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Long distance runners at risk of low bone density

January 24, 2003

Long distance running increases the risk of low bone density, shows research in the British Journal of Sports Medicine. Accepted wisdom is that running, as a weight bearing exercise, increases bone density and therefore reduces the risk of osteoporosis and bone fracture.

Researchers measured the bone density of the spine and hip (femoral neck) in 52 women who ran anything from 5 to 70 km a week for pleasure or to reach competition level. The women were all aged between 18 and 44 and had not started the menopause.

The women 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. They were also weighed and the extent of body fat was assessed. Other potentially important factors, such as menstrual cycle irregularities and contraceptive hormone use were included in the analysis.

The results showed that the heavier women had greater bone density in both the spine and hip, but only if this weight was primarily lean - that is, they had more muscle than fat.

But after taking account of body size and age, the results showed an association between lower bone density in the spine and hip and greater distances run. Although not directly measured in the study, there seemed to be a small percentage drop in bone density with every further 10 km run in a week, regardless of the age at which the women started training.

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.

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.

The authors explain that although running involves repeated impact of the feet against the ground, the lower forces do not stimulate bone growth to the same degree. The magnitude of the force applied is likely to be critical, they say.

British Medical Journal (BMJ)