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Magnesium Oxide Delivers More Magnesium with Far Fewer Pills

It’s true that magnesium oxide is less soluble than several other forms of supplemental magnesium.

However, magnesium oxide is at least as effective (if not better) at restoring and maintaining a healthy level of magnesium in your cells and bones.

How is this possible? I’ll explain!

But before I do, we need to understand how magnesium is absorbed.

In all its natural forms, magnesium is bound to another chemical compound, for example: carbonate; gluconate; citrate; lactate; aspartate; or oxide. The combinations of magnesium and one of these compounds are called magnesium salts.

When you consume any magnesium salt, the magnesium must be freed from its partner compound during digestion and made soluble (ionized). Once it is made soluble, your body can absorb the magnesium.

For this to happen effectively, stomach acid has to be present. So it’s also helpful to remember that:

  1. It’s best to take your supplements with a meal because your stomach responds to your consumption of food by secreting stomach acid.
  2. Medications that prevent stomach acid production impair your ability to digest and absorb minerals – and virtually all other nutrients.

Why Magnesium Oxide Provides More Elemental Magnesium

It’s true that magnesium oxide is not as easy to render soluble as some other magnesium salts. It’s therefore not absorbed as rapidly as other forms of magnesium, e.g. magnesium citrate, gluconate, lactate, aspartate.

But although magnesium oxide has a lower rate of absorption it contains a MUCH greater amount of actual magnesium to begin with. More than what is provided by other magnesium salts.

Therefore, even with the lower rate of absorption, magnesium oxide still delivers more magnesium per tablet. So more magnesium gets into your bloodstream to deliver all its many benefits to your bones and body!

This is illustrated in the table Comparison of Magnesium Salts below: 1-2

Comparison of Magnesium Salts
MG Salt % Elemental MG % Elemental MG Absorbed Mg’s of Elemental MG Absorbed per 100 mg of MG Salt
Oxide 60% 23%* 13.8
Carbonate 28% ~20% ** ~5.2
Chloride 26% 20% 5.2
L-lactate 12% 42% 5
Citrate 11% 30% 3.3
Aspartate 8% 42% 3.3
Glycinate 14% 23% 3.2
Gluconate 6% 19% 1.14

*On the web, you’ll see many sites claiming that only 4% of the magnesium in magnesium oxide is absorbed. This claim is based upon a small study by Firoz et al. (2001), which is discussed fully below. The actual data in this study shows something very different. The stated percentage of elemental magnesium absorbed from magnesium oxide in the above table — 23% — is the statistic provided in a review and meta-analysis of numerous studies by Ranade et al., (2001), cited below.

**Magnesium carbonate is nearly insoluble, but in the presence of stomach acid (HCl), magnesium carbonate is converted into magnesium chloride, which allows 20% of elemental magnesium to be absorbed.

As you can see, magnesium oxide contains the most elemental magnesium of 60%. And 23% of this can be absorbed.

Magnesium citrate, for example, contains only 11% elemental magnesium. And only 30% of this is absorbed.

So the percentage of elemental magnesium to begin with is just as important as the rate of absorption.

What Does This Mean For You, In Practical Terms?

You can get comparable or better health benefits with magnesium oxide. And you’ll also require fewer capsules, which will save you time and money.

The research most often cited to disparage magnesium oxide is a small study by Firoz et al. of 16 volunteers. The aim of this study was to test the claim that organic (chelated) magnesium salts (e.g., aspartate, lactate) are more easily absorbed than non-organic salts (e.g., oxide, chloride).3

The 16 volunteers were given four commercially-available magnesium salts: 2 organic (aspartate, lactate) and 2 non-organic (oxide, chloride), after which the amount of magnesium in their urine was checked. More magnesium excreted in the urine is thought to indicate that more magnesium has been absorbed; less magnesium excreted is thought to indicate that less magnesium has been absorbed. This rationale is based on the principle that the body works hard to maintain a steady state of magnesium balance, and in a steady state of magnesium balance, any extra magnesium absorbed will be excreted in the urine.

In this study, those taking magnesium oxide excreted less magnesium on average. So magnesium oxide was said to have a low fractional absorption rate of 4%. However, there are 2 important points not shared in this study:

  1. Half of the volunteers in this study excreted more or very comparable amounts of magnesium when taking magnesium oxide as they did when taking the other magnesium salts. Take a look for yourself at the table below – Urinary Magnesium, mg/day. The data in the following table is taken directly from Firoz M, et al’s 2001 article in Magnesium Research.

Averaging out the data produces a result that does not reflect what actually happened in the participants. Yet, what was reported is repeatedly cited on the internet. This is why I always read the full paper of any study I am going to rely upon for information.

2. Most importantly, the key practical discovery made in this study was: “Magnesium excretion increased with all magnesium supplements.” What this means is that all the magnesium salts tested, including magnesium oxide, delivered more magnesium than was needed to meet the body’s magnesium balance requirement.

Urinary Magnesium, mg/day
Study participant MG oxide MG chloride MG aspartate MG lactate Control 1* Control 2*
A 100 156 95 114 77 70
B 95 43 78 65 68 43
C 101 89 103 137 75 32
D 66 120 137 161 100 103
E 78 166 101 119 134 65
F 63 144 88 48 56 67
G 84 78 76 67 74 61
H 142 182 214 198 128 129
I 78 39 55 62 102 60
J 114 112 110 76 53 86
K 77 106 177 116 80 32
L 114 94 137 117 109 92
M 66 87 51 81 104 115
N 82 82 82 163 76 85
O 162 133 85 106 75 75
p 34 120 58 87 28 57

*Controls did not receive any supplemental magnesium.

