes into the blood (NIH), more when intake is low (Jahnen-Dechent and Ketteler, 2012).
- humanKidneys. About 2,400 mg pass through the kidneys' filter each day; 95% go back into the blood, about 100 mg leave in the urine (Jahnen-Dechent and Ketteler, 2012).
- hypothesisGauges. A full gauge bar equals 62.5% of the body's magnesium; the bone bar sits in the middle of 50 to 60%; the Mg dots show the direction of travel, not the amounts.
How does magnesium help muscle relax?
CaCalcium: triggers contraction
A sarcomere, the smallest unit that contracts, greatly magnified; number of molecules and speeds are illustrative.
The steps in text, and where these numbers come from
| Step | Number | What happens |
|---|---|---|
| 1. At rest, calcium stays stored away | 0.5 to 1 mmol/L of free magnesium in the cell | At rest, the pump keeps calcium locked in the reticulum. Each myosin head holds an ATP bound to a magnesium: it is cocked, but does not touch the actin. |
| 2. The nerve gives the signal: calcium comes out | × 100 to 200 free calcium rises inside the cell | The nerve signal opens the reticulum's channel. Calcium comes out, lands on the actin and uncovers the spots where the heads can grab on. They pull the actin toward the center: the muscle contracts. |
| 3. Magnesium-ATP makes the heads let go | 1 Mg-ATP per stroke of each head | After each stroke, a head lets go of the actin only if a new magnesium-ATP binds to it. Without ATP, the heads stay stuck: this is the stiffness of muscles after death. |
| 4. The pump puts calcium away: the muscle relaxes | 2 calcium stored for each Mg-ATP used by the pump | When the signal stops, the pump sends calcium back into the reticulum; it too runs on magnesium-ATP. Calcium leaves the actin, the heads no longer grab on: the muscle relaxes. |
| 5. Magnesium puts the brakes on calcium | ≈ 1 mmol/L of magnesium keeps the exit channel more closed | At rest, magnesium sits on the exit channel and keeps it closed; the nerve signal lifts this brake. It also takes some of calcium's places. It acts as a natural brake on contraction. |
| 6. When the blood is truly low | < 0.75 mmol/L in the blood: this is called a deficiency (NIH) | If magnesium is truly lacking, the brake gives way: calcium comes out too easily, nerves and muscles get overexcited. This causes cramps, tremors, sometimes spasms. But for ordinary cramps, taking magnesium barely helps (“Sleep and cramps” tab). |
- humanContraction. Calcium released by the reticulum lets the myosin heads grab onto the actin; each head needs a new ATP, carried by magnesium, to let go. Without ATP, the heads stay attached: this is the stiffness after death (Squire, 2019). Each stroke of a head costs one ATP (Herzog, 2017).
- humanRelaxation. Magnesium helps the reticulum's pump take the calcium back (Jahnen-Dechent and Ketteler, 2012): on each turn, it takes 2 calcium and 1 ATP, and one magnesium lets it use the ATP (Aguayo-Ortiz and Espinoza-Fonseca, 2020).
- humanThe brake. Magnesium acts as a “natural calcium antagonist” (Jahnen-Dechent and Ketteler, 2012). At around 1 mmol/L, the amount found in the cell, it almost always occupies a site on the exit channel (RyR1) that slows it down (Laver et al., 1997); the nerve signal lifts this brake. Signs of a real deficiency: tremors, cramps, spasms (Jahnen-Dechent and Ketteler, 2012; NIH).
- animalIn animals. In mouse fibers, free calcium goes from about 0.1 µmol/L at rest to 10 µmol/L (slow fibers) or 18 µmol/L (fast fibers) at the peak of a twitch, 100 to 200 times more (Baylor and Hollingworth, 2011 and 2012). In toad fibers, lowering magnesium from 1 to 0.05 mmol/L is enough to set off contractions (Lamb and Stephenson, 1991).
- hypothesisDrawing. One sarcomere, greatly magnified, with a few heads; in reality, hundreds of heads work out of step.
Does magnesium help you sleep or prevent cramps?
Each bar shows the difference from a placebo and its margin of uncertainty. To the right of the line: better than with the placebo. A bar that touches the line shows no clear effect.
