The following information is taken from the NOS website.
Phytates and Oxalates
A wide range of foods contain dietary phytates and oxalates, which bind to calcium and decrease absorption. The presence of phytates and oxalates are not a significant contributor to calcium loss unless consumed in excessive quantities, and the foodstuffs they are present in remain an essential part of a well-balanced diet.
Phytic acid is found in bran, wholegrain cereals and the seed coats of beans and grains. Oxalic acid is found in particularly high quantities in rhubarb and spinach. Although spinach and rhubarb are naturally high in calcium, the oxalic acid it contains affects the amount of calcium that can be absorbed and many portions of these foods would be needed to give the equivalent amount of calcium found from other sources.
Some slimming programmes suggest adding bran to food, and some people may add bran to ease constipation. Instead, try eating plenty of high fibre foods, such as fruit and vegetables, drinking lots of water and increasing exercise and mobility.
Those people who are advised to eat wholemeal products to ease constipation and aid bowel health are often worried that this may inhibit calcium absorption. However, if calcium is taken at other times of the day and not all at the same time as the wholemeal product then calcium intake is unlikely to be adversely affected, and both bone and bowel health will be maintained.