HHS, National Institute of Mind Health, (NIMH) Dr. Thomas Insel confirms "Soy has endocrine disruptor properties" while stopping short of confessing mass evidence that according to overwhelming studies conclude, endocrine disruptors cause autism, behavioral disorders, seizures, reproductive disorders, gender chaos, gastrointestinal distress, cancer, and more.
It is true that endocrine disruptors are highly toxic brain and body chemicals, of which soy is a confirmed endocrine (hormone) disruptor that we insert into our mouths and swallow. It is conclusive that endocrine disruptors, such as soy are physiologically, reproductive, and neurologically poisonous especially during most fragile fetal, infant, and child exposure. U.S. FDA states, "Given the DES tragedy, it would be foolish to ignore the possibility that some phytoestrogens (soy genistein, daidzein, equol, glycitein) constitute a developmental hazard." Yet the FDA does just that, they completely ignore the overwhelming importance to label soy food, fillers, and formulas as toxic, in accordance with multiple food and infant formula laws. Janice Oliver, deputy Director of Center for Food Safety and Applied Nutrition states, "FDA acknowledges that concerns have been voiced about possible effects of isoflavones in soy infant formula on; sexual development, neuorbehavioral development, immune function and thyroid disease." ......again while refusing legitimate soy warning labels. In fact the FDA has well over 700 studies proving soy estrogenic endocrine disruptors, and multiple toxic soy components can cause a host of adverse brain and body effects, while refusing to properly label the truth, as is the FDA promised duty to a trusting American public. (http://toxicstudylist.blogspot.com, http://fetaltoxic.blogspot.com). National Institute of Environmental Health Sciences (NIEHS) Presents: FDA SCIENTISTS AGAINS SOY:
Researchers Daniel Doerge and
Daniel Sheehan, two of the Food and Drug Administration's experts on
soy, signed a letter of protest, which points to studies that show a
link between soy and adverse health effects in certain animals and humans. These NIEHS Scientists try in vain to stop the FDA approval of soy as beneficial, while true to fact soy is quite the contrary, with multiple studies proving toxic effects.
The text of their letter follows. DEPARTMENT OF HEALTH and HUMAN SERVICES Public Health Service Food and Drug Administration National Center For Toxicological Research Jefferson, Ark. 72079-9502 Daniel M. Sheehan, Ph.D. Director, Estrogen Base Program Division of Genetic and Reproductive Toxicology and Daniel R. Doerge, Ph.D. Division of Biochemical Toxicology February 18, 1999 Dockets Management Branch (HFA-305) Food and Drug Administration Rockville, MD 20852 To whom it may concern, We are writing in reference to Docket # 98P-0683; "Food Labeling: Health Claims; Soy Protein and Coronary Heart Disease."
We
oppose this health claim because there is abundant evidence that some
of the isoflavones found in soy, including genistein and equol, a
metabolize of daidzen, demonstrate toxicity in estrogen sensitive
tissues and in the thyroid. This is true for a number of species,
including humans.
Additionally,
the adverse effects in humans occur in several tissues and, apparently,
by several distinct mechanisms. Genistein is clearly estrogenic; it
possesses the chemical structural features necessary for estrogenic
activity ( Sheehan and Medlock, 1995; Tong, et al, 1997; Miksicek, 1998)
and induces estrogenic responses in developing and adult animals and in
adult humans. In rodents, equol is estrogenic and acts as an estrogenic
endocrine disruptor during development (Medlock, et al, 1995a,b). Faber
and Hughes (1993) showed alterations in LH regulation following
developmental treatment with genistein.
Thus,
during pregnancy in humans, isoflavones per se could be a risk factor
for abnormal brain and reproductive tract development. Furthermore,
pregnant Rhesus monkeys fed genistein had serum estradiol levels 50- 100
percent higher than the controls in three different areas of the
maternal circulation (Harrison, et al, 1998).
Given
that the Rhesus monkey is the best experimental model for humans, and
that a women's own estrogens are a very significant risk factor for
breast cancer, it is unreasonable to approve the health claim until
complete safety studies of soy protein are conducted. Of equally grave
concern is the finding that the fetuses of genistein fed monkeys had a
70 percent higher serum estradiol level than did the controls (Harrison,
et al, 1998). Development is recognized as the most sensitive life
stage for estrogen toxicity because of the indisputable evidence of a
very wide variety of frank malformations and serious functional deficits in experimental animals and humans.
In
the human population, DES exposure stands as a prime example of adverse
estrogenic effects during development. About 50 percent of the female
offspring and a smaller fraction of male offspring displayed one or more
malformations in the reproductive tract, as well as a lower prevalence
(about 1 in a thousand) of malignancies. In adults, genistein could be a
risk factor for a number of estrogen-associated diseases.
Even
without the evidence of elevated serum estradiol levels in Rhesus
fetuses, potency and dose differences between DES and the soy
isoflavones do not provide any assurance that the soy protein
isoflavones per se will be without adverse effects. First, calculations,
based on the literature, show that doses of soy protein isoflavones
used in clinical trials which demonstrated estrogenic effects were as
potent as low but active doses of DES in Rhesus monkeys (Sheehan,
unpublished data). Second, we have recently shown that estradiol shows
no threshold in an extremely large dose-response experiment (Sheehan, et
al, 1999), and we subsequently have found 31 dose-response curves for
hormone-mimicking chemicals that also fail to show a threshold (Sheehan,
1998a).
Our conclusions are that no dose is without risk; the extent of risk is simply a function of dose. These two features support and extend the conclusion that it is inappropriate to allow health claims for soy protein isolate.
Additionally,
isoflavones are inhibitors of the thyroid peroxidase which makes T3 and
T4. Inhibition can be expected to generate thyroid abnormalities,
including goiter and autoimmune thyroiditis. There exists a significant
body of animal data that demonstrates goitrogenic and even carcinogenic effects of soy products (cf., Kimura et al., 1976).
