Bone risk for runners

Running, as a weight bearing exercise, increases bone density and is therefore cited as an exercise that helps reduces the risk of osteoporosis and bone fracture.

But the latest research in the UK suggests that is not the case - and in some cases long distance runners may actually have weaker than average bones.

" If you run you need to take in more energy because you expend more energy "
Dr Melanie Burrows

Researchers measured the bone density of the spine and hip in 52 women who ran anything from 5km to 70km a week for pleasure or to reach competition level.

The women were all aged between 18 and 44 and had not started the menopause.

They also kept food diaries for seven days to provide information on nutrient and mineral intake, especially zinc, magnesium, and calcium, all of which affect bone turnover.

Other factors such as body weight, menstrual cycle irregularities and use of hormonal contraceptives were also taken into account.

The results showed women who ran longer distances were most likely to have the lower bone density.

There seemed to be a small percentage drop in bone density with every further 10km run in a week.

Nutrients

This could be the result of a diet lacking a healthy balance of essential nutrients.

None of the dietary factors seemed to have a significant impact on the bone density of the spine.

But higher magnesium and lower zinc intakes were associated with greater thigh bone density.

This could be because magnesium is important for thyroid hormone activity, low levels of which would decrease calcium uptake and therefore bone composition, say the authors.

And it is possible that zinc stimulates the immune system to start breaking down bone.

Athletes who do sports such as gymnastics, weightlifting, and volleyball, where the forces applied are more than 10 times bodyweight have higher bone density than those who do sports where forces are in the range 5 to 10 times bodyweight. This includes long distance running.

Although running involves repeated impact of the feet against the ground, it seems the lower forces do not stimulate bone growth to the same degree.

Restricted diet

Lead researcher Dr Melanie Burrows, of the University of East London, told BBC News Online, more work was needed to pin down the exact link between long distance running and low bone mineral density.

But she said the problem was possibly that long distance runners tended to restrict their dietary intake because they thought excess weight would damage their performance.

"If you run you need to take in more energy because you expend more energy.

"It might be that other people also have the same dietary inadequacies, but they might get away with it because the effects do not show up if you have a less active lifestyle.

"I would never suggest that people should not go long distance running. It is better to do exercise because there are a whole myriad of health benefits."

The research is published in the British Journal of Sports Medicine.