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.