Is it saying that our best option for estrogen absorption is vaginally?
Noooooooooo, not at all. In the study summarized in the first link, the subjects were given 2mg of estradiol orally or 2mg of estradiol vaginally. The resulting endometrial and serum levels were higher for the ones who used the 2mg vaginally. Well, DUH!!!!
Right off the bat, 95% of the 2mg of estradiol taken orally is lost in the digestive process, so they're down to .05mg, versus 2mg vaginally. Secondly, they measured endometrial
tissue concentrations. Well, and how did the estradiol get there? You can be pretty sure most of it did not get there by absorbing into the vaginal walls and entering the bloodstream, traveling throughout the body and somehow magically winding up in a fairly high level in the uterus. It just took the shortcut right up the vagina, through the cervix and there you go! Then it would absorb directly into the walls of the uterus in much the same way it does into the walls of the vagina. It would have been much better if they had used .05mg vaginally (the transdermal equivalent of 2mg oral) and measured tissue levels somewhere else that didn't have a direct connection to the vagina. I suspect that then they would have found that the levels were higher with the oral group because of the low permeability of the vaginal walls.
Of course, imho, I think anyone who would participate in a study which administers 2mg daily of estradiol vaginally and still has a uterus ought to have her head examined... that is a HUGE dose to be using vaginally and unopposed by some form of progesterone or progestin. I wonder how many of those women will wind up with endometrial cancer in 5-10-15 years?
Yes, women who still have a uterus may use a vaginal estrogen like Vagifem or Estring to treat vaginal dryness in natural menopause, but the doses are much, much lower - so low that hardly anything gets past the walls of the vagina. Those products are (imho intelligently) designed precisely with that aim in mind. Vagifem, for example, delivers only 0.025mg of estradiol (1/8 of the dose used in the study) and is typically used twice a week after the initial treatment period.
About the second link... again, it is no big surprise that progesterone would travel right up the vagina, through the cervix and into the uterus much more efficiently than it would get there if taken intramuscularly. That is precisely why, 14 years ago when I was having trouble conceiving because my cycles were too short to allow for implantation (19-21 days), my GYN prescribed natural progesterone vaginal suppositories rather than Prometrium or injections. It's a much more direct route that doesn't rely on the entire body having a very high concentration of progesterone to get the levels up to where they'll do some good in the uterus.
Of course, I don't see where either of these studies is relevant to those of us here who no longer have a uterus... do we care what the endometrial levels would be? No... that's not where our symptoms are coming from.
I'm waiting for research to be done in women who have had a hyst/bso and are using reasonable dosages of bio-identicals for symptom relief.