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Linkage of Pesticides and Breast Cancer through Lactation Studies:

Important Public Health Issues

Peggy Land

In July í97, much research implicating environmental factors was presented at the first World Conference on Breast Cancer in Kingston. In Ottawa, July í99 more evidence was presented at the second conference. However, like tobacco and lung cancer, the challenging issues of establishing absolute proof, and the economic implications, have combined to make policy-makers very cautious. Meanwhile, The Canadian Cancer Society says that in 1999, one in nine Canadian women has a lifetime risk of developing breast cancer. So we need more than ever to know which risk factors are avoidable and which behaviors are protective. Some risk factors such as family history of breast cancer and alcohol intake, for example, are already well established. The importance of other possible factors such as breast-feeding history and exposure to environmental toxins, however, have been more difficult to establish. A review of the literature on these two possible factors, with commonly used organochlorine pesticides as the environmental toxins being examined reveals linkage between the two, with far-reaching implications for women to consider. In this regional municipality of Ottawa Carleton, breast-feeding and encouragement to seek alternatives to domestic-use pesticides are actively promoted by the Department of Health. This paper suggests that a possible result, in common to both practices, could be a protective effect from breast cancer:


Laboratory evidence using experimental animals has shown that some organochlorine pesticides and organochlorine pesticide contaminants can cause or promote growth of mammary tumors. These chemicals can also damage human reproductive systems. According to the International Agency for Research on Cancer (IARC), World Health Organization, since 1993 there has been "sufficient" evidence that pesticides Chlordane and DDT, and dioxins and related components cause cancer in humans and/or animals. (1) There is evidence as well that Atrazine, beta-HCH, Chlordane, Dieldrin, 2,4-D and 2,4,5-T,(1) and DDT(2), (3), can cause malignant mammary tumors in rodents. As well, all of the preceeding plus seventy-eight other pesticides are considered to be known or suspected estrogen-like hormone disrupters (4) (5), (6), (7), (8). Such chemicals can permanently damage human endocrine, reproductive and central nervous systems (5). Atrazine, Endosulphan and Lindane, all still used in Ontario, have been linked specifically with breast cancer. (9).


Bio-accumulation and long term persistence in human tissues of some toxic organo-chlorine-type pesticides and organochlorine pesticide components is well documented. Such widely used pesticides as DDT/DDE, Lindane, Atrazine, Dieldrin, Chlordane, Mirex and b-HCH are fat-soluble and have accumulated up the food chain into our (breast and other sites)fat (1), (10), (11), (12), (13), (16) and breast milk (14), (15), (17), (18). Of this list, only DDDT, Dieldrin and Mirex and Chlordane have been banned in Canada, but all are considered to be at least "possibly" carcinogenic, and some are "probably" carcinogenic (19). The IARC has confirmed that the dioxin 2,3,7,8- TCDD is a known carcinogen (20). In addition, the dioxin contaminants of 2,4,-D and 2,4,5-T are (as are all dioxins) fat-soluble, "highly persistent, bio-accumulative and toxic" (21). And although 2,4,5-T has been banned in Canada, 2,4-D is a very widely used herbicide for agricultural and domestic purposes, and is a suspected estrogen-mimicker. It can it can be tracked into our homes before it has degraded and persist as household carpet dust for as much as a year. (22,23,24)


Women with breast cancer tend to have higher levels of some organochlorine pesticides including DDT, Dieldrin, Mirex (74) and b-HCH, in their blood and breast fat, than those without breast cancer. (4), (16), (18), (25) (26), (27). Also, it has recently been reported that when some estrogen-mimicking organochlorine pesticides are present in human breast cells, cancerous growth is accelerated, and so it is now believed that there is sufficient evidence to say that such (bio-accumulated) environmental estrogen is a contributing risk factor to breast cancer (28).


Fortunately, lactation, or breast feeding, is an effective way to flush out organochlorine toxins such as pesticides which have been accumulating in breast fat and breast milk, (17), (29), (30), (31), making it now the most contaminated of all human food. (70). Concentrations, decline on successive feedings (17), (32), (70) Despite concerns about this unloading effect on the baby, the benefits of breast-feeding still outweigh the risks, including the tendency of breast-fed babies to have fewer bouts of infectious diseases (70), to be less likely to develop breast cancer (34), (35) and to live longer (30), (36). It has long been known that cholostrum in breast milk has potent immmune-system properties and this may be why it is even now, still so good for our babies. But how long will it be until women are told not to give their milk to their babies because it is simply too contaminated? Breast milk of women in the High Arctic who eat a diet high in animal fat has unfortunately become seriously contaminated, and may have already exceeded limits which justify giving it to their children. (71)


