Many postmenopausal women are looking for alternatives to hormone
therapy, especially in light of the recent Women's Health Initiative
research findings concerning the risks of combined estrogen and
progestin therapy. Of particular interest are phytoestrogens, which
have been gaining popularity due to their "natural" status, alleged
health claims, and availability in a wide range of foods and
supplements.
What are Phytoestrogens?
Phytoestrogens are naturally occurring plant compounds that have some
similarities to estradiol, the most potent naturally occurring
estrogen. However, phytoestrogens tend to have weaker effects than most
estrogens, are not stored in the body, and can be easily broken down
and eliminated.
Observational studies have found a lower prevalence of breast cancer,
heart disease and hip fracture rates among people living in places like
Southeast Asia, where diets are typically high in phytoestrogens. In
North America, knowledge of these reported health effects has
stimulated great interest in the health benefits of phytoestrogens.
According to the Food and Drug Administration, the sale of soy foods, a
major source of phytoestrogens, has increased dramatically in the past
decade.
Dietary Sources of Phytoestrogens
Phytoestrogens consist of more than 20 compounds and can be found in
more than 300 plants, such as herbs, grains and fruits. The three main
classes of dietary phytoestrogens are isoflavones, lignans and
coumestans:
1. Isoflavones (genistein, daidzein, glycitein and equol) are primarily
found in soy beans and soy products, chickpeas and other legumes.
2. Lignans (enterolactone and enterodiol) are found in seeds (primarily
flaxseed), cereal bran, legumes, and alcohol (beer and bourbon).
3. Coumestans (coumestrol) can be found in alfalfa and clover. Most
food sources containing these compounds typically include more than one
class of phytoestrogens.
The Skeletal Effects of Phytoestrogens
Much of the evidence concerning the potential role of phytoestrogens in
bone health is based on animal studies. In fact, soybean protein, soy
isoflavones, genistein, daidzein and coumestrol have all been shown to
have a protective effect on bone in animals who had their ovaries
surgically removed.
In humans, however, the evidence is conflicting. Compared to Caucasian
populations, documented hip fracture rates are lower in countries such
as Hong Kong, China and Japan where dietary phytoestrogen intakes are
high. Yet reports suggest that Japanese women have a greater risk of
sustaining a vertebral fracture than Caucasian women.
Several studies have explored the effects of soy isoflavones on bone
health, but results have been mixed, ranging from a modest impact to no
effect. Most of these studies have serious limitations, including their
short duration and small sample size, making it difficult to fully
evaluate the impact of these compounds on bone health.
Ipriflavone Supplements
Ipriflavone, a synthetic isoflavone, has shown some promise in its
ability to conserve bone in postmenopausal women. Ipriflavone has also
been shown to have a protective effect on bone density in
pre-menopausal women taking gonadotropin-releasing hormone (GnRH), a
treatment for endometriosis that triggers bone loss.
However, a definitive three-year study of more than 400 postmenopausal
women concluded that ipriflavone did not prevent bone loss.
Additionally, the compound was linked to lymphocytopenia (a reduction
in lymphocytes) in a significant number of study participants.
Lymphocytes are a type of white blood cell that helps the body fight
infection.
Risks and Benefits Are Unclear
Some studies suggest that, unlike estrogen, phytoestrogens do not
appear to target breast or uterine tissue. This suggests that they may
act more like SERMS (selective estrogen receptor modulators such as
raloxifene and tamoxifen) than actual estrogens. However, in other
studies high isoflavone levels have been linked to an increased risk of
breast cancer.
Clearly, additional research is needed to further evaluate the effects
of phytoestrogens before judgments regarding their safety and
usefulness can be made.
Key Points
Based on information available at this time, it is reasonable to make
the following conclusions concerning phytoestrogens and bone health in
postmenopausal women:
1. Moderate amounts of foods containing phytoestrogens can be safely
included in the diet but do not expect it to help build bone. Keep to
the basic rule - eat the least processed forms.
2. Due to a lack of evidence and concerns about safety, supplementation
with synthetic isoflavones (ipriflavone) is in question.
3. Postmenopausal women are encouraged to view evidence concerning
phytoestrogens and bone health as conflicting and incomplete. For women
who are estrogen dominant increasing their phytoestrogen intake may not
improve their bone position.