Moreover, there are significant reports of goitrogenic effects from soy consumption in human infants
(cf., Van Wyk et al., 1959; Hydovitz, 1960; Shepard et al., 1960;
Pinchers et al., 1965; Chorazy et al., 1995) and adults (McCarrison,
1933; Ishizuki, et al., 1991). Recently, we have identified genistein
and daidzein as the goitrogenic isoflavonoid components of soy and
defined the mechanisms for inhibition of thyroid peroxidase
(TPO)-catalyzed thyroid hormone synthesis in vitro (Divi et al., 1997;
Divi et al., 1996). The observed suicide inactivation of TPO by
isoflavones, through covalent binding to TPO, raises the possibility of
neoantigen formation and because anti-TPO is the principal autoantibody
present in auto immune thyroid disease.
This hypothetical mechanism is consistent with the reports of Fort et al. (1986, 1990) of a doubling of risk for autoimmune thyroiditis in children who had received soy formulas as infants
compared to infants receiving other forms of milk. The serum levels of
isoflavones in infants receiving soy formula that are about five times
higher than in women receiving soy supplements who show menstrual cycle
disturbances, including an increased estradiol level in the follicular
phase (Setchell, et al, 1997).
Assuming
a dose-dependent risk, it is unreasonable to assert that the infant
findings are irrelevant to adults who may consume smaller amounts of
isoflavones. Additionally, while there is an unambiguous biological
effect on menstrual cycle length (Cassidy, et al, 1994), it is unclear
whether the soy effects are beneficial or adverse.
Furthermore, we need to be concerned about transplacental passage of isoflavones as the DES case has shown us that estrogens can pass the placenta.
No such studies have been conducted with genistein in humans or
primates. As all estrogens which have been studied carefully in human
populations are two-edged swords in humans (Sheehan and Medlock, 1995;
Sheehan, 1997), with both beneficial and adverse effects resulting from
the administration of the same estrogen, it is likely that the same
characteristic is shared by the isoflavones. The animal data is also
consistent with adverse effects in humans.
Finally,
initial data from a robust (7,000 men) long-term (30+ years)
prospective epidemiological study in Hawaii showed that Alzheimer's
disease prevalence in Hawaiian men was similar to European-ancestry
Americans and to Japanese (White, et al, 1996a). In contrast, vascular
dementia prevalence is similar in Hawaii and Japan and both are higher
than in European-ancestry Americans. This suggests that common ancestry
or environmental factors in Japan and Hawaii are responsible for the
higher prevalence of vascular dementia in these locations. Subsequently,
this same group showed a significant dose-dependent risk (up to 2.4
fold) for development of vascular dementia and brain atrophy from
consumption of tofu, a soy product rich in isoflavones (White, et al,
1996b). This finding is consistent with the environmental causation
suggested from the earlier analysis, and provides evidence that soy
(tofu) phytoestrogens causes vascular dementia.
Given
that estrogens are important for maintenance of brain function in
women; that the male brain contains aromatase, the enzyme that converts
testosterone to estradiol; and that isoflavones inhibit this enzymatic
activity (Irvine, 1998), there is a mechanistic basis for the human
findings. Given the great difficulty in discerning the relationship
between exposures and long latency adverse effects in the human
population (Sheehan, 1998b), and the potential mechanistic explanation
for the epidemiological findings, this is an important study. It is one
of the more robust, well-designed prospective epidemiological studies
generally available. We rarely have such power in human studies, as well
as a potential mechanism, and thus the results should be interpreted in
this context.
Does the
Asian experience provide us with reassurance that isoflavones are safe?
A review of several examples lead to the conclusion "Given the
parallels with herbal medicines with respect to attitudes, monitoring
deficiencies, and the general difficulty of detecting toxicities with
long latencies, I am unconvinced that the long history of apparent safe
use of soy products can provide confidence that they are indeed without
risk." (Sheehan, 1998b).
It
should also be noted that the claim on p. 62978 that soy protein foods
are GRAS is in conflict with the recent return by CFSAN to Archer
Daniels Midland of a petition for GRAS status for soy protein because of
deficiencies in reporting adverse effects in the petition. Thus GRAS status has not been granted.
Linda Kahl can provide you with details. It would seem appropriate for
FDA to speak with a single voice regarding soy protein isolate.
Taken
together, the findings presented here are self-consistent and
demonstrate that genistein and other isoflavones can have adverse
effects in a variety of species, including humans. Animal studies are
the front line in evaluating toxicity, as they predict, with good
accuracy, adverse effects in humans. For the isoflavones, we
additionally have evidence of two types of adverse effects in humans,
despite the very few studies that have addressed this subject. While
isoflavones may have beneficial effects at some ages or circumstances,
this cannot be assumed to be true at all ages. Isoflavones are like
other estrogens in that they are two-edged swords, conferring both benefits and risk (Sheehan and Medlock, 1995; Sheehan, 1997).
The
health labeling of soy protein isolate for foods needs to considered
just as would the addition of any estrogen or goitrogen to foods, which
are bad ideas. Estrogenic and goitrogenic drugs are regulated by FDA,
and are taken under a physician's care. Patients are informed of risks,
and are monitored by their physicians for evidence of toxicity. There
are no similar safeguards in place for foods, so the public will be put
at potential risk from soy isoflavones in soy protein isolate without
adequate warning and information.
Finally, NCTR is currently conducting a long-term multigeneration study of genistein administered in feed to rats.
The analysis of the dose range-finding studies are near-complete or
complete now. As preliminary data, which is still confidential, may be
relevant to your decision, I suggest you contact Dr. Barry Delclos at
the address on the letterhead, or email him.
Sincerely, Daniel M. Sheehan Daniel R. Doerge 870-543-7561 Fax 870-543-7682 DSHEEHAN@NCTR.FDA.GOV 870-543-7943 Fax 870-543-7136 DDOERGE@NCTR.FDA.GOV AND CARE TO READ:
Front Neuroendocrinol. Author manuscript; available in PMC 2011 April 12.
Published in final edited form as:
|
THE PROS AND CONS OF (Soy) PHYTOESTROGENS
NIEHS Scientists: Heather B. Patisaul* and Wendy Jefferson
Department of Biology, NC State University, Raleigh, NC 27695, United States
Laboratory
of Reproductive and Developmental Toxicology, National Institute of
Environmental Health Sciences, Research Triangle Park, NC 27709, United
States
*Corresponding author. Email: heather_patisaul@ncsu.edu (H.B. Patisaul)
The publisher's final edited version of this article is available at Front NeuroendocrinolPhytoestrogens are present in numerous dietary supplements and widely marketed as a natural alternative to estrogen replacement therapy. Soy infant formula now constitutes up to a third of the US market, and soy protein is now added to many processed foods. As weak estrogen agonists/antagonists with molecular and cellular properties similar to synthetic endocrine disruptors such as Bisphenol A (BPA), the phytoestrogens provide a useful model to comprehensively investigate the biological impact of endocrine disruptors in general.