If lactation has a protective effect against breast cancer because of this unloading effect, at least temporarily, then of the thirty studies found comparing breast-feeding to non-breast-feeding women, most could be expected to show less breast cancer in the breast-feeding group. Indeed, most do, showing that the protective effect tends to be strongest in women of pre-menopausal age. (17, 29, 33, 34, 37-53). A recent study showed a higher protection rate in women over 50 (72). Other studies show no clear association (54-63) and none could be found indicating an inverse relationship. It has also been it noted by researchers that the inconclusive studies were largely confined to post-menopausal women, and most failed to precisely define lactation (64), or failed to examine the total duration of lactation or included few women with prolonged lactation (51), or used a wide variety of methodologies for reporting lactation history (42, 63). Also of note is that the inconclusive studies were on average done ten years before the studies which show protective effects of breast-feeding. Certainly, though, the majority of studies indicate that breast-feeding is protective against breast cancer mainly in women of premenopausal age. With no evidence to the contrary, why then is this not a recognized protective factor by the Canadian Cancer Society?


Researchers have suggested three possible mechanisms to explain the apparent protective effect of breast-feeding against breast cancer. They are not mutually exclusive. The first theory is that lactation suppresses production of ovarian estrogen, considered to be essential for the initiation or growth of some types of breast cancer (14, 38, 42, 48, 51, 53, 65, 66). The second is that physiological changes in the cells of the mammary ducts, on reaching functional maturity with lactation, are in some way then able to confer some protection against the development of breast cancer ( 37, 38, 42, 48, 51, 63, 71). The third theory goes further in suggesting that lactation unloads breast tissue of carcinogens including highly suspect organochlorine (and estrogenic) pesticides and is therefore protective ( 25, 32, 38, 48, 63,). This would explain why functionally mature (and toxin-unloaded) breast tissue is less likely to become cancerous. This third theory is supported by animal research (66, 67, 68), and also by a study which showed that women who had had babies and who were found to have Mirex in their breast tissue were more likely to develop breast cancer if they had not breast fed.(74)


In summary, the literature supports that exposure to certain organochlorine pesticides, especially those with estrogen-mimicking abilities could be a risk factor for the development of breast cancer. Studies on lactation have varied greatly in design making it difficult to establish exact predictions of degree of protection, but despite this, it appears that lactation does have a protective effect in women up to menopause and possibly beyond. The connection between these two factors is that lactation is an effective way to unload breast tissue from chemical carcinogens including highly suspect organochlorine estrogen-mimicking pesticides. It is up to decision-makers and educators to acknowledge that these two factors are important for women to know about, and also that estrogen-mimicking chemicals in our environment should, as in the U.S., be officially declared toxic. Recently the Federal Commissioner for the Environment and Sustainability, Brian Emmet, reported that the vast majority of pesticides have not been re-evaluated for safety by current standards. (73). Following this came the announcement that all pesticides would be re-evaluated for their effects on children, but not until the year 2006. Unfortunately, by then, mothersí milk may be fit only for toxic waste disposal facilities.

Abstract: Linkage between Pesticides and Breast Cancer through Lactation Studies

With high odds of developing breast cancer (one in nine in Canada), women need to know how to better their chances for prevention. An extensive search of the literature supports that exposure to commonly used organochlorine pesticides, especially those which are persistent and estrogen-mimickers, should be avoided as a preventative measure, and that breast-feeding is protective against breast cancer up to menopause and possibly beyond. Studies on pesticides show complex and indirect linkage with breast cancer. Lactation studies vary greatly in method and design, but do indicate that lactation is an effective way to flush out toxins from breast tissue. Fortunately, breast-fed babies still tend to be healthier than their formula-fed peers. Lactationís protective effect on breast cancer may well be due to the achievement of functional maturity combined with the subsequent flush-out of toxins including highly suspect organochlorine, and estrogen-mimicking pesticides. Avoidance of unnecessary use of pesticides, and promotion of lactation are important public health issues, and persistent estrogenic chemicals should be classed as toxic in Canada, as they are already in the U.S.

Peggy Land, B.Sc. P.T.,
Co-chair with Region of Ottawa -Carleton Councillor Dianne Holmes: Working Group on Health Dangers of Urban-Use Pesticides,
Director of Campaign for Pesticide Reduction, Sierra Club of Canada



References available on request



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