The relationship between physical exercise and osteoporosis - the bone-thinning disease which leads to height loss and fractures in middle and old age - is strong but far from straightforward. For while too little exercise predisposes to osteoporosis and moderate exercise protects against it, too much exercise can have the opposite effect, particularly in combination with amenorrhoea (absent menstruation) and an eating disorder.
Indeed the combination of amenorrhoea, eating disorders and osteoporosis are now so prevalent in adolescent and young adult female athletes that it is known collectively as'the athletic triad', according to Professor Moira O'Brien, delivering the 7th Samuel Haughton Lecture to the Royal Academy of Medicine in Ireland earlier this year.
Osteoporosis in young female athletes is sport-specific, she points out, with a much higher incidence in'appearance sports', such as diving, figure skating, gymnastics, ballet and synchronised swimming as well as endurance sports, particularly long distance and marathon running. Also at risk are athletes from sports with weight classification, including jockeys, boxers, wrestlers and rowers.
The best way to prevent osteoporosis is to maximise bone mass in the first three decades of life when bone is being formed at a greater rate than it is being lost - and exercise plays a key role in this process. But different types of exercise have different effects. As O'Brien explains:'Weight-bearing exercises incorporating gravity stimulation (eg running, gymnastics and jumping) cause a greater increase in bone density than non-gravity, non-weight-bearing exercise such as swimming.'
Also, the skeletal response to exercise is site-specific, being greatest at the site of maximum stress. So, for example, professional tennis players have up to 30% greater bone density in their playing arms, runners have increased density at the femoral neck, while weight-lifters have 19-35% greater density of the lumbar spine.
In terms of preventing osteoporosis, the best time to start an exercise programme is during the early teenage years, when bone growth is at its peak. Amazingly, according to O'Brien, as much as 60% of bone growth occurs during adolescence.
'The adolescent growth spurt is when the osteotrophic effect of physical activity is greatest,' she says. And other researchers have found that weight-bearing activity during adolescence and early adulthood is a far more important predictor of peak bone mass than calcium intake, possibly because growing bone has a much greater capacity to add new bone to the skeleton than mature bone.
A particularly telling finding in this respect is that female tennis players whose careers began before the onset of menstruation had a 2-4 times greater bone mineral density in the humerus of their dominant arm than those who started 15 years later.
'The age at which physical activity begins affects BMD,' O'Brien concludes.'High school athletic participation is a significant predictor of BMD of the femora neck and possibly of the spine.'
Ir J Med Sci 2001 Jan 170(1), pp 58-62

Isabel Walker