I don't remember a lot of detail- I read about it in one of those Rodale Press bike rags we all think are so boring about ten years ago. This is not an estrogen/ womens thing- men get it too. I could feel it after a summer of hard training and I had plenty of fat! Although that was going pretty fast too. This isn't after you get emaciated and lose muscle; It's just a training adaptation. I was around 160 and a hardbody when this caught up with me.
If it's asked to move fast and long, your body will shed ballast to lighten the load. It's not just about burning fat for energy; the very fit get very slender because the body is trying to get as efficient as possible. As Covert Bailey says- "Have you ever seen a fat fox?"
For runners and such, the bones take a pounding so I don't think they lose bone mass; but in a cyclist who by her nature is bird st heart losing bone density is really effecient training adaptation. Birds have hollow bones. And a rider with this, well it doesn't exactly protect you from breakage in a crash does it?
We're talking lots of riding hours here, and some people get this bad and some never get it at all. Weight training and/ or crosstraining, anything with resistance will reverse it. I'm not an expert- I just know it happens. Not all adaptations are all good.
It's very simple. It's a training adaptation, and you should be aware of it and hit the weights. You need the crosstraining for better performance anyway.

Lizzy