Bone calcium has to become ions first; vitamin D opens the active carrying path.
Full mechanism, in text
Calcium and vitamin D, bones included
1 · The calcium in the bone first has to become ions
Calcium in the soft bones is locked in hydroxyapatite grown with collagen; once chewed apart and soaked in stomach acid, the crystal surface starts to dissolve into absorbable ions.Swallow them whole and too little surface meets the acid.
2 · Two paths across the small-intestine wall
Calcium crosses the epithelium by two paths: passively through the gaps between cells, or actively through the cells themselves, a path whose capacity is set by vitamin D.A layer of epithelium sits between the lumen and the blood. Calcium can squeeze through the gaps between cells, down the gradient — passive, unregulated. Or it can go through the cell, handed along in relays — active carrying. The capacity of that path is set by vitamin D.
3 · Vitamin D raises three proteins, and the door opens
Active vitamin D, via its nuclear receptor, raises a calcium channel, a calcium-binding protein and a calcium pump, and all three must rise for the active path to gain capacity.Active vitamin D enters the duodenal epithelium, finds its nuclear receptor, and raises three proteins: the lumen-facing calcium channel, the calcium-binding protein inside the cell, and the blood-facing calcium pump. All three rise, and the active path's capacity goes up. When D is short, only the passive gap is left.
4 · Material and switch packed into the same bite
Calcium is the material and vitamin D the switch; neither works alone, and while most foods give one end, bone-in sardines pack both into the same bite.Calcium without vitamin D: the switch is off. Vitamin D without calcium: the switch is on, but there is no material. Most foods give one end of the chain — milk gives calcium, sunlight gives D. A bone-in can packs both into the same bite.