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Understanding Competitive Inhibition
I'll explain this effect by discussing the various estrogens and
by Kevin Pezzi, MD
and how they interact with one another in the process known as competitive
Most people think of estrogen as if it were a single
hormone. In reality, there are three main hormones in women that possess
estrogen-like effects. These include:
estradiol, the primary estrogen in nonpregnant
premenopausal women; produced in the ovaries.
estrone, the primary estrogen after menopause,
but also present in premenopausal women. Estrone is produced in the
ovaries and in other tissues, such as the liver and fat, by the
biotransformation of estradiol. As a consequence, obese women produce more
of this hormone. Incidentally, males also make estrogen in adipose tissue.
estriol, the primary estrogen during pregnancy
during which it is produced by the placenta; also produced in nonpregnant
women by conversion in the liver from estradiol and estrone.
Each of these endogenous estrogens has unique properties.
For example, while estrogen is often considered to be a risk factor for breast
cancer, estriol may reduce the cancer risk.
To complicate matters, women have a number of other
chemicals in their bodies that exert estrogen effects, including:
- Phytoestrogens are chemicals with estrogen activity
that occur naturally in a variety of plants such as soybeans, some other
beans, flaxseed oil, licorice, beer, bourbon, rye, cabbage, spinach,
cashews, peanuts, dates, oats, corn, wheat, apples, almonds, carrots,
garlic, fennel, anise, celery, and certain herbs. You may have heard that
eating yams is effective as a means of estrogen replacement. This is not
true. The "yams" sold in the United States are usually sweet
potatoes, which do not have significant estrogen or progesterone activity.
True yams contain diosgenin, which can be converted in a laboratory—but not
your body—into progesterone. Yams are virtually inedible because of
their soapy, bitter taste. American yams are edible, but do not contain
On a milligram-for-milligram basis, phytoestrogens are
much weaker than natural estrogens. However, phytoestrogens can still exert
significant effects because they are sometimes consumed in large quantities,
hundreds or thousands of times more than the endogenous production of
estrogen. Furthermore, phytoestrogens can exert a powerful effect by
blocking the effects of stronger hormones.
Some phytoestrogens are produced in the intestine when
bacteria normally resident there metabolize certain plant substances into
compounds with estrogenic activity. Since oral antibiotics can suppress the
normal bacterial flora in the intestines, antibiotics can indirectly affect
- Xenoestrogens are man-made chemicals that mimic
- Designer estrogens are drugs, such as tamoxifen or
raloxiphene, given to women to achieve specific effects such as treating
breast cancer or preventing osteoporosis.
- Exogenous estrogens are animal estrogens introduced
into women either purposely (e.g., Premarin®, intended as an
estrogen supplement after menopause) or unintentionally, as from ingestion
of food containing animal estrogens. Premarin® is derived from
the urine of pregnant mares and contains estrone (also a human estrogen) and
equilin (a horse estrogen) and other estrogens in smaller amounts.
Not all women have designer estrogens, but virtually every
woman and man has phytoestrogens, xenoestrogens, and exogenous
estrogens in their bodies that interfere with the action of natural estrogen
and testosterone. The interactions between this slew of estrogens is complex,
occasionally producing beneficial results but oftentimes creating bothersome
or even dangerous effects.
All of the estrogens achieve their effects by binding to
receptors. You can think of the various estrogens (including phytoestrogens,
xenoestrogens, designer estrogens, and exogenous estrogens) as being
different keys, and the receptors as being different locks. When the correct
key (estrogen) fits into a lock (receptor), it can open it up (trigger an
estrogen response, such as softening of the skin). In reality, the various
estrogens can have effects beyond just triggering or not triggering a
response. An estrogen can bind to a receptor and trigger a full response,
trigger a partial response, or simply just sit there and block other
potentially more active estrogens from binding and achieving an effect. This
is the crux of competitive inhibition.