One group of compounds that has received considerable attention is the
phytoestrogens, many of which are now recognized to be endocrine
disruptors. Although they behave similarly to man-made endocrine
disrupting compounds (EDCs) on numerous molecular and cellular targets,
the attitude of the general public and clinicians toward soy
phytoestrogens are generally positive, while their synthetic
counterparts are increasingly the subject of mounting public and
congressional concern.
A growing body of work now cautions that the health benefits frequently attributed to soy may be overstated.
Clinical and experimental studies examining the impact of soy or soy
phytoestrogen consumption on human health have produced mixed and often
conflicting results. Of even greater concern is that emerging evidence
suggests that exposure to these compounds may, in fact, pose a risk to
some groups, particularly infants and the unborn.
So are they helpful or harmful? The answer is undoubtedly complex and
may ultimately depend on age, health status, level of consumption, and
even the composition of an individual’s intestinal microflora.
(Soy Phytoestrogens) Daidzein and genistein are the two most well characterized isoflavones
and human exposure to these compounds occurs primarily through the
consumption of soy-based food and beverage products. Unbeknownst to most
consumers, in addition to well recognized soy products such as soy
milk, tofu and tempeh, soy is found in upwards of 60% of processed foods.
Textured soy protein (50–70% soy protein) is a meat substitute found in
hotdogs, hamburgers, sausages and other meat products while soy protein
isolate (90% soy protein) is used to enrich energy bars, sports drinks,
infant formula, cereals, granola bars, imitation dairy products, ice
cream, cheese and even doughnuts.
Soy formula has been estimated to constitute approximately 25% of the infant formula market and at least one report suggests that for women who would prefer to
breast feed, but are unable, soy formula is their substitute of choice
Some isoflavones, most notably genistein, inhibit pathways important for
cell growth and proliferation, an effect which affects multiple organ
systems. Genistein inhibits the activity of protein tyrosine kinases
(PTKs) in numerous tissues. PTKs are highly expressed in several brain regions, including the
hippocampus, and phosphoregulation of PTKs is critical for numerous
brain responses including synaptic plasticity, neurode-generation and
response to neuronal injury. It can also down-regulate the
expression of vascular endothelial growth factor (VEGF) along with other
related growth factor genes.
Perhaps the most well characterized mode of
phytoestrogen action is estrogen receptor (ER) binding. There are two
major ER subtypes in mammals, ERα and ERβ (also referred to as ESR1 and
ESR2, respectively). As such, phytoestrogens, particularly the
isoflavones, fit the Environmental Protection Agency’s definition of an
endocrine disruptor which characterizes these compounds as those which,
“alter the structure or function(s) of the endocrine system and cause
adverse effects.” This definition includes disruption of lactation, the
timing of puberty, the ability to produce viable, fertile offspring, sex
specific behavior, premature reproductive senescence and compromised
fertility. In animal models, isoflavones produce all of these effects.
Recognition of the endocrine disrupting properties of phytoestrogens
dates back to the 1940’s when ewes grazing on clover rich pastures in
Australia were observed to have abnormally high rates of infertility,
abortion, and reproductive abnormalities in their offspring. It was ultimately determined coumestrol was primarily responsible for the observed effects.
Decades later, a singular case of infertility and liver disease in
captive cheetahs placed on a soy-based diet was ultimately attributed to
isoflavones.
These incidents have raised concerns that isoflavone intake, by
mimicking or interfering with endogenous estrogens, could pose a risk to
human reproductive health.
In vitro
assays have found that, although most phytoestrogens, including the
isoflavones, bind both ERα and ERβ, and activate ER-dependent gene
transcription through both subtypes, they generally have a higher
relative binding affinity for ERβ than ERα. Genistein is 7- to 48-fold more selective for ERβ than ERα, depending on the assay used.
The relative estrogenic potency of genistein for ERβ is approximately
30-fold higher than for ERα. Potency estimates vary considerably
depending on the assay used,
but as a general principle, most isoflavones bind and activate both ERα
and ERβ more readily than synthetic EDCs including BPA.
Once bound, isoflavones do not act like typical estrogen agonists, but
rather more like selective estrogen receptor modulators (SERMS) such as
the breast cancer drug tamoxifen which is an ER agonist in the uterus
and bone but an antagonist in the breast.
The
fact that most phytoestrogens bind ERβ more readily than ERα is likely
functionally significant because ERα and ERβ are differentially
distributed throughout the body and the brain and appear to upregulate
different gene families. In addition to the breast, ERβ is strongly expressed in bone, the
cardiovascular system, uterus, bladder, prostate, lung, ovarian
granulosa cells and testicular Sertoli and germ cells.
This distribution can change over the lifespan and is sexually
dimorphic, particularly in the brain, suggesting that the two ER
subtypes regulate different aspects of reproduction, behavior and
neuroendocrine function and likely have differential roles across the
lifespan. ERβ is also expressed at higher levels than ERα in the basal forebrain, hippocampus and cerebral cortex in the adult, all brain regions critical to memory function and vulnerable to Alzheimer’s disease.
Phytoestrogens
can also manipulate steroid biosynthesis and transport by, for example,
stimulating hormone-binding globulin (SHBG) synthesis in liver cells, and competitively displacing either 17β-estradiol or testosterone from plasma SHBG.
This subtle deflection of the quantity or availability of SHBG by
phytoestrogens changes the free fraction of endogenous hormones in
circulation, either systemically or locally. Phytoestrogens can also
manipulate endogenous hormone levels by interfering with the enzymes
needed for steroid biosynthesis.
