No data exists in the scientific literature showing that any type of exercise weakens bones. Bone growth depends on the forces exerted on them by gravity and contracting muscles. So any activity or exercise that causes you to contract your muscles will strengthen bones (Medicine & Science in Sports & Exercise, November 2009).
Previous studies showed that world-class cyclists had reduced bone densities in their spines. However, bone density tests do not measure bones strength. They measure how much bones block X-rays that try to pass through them. The only way to measure bone strength is to see how much force it takes to break a bone.
The most likely explanations for broken bones in cyclists are high-impact crashes and/or lack of vitamin D.
Get a Regular Vitamin D Test
I recommend that all cyclists get a blood test called Vitamin D3 in December or January. If it is below 75 nmol/L, then the cyclist is deficient in vitamin D and at increased risk for breaking bones.
To prevent fractures, you should do winter training in the southern sunbelt or take at least 800 IU of Vitamin D3 per day.
A recent review of 12 blinded, controlled scientific studies showed that oral vitamin D reduced non-vertebral and hip fractures in patients over 65 years of age (Evidence-Based Medicine, October 2009).
Blood levels of vitamin D below 75 nmol/L cause parathyroid hormone levels to rise too high, which causes osteoporosis. A main function of vitamin D is to increase calcium absorption from the intestines into the bloodstream.
When blood levels of vitamin D fall below 75 nmol/L, levels of ionizable calcium drop. This causes the parathyroid gland to produce large amounts of its hormone. Higher than normal blood parathyroid hormone levels take calcium out of bones to cause osteoporosis.
Gabe Mirkin, M.D., is a sports medicine doctor and fitness guru. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin has run more than 40 marathons and is now a serious tandem bike rider with his wife, Diana. His website is http://drmirkin.com/. Click to read Gabe’s full bio.
OK, since you CAN’T measure the force needed to break a bone, and you CAN measure bone density (and know that results correlate with risk of fracture), shouldn’t you be concerned about decreased bone density, whether it’s a world class cyclist, a recreational cyclist, or a thin, elderly, female smoker?
Some good advice about Vit D testing, but I know of no evidence that Vit D deficiency per se is a major reason for bone fractures in older cyclists. Studies finding decreased bone density in older cyclists should not be dismissed lightly. It is well accepted that bone density measurement correlates with the risk of bone fractures.
http://www.niams.nih.gov/Health_Info/Bone/Osteoporosis/overview.asp
it is perfectly obvious to me why cyclists would be more suscepible to osteopenia and osteoporosis than noncyclists. They can sweat for hours a day, far more than any other sport, loosing more calcium per day than any other sport would allow. More calcium can be lost in a day of training can can be possibly replaced. This chronic calcium depletion would lead to thinning of the bones over a prolonged period of time.