The amount of calcium absorbed in the intestine depends on habitual calcium intake. When
intake is low, active transcellular calcium transport in the duodenum is upregulated and a
larger proportion of calcium is absorbed by the active process than by the passive
paracellular process that prevails in the jejunum and ileum. Bioavailability of the calcium
source—digestibility and solubilization—plays a role under conditions of low calcium intake
but is relatively unimportant when calcium intakes are high (eg> 800 mg/d in people) …