Determining if phytoestrogens increase or reduce the risk of developing
breast cancer has proven to be one of the most challenging human health
impacts to address. It is well established that estrogens promote breast
tumorigenesis, and that parameters which increase lifetime estrogen
exposure (such as early menarche, short duration breastfeeding, and low
parity) are associated with elevated breast cancer risk. Because they
bind ERs with relatively high affinity, some researchers and clinicians
are concerned that high phytoestrogen intake may increase the risk of
carcinogenesis and put breast cancer survivors at risk for reoccurrence. Depending on the assay used, levels of endogenous estrogen present,
life stage, and tumor type, genistein can act as both a proliferative
and an antiproliferative agent. For example, in vitro,
genistein can inhibit proliferation of ER-positive and ER-negative
breast cancer cells at high doses (>10 M), but, paradoxically,
promote tumor growth at lower, more physiological doses. The SERM-like activity of soy phytoestrogens makes dietary guidelines particularly difficult to issue with confidence.
Phytoestrogens may have the biggest impact on
lifetime risk when exposure occurs prior to puberty and possibly before
birth. Although not an initial goal of the study, a Hawaiian research
group found an association between high soy intake during early life and
increased breast density, a risk factor for breast cancer.
The study consisted of 220 pre-menopausal women and was designed to
determine if consumption of approximately 50 mg of isoflavones over 2
years in adulthood could reduce breast density. This intervention failed
but life history data obtained during the process led the authors to
conclude that Caucasian women who ate more soy over their lifetime had
denser breast tissue than those who did not.
Results
from perinatal exposure in animals have also been mixed. A more recent study found that neonatal, subcutaneous
administration of 5 or 50 mg/kg genistein stunted mammary gland
development and the animals, particularly those given the higher dose,
exhibited abnormal ductal morphology including reduced lobular alveolar
development, and focal areas of “beaded” ducts lined with hyperplastic
ductal epithelium.
The biphasic effect of genistein on breast tissue
development and differentiation indicates that dose may be an important
factor when considering risk. The hypothesis that exposure to soy
phytoestrogens early in life can alter the timing and character of
breast development is supported by a 2008 cross-sectional study of 694
girls in Israel, which found increased prevalence of breast buds in
2-year old girls fed soy formula as infants.
It is unclear how this may impact their lifetime risk of developing
breast cancer but argues for a more thorough investigation of the
possible relationship between early life phytoestrogen exposure,
premature thelarche, and breast cancer risk.
CONS: The endocrine disrupting properties of phytoestrogens in the reproductive tract, and adult brain:
In
a 2008 clinical case report, physicians at SUNY Downstate Medical
Center treated three women (aged 35–56) for a similar suite of symptoms
including abnormal uterine bleeding, endometrial pathology and
dysmenorrhea. In all three cases, symptoms ameliorated after soy intake
was reduced or eliminated, demonstrating that consumption of
particularly high isoflavone levels can compromise female reproductive
health.
The youngest of the three had been on a soy-rich diet since age 14 and
was experiencing secondary infertility, a condition that resolved and
resulted in a pregnancy once she reduced her soy consumption. Isoflavone
intake was not quantified, but estimated to exceed 40 g per day in the
oldest of the three patients. It remains to be determined if these cases
are atypical or sentinels of a legitimate public health concern.
Because soy consumption is increasing so rapidly, and so many products
now contain soy, along with its isoflavones and other phytoestrogens,
this possibility clearly warrants greater attention.
Animal and human studies evaluating phytoestrogen effects on the adult hypothalamic–pituitary–gonadal (HPG) axis following adult exposures have been fairly consistent and reveal the potential for suppression. Multiple studies have documented the estrogenic activity of phytoestrogens in ovariectomized rodents and, in humans, it is generally accepted that consumption of isoflavones-rich soy foods suppresses circulating estrogen and progesterone levels and can attenuate the preovulatory surge of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). A 2009 meta-analysis concluded that, in pre-menopausal women, isoflavone intake increases cycle length and suppresses LH and FSH levels. This conclusion is consistent with the clinical case report from SUNY Downstate Medical Center and indicates that use of soy foods should be approached with caution in women attempting to become pregnant or experiencing menstrual cycle irregularities.
Behavior:
Our
research has revealed that isoflavone intake can suppress female sex
behavior in rats. Administration of a commercially prepared
phytoestrogen supplement to adult female rats, at a dose that results in
serum levels between those seen in Western and Asian (human) adults,
attenuated lordosis to the same degree as the SERM tamoxifen.
The supplement treated group displayed significantly fewer proceptive
behaviors than the tamoxifen treated group, demonstrating the potential
for soy isoflavones to suppress sexual motivation.
Other
behaviors may be affected as well including social, aggressive, and
anxiety-related behaviors. Increased aggression and circulating
testosterone levels have been reported in male Syrian hamsters
maintained for 5 weeks on the soy-rich Purina 5001 diet compared to
control animals fed a soy-free diet.
Animals on the phytoestrogen- rich diet also had lower vasopressin
receptor (V1a) expression in the lateral septum but higher V1a
expression in lateral hypothalamus indicating that the altered behavior
might result from changes within vasopressin signaling pathways.
Similarly, male rats maintained on a diet containing 150 µg/g genistein
and daidzein displayed increased anxiety and elevated stress-induced
plasma vasopressin and corticosterone levels.
Elevated hypothalamic vasopressin has also been reported in rats fed a
diet containing 1250 ppm genistein across the lifespan.
Anxiolytic effects of phytoestrogen-rich diets have also been reported
in gonadally intact male and female rats exposed over their entire
lifetimes but not when administered briefly in adulthood.
Phytoestrogen intake can also affect behavior in non-human primates.
Male cynomolgus monkeys fed soy protein isolate containing 1.88 mg
isoflavones/g protein over 18 months demonstrated higher frequencies of
intense aggressive (67% higher) and submissive (203% higher) behaviors
as well as a decreased proportion of time (68% reduction) spent in
physical contact with other monkeys.
ER-dependent gene expression in the brain:
Phytoestrogens have widespread effects in the adult brain which have previously been reviewed in detail elsewhere. Both the PVN and the ventromedial nucleus (VMN) are critical nuclei for the initiation and regulation of sex behavior and each contains primarily one ER subtype. Only ERβ is expressed in the PVN while the VMN contains primarily ERα . Coumestrol and genistein stimulate ERβ mRNA expression in the PVN, an effect opposite to that of 17β-estradiol.
