Scientists in America have discovered that the bones of hibernating bears increase in density while the bears are asleep, although their body weight reduces.
They believe that calcium secretion is at the centre of the problem and are trying to understand the chemical processes that take place when the bears are not moving.
Dr Ralph Nelson, research director of the Carle Foundation, in Urbana, Illinois, said they had found that the bears are 'recycling rather than secreting' their bodies' waste materials. This process in some way stimulates their bone growth, although their muscles reduce.
The picture is complicated because other research has shown that activity and sport increases bone density in adult women.
The American research is being closely monitored by the National Osteoporosis Society, whose spokeswoman warned that any advances from the bear experiments were 'a long way off'.
Osteoporosis affects more than two million people in Britain. Bone loss is a feature of ageing but the disease is common in women after the menopause when their bones become brittle.