More than a bone mineral: a tightly controlled signal that runs muscles, nerves and clotting.
Mechanisms, studies and evidence grades for each claim above.
About 99% of the body's calcium, roughly 1 kg, is in bone and teeth. It is bound to phosphate as hydroxyapatite, a hard crystal laid down along collagen fibres. Collagen keeps bone from shattering, and the mineral keeps it from bending.1
Bone is also a calcium bank. Blood calcium is held in a very tight range (about 8.5–10.5 mg/dL) because nerves and heart depend on it. When intake falls short, the parathyroid glands release PTH, which does three things. It pulls calcium out of bone, makes the kidneys keep more, and switches on vitamin D so the gut absorbs more. Blood calcium stays normal while the skeleton pays. That's why a normal blood calcium says almost nothing about bone, and why long-term shortfalls show up as osteoporosis decades later instead of symptoms now.1,2
What intake actually does. Getting enough matters while bone is being built, in childhood and adolescence. In adults, the trial evidence is more modest than the advice suggests. A BMJ systematic review found that dietary calcium intake was not linked to fracture risk. In the supplement trials at lowest risk of bias (44,505 people), calcium did not prevent fractures at any site. The one clear benefit was in frail elderly women in care homes who started low in both calcium and vitamin D.3 Bone depends on the whole system working: vitamin D, magnesium, vitamin K2, protein and loading through exercise. Pushing intake higher than the RDA doesn't add more.
Food over pills. A meta-analysis of randomised trials found calcium supplements (500 mg/day or more, without vitamin D) raised heart attack risk by about 27–31%.4 A likely explanation is that a pill causes a sharp spike in blood calcium, which food doesn't. Dairy, sardines and greens deliver calcium slowly, alongside the cofactors that direct it into bone.
↑ back to summaryA resting muscle fibre keeps calcium locked away in an internal store, the sarcoplasmic reticulum. Free calcium inside the fibre is about 10,000 times lower than in blood. When a nerve signal arrives, the store's release channels (ryanodine receptors) open and calcium floods the fibre. Calcium binds troponin, which shifts its partner protein tropomyosin out of the way and exposes binding sites on actin. Myosin heads then grab actin and pull, using ATP. That is the contraction. Relaxation needs calcium to be pumped back into the store by the SERCA pump, which runs on Mg-ATP.5
The heart adds a step. Its trigger is a small amount of calcium coming in from outside the cell, which sets off the much larger release from inside ("calcium-induced calcium release"). How much calcium is released on each beat largely sets how hard the heart squeezes. Calcium channel blockers lower blood pressure and slow the heart by limiting that trigger.5
Low blood calcium (more precisely, low ionised calcium) does something that seems backwards. It makes nerves near muscles more excitable, because calcium normally stabilises sodium channels. The result is tingling around the mouth and in the fingers, muscle twitches and, in severe cases, tetany (sustained spasm).1
↑ back to summarySodium carries the electrical signal down a nerve, but calcium is what turns it into a chemical message at the next cell. When the signal reaches a nerve ending, voltage-gated calcium channels open and calcium rushes into a tiny space next to vesicles full of neurotransmitter. A calcium sensor protein, synaptotagmin, binds calcium and within less than a millisecond drives the vesicles to fuse with the membrane and release their contents. Thomas Südhof won a Nobel Prize for working out this machinery.6
Release scales steeply with calcium: doubling calcium at the nerve ending raises release about 16-fold. So small changes in calcium entry adjust how strongly a synapse fires, and that's one of the main ways synapses strengthen with learning. Inside neurons and almost every other cell, calcium also works as a second messenger. Short calcium pulses switch on enzymes (through calmodulin), gene programs and hormone secretion, including insulin release from the pancreas.6
↑ back to summaryCalcium is "factor IV" in the classic list of clotting factors. Several clotting proteins (factors II, VII, IX and X) have a cluster of special amino acids called Gla residues at one end. Vitamin K is needed to make them. Calcium ions bridge those Gla residues to the negatively charged surface of activated platelets. That parks the clotting enzymes right where the clot needs to form, which speeds the reactions by many thousands of times.7
This is also why blood collection tubes for clotting tests contain citrate. Citrate binds calcium, so the blood can't clot until calcium is added back in the lab. In practice, blood calcium never falls low enough to impair clotting before it causes heart and nerve problems, so diet doesn't affect this role. But it shows how calcium and vitamin K depend on each other.
↑ back to summaryThe RDA is 1,000 mg/day for most adults, and 1,200 mg/day for women over 50 and men over 70. The gut only absorbs about 25–35% of what you eat, and that depends heavily on vitamin D (as calcitriol), which switches on the gut's calcium transporters.2 Magnesium is needed for the parathyroid glands to release and respond to PTH, so low magnesium can cause low calcium that won't correct until magnesium is replaced.8
Low intake over years usually causes no symptoms while bone density slowly falls. Low blood calcium (hypocalcemia) is a different problem and usually comes from low vitamin D, low magnesium, or parathyroid or kidney disease, not diet. It shows up as tingling, muscle cramps and twitching, tetany and, when severe, seizures and heart rhythm problems.1
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