It is not yet clear what the functional significance of this might be
but the PVN is the primary site of oxytocin (OT) production, a peptide
hormone important for social behavior and the facilitation of sexual
behavior. Estrogen stimulates OT production, a process that is regulated through ERβ. Oxytocin then binds to its receptor (OTR) in the VMN, a nucleus critical for mediating the lordosis response in females. ERα is required for the upregulation of OTR.
Consumption of the commercially prepared phytoestrogen supplement
described previously, attenuated the estrogen-dependent up-regulation of
OTR in the rat VMN, an effect which was accompanied by reduced lordosis.
Evidence for endocrine disruption during development:
A
possibility of increasing concern is that phytoestrogens may interfere
with the organizational role of estrogen in the developing brain and
reproductive system.
Regardless of animal model used, manipulation of estrogen during
specific critical windows of development throughout gestation and early
infancy leads to a myriad of adverse health outcomes including
malformations in the ovary, uterus, mammary gland and prostate, early
puberty, reduced fertility, disrupted brain organization, and
reproductive tract cancers.
These effects mirror some very disturbing public health trends in
Western nations. For example, in the United States and Europe median age
at menarche, first breast development, and sexual precocity has
steadily advanced, especially among minority populations. Similar trends have been documented among children adopted from developing countries by Western parents.
There are also indications that female fecundity is declining, even
among young women, although the rate and degree to which this is
occurring has been difficult to quantify. Among men, sperm counts in the
United States and Europe appear to have declined by roughly half over
the past 50 years.
Rates of testicular cancer also appear to be increasing. Another study
showed that infant boys born to vegetarian mothers had increased
incidence of hypospadias (malformation of the male external genitalia) suggesting that dietary components (perhaps phytoestrogens) cross the
placenta and cause adverse effects on the developing fetus. The causes
of these reproductive health trends are likely complex and
multi-faceted, but rapidity of the increase in reproductive and
behavioral disorders suggests an environmental, endocrine disrupting
component.
Whether or not isoflavone phytoestrogens could be one such component is
now the subject of rigorous debate and has caught the attention of
public health officials in the US and abroad.
Soy-based infant formulas: prevalence and phytoestrogen content:
Isoflavones
can pass from mother to fetus through the placenta, and have been found
in human umbilical cord blood and amniotic fluid at levels comparable
to concentrations seen in maternal plasma, indicating that fetal
exposure is possible.
These levels are considerably lower, however, than blood levels in
infants exclusively fed soy formula. Initially developed as an
alternative to bovine milk formulas for babies with a milk allergy, use
of soy infant formula in the US has steadily risen in popularity. An
estimated 25% of US infants, approximately one million each year, are
now raised on soy formula, largely because of perceived health benefits
or to maintain a vegetarian lifestyle, rather than concerns about cow
milk allergies, colic, or other health concerns.
The widespread prevalence of popular media articles touting the
beneficial effects of soy have undoubtedly contributed to its selection
by mothers trying to make the most healthful choice for their babies.
Total
isoflavone content in soy infant formula varies widely due to
environmental and genetic differences between batches and sources, but
is consistently higher than in most other food sources.
Infants on soy formula consume approximately 6–9 mg isoflavones per kg
body weight per day, an amount, when adjusted for body weight, that is
up to seven times higher than for adults meeting the FDA soy consumption
guideline, or Asians consuming a traditional soy-based diet (0.3–1.2
mg/kg per day). Moreover, plasma isoflavone levels are an order of magnitude higher in
infants than adults, even when levels of intake are similar.
Infants fed soy formula have circulating phytoestrogen concentrations
of approximately 1000 ng/ml, 13,000–22,000 times higher than their own
endogenous estrogen levels, 50–100 times higher than estradiol levels in
pregnant women and 3000 times higher than estradiol levels at ovulation.
These blood levels are high enough to produce many of the physiological
effects observed in research animals and human adults. In addition,
they are at least a level of magnitude higher than those reported for
other endocrine disruptors including BPA and the phthalates.
A recent prospective study in human infants observed that female
infants on soy-based infant formulas exhibit estrogenized vaginal
epithelium at times when their breast fed or cow based formula
counterparts did not, suggesting estrogenic activity of the soy infant
formula. Determining if use of soy infant formulas can have long term reproductive health effects is a public health imperative.
Diethylstilbestrol (DES): lessons from history:
Effects
of endocrine disruption in the developing fetus are likely to be subtle
and not readily apparent at birth, a lesson learned from the tragic
case of diethylstilbestrol (DES), a synthetic estrogen prescribed to
upwards of 10 million women between 1938 and 1971. Unfortunately, many children, both male and female, born to women
prescribed DES during pregnancy subsequently developed reproductive
health problems as adults.In general, the severity of the health
effects correspond with timing and level of exposure, an observation
which was the first, clear demonstration of how important it is to
consider “critical windows of exposure” when attempting to predict
potential consequences of human exposure to endocrine disruptors, such
as the phytoestrogens. So
is another reproductive health tragedy silently unfolding with the use
of soy infant formula? Most of the reproductive outcomes following fetal
exposure to DES were predicted by or replicated in animal models emphasizing the importance of animal models for adumbrating potential adverse effects of endocrine disruptors in humans.
Animal data: Disruption of sexual brain differentiation:
In
the rat, genistein readily crosses the placenta and in the fetal brain,
the bioactive aglycone form is present at levels comparable to
circulating levels in the dam.
In addition, the transfer of genistein to the brain from systemic
circulation appears to be more efficient in prenatal animals than adults
indicating that it and other isoflavone phytoestrogens could interfere
with the organization of estrogen sensitive neuroendocrine signaling
pathways. Hormone mediated architectural and functional changes within
the HPG axis occur during a series of well defined critical periods
spanning gestation through puberty, resulting in sex specific physiology
and behavior in the adult animal.
Interference with the hormone-sensitive formation of these pathways
could result in irreversible developmental defects, potentially making
development one of the most susceptible periods for phytoestrogen and
EDC (Endocrine Disrupting Chemicals) exposure over the lifespan.
