Hi Denver! Yes, my sister has had the same documented improvement! Since I retired I haven't kept up with the various bone dinsitometry stuff, although some machines measure vertebral body densities, some hips, some metatarsals, some forearms, etc.
I always thought it would be an interesting research project for a sportsmedicine Master's or Phd candidate to study types and locations of osteoporosis in bikers. Yes, biking doesn't do anything for hips, but on the same premise that a bone that is used heavily gets strong and stays strong I would wonder if time spent biking and supporting increased weight on the arms might have a therapeutic/preventative effect on radius/ulna bone density. Also, by riding and "exercising" the spine with the weight of the upper body and increased movement and working of the spine, if that could have an effect on spinal density as well. Sign up a bunch of 'over 50 bikers' and a couch potato control group for long term follow up and check bone density every year or two in various parts of the body, then compare the two groups after 5 - 10 years.
The answer would be interesting and probably not too practical from that point of view. Probably wouldn't prove anything other than biking is good for you, and we all know that already! :)

This is just from my own experience.....

During summer months I average 100 miles a week on the bike. I also have a tendency to slide on working out with the weights during this time:( Over the past three years the bone density in the spine and hip area has improved but there has been no significant change in the forearm and wrist areas. These remain my problem areas.

If I can find it again I'll post a link to a study on cycling and osteoporosis. It simply stated that cycling wasn't the best exercise for the condition but it wasn't as bad as originally thought. My doctor's main objection to cycling has been the increase risk of falling but after 10 years and no major crashes he doesn't mention it.

Everyone gave good advice. I was just diagnosed with osteopenia last year. The doctor said take calcium with vitamin D, walk, lift weights, drink milk or soy, eat yogurt, veggies, fruit, etc. He says when estrogen stops (menopause or surgical menopause), that's when you have to worry. He recommends another scan in a year.

I am a lifelong walker, don't drink soda, love milk & yogurt, lifted weights up until two years ago, but still have the "o" diagnosis. I only took up bike riding again this May to supplement my walking because my feet & hips get tired after an hour of walking. Guess I need to start weights again.