Hormones and breast cancer

Information on this webpage is drawn from our 2005 report: Breast cancer - an environmental disease: the case for primary prevention, available free as a pdf, see Downloads. For current statistics and data, see our homepage.

Today, breast, ovarian and endometrial uterine cancers are clinically categorized as 'hormone dependent' cancers.
(Kelsey & Whittemore 1994)

Hormones are the key chemicals involved both in the development and the function of the breast. Breast development which precedes menstruation by individually variable times, and the changes in breast tissue which begin with the first menstrual cycle, will depend on a complex interplay of hormones, mainly oestrogen, progesterone, prolactin and other growth factors. 'The breast is fairly quiescent from infancy until puberty, then, under the influence of sex steroid hormones, remarkable changes occur … From here on in, the activity of the breast is firmly locked into our hormonal cycles.'
(Read 1995)

Oestrogen and breast cancer

Throughout the life cycle, the hormonal environment plays a critical role in the development of breast cancer.
(Brody & Rudel 2003)

Oestrogen, the primary female sex hormone, has been associated with breast cancer since the 19th century when Scottish surgeon, George Beatson, observed that removal of the ovaries (the main source of oestrogen production) reduced incidence of the disease in women.

'Estrogen may be implicated in breast cancer because of

its role in stimulating breast cell division
its work during the critical periods of breast growth and development
its effect on other hormones that stimulate breast cell division, and
its support of the growth of estrogen-responsive tumors.'
(Cornell University fact sheet 9)

A woman's lifetime exposure to oestrogen is influenced by her age at three stages in her reproductive history:

onset of menstruation (menarche)
first full-term pregnancy
onset of menopause.

'A study of breast cancer risk based on oestrogen levels in 15,000 women found that women with higher oestrogen levels were more likely to go on to develop breast cancer.'
(Toniolo et al 1993)

'Because total estrogen exposure is the single most important risk factor for breast cancer, estrogenic chemicals, which would add to this lifelong exposure, are an obvious suspect when searching for the cause of rising rates (of breast cancer) over the past half century.'
(Colborn et al 1996)

Hormone disruptors and mimics

Strong toxicologic evidence points to a large number of ubiquitous pollutants that are plausibly linked to breast cancer because they mimic or disrupt hormones known to affect breast cancer risk.
(Brody & Rudel 2003)

Man-made chemical compounds with the ability to mimic hormones produced by the body and to interfere with the hormonal messaging systems that direct and regulate our biological development and function are variously known as xenoestrogens, xenohormones, environmental oestrogens, hormone disruptors, hormone mimics or hormonally active agents (HAAs), and endocrine disrupting chemicals (EDCs).

Authors of 'Our Stolen Future' describe EDCs as: 'thugs on the biological information highway that sabotage vital communication. They mug the messengers or impersonate them. They jam signals. They scramble messages. They sow disinformation. They wreak all manner of havoc. Because hormone messages orchestrate many critical aspects of development, from sexual differentiation to brain organization, hormone-disrupting chemicals pose a particular hazard before birth and early in life.'
(Colborn et al 1996)

'EDCs are found in a large number of frequently used products such as weedkillers, foods, petrol, insect sprays, cosmetics, shampoos, disinfectants, plastic linings of food cans, plastic bottles and some medicines. Another main source of hormone disrupting chemicals are dioxin emissions from waste incineration plants. In most Western European countries 95% of dioxins which humans absorb, enter our bodies in the form of food, particularly milk products and fish oil.'
(van Dooren 1997)

Synthetic oestrogens are also found 'in pesticides, in the growth hormones fed to chickens, cows and other animals, and in the contraceptive pill and Hormone Replacement Therapy prescribed to women.'
(Hoult 1996)

Man-made hormone mimics differ in fundamental ways from oestrogens produced by plants and humans. One important difference is the long-evolved ability of the body to break down and excrete the natural oestrogens, whereas 'many of the man-made compounds resist normal breakdown and accumulate in the body, exposing humans and animals to low-level but long-term exposure. This pattern of chronic hormone exposure is unprecedented in our evolutionary experience.'
(Colborn et al 1996)

Being fat-soluble compounds, most EDCs are not excreted by normal body processes but are stored in body fat, thus impacting on cells in the body over many years.

