Endocrine Disruption

Bisphenol A

SUMMARY AND COMMENTS ON THE LOW DOSE BPA SPREADSHEET

September, 2009

INTRODUCTION

Bisphenol A (BPA) is a chemical building block used to make the polycarbonate plastic and epoxy resins used in a wide variety of commercial products. Some of these include digital media (e.g., CDs, DVDs), electrical and electronic equipment, automobiles, airplanes, construction material, sporting goods, food and drink can liners, dental sealants, and many other products. Global production of BPA in 2006 was approximately 3.8 million tons1 and 92.6% of adults in the US have detectable levels of BPA in their urine averaging 2.4 μg/L in females and 2.9 μg/L in males.2

A 1997 study reported significant changes in the male reproductive system of offspring exposed in utero to low levels of BPA. Prior to this research, few BPA studies had been conducted at levels below EPA’s lowest observed adverse effect level (LOAEL) of 50 mg/kg/day.3 The changes reported in the 1997 study were found at much lower doses.

The above research posed two challenges to the assumptions of BPA’s safety: first, significant changes in the male reproductive system (prostate weight, daily sperm production and epididymis weight) resulted from exposure to BPA at doses well below levels deemed safe for human exposure. Second, the findings suggested BPA’s activity may be much more potent and diverse than previously assumed. These assumptions initiated a series of low dose BPA studies that continue today.

As a tool for organizing the extensive number of studies that followed, we produced a spreadsheet on low dose BPA. We selected all papers in the published, peer-reviewed literature that include doses at or below 1 mg/kg/day (1 ppm), including in vivo and in vitro studies.4 This dose range is more stringent than that suggested by the National Toxicology Program’s (NTP) Low Dose Peer Review Panel (2001).5 The NTP Panel suggested using doses of 5 mg/kg/day or less as the low dose range, less than a tenth of the LOAEL. We chose to be more conservative.

The spreadsheet is designed to allow the user to sort the data as desired. The results (outcomes) of the studies are categorized as either expressing or not expressing change. Outcomes cover a wide range of parameters from the gene, to the molecular, cellular, tissue, organ, and system level. The outcomes are grouped into broad categories such as male reproductive system, female reproductive system, brain and behavior, organ systems, etc. Each broad category is broken into specific outcomes which then can be broken out further. For example, the broad category, male reproductive system includes the outcome, sperm, which is comprised of a diversity of measurements including sperm count, sperm production, sperm motility, and damage to sperm. When an organ is the listed outcome, multiple changes in the organ, including structural, functional and genetic changes are included. The following summary is based on the TEDX Low Dose BPA Spreadsheet. 

SUMMARY

As of June 2009, 391 studies examined BPA effects at less than 1 ppm in this spreadsheet. Currently there are 50 different outcomes in this spreadsheet that range from the cell to the system level. The following table contains a brief overview of the data.

Total number of studies	391
Studies with significant effects	319	82%
Studies with no effects	72	18%
Studies using mammals or mammalian tissue	277	70%
Studies using fish, amphibians, birds, invertebrates, and other wildlife species or their tissues	115	30%
Studies using in vitro methodology	132	34%
Studies using in vivo methodology	240	61%
Studies using both in vivo and in vitro methodologies	19	5%

 

The purpose of this summary is to provide a general overview of the data through June 2009. Some of the studies contain more than one outcome; consequently the number of studies in the following charts will exceed the total number of studies in the spreadsheet. The total number of studies reporting a specific outcome is listed, as well as the percent of studies that did and did not find statistically significant changes. Click here to see bar graphs of the tables below. Click here to see summary tables containing more specific information on the results of these studies.

The following provides a brief overview of the data when examined by animal model, timing of exposure, in vivo and in vitro.

In vivo mammalian effects

Gestational exposure

Most mammalian studies (109 studies) examined effects from exposure to developing animals during a period equivalent to human fetal development, which includes weaning in rodents. When animals are exposed at this stage, the effects of BPA are far ranging. Multiple changes in genes, gene methylation, and gene expression were found in the sex organs, internal organs and the brain, resulting in structural, organizational and functional changes. Adverse changes were also found in immune system functioning and in cholesterol and triglyceride levels in blood. Three areas have been most frequently studied: male and female reproductive organs, the breast, and the brain. 

In the male, BPA harmed the testes, penis and prostate. The changes found in the prostate are known cancer precursors. Female exposure caused changes in the structure and function of the female reproductive organs, and adversely affected the development of eggs and embryos. Multiple changes were found in developing breast tissue, many of which have been linked to the formation of breast cancer in later life. This chemical altered the organization and functioning of brain tissue, and the behavior of animals. Some of the changes seen were hyperactivity, changes in responses to morphine and amphetamines, and the elimination of sex specific behaviors. BPA did not show any significant changes in adrenal and pituitary outcomes.

