erig, the point is not how much effort you put out. The problem, people theorize, is that your body is not supporting its own weight. Unless you have that 200 lbs of groceries on your back and you are out of the saddle the entire time, it is not helping you.
Astronauts lose bone mass in space because there is never any pressure on their bones, so basically, the bones atrophy.
I don't quite understand the way weight bearing on a bike differs from standing. Cyclists absolutely put huge forces on their legs, but the force on a cyclists hips is much less than say a runner..... I don't understand the mechanism, but bone loss in professional cyclists is very real.
People also theorize that it has to do with exercising with a calorie deficit. Professionals clearly can't feed their bodies enough calories to make up for what they expend during a long race. The body could be pulling nutrients from other parts of the body to make up for that. Hence bone loss.
I was hoping to find some people who have really looked into this issue and dealt with it. Speculation really doesn't help anything. But, my question was 'do people worry about bone loss' and it seems the answer so far is "No."