Women who cycle regularly appear to be at reduced risk of developing breast cancer, research has found.

Just three hours of moderately intensive cycling a week is linked to a 34% reduction in risk, they found.

German researchers found the benefits of cycling appeared to increase the more women did.

Previous research has suggested exercise can reduce cancer risk.

This study suggests cycling could have particular benefits.

But the researchers admit this could be partly because those questioned had a better recall of bike riding compared to other forms of exercise.

It's important not to read too much into this Clare Stevenson, Cancer Research UK

Just under 400 pre-menopausal women under 45 who had already had breast cancer were studied, along with 880 healthy volunteers.

They were asked about the kind of exercise they took between the ages of 12 and 19, and 20 to 30 and whether it was light, moderate or heavy.

Immune boost

In the study, published in the American Journal of Epidemiology, researchers from the German Cancer Research Centre in Heidelberg said: "We found decreasing risks with increasing cycling activity levels.

They added: "For cycling, we found a significant protective effect, and sports appeared to have some protective effect among otherwise less active women."

Martina Schmidt, who worked on the study, told BBC News Online: "It could be that cycling improves your immune system."

But she said more research was needed before their finding could be confirmed.

"It's too early to give advice."

The researchers say future studies should look at national differences in exercise habits.

They say that in Germany, cycling is used for transportation as well as a recreational activity, unlike other countries such as the US.

Activity

Clare Stevenson, a Cancer Research UK researcher at the University of Bristol said studies had shown a "robust" protective link between exercise and cancer in postmenopausal women.

But she said the evidence was far less clear in premenopausal women.

She also said it was unlikely that cycling was much better than any other form of exercise.

"It could be that when people are asked to recall how much physical activity they've done, they may have had more accurate recall of cycling - because they cycled to school or work.

"It's important not to read too much into this and take away the message that cycling is particularly good for you."

Ms Stevenson said there were indications that physical activity could help protect against cancer, but she said scientists had not yet pinned down the explanation.

Theories including it bolstering the immune system, and reducing levels of the hormone oestrogen have been put forward.