How Long Magnesium Supplements Stay In The Body + When To Take Different Forms

Magnesium is important for everything from energy production to blood sugar regulation, so it makes sense that it's one of the most popular dietary supplements in the U.S. Those who take a magnesium supplement might be wondering how long magnesium stays in your body and how much of it gets flushed out quickly.

The human body does not produce magnesium on its own. Rather, it's on us to get our recommended daily amount1 (320 milligrams for women and 420 milligrams for men) through diet. "It's important to note that this essential macromineral's nutritional requirement increases to 360 milligrams a day during pregnancy," adds nutrition scientist Ashley Jordan Ferira, Ph.D., RDN.

A large portion of the population in Western countries2 does not get enough magnesium from food alone, hence why dietary supplements have utility and have also become so popular3. "It's estimated that 43% of U.S. adults4 currently fail to meet their daily magnesium needs through diet alone, which is over 100 million adults, to put this nutrient gap in perspective," Ferira expounds.

The amount of magnesium that is absorbed by the body when we take a supplement depends on the form (i.e., complex) the magnesium is delivered in (think glycinate, oxide, etc.) but also our internal magnesium levels, or status. Someone who is magnesium deficient (has a blood serum level below 0.75 mmol/L) will generally absorb more of the mineral than someone who is not.

Infusing a bit more nuance into the assessment of magnesium levels in the body, Ferira shares that, "some health care practitioners, especially those with a functional or integrative focus, choose to assess magnesium status with red blood cell (RBC) levels5 instead of plasma or serum due to the higher magnesium content in RBCs, but even that test has its challenges in capturing the whole-body magnesium status situation."

It's a complex mineral. Utilized for over 300 essential cellular pathways in the body, it's no wonder we would benefit from a sufficient supply of magnesium daily. "Your body is constantly using up magnesium and the amount that you store is regulated," registered dietitian Tracey Frimpong, R.D., tells mbg. On average, she adds, about 40% of the magnesium we consume is absorbed in the upper GI tract, while 5% is absorbed lower down in the large intestine (i.e., colon).

One of the reasons it can be challenging to accurately measure magnesium levels in the blood is because of where it preferentially concentrates in the body. "Interestingly, 99% of the mineral6 is located in our bones (about 50 to 60%), muscles, and other soft tissues," explains Ferira.

This ongoing process of absorption and utilization is pretty quick, so it's important to make sure you're always giving your body the magnesium it needs. "Most magnesium will stay in the body for anywhere from 12 to 24 hours. So it's not something you can just take once, see results, and then never take again," explains registered dietitian Amanda Li, R.D.

This means that as you're looking to fulfill your nutritional requirements through rich dietary sources (like green leafy vegetables, legumes, nuts, seeds, and whole grains) and high-quality supplements, you would want to take advantage of that magnesium supplement at least once a day.

When it comes to the best time of day to take a magnesium supplement, functional dietitian Selva Wohlgemuth, M.S., RDN, says that it depends on the magnesium form and intended effect.

For a little context: Since magnesium is an insoluble mineral on its own (as a mineral element), "it needs to be paired with another compound to create organic or inorganic salts or complexes, to make the mineral more soluble5 for the body to absorb," Ferira shares. It's that secondary compound that you'll want to look at when you're deciding on timing and a personalized supplement regimen.

Again, this will depend on the form you're taking, your baseline magnesium status, your personal biology (always a source of variability for any nutritional factor), and the reason you're taking it.

Registered nurse and master nutrition therapist Ella Soderholm R.N., MNT, explains that most magnesium supplements (particularly, magnesium bisglycinate) have a calming effect on the body that you can feel relatively quickly—within an hour or so.*

And those less absorbable forms like magnesium citrate and oxide tend to get the bowels moving within a few hours also. So in this sense, magnesium supplements work quickly.

Again, not a dietary supplement at all, transdermal magnesium is an interesting topic nonetheless. "The notion that Epsom salts or other magnesium applied cutaneously (i.e., via the skin) can be rapidly absorbed because the mineral supposedly bypasses your GI tract and enters the bloodstream rapidly is a nice theory but not yet supported by robust science8," Ferira explains. She goes on to say that, "while limited epidermal absorption of magnesium may be possible in certain, limited areas of the body (like hair follicles and sweat glands), the absorption efficiency is thought to be quite limited."

While more research is warranted in the magnesium topical arena, Ferira concludes that people can, "feel free to soak in your salt bath, but this is not remotely relevant for achieving your daily magnesium needs in a nutritional sense." That is where a nutrient-diverse, plant-centric diet and smart supplementation come in.

It's important to set expectations for the timeline to improve magnesium status. If you're taking a supplement with the intention to address a magnesium deficiency (a clinical issue thought to affect 20% or more9 of the population), that won't happen right away.* It's going to take longer to get to the root of the problem, Soderholm says, noting that it can be up to three to six months before levels start to rebound. "Most people with deficiency need to take magnesium every day, and it doesn't work overnight—but that's true with most things,"* she adds.