Magnesium Oxide is Effectively Absorbed

Despite what you may have read on various websites, the peer-reviewed research on PubMed shows that magnesium oxide is effectively absorbed:

In a study involving 18 men, 24 hours after consumption of magnesium oxide supplements, magnesium levels in the urine increased about two times the normal amount. And, after 48 hours, magnesium levels had increased to roughly four times normal. The conclusion drawn by the researchers: these results clearly show that “magnesium oxide is effectively absorbed and elevates the biologically-active levels of magnesium in the bloodstream.”4

It’s important to note here that magnesium oxide did not increase urinary excretion of magnesium within a couple of hours. This is an important point which I’ll explain below in “Higher bioavailability does not translate to more effective delivery to tissues.”

All forms of magnesium salts used in animal studies have been found to effectively restore magnesium levels in magnesium-depleted animals.

In one such study in 2005, rats were first given a magnesium-depleted diet and then provided with the same diet supplemented with 10 different magnesium salts: magnesium oxide, chloride, sulfate, carbonate, acetate, picolate, citrate, gluconate, lactate or aspartate. The results showed that while magnesium absorption values varied from 50% to 67%, all the magnesium salts used were equally efficient in restoring magnesium levels.5

The researchers also noted that “the quantity of magnesium in the digestive tract is the major factor controlling the amount of magnesium absorbed.” And as you now know, magnesium oxide contains a far greater amount of elemental magnesium than any of the other magnesium salts .5

A more recent animal study confirmed and strengthened these findings. In this one, for 5 weeks, rats were fed one of eight test diets supplemented with phytic acid (5 g/kg diet) and magnesium (155 mg elemental Mg/kg diet) from the following magnesium salts: magnesium oxide; magnesium sulfate; magnesium chloride; magnesium citrate; magnesium gluconate; magnesium orotate; and magnesium malate or EDTA magnesium salt.

The addition of phytic acid to the animals’ diet is important because phytic acid binds strongly to minerals and forms an insoluble complex. This means less of the minerals consumed can be absorbed. Despite the addition of phytic acid to the animals’ diet, all the magnesium salts tested were found to be comparably bioavailable: “The results indicate that any differences in the magnesium bioavailability of the compounds were small and physiologically irrelevant.” 6

woman put effervescent tablet in glass

Higher Bioavailability Does Not Necessarily Translate to More Effective Delivery to Tissues

When a magnesium supplement is highly bioavailable, it will be rapidly absorbed from the small intestine into the bloodstream, and magnesium blood levels will quickly rise. This sounds good, but when blood magnesium levels exceed a critical threshold, the excess is rapidly excreted by the kidneys. The end result is a shorter duration of magnesium availability to tissues. Magnesium salts with the greatest water solubility, bioavailability and rate of absorption are also more rapidly excreted in urine.

The lower parts of the small intestine are the primary sites of magnesium absorption. However, magnesium is also absorbed throughout the entire intestinal tract. Magnesium does not have special carriers that pull it into the bloodstream. Its absorption is a passive transcellular process.

For this kind of absorption process, the quantity of magnesium that passes through the digestive tract is the major factor controlling the amount of magnesium that is absorbed.

Because magnesium oxide contains more elemental magnesium and is less soluble, it’s absorbed into the bloodstream more slowly as it passes through the digestive tract. It will not raise blood levels excessively, and will therefore not be rapidly excreted. 5,7.8

The Research-Backed Bottom Line on Magnesium Oxide

You can get the same benefits from magnesium oxide with fewer pills. Magnesium oxide is the most effective magnesium supplement – and that is why it was selected for use in AlgaeCal Plus.


    1. Molecular weights: For the other magnesium salts, use the search box in the top right corner of the page.
    2. Ranade VV, Somberg JC. Bioavailability and pharmacokinetics of magnesium after administration of magnesium salts to humans. Am J Ther. 2001 Sep-Oct;8(5):345-57. PMID: 11550076
    3. Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res. 2001 Dec;14(4):257-62. PMID: 11794633
    4. Altura BT, Wilimzig C, Trnovec T, et al. Comparative effects of a Mg-enriched diet and different orally administered magnesium oxide preparations on ionized Mg, Mg metabolism and electrolytes in serum of human volunteers. J Am Coll Nutr. 1994 Oct;13(5):447-54. PMID: 7836622
    5. Coudray C, Rambeau M, Feillet-Coudray C, et al. Study of magnesium bioavailability from ten organic and inorganic Mg salts in Mg-depleted rats using a stable isotope approach. Magnes Res. 2005 Dec;18(4):215-23. PMID: 16548135
    6. Bertinato J, Plouffe LJ, Lavergne C, et al. Bioavailability of magnesium from inorganic and organic compounds is similar in rats fed a high phytic acid diet. Magnes Res. 2014 Oct-Dec;27(4):175-85. doi: 10.1684/mrh.2014.0374. PMID: 25635418
    7. Ranade VV, Somberg JC. Bioavailability and pharmacokinetics of magnesium after administration of magnesium salts to humans. Am J Ther. 2001 Sep-Oct;8(5):345-57. PMID: 11550076
    8. Musso CG. Magnesium metabolism in health and disease. Int Urol Nephrol. 2009;41(2):357-62. doi: 10.1007/s11255-009-9548-7. Epub 2009 Mar 10. PMID: 19274487

Author: Lara Pizzorno, MDIV, MA, LMT

Best-selling author of “Your Bones: How You Can Prevent Osteoporosis and Have Strong Bones for Life – Naturally” and a member of the American Medical Writers Association with 30+ years of experience specializing in bone health. Lara is the Editor of Longevity Medicine Review ( as well as a Senior Medical Editor for SaluGenecists Inc., and Integrative Medicine Advisors, LLC.