The sexually dimorphic brain region that is most frequently used as a “biomarker” of endocrine disruption in rats is the sexually dimorphic nucleus of the preoptic area (SDN-POA). The volume of the SDN-POA is enhanced by estradiol aromatized from perinatal, testicular androgen , and is five to six times larger in males than females. Both ERα and ERβ are expressed in the SDN-POA across the lifespan but ERα appears to play a dominant role in masculinizing SDN-POA morphometrics. In rats, numerous studies have been undertaken to determine the extent to which phytoestrogens and other EDCs can alter SDN-POA volume in both sexes. Although not always in complete accordance, the data have generally shown the potential for genistein and other phytoestrogens to act as estrogen agonists in the brain. When administered prenatally through adulthood, genistein increases SDN-POA volume in males but not females. Effects in female rats have been less robust but consistent with an estrogenic effect of genistein, at least at high doses. Masculinizing effects on female SDN-POA volume have been reported following subcutaneous administration on PND 0–10 of 500 or 1000 µg of genistein but not lower amounts.
The sexually dimorphic brain region that is most frequently used as a “biomarker” of endocrine disruption in rats is the sexually dimorphic nucleus of the preoptic area (SDN-POA). The volume of the SDN-POA is enhanced by estradiol aromatized from perinatal, testicular androgen , and is five to six times larger in males than females. Both ERα and ERβ are expressed in the SDN-POA across the lifespan but ERα appears to play a dominant role in masculinizing SDN-POA morphometrics. In rats, numerous studies have been undertaken to determine the extent to which phytoestrogens and other EDCs can alter SDN-POA volume in both sexes. Although not always in complete accordance, the data have generally shown the potential for genistein and other phytoestrogens to act as estrogen agonists in the brain. When administered prenatally through adulthood, genistein increases SDN-POA volume in males but not females. Effects in female rats have been less robust but consistent with an estrogenic effect of genistein, at least at high doses. Masculinizing effects on female SDN-POA volume have been reported following subcutaneous administration on PND 0–10 of 500 or 1000 µg of genistein but not lower amounts.
Within the male rodent brain, testicular testosterone is converted to
estrogen by the enzyme aromatase, and it is this estrogen that is
primarily responsible for defeminizing/masculinizing the hypothalamic
nuclei of the HPG axis during development. At birth, if endogenous
estrogen is blocked in males, either by castration, by blocking the
action of aromatase, or by blocking hypothalamic estrogen receptors, the
HPG axis fails to defeminize and the capacity to elicit a gonadal surge
remains.
In contrast, neonatal estrogen can defeminize the female HPG axis and
thus eliminate the future emergence of the female estrous cycle.
Therefore, interference with estrogen at birth, either by blocking
estrogen activity in males or by triggering estrogen signaling in
females, can result in the improper differentiation and function of the
HPG axis across the lifespan.
We have found that subcutaneous administration of 10 mg/kg genistein, a
dose that is approximately equivalent to the total amount of isoflavones
ingested by infants fed soy formula, over the first 4 days of life,
advanced vaginal opening and compromised the ability to maintain a
regular estrous cycle in female rats, effects which have also been observed by other research groups.
These physiological changes were accompanied by an impaired ability to
stimulate GnRH neuronal activity (as measured by the immunoreactivity of
both of GnRH and FOS) following ovariectomy and hormone priming.
This finding indicates that the capacity to generate a GnRH surge is
compromised. Because it has long been appreciated that estrogen
administration during this neonatal critical period typically induces a
similar suite of effects, our data indicate that neonatal genistein
exposure has a defeminizing effect on the organization of the female rat
HPG axis.
It is important to emphasize that in humans
androgen, rather than estrogens, is thought to be most important for
masculinizing the brain during development.
This species difference makes organizational neuroendocrine effects in
animals difficult to apply to human risk assessment because it is not
readily apparent how estrogenic compounds, like the phytoestrogens,
might impact the sexual differentiation of the human hypothalamus or
other brain regions. Our lab has tried to address this by exploring the
impact of perinatal phytoestrogen exposure on a neuroendocrine system
that was initially discovered in humans: the kisspeptin system.
It
has rapidly become apparent that neurons which express the kiss1 gene
are the primary gatekeepers of GnRH release in many species, including
rats and humans.
This gene codes for a family of peptides called kisspeptins (previously
called metastins). In rodents, kisspeptin neurons lie predominantly
within two sexually dimorphic hypothalamic regions, both of which were
already well appreciated for their importance in regulating GnRH
secretion: the AVPV and arcuate (ARC) nucleus.
AVPV kisspeptin neurons are more numerous in females than males and are
thought to be essential for steroid positive feedback and the
initiation of the preovulatory GnRH surge.
In contrast, kiss1 mRNA expression in the ARC is not thought to be
sexually dimorphic and appears to be important for the regulation of
steroid negative feedback and, possibly, pubertal onset.
We recently showed that neonatal exposure to 10 mg/kg genistein
significantly decreases the density of neuronal fibers immunoreactive
for kisspeptin in the AVPV but not the ARC of female rats.
This was accompanied by early vaginal opening (a hallmark of puberty in
the rat), premature anestrous and blunted GnRH activation. Our findings
suggest that disrupted organization of kisspeptin signaling pathways by
genistein may be a novel yet comprehensive mechanism underlying a suite
of reproductive abnormalities in females.
Animal data: Abnormal development of the female reproductive tract:
Altered
timing of pubertal onset and estrous cyclicity following perinatal
phytoestrogen exposure has been shown by us and others in rodents.
Mice or rats treated perinatally with lower doses of genistein (0.5–10
mg/kg) advance the timing of vaginal opening while higher doses (50
mg/kg) have no impact or delay it. Estrous cyclicity is also disrupted
following developmental exposure to genistein. Mice or rats treated
neonatally with genistein spend significantly longer periods of time in
the estrous phase of the cycle; this abnormality increases in severity
with increasing dose as well as increasing age.
Other investigators have shown similar estrous cycle alterations in
experimental animal models including a study by Nikaido et al. which
reported developmental exposure to numerous environmental estrogens,
including genistein, resveratrol, zearalenone, and Bisphenol A, resulted
in extended estrous cycles when the animals became adults. This is similar to mice exposed perinatally to DES further confirming the idea that developmental exposure to estrogens
causes disruptions in estrous cyclicity. These abnormalities could
result from organizational disruptions anywhere within the HPG axis,
including the ovary. Other aspects of reproductive tract development may
be vulnerable as well.
