Low bone density in cyclists

Elite cyclists have low bone mineral density (some cyclists may have the bone density expected in a 70 year old) and this has been linked to an increased risk of breaking bones in crashes, and a higher risk of osteoporosis in later life. However, what is the evidence that there are more breaks and that these effects are long term effects? Also, what causes this low bone mineral density? We recently published a viewpoint in the Journal of Applied Physiology (1) and here is a summary.



Luuk Hilkens was the lead author on the viewpoint (1) that shows that there is accumulating evidence to suggest that most elite cyclists have lower bone mineral density values compared with non-elite cyclists (2).


Causes

There may be several causes of this observation and it is likely that several factors are responsible. First cycling is not a weight baring activity and thus there is no mechanical loading of the skeleton. This loading is essential to stimulate bone growth. However, elite cyclists spend a large part of the day on a bike (20–30 h/week) and the rest of the day resting and recovering with relatively little walking and even less running. The stimulus through mechanical loading is therefore minimal.


There may be several causes of this observation and it is likely that several factors are responsible.

A second factor is that many cyclists are weight conscious and have very low body fat. In an attempt to reduce body weight or maintain low body weight, they may have low energy availability, especially at the elite level. Low energy availability is defined as the energy that is available for basic processes in the body after training energy expenditure is deducted from the energy intake. Making bone costs energy and low energy availability means there is less energy available for bone formation.


Another factor could be the chronic training stress. So rather than energy availability it could be the actual stress response, the changes in hormonal, the inflammation reaction, circulating cytokines or other factors that could interfere with normal bone growth. Evidence for this is limited.



More fractures?

Although as a theory it is attractive to think that stronger bones will result in fewer bone fractures, whether this is actually true remains to be seen. In cycling almost all fractures are as a result of crashes. In fact it is the most common cause for withdrawal from the Tour de France between 2010 and 2017. These crashes, however, are inherent to cycling races, and we don’t know if stronger bones would reduce the risk of bone fractures due to crashes. There simply isn’t any evidence (yet).


Fractures from crashes are the most common cause for withdrawal from the Tour de France between 2010 and 2017


Problems later in life?

However, the progression and/or regression of impaired bone status during and after the cyclists’ active career remains to be established, and no (anecdotal) evidence is available that indicates a higher prevalence of bone fractures in retired elite cyclists.


So, what could we do about it?

Resistance exercise training and impact training (e.g. jumping or bounding) are generally prescribed as the more effective exercise strategies to increase BMD but whether these would be effective in cyclists and how much this would interfere with their regular training is unknown. This will largely depend on how much of this training is needed in order to achieve a significant change and the answer to this question is unknown (especially in this population).


Taking in enough energy (kcal), calcium, vitamin D and protein are among the major nutritional factors that should be considered. Collagen supplementation (in combination with loading exercise) has also been suggested as a strategy and there is some evidence for this to be effective. However, if this is more effective than a corresponding increase in protein intake is unknown. A standard intervention that will work in this population, however, is unknown and research should address this in the coming years.


Taking in enough energy (kcal), calcium, vitamin D and protein are among the major nutritional factors that should be considered

Should we treat it?

Perhaps the more controversial question is should we treat it? What is the evidence that there are long term consequences for these athletes? What is the evidence that there are more breaks? Quoted from the viewpoint:


“The answer to the question whether low bone mineral density in elite cyclists should be treated may not be as clear-cut as initially thought. Of course, it is concerning that elite cyclists have a low bone mass at an age where peak bone mass is normally achieved. However, the potential short and long-term consequences of impaired bone health in terms of health and performance are unclear in this specific population. Although bone mineral density can generally be increased by exercise and/or nutritional interventions, the feasibility, effectiveness and potential side-effects of such interventions remain to be established in this population."


Next steps

As a next step, studies are needed to establish whether ex pro cyclists still have lower bone mineral density and/or suffer from more bone fractures. Of course, cyclists and staff working in this sport should be aware of these issues, monitor the status of bone density and ensure good nutritional practices.



References

  1. Hilkens L, Knuiman P, Heijboer M, Robert Kempers, Jeukendrup AE, van Loon LJC, and van Dijk J-W. Fragile bones of elite cyclists: to treat or not to treat? J Appl Physiol 2021.

  2. Mojock CD, Ormsbee MJ, Kim JS, Arjmandi BH, Louw GA, Contreras RJ, and Panton LB. Comparisons of Bone Mineral Density Between Recreational and Trained Male Road Cyclists. Clin J Sport Med 26: 152-156, 2016.

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