Vitamin D metabolites stimulate calcium and phosphate absorption from the gastrointestinal tract, and mineralisation of bone by osteoblasts. Vitamin D also decreases renal excretion of calcium and phosphorus. Dietary deficiency of vitamin D can cause rickets in growing animals, particularly if the diet is also deficient in calcium and phosphorus. Rarely diagnosed in small animal practice, rickets is characterised by widening of the epiphyseal growth plates, often accompanied by bending of the shafts of long bones and reduced bone density. Absorption of calcium from the intestine Calcium is absorbed in the ionised form, so the absorption is directly related to the amount that is soluble and ionised in the small intestine.
Passive diffusion is the more important route in growing animals.
The availability of calcium in the diet is a major factor influencing calcium requirements. Many calcium salts such as phosphate and carbonate are insoluble at neutral pH and but digestion of foods in the acid environment of the stomach greatly increases the amount of soluble calcium. Calcium absorption is also depressed by corticosteroids, thyroid hormones and oestrogens. Balance of calcium to phosphorus Calcium and phosphorus in the body are so closely interrelated that excessive intake of phosphorus relative to calcium leads to signs of calcium deficiency, due to decreased calcium absorption, even if dietary calcium is adequate. During gestation or lactation the optimum range is broader.