Calcium vs Magnesium

The honest answer to "calcium or magnesium" is that the question contains a false premise. These two are not substitutes for one another in any biological sense — a body short of one is not helped by more of the other, and the jobs they do barely overlap.

What is genuinely decidable, and what most pages get wrong, is the form. A gram of calcium carbonate and a gram of calcium citrate do not contain the same amount of calcium, and the same goes for the magnesium salts by an even wider margin. That arithmetic is where the useful part of this comparison lives.

They are not doing the same job

Calcium is the most abundant metal in the human body by a large distance — somewhere over a kilogram in an adult, and about 99% of it is locked into the hydroxyapatite crystal of bone and teeth. It is a construction material first.

The remaining one per cent does something entirely different and arguably more remarkable. Cells hold their internal calcium concentration roughly ten thousand times lower than the fluid outside, and a brief, controlled collapse of that gradient is what makes a muscle fibre shorten, what tips a neurotransmitter into a synapse, and what marks the instant an egg is fertilised. Calcium is both the framework and the trigger.

Magnesium is present in far smaller quantity — around twenty-five grams — and almost none of it is structural in the same sense. Its role is catalytic. ATP is not biologically active as a bare molecule; it works as a magnesium complex, so every reaction in your body that spends energy involves a magnesium ion at the active site. DNA and RNA polymerases need it. Ribosomes are held together partly by it.

Outside animals the division is just as sharp: magnesium is the atom at the centre of the chlorophyll molecule, so every green leaf on the planet depends on it, while calcium in plants is mostly cell-wall cement — which is why a calcium-starved tomato rots at the blossom end while its leaves stay green.

The number on the label is not the number in the capsule

This is the part worth doing arithmetic on, because supplement labels are quoted inconsistently and the differences are large.

• Calcium carbonate is 40.0% calcium by mass. It is the cheapest and densest source, and it depends on stomach acid to dissolve.
• Calcium citrate, sold as the tetrahydrate, is about 21% calcium. Less than half of the carbonate's density per gram, but it dissolves without needing gastric acid.
• Magnesium oxide is 60.3% magnesium — the highest of any common magnesium salt, and the worst absorbed by a wide margin. Much of what is swallowed is never taken up.
• Magnesium citrate is about 16% magnesium, and magnesium glycinate about 14%, both of them absorbed far better than the oxide.

Put those together and the trap is obvious. A magnesium oxide tablet declaring 400 mg of the compound contains around 241 mg of magnesium and delivers a small fraction of it, while a glycinate tablet declaring 400 mg of the compound contains around 56 mg of magnesium and delivers much more of that. Reputable labels state the elemental figure. The ones that do not are quoting the heavier number on purpose.

Absorption efficiency adds a second correction on the calcium side: the fraction absorbed falls as the dose rises, which is the reason for the standard advice that large calcium intakes are split across the day rather than taken at once.

Magnesium is the gatekeeper for three other things

The interactions between these two elements run mostly in one direction, and they are more interesting than the competition people assume.

Magnesium behaves as a natural calcium antagonist. It blocks the pore of the NMDA receptor and competes with calcium at voltage-gated channels, damping calcium-driven excitability. That is not a theoretical property: intravenous magnesium sulfate is the established treatment for eclampsia, established by the Magpie trial in 2002, and it is used in severe acute asthma for the same smooth-muscle reason.

Magnesium also controls calcium indirectly. Severe magnesium depletion causes a low blood calcium that cannot be corrected by giving calcium — parathyroid hormone secretion and its action both fail without magnesium, so the magnesium has to be fixed first. The same is true of potassium depletion. And the enzymes that hydroxylate vitamin D into its active form are magnesium-dependent, so a magnesium shortfall quietly undermines the vitamin that calcium absorption relies on.

One deficiency is silent, the other is not

The two deficiencies present in completely different ways, which is why they are found at different stages.

Blood calcium is defended fiercely. If dietary intake falls, parathyroid hormone withdraws calcium from the skeleton to hold serum levels steady, so a chronic shortfall produces normal blood tests and a slowly thinning bone architecture that announces itself decades later as a fracture.

Magnesium status is hard to measure for the opposite reason: under one per cent of body magnesium is in serum, so a normal serum magnesium is compatible with genuine depletion in the tissues. When it does show, it shows quickly, as neuromuscular irritability, tremor and arrhythmia rather than as anything structural.

Safe intakes differ in structure too. Calcium has an upper limit driven by kidney stone risk and by an unresolved argument about whether supplemental calcium — as distinct from dietary calcium — affects cardiovascular risk, an argument in which good studies genuinely disagree. Magnesium's upper limit applies only to supplements, because dietary magnesium is self-limiting: the gut simply stops absorbing it and the excess acts as an osmotic laxative, which is precisely what magnesium sulfate and magnesium hydroxide are sold to do.

Both of them are in your kettle

Hard water is a solution of these two ions and nothing else of consequence. Bicarbonate hardness decomposes on heating and deposits calcium carbonate as limescale, with magnesium hydroxide joining it at higher temperatures. The grey film on a shower screen is the calcium and magnesium salts of the fatty acids in soap, which is also why soap lathers poorly in hard water and detergents were invented to get around it. An ion-exchange softener does not remove hardness so much as trade it, swapping both ions for sodium.

Their solubility patterns diverge in a way that follows the group. Magnesium hydroxide is essentially insoluble — hence the suspension sold as milk of magnesia — while calcium hydroxide dissolves slightly, enough to make limewater. Sulfates run the other way: magnesium sulfate is freely soluble, while calcium sulfate is only sparingly so, which is why gypsum scale is a persistent nuisance in desalination plants and boilers.

One builds cement, the other lines the furnace

The industrial split is as clean as the biological one. Calcium carbonate as limestone is the feedstock for Portland cement, which is made in enough quantity that its calcination step alone is a significant share of world carbon dioxide emissions. Gypsum, chalk, marble and quicklime are all the same element in different dress.

Magnesium's compound industry is refractories. Magnesia bricks and monolithics line the steel furnaces and cement kilns that the calcium industry runs, because magnesium oxide melts near 2,850 °C and is chemically compatible with basic slags. Dolomite, the mixed carbonate of both elements, is the one raw material they share.

Which one, and in what form

• Building or maintaining bone mass — calcium, with vitamin D, and magnesium only as far as it is needed to make the vitamin D work.
• A cheap, compact calcium source taken with food — calcium carbonate, for the 40% content.
• Calcium for someone with reduced stomach acid — citrate, accepting half the elemental density per gram.
• Magnesium intended to be absorbed — citrate or glycinate; the oxide's headline percentage is misleading.
• Magnesium intended to act in the gut — the oxide, hydroxide or sulfate, which is what they are good at.
• A furnace lining — magnesia. A bag of cement — limestone. The two elements have not competed for a single industrial job in centuries.