chapter  4
5 Pages

Biochemical changes

WithJohn C Stevenson, Michael S Marsh

In the normal person whose bone mass is being maintained, approximately 6 mmol of calcium enters and leaves the skeleton each day, and the amount of calcium leaving the body in the urine and digestive juices is matched by the amount absorbed from the gut. Parathyroid hormone secretion increases in response to a fall in plasma calcium and acts directly on the kidney to increase tubular calcium reabsorption. Calcium metabolism changes considerably around the time of the menopause. Both bone resorption and bone formation increase, but with formation less so than resorption, leading to a negative calcium balance of approximately 50 mg/day. Fasting urinary calcium-to-creatinine ratios increase after the menopause and fall in response to antiresorption therapy. Biochemical markers of bone formation and resorption can be useful in indicating the state of bone turnover. However, attempts to combine markers of bone turnover to improve the sensitivity and specificity of detection of low bone mineral density have proved disappointing.