The Complete Guide to Magnesium Forms
Magnesium drives more than 300 enzyme reactions and low intake is genuinely common (NHANES data put 45–48% of U.S. adults below the Estimated Average Requirement), which is why correcting a shortfall can help with muscle cramps, sleep, anxiety, blood pressure, and glucose control. The honest headline of this guide is that the form matters less than the dose and your starting status — cheap magnesium oxide is only about 4% absorbed and works mainly as a laxative, so it is the weakest choice for raising magnesium. Where form does matter, match it to the goal: glycinate is gentle and calming, citrate is a solid general-purpose option, and L-threonate is the one marketed for the brain. Start around 200–400 mg of elemental magnesium and ease off if you get loose stools.
Magnesium Oxide: Cheap but Poorly Absorbed
Magnesium oxide is mostly elemental magnesium by weight (about 60%), but the body absorbs only about 4% of it (Firoz & Graber 2001). It works mainly as a laxative or antacid. For fixing low magnesium status, oxide is the weakest choice, even though it dominates the cheapest multivitamins.
Magnesium Citrate: The General-Purpose Choice
Magnesium citrate dissolves easily in water and is one of the better-absorbed forms in head-to-head studies (Walker 2003, Lindberg 1990). It has a mild osmotic laxative effect, which helps with constipation. Citrate is the most cost-effective all-rounder for general use: affordable, reasonably absorbed, and well-studied.
Magnesium Glycinate: The Calmer Choice
Magnesium glycinate (also called magnesium bisglycinate) pairs magnesium with the amino acid glycine. It is generally well tolerated and causes less diarrhea than citrate or oxide at similar doses. Direct trial evidence that glycinate beats other forms for sleep or anxiety is thin, but Schuchardt and Hahn's 2017 review notes that the type of magnesium salt matters less than the dose and your starting status. Glycinate is a sensible choice if other forms upset your gut.
Magnesium L-Threonate: The Brain Form
L-threonate was designed to raise magnesium in the brain. Animal studies in rats showed clear cognitive gains. Human data are limited to one small RCT — Liu 2016 (PMID 26519439), 44 adults aged 50–70 with cognitive complaints, 12 weeks of MMFS-01 (magnesium L-threonate) — which reported gains in a composite cognitive score versus placebo. Threonate runs $50–$80 a month and is the most expensive form. Larger trials have not been published.
Magnesium Malate: For Muscle Function
Magnesium malate combines magnesium with malic acid, a small molecule used in cellular energy production. Limited trials have looked at malate in fibromyalgia, with mixed results. Absorption is similar to citrate. It is a reasonable option, but the muscle-specific advantage is more theoretical than proven.
For most adults, magnesium citrate or glycinate at 200–400 mg/day of elemental magnesium is an effective, affordable way to correct a common dietary gap. Doses above 350 mg/day from supplements (the IOM tolerable upper intake) can cause loose stools, so split the dose with meals.
Sources
- Schuchardt JP, Hahn A. “Intestinal absorption and factors influencing bioavailability of magnesium — an update.” Curr Nutr Food Sci, 2017. PMID 29123461.
- Firoz M, Graber M. “Bioavailability of US commercial magnesium preparations.” Magnes Res, 2001. PMID 11794633.
- Liu G, et al. “Efficacy and safety of MMFS-01, a synapse density enhancer (magnesium L-threonate), for treating cognitive impairment in older adults.” J Alzheimers Dis, 2016. PMID 26519439.
- Slutsky I, et al. “Enhancement of learning and memory by elevating brain magnesium.” Neuron, 2010. PMID 20152124.
- Institute of Medicine. “Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D and Fluoride.” National Academies Press, 1997 (UL 350 mg/day from supplements).