Here, dietitians and nutrition experts share the basics of magnesium absorption and explain how long the supplement typically stays in the body—and what that means about when to take it. 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. 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. Though the exact cadence will depend on the form you’re taking—be it magnesium bisglycinate, citrate, chloride, etc., or even a comprehensive multivitamin that contains magnesium—and what you’re taking it for. 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. Here are a few of the main forms (i.e., complexes) of magnesium you’ll find, and some best practices on when to take them during the day: 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. To ensure that you’re giving your magnesium supplement the best chance of working for you, experts offer up these tips for optimal absorption and utilization in the body: Emma received her B.A. in Environmental Science & Policy with a specialty in environmental communications from Duke University. In addition to penning over 1,000 mbg articles on topics from the water crisis in California to the rise of urban beekeeping, her work has appeared on Grist, Bloomberg News, Bustle, and Forbes. She’s spoken about the intersection of self-care and sustainability on podcasts and live events alongside environmental thought leaders like Marci Zaroff, Gay Browne, and Summer Rayne Oakes.
