Osteoporosis ('brittle bones') isn't just a problem for post menopausal women. Both younger women and men are at risk from this disease. Once osteoporosis is in process, it is irreversible; therefore prevention is of paramount importance. Experts believe that strategies for preventing bone loss should begin in childhood and continue throughout the lifespan. The three most important factors contributing to bone health are hormonal, nutritional and mechanical..

Mechanical stressing, which occurs during exercise, can contribute to an increase in bone mass when nutritional and hormonal factors are also favourable. To put the effect of activity into context, measurements have found that a week's bed rest for a young adult can lead to a 1 per cent loss of spinal bone density; a loss which can take up to 4 months of remobilisation to regain..

Cross sectional studies of athletes show that the highest bone mass density values occur in strength trained athletes..

An additive effect of exercise and diet has been shown for both premenopausal and postmeno-pausal women. An Australian study investigated three different approaches to preventing osteo-porosis in postmenopausal women with low bone density ('Prevention of postmenopausal osteo-porosis', Prince et al, New England Journal of Medicine, vol 325, pp1189 1195). One group of women was given an exercise regime, another exercise plus calcium supplementation, and a third exercise plus hormone replacement (oestrogen and progesterone). After two years, it was found that bone loss was slowed or prevented by exercise plus calcium supplementation, or exercise plus hormone replacement. Exercise alone was ineffective in slowing bone loss..

Although exercise can be beneficial, the level and quantity is important, as high levels of activity can interfere with hormone levels. Female athletes whose activity is at a level high enough to interfere with their menstrual cycle (leading to cessation or irregularity of periods) are found to have lower bone density than women with normal (regular) menstruation, despite higher levels of physical activity. Two studies suggest that bone mass in such women does not return to normal even after the resumption of regular menstruation. Three studies have reported low vertebral bone mass density for male runners, which may be related to reduced levels of reproductive hormones found in some male endurance athletes..

If physical exercise is kept within limits that do not adversely affect reproductive hormone levels, it appears to be beneficial, as long as adequate calcium is being consumed. One study found that the threshold calcium intake necessary to prevent accelerated bone loss was at intakes below 405 mg/day (current UK Reference Nutrient Intake is 700mg/day)..