In
rodents, neonatal exposure to genistein alters ovarian differentiation,
reduces fertility and causes uterine cancer later in life. Most of these
studies have been done with subcutaneous injections of the aglycone
form of the compound, genistein. Mice treated by subcutaneous injection
of genistein at a dose of 50 mg/kg/day have peak serum circulating
levels of genistein of 3.4–6.2 µM. These levels are similar to infants on soy-based infant formulas (1– 5.4 µM) .
This level of genistein exhibits estrogenic activity in the neonatal
mouse as measured by uterine wet weight gain during the time of
treatment.
Neonatal,
subcutaneous injection of genistein at doses of 0.5, 5 and 50 mg/kg/day
produced a dose-dependent increase in the number of mice with
multi-oocyte follicles (MOFs) in the ovary prior to puberty with almost
all of the mice in the highest treatment group having MOFs. This effect is mediated through ERβ since mice lacking ERβ do not develop MOFs following neonatal treatment with genistein. The formation of MOFs was further characterized by observing ovarian development during the time of neonatal treatment.
Treatment with genistein at a dose of 50 mg/kg on days 1–5 inhibits
this differentiation process leaving the oocytes together in nests and
still attached to each other by intercellular bridges. This study also
showed a higher percentage of unassembled oocytes (those not completely
surrounded with granulosa cells) further supporting the limited
differentiation of the ovary in genistein treated mice. MOFs have also
been found in rats treated during development with genistein suggesting
that this occurrence is not limited to mice.
Further, the presence of MOFs has also been noted in humans, supporting
the idea that effects observed in animal models translate to humans,
although the cause in humans is still not known. Several other
estrogenic compounds have also been found to cause MOFs if exposure
occurs during development including 17β-estradiol, DES and Bisphenol A
confirming that estrogenic substances alter ovarian differentiation.
Altered ovarian function
has also been observed following developmental exposure to genistein. A
recent study from our laboratory showed the complete lack of corpora
lutea (CL) and anovulation at 4 months of age following neonatal
exposure to genistein at 50 mg/kg on days 1–5 indicating ovarian
function was disrupted.
Doses of genistein lower than 50 mg/kg showed enhanced ovulation rates
as evidenced by increased numbers of oocytes ovulated following
exogenous gonadotropins at 26 days of age as well as increased numbers of CLs at 4 months of age. In addition, higher neonatal doses of
genistein exposure were associated with decreased pituitary
responsiveness and suppressed LH production in response to GnRH
stimulation.
The LH surge is necessary for ovulation so lower levels of LH may
explain the lack of ovulation seen in the high dose treated mice.These data taken together, suggest that the HPG axis is disrupted following developmental exposure to genistein.
Female
fertility is also disrupted in rodents following developmental exposure
to genistein. Female mice treated with lower doses of genistein (0.5
and 5 mg/kg) showed no difference in the numbers of mice delivering live
pups compared to controls at 2 and 4 months of age. However, by 6
months of age, a reduction in the percentage of mice delivering live
pups in both treatment groups compared to controls was seen as well as a
reduction in the number of live pups in the mice that delivered; these
findings suggest early reproductive senescence.
Mice treated neonatally with genistein (25 mg/kg) exhibit reduced
fertility at 2 months of age with only 4 out of 8 (50%) plug positive
mice delivering live pups. Female mice treated neonatally with genistein
(50 mg/kg) did not deliver any live pups at 2 months of age (0/8 mice)
suggesting mice exposed developmentally to this dose are infertile. A
study from another laboratory supports our findings since rats treated
with genistein (100 mg/kg) also showed disruption of fertility.
Since mice treated with genistein 50 mg/kg did not deliver live pups,
additional studies were conducted to characterize the source of
infertility. Less than half of the genistein treated mice showed signs
of pregnancy following vaginal plug positive compared to almost all of
the controls. In addition to low numbers of pregnancies, the females
that were pregnant had smaller and fewer implantation sites compared to
controls. There were also visible reabsorption sites in some of the
genistein treated mice. One possible explanation for these implantation
defects is that the environment of the uterus or the hormonal milieu is
not suitable for implantation. However, serum hormones measured during
pregnancy did not reveal any deficiencies in hormones needed to maintain
pregnancy such as progesterone and estradiol suggesting that these are
not a likely cause of the implantation problems.
There is a 50% loss of embryos
during transit through the oviduct of genistein treated mice and embryo
transfer experiments showed that the uterus of genistein treated mice is
not capable of sustaining pregnancy even if the blastocysts are from
control mice.
These data suggest that there is a permanent change in the function of
the female reproductive tract that leads to complete infertility in
these mice.
Female mice
treated orally with genistein at doses of 6.25, 12.5, 25 and 37.5 mg/kg
developed multi-oocyte follicles, altered vaginal opening and altered
estrous cyclicity in a dose dependent manner similar to previous studies
using subcutaneous injections of genistein.
These mice also developed subfertility increasing in severity with
increasing dose and over time similar to mice exposed by subcutaneous
injection of genistein. This study confirms that it is not the route of
exposure that is important but the amount of biologically active
compound that gets to the target.
Animal data: Abnormal development of the male reproductive tract:
There is a surprising paucity of data on the developmental effects of phytoestrogens in males (reviewed in 2009). There was a large multi-generational
study conducted in rats exposed to genistein through the diet throughout
the lifetime of the animal.
The dose range finding portion of this study exposed rats to genistein
in the diet starting on gestational day 7 and through lactation until
weaning and then in the diet until postnatal day 50 (adulthood). The
doses used in that study were approximately 0.3, 1.7, 6.4, 16, 38, and
72 mg/kg to the dam and 0.6, 3, 11, 29, 69, and 166 mg/kg to the pups
after weaning. In addition to females developing ductal/alveolar
hyperplasia in the mammary gland, males in this study developed mammary
gland hypertrophy at doses at or above 11 mg/kg and mammary gland
hyperplasia at doses at or above 29 mg/kg. Males in this study had
reduced prostate weight following the highest dose. There were very few
additional effects on the male reproductive tract in this study.