Better than with a placeboNo clear difference
The trials in text, and what these numbers are worth
| Measure | Study | Difference from placebo |
|---|---|---|
| Falling asleep | Mah and Pitre, 2021: 3 trials, 151 people | 17 minutes less to fall asleep (from 7 to 27). Low-quality evidence. |
| Sleep duration | Mah and Pitre, 2021: 3 trials, 151 people | 16 minutes more on average, but the margin runs from 6 minutes less to 38 more: no clear difference. |
| Insomnia score | Schuster et al., 2025: 1 trial, 155 people, bisglycinate | 1.6 points better on the insomnia score (from 0 to 3.3): a small effect, just barely clear. Sleep stayed in the mild insomnia range. |
| Cramps in older adults | Garrison et al., 2020 (Cochrane): 5 trials, 307 people | 0.18 fewer cramps per week (from 0.49 more to 0.84 fewer): nothing clear. |
- humanSleep, older adults. Review of 3 trials, 151 older adults; in the largest, 500 mg a day for 8 weeks in people who ate little of it (Abbasi et al., 2012). Quality of evidence rated low to very low (Mah and Pitre, 2021).
- humanSleep, adults. 250 mg of magnesium as bisglycinate for 4 weeks, in a university-funded trial: small effect on the insomnia score, nothing on the other measures (Schuster et al., 2025). A trial with L-threonate, funded by a company in the industry, finds a better deep sleep score on a smart ring, mostly because it dropped in the placebo group; sleep duration and the insomnia score do not differ from placebo (Hausenblas et al., 2024).
- humanCramps. In older adults with cramps of no known cause, magnesium does not reduce the number of cramps in a useful way (Garrison et al., 2020).
- hypothesisWhy the effect stays small. In someone who is not low in magnesium, adding more does not change the magnesium in the cell: the kidneys get rid of the excess.
Which foods provide the most magnesium?
Tap the foods for one day: the bar shows the magnesium they provide, the gold line the recommended intake.
The food table, and where these numbers come from
| Food | Serving | Magnesium |
|---|---|---|
| Roasted pumpkin seeds | 30 g | 156 mg |
| Chia seeds | 30 g | 111 mg |
| Roasted almonds | 30 g | 80 mg |
| Cooked spinach | 90 g | 78 mg |
| Cashews | 30 g | 74 mg |
| Cooked black beans | 90 g | 60 mg |
| Cooked edamame | 80 g | 50 mg |
| Peanut butter | 2 tablespoons | 49 mg |
| Baked potato, with skin | 100 g | 43 mg |
| Cooked brown rice | 100 g | 42 mg |
| Plain yogurt | 225 g | 42 mg |
| Banana | 1 medium | 32 mg |
| Cooked salmon | 85 g | 26 mg |
| Milk | 250 mL | 27 mg |
| Whole wheat bread | 1 slice | 23 mg |
| Cooked broccoli | 80 g | 12 mg |
- humanAmounts. Values from the NIH (food table, based on the USDA database); US servings converted to grams by us.
- humanRecommended intakes. Adults over 30: 320 mg a day for women, 420 mg for men (NIH). In Europe: 300 and 350 mg (EFSA, 2015); in France: 300 and 380 mg (ANSES, 2021).
- humanSupplements. Limit for magnesium from supplements only: 350 mg a day for adults (NIH); 250 mg in Europe (EFSA) and in France (ANSES). Magnesium from food has no limit for a person with healthy kidneys.
With care: this page explains, it does not replace medical advice. If you have kidney disease, do not take magnesium supplements without medical advice: your kidneys could no longer get rid of the excess. Above 350 mg a day from supplements, diarrhea and stomach cramps are common. Magnesium can interfere with some medicines, such as some antibiotics and bone medicines: ask your pharmacist how to space out the doses.
Frequently asked questions about magnesium
Does magnesium really help you sleep?
A little, maybe, and mostly in older adults who sleep poorly: in 3 small pooled trials, they fell asleep about 17 minutes faster than with a placebo, without sleeping clearly longer. The evidence is still low quality (Mah and Pitre, 2021).
Does magnesium prevent cramps?
Not ordinary cramps: in older adults with cramps of no known cause, it does barely better than a placebo (Garrison et al., 2020). It helps when the blood is truly low in it, which only a doctor can check.
How much magnesium do you need per day?
For adults, 310 to 320 mg for a woman and 400 to 420 mg for a man, depending on age (NIH). Europe advises a little less: 300 mg for a woman, 350 mg for a man (EFSA, 2015). A handful of pumpkin seeds already provides half of that.
How can you tell if you are low in magnesium?
It is hard: the blood holds less than 1% of it, and blood levels stay normal as long as the bones and kidneys make up for it. Clear deficiencies come mainly from long-lasting diarrhea, alcohol, some diuretics, long-term use of acid-reducing medicines, or poorly controlled diabetes (NIH).
Which form of supplement should you choose?
