I haven't come across any of the research papers you mention that say any amount is protective - can you point me in the right direction?
Here:
https://www.webmd.com/osteoporosis/news/20040504/ultra-low-dose-estrogen-patch-helps-bones"The study enrolled more than 400 postmenopausal women aged 60 to 80. All had thinning bones and were at higher risk of having a spine and/or hip fracture -- as indicated by bone mineral density tests. The women also had blood tests to look for markers of increased bone turnover.....Researcher Bruce Ettinger, MD, of Kaiser Permanente in Oakland, Calif., tells WebMD that the ultra-low dose patch significantly increases bone density in the spine and at the hip.
"The dose in the patch is only about "
one-fourth the traditional 0.625-milligram dose" contained in most estrogen pills used in menopausal hormone therapy,""... The study results are good news says Nahum. He tells WebMD that the new study confirms something that he and other obstetrician/gynecologists have long suspected: "A little bit of estrogen goes a long way."
The patch which they made is now on the market and is called Menostar. It is just 14mcg of estrogen:
https://www.rxlist.com/menostar-drug.htm#indications I think I'm right in saying that it actually doesn't need any progesterone to oppose it because the estrogen is so low, but don't quote me on that - I read it in one of the research papers.
And here:
https://pubmed.ncbi.nlm.nih.gov/16607111/And here:
https://www.uptodate.com/contents/menopausal-hormone-therapy-in-the-prevention-and-treatment-of-osteoporosis"Women who seek MHT for menopausal symptoms in their late 40s or early 50s will have the additional benefit of a reduced risk of bone loss and fracture [3,4]. In such women, a separate first-line drug for prevention or treatment of osteoporosis is usually not required with estrogen (estradiol) doses equivalent to or higher than
25 mcg/day of transdermal or 0.5 mg/day oral. Lower doses of transdermal estradiol (14 mcg) have also been shown to have skeletal benefits. With this ultra-low dose of transdermal estrogen, bone density should be monitored in women at high risk for osteoporosis and fracture as another agent (eg, a bisphosphonate) may be needed."
And here:
https://academic.oup.com/jcem/article/85/12/4462/2852143 "The purpose of this study was to examine the effects of three doses (0.25, 0.5, and 1.0 mg/day) of micronized 17β-estradiol on bone turnover, sex hormone levels, and side effects compared with placebo in healthy older women...All markers of bone resorption significantly decreased at 12 weeks on treatment compared with placebo and returned toward baseline at 12 weeks posttreatment. ... Based on equivalence testing,
the response of markers of bone turnover to therapy with 0.25 mg/day was similar to that seen with 1.0 mg/day.... We conclude that low dose of estrogen (0.25 mg/day 17β-estradiol) reduced bone turnover to a similar degree as that seen with usual replacement therapy (1.0 mg/day 17β-estradiol), but had a side effect profile similar to that of placebo. In our study additional increases in estradiol levels, as seen with 0.5 and 1.0 mg/day 17β-estradiol treatment, resulted in more side effects without evidence of additional benefit to bone. These data suggest that 0.25 mg/day 17β-estradiol may be an effective and tolerable agent for the treatment of osteoporosis in older women. "
And there are many more.... Most of the women in these studies were post-menopausal and 60+ so whether this translates across to younger women with earlier menopauses, I'm not sure. But it's promising.