Magnesium Forms Compared: What the Absorption Data Show

A magnesium shelf is a list of second words. Oxide, citrate, chloride, glycinate, malate, threonate, aspartate, lactate. The first word is the same in every case, and the marketing usually implies that the second word is a minor technical detail.

It is not. But the published data separate these forms far more clearly on one axis than the other, and keeping the two apart is the exercise.

Two measurements of absorption

The first study compared four commercially available US preparations by measuring the increment in urinary magnesium excretion in normal volunteers given approximately 21 milliequivalents per day, on the reasoning that magnesium which gets absorbed has to be excreted.¹

Magnesium oxide showed relatively poor bioavailability, with fractional absorption of 4 percent. Magnesium chloride, lactate, and aspartate showed significantly higher and equivalent bioavailability to each other. The authors’ conclusion is worth noting for what it rejects: inorganic magnesium salts, depending on the preparation, may have bioavailability equivalent to organic ones.¹ Magnesium chloride is inorganic and performed like the organic salts, which undercuts the claim that chelated forms are categorically better absorbed.

The second study was a randomized, double-blind, placebo-controlled parallel trial in 46 healthy individuals, comparing amino acid chelate, citrate, and oxide at 300 milligrams of elemental magnesium per day for 60 days, with urine, blood, and saliva sampled at baseline, 24 hours after the first dose, and at 60 days.²

Citrate and amino acid chelate both showed greater absorption than oxide at 60 days by 24-hour urinary excretion. Citrate produced the highest mean serum magnesium after both acute and chronic supplementation, and the highest mean salivary magnesium after chronic supplementation. Erythrocyte magnesium showed no differences between groups. Magnesium oxide produced no differences compared with placebo.²

Two studies, two methods, one consistent finding: oxide is the poorly absorbed one. Poorly absorbed is not unabsorbed, though. A study summarized in a 2021 review gave 404 mg per day of magnesium oxide to healthy volunteers and found no significant change in blood magnesium but a 40 percent increase in urinary magnesium excretion, so some of it did cross.⁴

Why a poorly absorbed form is the laxative

Magnesium that is not absorbed does not disappear. In the intestinal lumen, poorly absorbable magnesium ions, along with accompanying ions such as sulphate, exert an osmotic effect and cause water to be retained in the lumen, which increases the fluidity of the intraluminal contents and results in a laxative action. The effect is thought to be primarily local, though released hormones such as cholecystokinin, or activation of constitutive nitric oxide synthase, may also contribute.⁶

For magnesium oxide specifically, the 2021 review sets out the chemical route: conversion to magnesium chloride under acidic conditions in the stomach, then to magnesium bicarbonate by sodium hydrogen carbonate from pancreatic secretion in the duodenum, and finally to magnesium carbonate. The bicarbonate and carbonate raise the osmotic pressure of the intestinal lumen fluid, promoting transfer of water into the lumen and increasing the water content and volume of the stool, and the swollen stool stimulates the intestinal wall and propulsive motor activity.⁴

The trial evidence, salt by salt

A prospective, double-blind, randomized, placebo-controlled trial assigned 90 patients, mean age 42 years, 93 percent women, mean symptom duration 9.9 years, to senna at 1.0 gram, magnesium oxide at 1.5 grams, or placebo for 28 consecutive days. The response rate for overall improvement was 11.7 percent with placebo, 69.2 percent with senna, and 68.3 percent with magnesium oxide. Changes in spontaneous and complete spontaneous bowel movements were significantly greater in both active groups than placebo, quality of life improved in both, and severe intervention-related adverse events occurred at a frequency of zero.³

A second salt has its own randomized evidence. A double-blind, placebo-controlled trial enrolled 244 women aged 18 to 60 with functional constipation by Rome III criteria, identified by 62 general practitioners across France. For four weeks they drank either 1.5 L of natural low-mineral water daily, or 0.5 L or 1 L of a magnesium sulfate-rich natural mineral water topped up with low-mineral water to the same volume. No significant effect appeared at week one. At week two, constipation was reduced in 21.1 percent of controls, 30.9 percent of the 0.5 L group, and 37.5 percent of the 1 L group, the last differing significantly from control at P equal to 0.013. The 1 L group also had fewer hard or lumpy stools and a substantial decrease in rescue medication use, and responses correlated with magnesium sulfate concentration.⁷

The comparison nobody has run

Two poorly absorbed magnesium salts each have randomized evidence of a bowel effect.³⁷ What does not exist is the study that would settle the shelf question directly.

The 2021 review is explicit about that gap. In European countries magnesium hydroxide, citrate, sulfate, and oxide are all used as saline laxatives, but there are only a limited number of studies comparing the different salt forms, and few actual cases of use. Its conclusion is that the decision of which magnesium salt to use as a laxative currently depends on the country in which it is prescribed: oxide took a central position in Japan, China, and Taiwan, while the magnesium preparation most commonly used in South Korea and the United States is magnesium hydroxide, and polyethylene glycol is the laxative of first choice in the United States.⁴

A US reader should sit with that: the form with the randomized trial above is not the one most used where they live.

Measured, and inferred

Measured: on urinary, serum, and salivary indices, citrate, chloride, lactate, aspartate, and amino acid chelate are better absorbed than oxide.¹² Measured: magnesium oxide and a magnesium sulfate-rich mineral water each produced a bowel effect against a control.³⁷

Inferred: the mechanism says an unabsorbed magnesium ion stays in the lumen and holds water there,⁶ and oxide is the least absorbed,¹² so a better absorbed salt should produce less of a bowel effect at the same elemental dose. That is a reasonable prediction, and it is still a prediction. The absorption studies measured magnesium in urine, blood, and saliva, not stool output or bowel movement frequency, and neither compared laxative effect at all.¹²

Note also which forms have been compared at all: oxide, citrate, chloride, lactate, aspartate, and an amino acid chelate.¹² Several heavily marketed forms appear in neither trial, and absence from a comparison is not evidence of doing well in one.

The safety caveat that matters most

The 2021 review calls magnesium oxide convenient, low in cost, and safe, while noting that its use should account for dose, serum concentration, drug interactions, and the potential for side effects, especially in older people and patients with renal impairment.⁴

A retrospective cross-sectional study puts numbers on that. Among 2,176 patients on daily magnesium oxide, 193 had serum magnesium assayed. Of those, 32 patients, or 16.6 percent, had a high serum magnesium concentration at or above 2.5 mg/dL, and 10, or 5.2 percent, had hypermagnesemia at or above 3.0 mg/dL. Hypermagnesemia was associated with chronic kidney disease grade 4 and with magnesium oxide dosage; high serum magnesium was associated with a dosage above 1,000 mg per day, chronic kidney disease grade 4, and concomitant stimulant laxative use. Age itself was not associated with either.⁵

Kidney function and dose, in other words, rather than being old.

The practical read

The second word on the label changes what has been measured. Oxide is the least absorbed of the forms that have been compared, and it is the form carrying a randomized placebo-controlled constipation trial.¹²³ Citrate, chloride, lactate, aspartate, and amino acid chelate are better absorbed on urinary, serum, and salivary measures, which is what matters if the goal is magnesium status rather than a bowel effect; whether they are correspondingly gentler on the bowel at the same elemental dose follows from the osmotic mechanism but has not been tested head to head.¹²⁴⁶

Decide which of those two goals you are after before comparing prices, check the elemental magnesium content rather than the compound weight, and take reduced kidney function as a reason to have the conversation with a clinician first.⁴⁵ Constipation that persists is a symptom worth evaluating rather than dosing around.