Small studies find that magnesium citrate, chloride, lactate, and aspartate get into the blood better than the oxide (NIH); in a 60-day trial, the oxide did no better than a placebo for the magnesium measured in urine, blood, and saliva (Walker et al., 2003). Without a proven deficiency, food is usually enough.
See also
What causes muscle cramps? What happens in the body during sleep? Why do bones become fragile with age? How do kidney stones form, and do they pass?
Sources
- Office of Dietary Supplements (NIH). Magnesium: fact sheet for health professionals. ods.od.nih.gov
- Jahnen-Dechent W, Ketteler M. Magnesium basics. Clinical Kidney Journal, 2012. doi:10.1093/ndtplus/sfr163
- Squire J. Special Issue: The Actin-Myosin Interaction in Muscle: Background and Overview. International Journal of Molecular Sciences, 2019. doi:10.3390/ijms20225715
- Herzog W. Skeletal muscle mechanics: questions, problems and possible solutions. Journal of NeuroEngineering and Rehabilitation, 2017. doi:10.1186/s12984-017-0310-6
- Aguayo-Ortiz R, Espinoza-Fonseca LM. Linking Biochemical and Structural States of SERCA: Achievements, Challenges, and New Opportunities. International Journal of Molecular Sciences, 2020. doi:10.3390/ijms21114146
- Laver DR et al. Reduced inhibitory effect of Mg2+ on ryanodine receptor-Ca2+ release channels in malignant hyperthermia. Biophysical Journal, 1997. doi:10.1016/S0006-3495(97)78222-5
- Lamb GD, Stephenson DG. Effect of Mg2+ on the control of Ca2+ release in skeletal muscle fibres of the toad. Journal of Physiology, 1991. doi:10.1113/jphysiol.1991.sp018483
- Baylor SM, Hollingworth S. Calcium indicators and calcium signalling in skeletal muscle fibres during excitation-contraction coupling. Progress in Biophysics and Molecular Biology, 2011. doi:10.1016/j.pbiomolbio.2010.06.001
- Baylor SM, Hollingworth S. Intracellular calcium movements during excitation-contraction coupling in mammalian slow-twitch and fast-twitch muscle fibers. Journal of General Physiology, 2012. doi:10.1085/jgp.201210773
- Mah J, Pitre T. Oral magnesium supplementation for insomnia in older adults: a systematic review and meta-analysis. BMC Complementary Medicine and Therapies, 2021. doi:10.1186/s12906-021-03297-z
- Garrison SR et al. Magnesium for skeletal muscle cramps. Cochrane Database of Systematic Reviews, 2020. PMID 32956536
- EFSA. Overview on Tolerable Upper Intake Levels as derived by the Scientific Committee on Food and the EFSA Panel, version 11, 2025. efsa.europa.eu
- EFSA NDA Panel. Scientific Opinion on Dietary Reference Values for magnesium. EFSA Journal, 2015. doi:10.2903/j.efsa.2015.4186
- ANSES. Actualisation des références nutritionnelles en vitamines et minéraux, opinion and report, 2021. anses.fr
- Romani AMP. Cellular magnesium homeostasis. Archives of Biochemistry and Biophysics, 2011. PMC3133480
- Abbasi B et al. The effect of magnesium supplementation on primary insomnia in elderly: a double-blind placebo-controlled clinical trial. Journal of Research in Medical Sciences, 2012. PMC3703169
- Schuster J et al. Magnesium Bisglycinate Supplementation in Healthy Adults Reporting Poor Sleep: A Randomized, Placebo-Controlled Trial. Nature and Science of Sleep, 2025. PMC12412596
- Hausenblas HA et al. Magnesium-L-threonate improves sleep quality and daytime functioning in adults with self-reported sleep problems: A randomized controlled trial. Sleep Medicine: X, 2024. PMC11381753
- Walker AF et al. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnesium Research, 2003. PMID 14596323
True or false?
Most of the body's magnesium is in the blood.
False. Less than 1% is in the blood; a little more than half is in the bones, and the rest mostly in the muscles and organs (NIH).
A normal blood magnesium level proves you are not low in it.
False. Blood levels stay in the normal range as long as the bones and kidneys make up for it: you can be low in magnesium with a normal blood test (NIH).
Magnesium helps the muscle relax.
True. It makes the myosin heads let go and keeps the pump that stores calcium running, which lets the muscle relax (Squire, 2019; Aguayo-Ortiz and Espinoza-Fonseca, 2020).
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Metabolive. "Where does magnesium go in the body?" Updated November 2, 2026. https://metabolive.com/en/everyday/magnesium.