Bikes and Bones – The effect of cycling on bone density
You’ve probably heard before that one of the beneficial effects of exercise is on the strength of your bones. Exercise can help to reduce the natural decline in bone density that occurs in ageing, lowering the risk of developing osteoporosis later in life.
But you may not be aware that cycling appears to have the opposite effect, and may actually accelerate bone loss. In this article Accredited Sports Dietitian Alan McCubbin reviews the scientific evidence for this phenomenon, what causes it, and how you can protect your bones whilst still enjoying your bike.
To find out more about Allan see the end of the article.
There’s plenty of health benefits to be gained from exercise, and bone strength is commonly thought to be one of them. Osteoporosis Australia encourages people of all ages to participate in regular exercise. But observations on the bone density of road cyclists show that unfortunately the opposite is true.
In 2000 a study compared the bone density of runners, cyclists, people who did both, and non-active people. The runners had greater leg and total bone density compared to the non-active people, whilst those who cycled and ran also had a greater bone density than non-active people. However the pure cyclists actually had a lower bone density at the spine in comparison to those people who did no exercise.
Other studies have since followed, including one in 2003 which compared masters cyclists (average age 51, had been training and racing for an average of 20 years) with younger cyclists (age 31 and training and racing for 10 years) and non-athletes of both ages. The masters cyclists had significantly worse bone density at the hip and spine compared to both the younger cyclists as well as being worse off than the non-athletes from the older age group.
More recent studies have measured the changes in bone density over a year in competitive cyclists, to track the rate of bone loss compared to non-athletes. The first of these, published in 2008 showed a loss of bone density in the cyclists of between 0.5-1.5% in a single season. Even when supplemented with either 250mg or 1500mg of calcium (well above the Australian Recommended Dietary Intake), there was no effect of supplementation on bone density.
So what’s causing the loss of bone in cyclists? Researchers at the University of Colorado (who wrote the 2008 study mentioned above) have suggested three key features of cycling, that may combine to create a “perfect storm” for bone loss:
1. Lack of weight bearing or impact exercise, which is known to stimulate bone strengthening. This may explain why similar results have been found in swimmers, whereas runners and triathletes have greater bone density than pure cyclists.
2. A prolonged calorie deficit (common in elite cyclists but less so for most recreational riders) caused by burning large amounts of calories and not adequately replacing them in the diet. When the total calories eaten is insufficient to fuel the body’s normal functions in addition to the training undertaken, the body is forced to ration calories and limit any bodily functions not essential to survival.
The main function affected in both men and women is reproductive function, meaning that oestrogen and testosterone levels fall. This has a flow-on affect because these hormones are involved in maintaining the normal balance of bone formation and breakdown.
3. A final theory has emerged that suggests that sweat calcium losses may play a role also. Endurance sports like cycling involve prolonged sweat losses, and calcium is one electrolyte lost through sweat. The theory being that as calcium is lost from the blood into sweat, the body is forced to break down bone to release calcium and top up blood levels.
This was first evidenced by an observation in 2008 that cyclists who had the highest levels of calcium in a one-off sweat test also had the lowest bone density at the time of testing. There is also evidence that various changes that occur in the body in response to calcium sweat losses do indeed occur during cycling exercise.
So what can you do to protect your bones? If you’re a pure cyclist, adding some weight bearing, impact or resistance exercise will help to stimulate bone formation and reduce bone losses. This could include any exercise that involves running or jumping (remember that triathletes have higher bone density than cyclists) or weight training. But many over 40’s take up cyclist specifically because it’s gentler on the body, so impact exercise is not always an option.
If you’re at the leaner end of the spectrum, avoid trying to look like Andy Schleck. Excessively restricting your diet to achieve a very low body weight can alter your testosterone level and increase the rate of bone loss. However in my experience it’s the younger riders who are more inclined to want to starve themselves, with dreams of emulating Schleck or Contador and winning the Tour of Tasmania or Tour of Bright.
The final thing to consider is the amount of calcium in your diet. Cyclists don’t appear to be particularly lacking in calcium compared to anyone else. In many studies the cyclists had very high calcium intakes yet still experienced accelerated bone loss, and as we saw earlier taking daily calcium supplements doesn’t seem to help either.
However the University of Colorado team have begun to look at whether taking calcium just before or during exercise can help to replace the sweat calcium losses, and prevent bone being broken down to top up the blood. So far they’ve only done this during a single exercise session, in cyclists with an average age of 37. They found that supplementing with 1000mg calcium 20 minutes before, or taking 250mg of calcium every 15 minutes during a 35km time trial, appears to have at least some effect on the hormone changes that are thought to cause bone loss.
Importantly the calcium supplements had no effect (good or bad) on performance in the time trial, so there’s no conflict between health and performance.
It’s too early to say whether calcium supplements (or adding loads of calcium to sports drinks) will improve the bone density in cyclists. The Colorado researchers are yet to complete a study that looks at calcium supplements before or during exercise over several months of training. Only then will we know whether this is a strategy that works. In the meantime weight bearing exercise and an adequate diet is all we can do to protect our bones whilst enjoying the sport we love.
Alan McCubbin is an Accredited Sports Dietitian specialising in endurance sports. He is the founder of Next Level Nutrition, Australia’s only exclusively online sports nutrition consultancy. Alan is also the Vice President of Sports Dietitians Australia and reviews submissions for the International Journal of Sports Nutrition & Exercise Metabolism. His clients have included summer and winter Olympians, cyclists, triathletes and ultra runners of all levels, and in 2012 consults to the search2retain cycling team. He writes for Cycling Tips and Trail Run Magazine, and also competes himself in endurance mountain biking.