It is known that man-made EDCs can:

cross the placenta
disrupt the development of the foetus
have serious effects that might not be evident until decades later and that the human body can mistake a man-made chemical for a hormone. (Colborn et al 1996)

Timing of exposure

Biologist and foetal toxicologist Dr Sandra Steingraber defines life periods when hormonal disturbances pose the greatest risks as 'windows of vulnerability'. These occur:

'when the foetus is developing in the womb and minute changes in hormone levels switch on the development of each organ system
when newborn babies still have incomplete immune systems and no blood-brain barrier
when puberty, triggered by hormonal changes measured in low parts per billion, leads to rapid cell division and DNA replication
in old age when the body's defence mechanisms weaken.'
(Steingraber/Lawrence Guardian 2004)

Bioaccumulation

A characteristic common to most EDCs is that they build up (bioaccumulate) in fatty tissues where they remain potentially active for long periods of time. It is therefore not surprising that 'a growing number of experts aware of the rise in oestrogen-mimicking chemicals are beginning to suspect that it is not so much the fat that predisposes a woman to breast cancer but the toxins which have accumulated in the fat that then build up in her body.'
(Kenton 1995)

The bioaccumulation of toxins in fat cells inevitably produces toxic effects in the body. UK scientist Peter O'Neill points out that such toxic effects 'may result after a relatively long time period either through levels rising above a threshold that induces adverse effects or because some stress event causes the chemical to be released from the fat.'
(O'Neill 1993)

Low-level effects

The relative strength of EDCs is much lower than the strength of hormones produced by humans. For this reason some scientists claim that EDCs cannot pose any significant threats to human health. However, reproductive biologist Fred vom Saal points out that 'vanishingly small amounts of free [natural] estrogen are capable of altering the course of development in the womb. Given this exquisite sensitivity, even small amounts of a weak estrogen mimic – a chemical that is one thousand times less potent than the estradiol made by the body itself – may nevertheless spell big trouble.'
(Colborn et al 1996)

'Natural estrogens operate at extremely low concentrations, measured in parts per trillion. In contrast, these so-called weak estrogens are present in blood and body fat in concentrations of parts per billion or parts per million – levels sometimes thousands to millions of times greater than natural estrogens. So even though the contaminant levels may seem miniscule, they are not necessarily inconsequential.'
(Colborn et al 1996)

EDCs acting in combination

Studies show that 'hormone-disrupting chemicals can act together and that small, seemingly insignificant quantities of individual chemicals can have a major cumulative effect' (Colborn et al 1996) and that 'multiple estrogenic chemicals can act together to produce an effect even when each individual component of the mixture is below a threshold for effect.'
(Brody & Rudel 2003)

Improved understanding of the number of ways in which EDCs affect breast cancer has developed from animal and laboratory tests and reveals that '[their] ability to bind to the oestrogen receptor (on a cell) appears to be a relatively common phenomenon. Once there, the mimics may act as agonists [aids] or antagonists [blocks], jamming these switches on or off. However, there are other ways in which chemicals can affect the hormone system. Some chemicals are anti-androgenic, blocking the androgen [male hormone] receptor. Some may interfere with the synthesis or metabolism of hormones, or with their transport in the body.'
(Chemistry & Industry 1996)

Because of the importance of hormones in developing and maintaining human health through all stages of life, man-made chemicals that can mimic and, therefore, interfere with the normal functions of hormones pose a serious threat to public health now and for the future.

Hormones of the wrong kind, hormones too soon in a girl's life, hormones for too many years in a woman's life, too many chemicals with hormonal action, and too great a total hormonal load. Another key is the kind of hormones, the foreign chemicals.
(Sherman 2000)