However, studies examining the actual dose of the chemical that entered
circulation revealed that there was very minimal exposure to the
neonates through lactation so the male rats were exposed primarily
prenatally and in adulthood but not during the neonatal period.
More studies should be conducted to determine potential adverse effects
in males during critical periods of development, particularly during
neonatal life. A pair of studies that have had a profound impact on the
field, and greatly contributed to health advisories in Europe was
conducted in marmosets. Twins were fed either soy formula or milk
formula. Males on the soy diet had lower serum testosterone
concentrations and higher numbers of Leydig cells at the discontinuation
of soy formula use. As adults, the soy fed marmoset had larger testes
and lower serum testosterone levels than its twin demonstrating that the
impacts were persistent.
Evidence for long term health consequences of soy infant formula in humans:
The
question of whether or not soy formula is safe has been extensively
debated across the globe for more than a decade. (The National Toxicology review of soy infant formula) is the same level of "minimal concern" initially expressed for BPA
until mounting evidence for wide ranging health effects pressed the FDA
to elevate that advisory to “some concern” of BPA in January, 2010. The
isoflavone phytoestrogens genistein and daidzien, as well as many
others, have far greater relative binding affinities for both ERα and
ERβ than BPA, raising concern that these compounds pose an
underappreciated threat to infant development.
In
2008 the American Academy of Pediatricians (AAP) concluded that in
terms of nutritional quality “isolated soy protein-based formula has no
advantage over cow milk protein-based formula” and that soy formula has
“no proven value in the prevention or management of infantile colic or
fussiness” but stopped short of recommending against its use.
Internationally, use of soy formula is viewed more cautiously than in
the US. For example, the United Kingdom, Australia and New Zealand all
caution against indiscriminate use.
Other nations offer it only by prescription and, consequently, use of
soy formula is customarily lower in these countries. These types of
advisories are confusing for parents and have done little to quash the
idea that soy formula is a healthful alternative to breastfeeding.
A near absence of
documented effects in soy-fed infants is not entirely reassuring,
however, because although soy infant formula has been widely available
for more than four decades, surprisingly little work has been done
regarding its potential long term effects on reproduction, fertility or
behavior. Historically, epidemiological studies have mainly focused on
nutritional status, growth parameters, and impacts on the thyroid system
because soy has long been recognized to induce hypothyroidism and
goiter when not counteracted with elevated iodine intake.
Very few have explored the possibility that soy formula use can impact
reproductive development or function and those which have, are hampered
by insufficient sample sizes and the absence of appropriately sensitive
measures. The task is not easy because, as with many endocrine
disrupting compounds, isoflavone effects, if present, may not manifest
for years, even decades, and are likely mild enough to escape clinical
detection.
The earliest
evidence for reproductive health effects came from two studies,
conducted in the mid 1980’s, which associated neonatal phytoestrogen
exposure with thelarche before age 2 in a population of Puerto Rican
girls. A number of confounding factors, however, including the
consumption of chicken that had been fattened with potent estrogens,
make the data problematic and difficult to interpret.
Within the last decade, a retrospective cohort study of 952 women found
that young women fed soy-based infant formula (248 women) as part of a
controlled, University of Iowa feeding study, reported longer menstrual
bleeding and menstrual discomfort than those who were fed a
non-soy-based formula (563 women).
At the time the study was conducted, the women were too young to
comprehensively examine pregnancy or fertility outcomes, but, now that
nearly a decade has past, this area is ripe for reevaluation. Most
recently, soy formula consumption has been linked to a greater risk of
developing uterine fibroids.
The study population included over 19,000 women enrolled in the Sister
Study, making it one of the largest and most highly powered
epidemiological evaluations of the long term reproductive health impacts
of soy formula.
In Asian
populations, soy consumption is high across the entire lifespan, except
for a brief 6–8 month neonatal breastfeeding window. In Westerners
feeding their babies soy infant formula the pattern is just the
opposite, and the highest consumption levels occur in the first year of
life then drop to near zero. In Asia, soy is consumed mostly in the form
of tofu, tempeh, and other unprocessed foods, not as dietary
supplements or products enriched with soy protein isolate. Asian
populations also eat considerably higher levels of seafood and low
levels of animal fat than Western populations. These variables make the
two populations quite distinct in terms of lifestyle, dietary habits,
and lifetime phytoestrogen exposure. Thus, phytoestrogen effects may
differ between the two groups, a possibility that should be taken into
account when interpreting epidemiological data.
Conclusions:
Phytoestrogens
are intriguing because, although they behave similarly to numerous
synthetic compounds in laboratory models of endocrine disruption,
society embraces these compounds at the same time it rejects use of synthetic endocrine disruptors in household products.
Thus, phytoestrogens both expand our view of environmental endocrine
disruptors and propound that the source of the compound in question can
influence the direction and interpretation of research and available
data. While the potentially beneficial effects of phytoestrogen
consumption have been eagerly pursued, and frequently overstated, the
potentially adverse effects of these compounds are likely
underappreciated. The opposite situation exists for synthetic endocrine
disruptors, most of which have lower binding affinities for classical
ERs than any of the phytoestrogens but can sometime produce similar
biological effects.
Animal data reveal that the isoflavones have a wide
range of molecular, cellular and behavioral effects at doses and plasma
concentrations attainable in humans. In vivo isoflavone
responses have been reported for a wider range of tissues and processes
than the endpoints generally used to evaluate most synthetic EDCs,
yet only minimal concern has been raised about their increasing use.
Infants fed soy formula have the highest exposure to any
nonpharmacological source of estrogen-like compounds, yet we know
virtually nothing about how the use of these phytoestrogen-rich formulas
might impact their future reproductive health. The health effects of endocrine disrupting
compounds in general are receiving more attention from public health
agencies, and the public at large.
As
with many other compounds, like alcohol or caffeine, there are many
pros and cons associated with moderate soy intake. Consumers should be
aware that soy contains endocrine disrupting compounds and make dietary
choices accordingly. Women who are pregnant, nursing, or attempting to
become pregnant should use soy foods with caution and be aware that soy
formula may not be the best option for their babies.
"Pros and Cons of Phytoestrogens" has been edited. You can read the entire study: www.ncbi.nlm.nih.gov/pmc/articles/PMC3074428/
