Fifty-three years after Rachel Carson, in her book Silent Spring, first raised concerns about the safety of the pesticide DDT, the chemical once again is in the news.
Public health researchers in California have published findings that connect maternal exposure to DDT during pregnancy to breast cancer—not in the exposed mothers but rather, 40 or 50 years later, in offspring exposed in utero. The article was published in the Journal of Clinical Endocrinology and Metabolism. In it, the authors looked at the rates of breast cancer in a group of 9,300 women born between 1959 and 1967. Of these, 118 developed breast cancer.
Incredibly, the group of pregnant moms had blood samples stored from more than 50 years ago as part of the Kaiser Foundation Health Plan (ah, the virtues of a single payer system!). The 21st-century researchers were able to analyze the blood samples taken during their pregnancy to determine DDT levels. They then connected the DDT level in Mom to the risk of breast cancer by age 52 in the daughters.
The authors used a well-established study approach called case-control: the 118 daughters with breast cancer (cases) were paired with 354 breast cancer-free daughters born at the same time (controls) and standard clinical variables extracted from each group. In this way, variables like race, maternal weight, lipid profiles, and breast-feeding history could be eliminated as likely causes of any difference in breast cancer rates.
By this analysis, the mothers with high rates of detectable DDT during pregnancy produced daughters who, 50+ years on, had three or four times more breast cancer than daughters of Moms with substantially lower DDT levels. A potentially very big deal for sure.
There is a large and important caveat in the analysis however. The investigators also examined the frequency of breast cancer in mothers, comparing the rate between cases and controls. Here, there was a large and very strong tilt: About 20 percent of daughters with breast cancer had a mom with the disease; in the controls, only 4 percent had a mother with breast cancer. Thus, some and perhaps most of the risk of breast cancer might be explained not by DDT exposure but genetic predisposition.
Which does not exclude DDT as an amplifying effect on genetic risk—but it does deflate some of the headline grabbing excitement the article already has claimed. But then DDT remains a hot-button item for both sides of the political spectrum. The Right has argued that malaria, which might have been prevented by DDT, will kill more people than DDT ever has whereas the Left has found evidence to suggest DDT as a cause of Alzheimer’s and various other cancers as well as low sperm counts in men and reduced fertility in women.
Though not fully proven (various agencies call the link “possible” or “probable”), the cancer connection to DDT is entirely plausible. DDT clearly is an endocrine (aka hormone) disruptor and therefore is a lead compound in a class of chemicals that clearly cause medical problems. Triclosan, for example, an established hormone disruptor just like DDT (and dioxin), has been banned from toothpaste because of its potential to harm animals, including humans.
Furthermore, the pathway by which a hormone disruptor could lead to a disease such as breast cancer is extremely plausible, given that many breast tumors have receptors for hormones such as estrogen. The presence of these receptors on so many tumors has led to a slew of effective treatments, such as tamoxifen, aimed at blocking the hormone-led promotion of tumor growth.
This latest dispatch from the DDT frontline unfortunately is most likely to be swallowed up into a political and not a scientific debate. As with global warming and evolution, the lack of 1000 percent proof of causation will allow the Right, always sticklers for Methodology and statistical assumptions, sufficient wiggle room to not believe, while the Left will trot out a flawed new entry into the debate and defend it exuberantly.
Which is a real tragedy—evidence that needs the oxygen supplied by the process of rigorous and neutral scientific inquiry will instead be suffocated by the oppressive gasping of people with a point to make.