The body’s ability to adapt to varying levels of calcium intakes, the lack of sensitive indicators of calcium concentration, and the complexity and slow progression of osteoporosis make it very difficult to establish the role of diet in this regard.
However, research has made it clear that diet composition and the chemical form of calcium in foods affect calcium bioavailability (the rate and extent of absorption for use). It has been observed that calcium (present in dairy products) is readily absorbed in the intestine. Various plant components like fibre, phytate, and oxalic acid (present in green leafy vegetables and whole cereals) may reduce calcium absorption rate. Also, high intake of protein increases urinary loss of calcium but this loss may be partially compensated by the phosphate association with most high-protein foods.
Calcium absorption from various supplement tablets is generally good. Absorption from salts such as calcium carbonate, which require acid for dissolution, may be poor in persons with achlorhydria (absence of free gastric acid in stomach) unless the tablets are consumed with a meal. Sometimes calcium absorption can be poor due to lack of vitamin D (especially in infants and the elderly) also.