However, while it is strongly implied, there is no proof that estrogen helps prevent heart disease--an issue that the Women's Health Initiative is attempting to clarify.
Meanwhile, says Herrington, women with heart disease should use cholesterol-lowering medications and other proven remedies for controlling heart disease instead of pinning their hopes on estrogen.
As for Alzheimer's disease and the effect of hormones, some important questions also remain unanswered. For instance, researchers do not know if long-term estrogen use can help women with dementia regain some cognitive function; the recent study from USC looked only at very short-term use.
There is also no proof that estrogen can prevent or delay the onset of Alzheimer's disease, although some laboratory work has indicated this may be possible.
Another major area of fierce controversy is how hormone use affects cancer risk.
What is clearly known is that a woman whose uterus has not been removed during a hysterectomy has a greatly increased risk of developing uterine cancer if she takes estrogen alone. Adding the hormone progesterone to the regimen, however, offsets that risk.
That's about all anyone agrees on when it comes to hormone therapy and cancer.
Over the last two decades, epidemiological studies on hormone use and breast cancer have produced a mixed bag of results. Some studies have found no increased risk, while others have found a small increased risk.
The recent studies from the National Cancer Institute and USC found a small increased risk, but one that accumulates over time.
The USC study found that women on both estrogen and progesterone--called combination therapy--had a higher increased risk of breast cancer than women who took estrogen alone. The authors projected that breast cancer risk rises 24% for every five years a woman takes both estrogen and progesterone, contrasted with a 6% rise per five years of estrogen-only use.
What all of this means is hotly debated.
"We've been worrying about progestins for more than 15 years, and this added a little, teeny bit to this," Judd says. "But these are epidemiological studies. Most of us feel it is not a particularly sharp instrument that can weed out and clear up issues."
However, the coauthor of the USC study, Dr. Ronald K. Ross, says that his study raises questions about the role of progesterone when used in estrogen therapy.
"I think there is still reasonably good evidence that estrogen might have fairly important health benefits," Ross says. "Because combination therapy is fairly recent therapy, and epidemiological studies take longer to conduct, it's not surprising that we are only now beginning to understand what progestins do to the risk-benefit equation. The emphasis has been on estrogen. But progestins obviously have an important role."
Critics of the two recent breast cancer studies point out that the relative risk increase is small and that the accumulated risk over many years is just a projection.
"When you are dealing with small risk estimates, there are questions about whether these effects are really [caused by the hormones]," Ross says. "But let's not bury our heads in the sand and ignore these findings. There are lots of reasons to believe these findings are real."
Another growing area of concern about progesterone is whether adding it to the replacement regimen will affect heart disease protection, Ross says.
"The epidemiological data are overwhelming that estrogen prevents heart diseases," he says. "The big question is, will combination therapy provide as much protection from cardiovascular disease as estrogen therapy? No one has any data on that."
Many women have hoped to skirt the confusing questions about progesterone by using so-called "natural" progesterone. Synthetic progesterones are known as progestins and include the drug Provera. "Natural" progesterones, which are derived from yams and come in pill form, vaginal gels or topical creams, may produce different side effects and different effects on cholesterol.
But there is virtually no strong scientific data to show that "natural" progesterone products will affect heart disease or breast cancer rates, experts say.
"Natural progesterone hasn't been studied as well as synthetic progesterone," says Dr. Donnica Moore, a gynecologist and expert in hormone therapy in Neshanio Station, N.J. While the substance may be absorbed better, there is little other scientific data on its effects, she says.
One facet of the Women's Health Initiative involves 27,000 women enrolled in a nine-year study examining hormones and breast cancer. But the issue is so complex that even the WHI may not offer firm answers, Judd says.
"There is a chance that the breast cancer issue will not be answerable by the WHI because the study may not be long enough," he says. Breast cancer is usually a slow-growing disease, and too few cases may accumulate to draw conclusions.