From what I've read as regards bone mass density in the elderly, walking (which involves more impact) may be better than cycling (which is less of an 'odd impact exercise'). Taking vitamins to increase calcium absorption may be a good idea but getting enough calcium the diet must come first so cycling probably helps there-- e.g., increasing the caloric requirement. The important thing to know about bones is that they will never get stronger than the body feels they need to be so... without the right kind of exercise, maintaining BMD can still be a problem no matter what you eat or how many vitamins you take.