Osteoporosis, or thinning of the bones, is the reduction of the amount of calcium carbonate or phosphate deposited in the bone matrix. This is usually the consequence of ageing, as a result of the cells which absorb bone being more active than those which deposit it, particularly in post menopausal women. In younger people it can occur after periods of immobility, such as may happen when a limb is immobilised in plaster after a fracture - because the continuous nature of the remodelling of our bones requires that they be stressed in order for new bone to be deposited: in the immobile bone and muscle mass shrinks like snow on a hot stove. It is more likely in association with some other conditions such as an overactive thyroid, lack of testosterone in a male or in those with a strong family history. It may also occur as a result of treatment with steroids for conditions such as asthma. Very rarely it is due to a deficiency of calcium in the diet. The likelihood of our developing osteoporosis in later life is reduced if during adolescence we take a diet high in calcium and lots of exercise, because this will give us more bone in the bank to start with.