This is because calcium is transported into the body by a special carrier protein which requires vitamin D for its synthesis. A number of substances can inhibit the absorption of calcium. Phytic acid, found in bran, whole cereals and raw vegetables is one of these. Uronic acid, a component of dietary fibre, and oxalic acid, found in certain fruits and vegetables can also bind calcium. However, diets habitually high in these acids are not thought to have a major effect on calcium absorption.
Saturated fats can also lessen calcium absorption.
Calcium is lost in the faeces, urine and sweat. Calcium loss is roughly equal to dietary calcium in adults. Calcium loss is reduced if dietary calcium is low. Adaptation to both high and low calcium intakes occur. Reduced intake leads to increased efficiency of absorption.