The risks
What are the risk factors for osteoporosis? The condition is not hereditary, but is linked to family factors. Therefore, when a mother, aunt or sister has broken a hip, the risk of being a victim of the same problem is multiplied by four for any woman related to them. “We estimate that 60% of bone mass depends on genetic factors and 40% on acquired factors”, explains Professor Reginster. Many of these pre-disposition genes have been identified (genes acting on collagen, intestinal calcium absorption and the efficiency of vitamin D), but no single one of them alone explains the disease and neither could they be the target of gene therapy.
There are numerous other risk factors. Advanced age is one; being of the female sex is another, family ancestors, poor acid-base balance in the organism are others. A low body mass index (BMI<20) exposes women to an increased risk of osteoporosis because they lose a lot of the protection granted to them by the transformation of androgens into oestrogen in the fatty tissues. The problem is particularly worrying in the case of anorexics. Similarly, early menopause (before the age of 45) is a formidable problem; the women concerned finding themselves in a situation of permanent deficiency in oestrogen. Tobacco, too, is harmful to the skeleton: it diminishes the activity of the osteoblasts. The same applies to alcohol when its daily consumption exceeds two units (glasses) in women and four in men.
A sedentary lifestyle and prolonged physical inactivity constitute other important risk factors. During physical exercise there is a transformation of the mechanical energy of the muscles into electrical energy – known as piezoelectric currents -, a process which supplies the osteoblasts with energy and helps them to synthesise bone. “In tennis players between the ages of 15 to 30 who practise the sport several hours per week, the bone density of the dominant arm is higher by twenty per cent than that of the other arm”, underlines Professor Reginster to illustrate his point.
The skeleton is made up principally of a bone matrix protein (90% collagen) covered in hydroxyapatite crystals (calcium). As a consequence, any deficiency in calcium or in protein has a damaging effect on bone formation. On the other hand, as vitamin D is necessary for the absorption of calcium in the intestines, any insufficiency thereof proves to be very damaging. “Three quarters of the Belgian population are in deficiency of vitamin D for nutritional reasons (2) or lack of exposure to sunlight. Calcium deficiency is also frequent”, Professor Reginster continues.
Some medicines can also pave the way for osteoporosis. This is particularly true of cortisone used over a long period, certain anti-depressants or diuretics. Various hormonal, metabolic or other diseases are also potential causes. For example, inflammatory rheumatic disorders such as rheumatoid polyarthritis due to the release of cytokines which are harmful to the skeleton. In the same way, ionic disturbances occurring in the kidneys can cause disorders in the metabolism of calcium, phosphorous and magnesium. Another example: during hyperthyroidism, the excessive secretion of the parathyroid hormone favours the resorption of the skeleton.