Competitive inhibition is a process in which two or more
different hormones compete for the same receptor, and post-receptor effects
are proportionately reduced with increasing concentration of the molecule
having a weaker receptor response. Stated another way, different molecules may
attach to the same receptor, but they do not usually cause the same degree of
response on the part of the receptor. Hormones, like estradiol, achieve their
effects through receptor binding and activation. However, if some other
substance with weak estrogen activity is able to bind to the receptor it
limits the ability of estradiol itself to bind. In the absence of estradiol or
another similar "strong" estrogen, any receptor binding by molecules
with weak estrogen activity will induce a weak estrogen response. However, if
a strong estrogen is present (such as in a premenopausal woman, or a
postmenopausal woman on estrogen replacement therapy [ERT]), receptor binding
by weak estrogens simply limits the ability of the stronger estrogen to bind
to estrogen receptors (since both molecules cannot occupy the same receptor at
the same time), thus blunting the ultimate estrogen effect.
Another way to conceptualize the competitive inhibition of
estrogens is to think of a strong estrogen (such as estradiol) as being black
paint, a weak estrogen (such as a phytoestrogen) as being gray paint, and a
severe estrogen-deficient state as being white paint. If gray paint is added
to white paint, it will make the white paint more gray. However, if gray paint
is added to black paint, it will make it whiter.
After menopause, the natural production of estrogen
plummets. Postmenopausal women sometimes supplement their diets with soybean
products as a source of phytoestrogens to reduce their hot flashes and other
undesirable symptoms. Competitive inhibition explains why phytoestrogens
(which are comparatively weak estrogens) may increase the overall estrogen
effect in postmenopausal women not on ERT, yet reduce the estrogen
effect in premenopausal women or postmenopausal women on ERT.
Most people—including many doctors—fail to consider how
the various estrogens interact with the different estrogen receptors present
in the various estrogen-responsive tissues of the body, such as the breast,
uterus, and vagina. While these interactions are complex, they can be boiled
down to one thing: are the effects additive, or does competitive inhibition
Let’s consider a simple example in which a woman has a
moderate amount of estradiol. If she were given more estradiol, her overall
estrogen effect would increase; this is an example of an additive effect. On
the other hand, if she were given estriol (a comparatively weak estrogen), her
overall estrogen effect would decrease because estriol would compete with
estradiol for space on receptors and hence exert a competitive inhibition
While this simplification is generally true, it must be
qualified by considering two more facts that are, fortunately, also easy to
The strength of a given estrogen is dependent upon not
just the properties of the estrogen but also of the receptors in the
target tissue. For example, while estriol is comparatively weak its effect
upon the vagina is much stronger than its effect upon the breast. This
concept is crucial and can be exploited to achieve very beneficial
effects. For instance, if a postmenopausal woman wished to reduce her risk
of breast cancer but also prevent vaginal atrophy and dryness, she could
Competitive inhibition can be effectively overturned if
the concentration of the weaker estrogen is sufficiently great. Let’s
again consider the example of a woman who has a moderate amount of
estradiol. If she were given a somewhat weaker estrogen, the weaker
estrogen could block the effects of the estradiol to some extent by
competitive inhibition and thus reduce the net estrogen effect. However,
if the concentration of the weaker estrogen is very high, it will still
partially block the effects of estradiol but yet stimulate the receptors
so much that the overall estrogen effect increases.
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You will have sex about 10,000 times during
Doesn't it make sense to read a book that can maximize
your enjoyment, and the enjoyment you give to your partner?
Cast away your preconceptions of sex books as
being a rehash of things you already know and hence a waste of time. By
reading this book, you will learn
many things that Dr. Ruth and other sexologists
have never considered.
The Science of Sex
Enhancing Sexual Pleasure,
Performance, Attraction, and Desire
by Kevin Pezzi, MD
Available in printed
and Adobe Acrobat e-book versions (will display on any computer)
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