Magnesium Oxide vs Magnesium Citrate

There are many reasons for taking magnesium supplements. For one, magnesium helps get rid of leg and foot cramps at night, helps you sleep better and wake up more rested and less stiff, prevents calf and other muscle cramps after exercise, lowers high blood pressure, regulates heart rhythm and prevents arrhythmia, extrasystoles and palpitations, calms nerves and reduces anxiety and stress levels. Not to mention all the other amazing, but less visible health benefits such as contributing to bone strength, regulating blood sugar in type 2 diabetes, preventing constipation, premature labor in pregnant women and pre-eclampsia and eclampsia. But in order to enjoy all of these wonderful benefits, you first have to know what magnesium form to choose.

There are quite a few types of magnesium supplements to choose from. There’s magnesium aspartate, carbonate, chloride, citrate, glycinate, gluconate, lactate, malate, orotate, pidolate or picolinate, sulfate, taurate, l-threonate and possibly also others. Each and every one of these forms is used to address magnesium deficiency and associated symptoms. However, not all have the same bioavailability – in other words, some formulas are better absorbed than others.

Magnesium oxide vs citrate absorption

Magnesium supplements absorption rate

Bioavailability of different magnesium supplements depends on several factors. On the one hand, organic magnesium salts are better absorbed than inorganic salts. For example, magnesium gluconate, lactate and aspartate are better absorbed than magnesium oxide, sulfate or carbonate. But it’s not just the magnesium form that affects bioavailability.

Taking a magnesium supplement at the same time with calcium can significantly inhibit and reduce the absorption of the former. The more calcium you take, the less magnesium you absorb. Other nutrients such as copper, iron, manganese, phosphorus or zinc may also compete with magnesium for absorption, whereas vitamin D facilitates its absorption.

Magnesium oxide versus citrate absorption

The two most common types of magnesium are oxide and citrate. Neither one of them is the most bioavailable magnesium form, but citrate is the best absorbed of the two. The citrate formula is basically magnesium bound to citric acid and citric acid is an organic acid, hence the relatively good absorption rate. Citrate is a chelated form, bound to the organic compound citric acid. The oxide formula is not bound to an organic acid such as citric acid in magnesium citrate, malic acid in magnesium malate or gluconic acid in magnesium gluconate, or to an amino acid such as aspartic acid in magnesium aspartate or glycine in magnesium glycinate. And, whilst bioavailable, meaning you do take up some of it, magnesium oxide absorption rate is quite low.

Magnesium oxide versus citrate

Magnesium Citrate form

  • Magnesium citrate absorption rate

Magnesium citrate bioavailability is good, with an estimated absorption rate of up to 30%. One of the best reasons for taking magnesium citrate is because it is bound to an organic acid (citric acid) which makes it more bioavailable than other forms and, of course, more bioavailable than the oxide form. Citric acid is naturally found in citrus fruits: lemon, lime, orange, ugli fruit, yuzu etc.

  • Magnesium citrate uses

Uses for the citrate form include correcting a deficiency, relieving constipation, preparation for medical procedures such as colonoscopies due to its laxative effect (it works by increasing water in the intestinal tract to trigger peristalsis and cause a bowel movement).

  • Best time to take magnesium citrate

If you are wondering when to take magnesium citrate, know that it depends on the reasons for which you are taking it. If it’s for constipation, then the best time is in the morning, on an empty stomach, with lots of water or juice. A good tip is to avoid eating for a few hours and just drink fluids. Or only have fresh fruit with plenty of water. This will help increase water levels in the intestines and encourage a bowel movement. Expect to have a bowel movement in up to 4-6 hours after taking magnesium citrate (although exact timing varies from person to person).

If you are taking magnesium citrate to correct a deficiency, then both in the morning and in the evening is best. You can divide your daily dose in two and have half in the morning when you wake up and half in the evening before sleeping. This is a good tip that works well because it helps ensure you are getting enough of the mineral throughout the day and enjoy the whole range of benefits, from better mood, reduced stress and less fatigue during the day to better sleep at night and fewer nighttime leg – calf or foot – cramps.

A good tip for avoiding stomach upset is to take the supplement after eating. Remember that drinking your magnesium citrate with lots of water, as instructed on the package, helps with its absorption since magnesium is dissolves in water.

Magnesium Oxide form

  • Magnesium oxide absorption rate

The bioavailability of magnesium oxide is the lowest of all magnesium forms, with an average absorption rate somewhere below 10%. Actually, the absorption rate of magnesium oxide has been found to be as low as 4% in certain instances. In part, this is owed to the fact it has a poor solubility in water.

  • Magnesium oxide uses

Main uses for magnesium oxide include correcting a deficiency (although not very effective), treating indigestion and acid reflux, relieving heartburn and constipation, treating migraines or reducing anxiety.

  • Side effects of magnesium oxide

A higher magnesium oxide dose has a laxative effect and can cause loose, watery stools. Abdominal cramps from the diarrhea and general stomach upset, nausea and vomiting are other common side effects. In rare cases, symptoms of an allergic reaction may occur. If you are experiencing a skin rash, itching or tingling sensation in the throat, swelling of the tongue, throat or face, lightheadedness, low blood pressure, breathing difficulties, fatigue, nausea, vomiting, fainting, seek medical help immediately.


What is the best magnesium to take of the two? Well, the citrate form is scientifically proven to be more bioavailable than the oxide form (up to 30% absorption rate) and thus the best of the two. However, while the magnesium oxide absorption rate is the lowest of all forms (as low as 4%), there is actually more magnesium in this supplement formula which makes it a relatively good source of the mineral.

If it’s the only one available to you, it’s better than nothing at all, especially since its absorption rate is close to 10% in some instances. The more severely deficient in magnesium you are, the more of the mineral you are likely to absorb, even if it’s from a form with an otherwise poor absorption rate.

This post was updated on Sunday / March 7th, 2021 at 11:34 PM