Physical activity when young increases bone density and size and potentially stave off osteoporosis, according to a new thesis.

The thesis, written at the Sahlgrenska Academy at the University of Gothenburg, Sweden, was based on a study of 3,200 men. The men had their bones and exercise habits examined and, of these, 2,300 18-year-olds were selected at random to have their heel bone examined too as it is directly impacted by exercise.

"In this group, we found that those who actively did sports, and also those who used to do sports, had greater bone density than those who had never done sports," said Martin Nilsson, physiotherapist and doctoral student at the Institute of Medicine.

The bone density and structure in the lower leg in around 360 19-year-old men who had previously done sports but had now stopped training was also looked at. They found that men who had stopped training more than six years ago still had larger and thicker bones in the lower leg than those who had never done sports.

Nilsson said: "This result is particularly important, because we know that a bone with a large circumference is more durable and resistant to fractures than a narrower bone."

The researchers also studied bone density in around 500 randomly selected 75-year-old men. Those who had engaged in competitive sports three or more times a week between the ages of 10 and 30 had higher bone density in several parts of the body than those who had not.

"The bones respond best when you're young, and if you train and load them with your own bodyweight during these years, it has a stimulating effect on their development. This may be important for bone strength much later in life too, so reducing the risk of brittle bones," said Nilsson.