Girls who do too much exercise may be putting themselves at the same risk of crumbling bones and a weak skeleton in later life as those who slouch in front of the television and travel everywhere by car, according to research.
A study that compared teenage ballet dancers who exercised for several hours a day with musicians from a performing arts school and girls from an ordinary academic school found the ballet dancers had the lowest bone density.
Bone density is the key factor in the bone thinning disease osteoporosis. One in three women and one in 12 men suffer from the disease during their lifetime but evidence is mounting that the years between the age of 10 and 20, when the skeleton is growing, are crucial to its prevention.
Dr Nicola Keay, a research fellow at Guy's and St Thomas' Hospital, who led the research presented to a conference of the National Osteoporosis Society this month, said: "There is definite evidence that if a girl gets it right when she is young she has a better chance later on. The teenage years are crucial - after the age of 20 the bones don't do much."
Adolescence is marked by growth spurts followed by periods of consolidation when the bones fill out. Between 50 and 70 per cent of bone density is determined by genetic factors but the remainder is influenced by diet, including the level of calcium, and exercise.
Conventional advice is that weight-bearing exercise - including running and walking but not swimming - is necessary to build up the bones. Dr Keay said: "Jumping up and down is good for you. But it seemed to me worth asking how much was good for you. If you carry on doing more and more, is it bad?"
She studied the diet, level of exercise, menstruation and bone density of 87 girls aged between 11 and 16 years. One group - "self-confessed slobs" according to Dr Keay - played an occasional game of netball but otherwise spent their free time hanging around with friends or watching television.
A second group of performers, training for West End musicals in London, did daily dance classes and a third group from a ballet school spent half of each day in dance classes.
The results showed the musicians who did regular but not excessive exercise had the highest bone density - higher than either the ballet dancers or the couch potatoes.
Dr Keay said: "The ballet dancers were eating regular amounts - they were not starving - but they were expending so much energy they were not in the right balance. Their periods started late, at age 14 rather than 12 or 13 in the other girls, and were erratic. There is a big surge in bone density when the periods start and bone is still accumulating two years later, which is why it is such a crucial time." Marathon runners who have failed to maintain an adequate diet have developed the bones of 80-year-olds and sustained fractures in minor falls, Dr Keay said. Ballet dancers would benefit from extra carbohydrate in their diet and variation in their exercise regime.
"Instead of doing slow graceful movements which are good for muscle control but not for the skeleton they need to do more high-impact stuff such as aerobics. The ballet schools are taking this on board and starting aerobics classes."
She added: "There is an onus on young girls' parents and coaches to get it right. My argument is that this is what they should be focusing on."