A rise in broken bones among adolescents has alarmed researchers who say a decline in milk drinking may be to blame.
Researchers at the Mayo Clinic in Rochester, Minnesota, examined data from 1969 to 1971 and compared it with information recorded between 1999 and 2001. They found that the incidence of forearm fractures had risen by 42 per cent. Most of the increase was among young people under 20.
Sundeep Khosla, an endocrinologist at the clinic and the lead author of the study, published in the Journal of the American Medical Association, said the "substantial increase" was of "some concern". The fracture rate rose to 469 per 100,000 young people in 1999-2001.
Most fractures occur around the time of the growth spurt during puberty and specialists have suggested that this could be because the bones become more porous when they are growing fast. That, though, does not explain the increase. The commonest age for fractures is between eight and 11 for girls and between 11 and 14 for boys.
Boys aged 12 had the highest fracture rate at 1,536 per 100,000, or 1.5 per cent.
The researchers classified the fractures according to their causes. They found that fractures caused by recreational activities, including team games such as baseball and individual sports such as skating, had doubled over the 30-year period.
Dr Khosla said: "Our study does not explain why these fracture rates increased, but the data raises concerns about whether bone mass development in today's children may be impaired by other lifestyle and dietary factors such as increased soft drink consumption, decreased milk consumption or changing patterns of physical activity."
In Britain, one third of schoolchildren consume less than the recommended three daily servings of dairy foods. Consumption of fizzy drinks has risen, with 60 per cent drinking at least one can a day and 22 per cent drinking four or more cans.
The Milk Development Council is running a "milk moustache" campaign, with the help of celebrities including the model Nell McAndrew, to persuade people to drink milk.
American specialists said that if the Mayo Clinic findings indicated that children living now had weaker bones, it could herald a health crisis when they reached middle and old age. Bone mass development in childhood is critical to preventing the bone-thinning disease osteoporosis in later life.