Immature exposure

Four studies exposed animals after weaning until puberty. Most of these studies did not find any changes. Increased neuronal development in the brain, changes in sexual response and play, and changes in hormone levels and ratios in the blood were reported.

Adult exposure

Fifty mammalian studies investigated outcomes as a result of exposure during adulthood. In the male, changes were found in numerous areas of sperm function and physiology. Changes in females were more widespread, with alterations in uterine structure and function from gene to cell to whole organ, changes in breast tissue, and in egg development. Exposure to BPA was correlated with polycystic ovarian syndrome, an in increase in miscarriages, and a decrease in successful births. The brain showed changes in cell structure and the activity of brain cells and neurotransmitters. The immune system was compromised and changes in blood chemistry were reported. Exposure to BPA changed pancreatic function and linked it with diabetes and cardiovascular disease.

Multigenerational exposure

Exposure to multiple generations (4 studies) resulted in changes in the weights of the ovaries, liver, uterus and testes. Decreased production of sperm and changes in the birth rates after the first generation were found. No measures of genetic changes or alterations in function were studied.

In vivo non-mammalian effects

Gestational exposure

In non-mammals (fish, amphibians, birds, etc.), exposure of eggs/embryos (17 studies) to BPA resulted in morphological changes and abnormal development. Eggs hatched earlier, growth was retarded, and abnormalities increased. An increase in the number of males was found, and an increase in the incidence of ovotestis in males. Multiple changes were noted in enzymes, hormones, and gene expression.

Immature exposure

Immature animals (15 studies) showed feminization of the males which included a reduction in the number of males, reduced testosterone levels, changes in gonad development, and increased numbers of ovotestis. Changes in sperm number and a decrease in reproductive success and offspring quality were documented. Again, maturation problems were noted, as were changes in gene activity and behavior. BPA also interfered with thyroid hormone genes, pathways and processes.

Adult exposure

In adult exposure experiments (56 studies), feminization of the males was again found, as was super-feminization of the females. The latter included a large increase in the number of eggs produced and the spawning mass, but a reduction in the successful growth of eggs and embryos. Both males and females displayed reproductive organ damage and malformation, a decline in the number and quality of embryos, and reduced sperm quality in all measured parameters. Multiple changes were found in gene expression and enzyme levels, especially in the liver.

Multiple generations or life stage exposure

Male organisms exposed through multiple life stages showed morphological changes in their reproductive organs and in sperm quality. The females also displayed reproductive organ changes and an increase in spawning mass and egg production. The weight and growth of larvae were reduced and disturbances in normal maturation noted. Gene expression was changed in multiple organs and processes. 

Exposure across multiple generations resulted in incremental decreases in egg production such that no eggs were produced in the sixth generation. Growth and development was retarded and the number of females decreased.

In vitro studies

This group of 151 studies included those that utilized only in vitro methods (132 studies), as well as those that studied both in vitro and in vivo effects (19 studies). Overall, BPA exposure altered the binding and transcriptional activities of estrogen receptors alpha and beta, the functioning of enzymes and proteins, and gene expression in many types of cells, organs and systems. Specifically in the reproductive organs of males, exposure resulted in cell proliferation, deformation and altered progesterone synthesis in the prostate, up-regulated androgen receptors, changed gene expression in Leydig cells, and changes in enzyme expression and progesterone biosynthesis in the testes.

In females, altered hormone production and hormone receptor activity was found. Cell viability decreased and cancer cell activity increased. BPA altered endometrial cell proliferation. In breast tissue, increased cell proliferation and decreased apoptosis were found along with multiple changes in genes and gene transcription. Exposure was linked to breast cancer and resistance to anti-cancer drugs. Egg development and maturation was adversely impacted, especially the division of the chromosomes in the egg.

BPA increased conversion of cells that maintain the structural integrity of connective tissues to adipocytes (fat cells) and enhanced glucose uptake in adipocytes. Decreased prolactin production and growth hormone content was found in the pituitary. In thyroid cells, gene expression and cell activity was altered. BPA also blocked T3 binding and bound to thyroid hormone receptors in a non-competitive manner. BPA increased insulin secretion in the pancreas, and altered glucose metabolism. Spleen cell proliferation, antibody levels, and the cytokines that regulate immune cells all increased.

In the brain, BPA altered intercellular CA++ levels and inhibited intercellular communication and signaling. It changed the ratio of neurons to support cells (astrocytes, oligondendrocytes) and activated those support cells in different areas of the brain. Dendritic growth increased along with dendritic density and activation patterns. BPA also caused neuronal death.

