Good point. I had bone loss the first year, similar to at menopause, and then it evened out. I had osteoporosis to begin with but still took an aromatase inhibitor for 5 years. Now I am on Tymlos. Many take Reclast during treatment but my doc did not want me to. In terms of day to day side effects that are FELT, I had almost none.

I won’t say I had no side effects but I can do hot flashes to avoid more cancer.
Bone loss is not a given, but it is a risk with loss of estrogen. I was told keep your vitamin D and calcium up, and walk a lot. Weight bearing is one of the best things for your bones. I did eventually get bone loss in the spine but still none in my hips. If I had been given that advice earlier, I might still not have any bone loss.

Accelerated bone loss is absolutely a given with AIs. Aromatase inhibitors deplete estrogen by preventing its production. Estrogen helps bone cell turnover [old bone sheds, new bone forms] and helps protect the coronary system in some ways, and has numerous other functions still being identified. [Cognitive health thought to be among them, partly by maintaining endothelial tissue, ergo arteries' ability to rebound. But not yet established as fact.] Women who took hormone-replacement therapy frequently took it for these purported benefits of "estrogen protection."

So ridding the body of estrogen affects these processes. What isn't a given, is the rate of loss of protection. Someone with osteopenia might have faster bone loss than someone with advanced osteoporosis according to my endocrinologist whose practice is primarily osteoporosis and diabetes. And who was formerly with the NIH bone-renewal research section.

I declined aromatase inhibitors because of what estrogen depletion does. Time will tell if that was a mistake but my OncotypeDX showed a 5% risk of recurrence if I didn't take them or tamoxifen, which translates to a 95% chance of no recurrence within 9 years anywhere in the body. Being older helped here. As I see it, going on anastrozole also means going on an osteoporosis treatment. Soon thereafter if not immediately, depending upon age. Or if pre-menopausal, taking tamoxifen, which does not deplete the body of estrogen, though also has side effects to be aware of.

There are no longitudinal studies of women with breast cancer who decline meds. My surgeon is collecting anecdotal data among the older patients who are increasingly rejecting them. And looking to design a study as there's very little data about breast cancer among women over 65, many of whom are not even getting mammograms. So this is a kind of data-deficient area...

That's exactly why I was prescribed tamoxifen. Osteoporosis