In non-mammals altered vitellogenin production and synthesis was found in the liver. Immune functioning was compromised with changes noted in macrophage and lymphocyte proliferation. Changes in cellular activity were found in bone and scale tissue.

COMMENTS

Exposure to BPA at 1 ppm or less resulted in changes across multiple outcomes and systems, in vivo and in vitro. While a number of studies relied only on traditional toxicological outcomes such as litter size, sex ratio, and organ weight, these do not fully reveal the effects of BPA. Some of the most profound effects were seen in morphological, histological, and/or functional changes in organs and the organism as a whole. Minute changes in the developing organism have consequences which are not often seen at birth, and may only become apparent as the organism matures and attempts to reproduce. These changes are often not addressed in the traditional toxicological paradigm, and as such, the subtle effects of BPA could have been overlooked or underestimated in some experiments in this spreadsheet.

BPA was recognized as an estrogen in the 1930s so it is not surprising that female reproductive system outcomes were among some of the most affected. Researchers have consistently found that BPA changes the pattern of growth and development in breast tissues - changes which have been implicated in the development of breast cancer. Aside from many changes in the female reproductive organs, egg/oocyte development has shown many adverse effects from BPA exposure.

Most of the male studies limited the outcomes to organ weight or size. For example, of the studies examining the prostate gland that did not find change, all of them primarily looked at prostate weight as the outcome. Of the studies that found changes, two thirds examined morphological or histopathological outcomes, such as prostatic intraepithelial neoplasia (PIN) lesions, cellular growth, proliferation, or androgen receptor changes. In the male reproductive system BPA may not interfere in the major testosterone pathways, and instead cause changes in the estrogen pathways that result in effects.

Almost all (97%) of the behavioral studies reported changes, ranging from feminization of masculine behavior and the masculinization of feminine behavior to changes in responses to addictive substances. Other consistent effects were changes in neuronal and brain structure, numbers of neurons, neuronal activity, hormone receptor activity, and neurotransmitters.

In the spleen and immune system, over 70% of the studies done found changes in phagocytes, macrophages, lymphocytes, splenocytes, and immunoglobulin. The results demonstrated the proliferative and/or suppressive effects of BPA and consistently showed interference in the normal functioning of the immune system.

Timing of exposure altered the results in many outcomes. One example is sperm count. Offspring dosed throughout gestation only and those exposed only in adulthood had reduced sperm counts. When timing of exposure was restricted to a narrow window (gestational days 11-17 or post natal day 1-12), sperm counts increased. No changes in sperm count were found when exposures ranged from gestation through weaning and in multi-generational exposures.

While there is a pattern of positive results in mammals and mammalian tissues, some of the most striking and consistent results were found in the non-mammalian species. Experiments performed on immature and adult non-mammals consistently found changes due to BPA exposure; changes that affected the way the animals developed and matured, how they functioned and their ability to reproduce.

Some of the results in the spreadsheet were discovered at doses 1,000 to 10,000 times lower than what is currently considered safe. Effects have been found at doses in parts per billion and trillion that are within ambient exposure levels. Dose response curves are often in the shape of a U, or an inverted U. These curves do not conform to the traditional expectations of toxicology, which states that an increase in dose is matched by an increase in effect. Because the endocrine system acts like a thermostat, through self-regulating feedback loops, BPA can harm systems at very low doses while at higher doses it will shut the system down before harm can occur. Very high doses, however, can overwhelm the system and cause damage and even death. It is the body’s responses to BPA at very low doses, operating well under traditional toxicology’s no observed effect level, that results in harm.

While the experiments on low dose exposure to BPA have been wide ranging, they are not comprehensive. Not all the systems in the mammalian body have been studied, nor have the results been consistent across all systems. There may be other effects that have yet to be seen or explored. Chemical exposures can impact an organism on many levels at the same time in synergistic or interlinking ways that result in diseases or dysfunction that the examination of only one system or organ cannot fully explain or even see.

The spreadsheet demonstrates that effects are expressed from exposure at every stage of life and in every organ and system examined. These effects range from changes in gene expression, to the operation of hormones and hormone receptors, lymphocytes, enzymes and proteins, which are then expressed as changes in the functional activity of organs, systems and the animal as a whole. Recent studies have opened the door to the concept that the origin of certain disorders, such as prostate and breast cancer, and behavioral and brain disorders, may be the result of prenatal exposure to BPA. The effects may vary from organ to organ, or system to system, but at these ambient exposure levels, there is no safe dose of BPA.