Why are young people getting colon cancer? A common weed killer may be linked, scientists say

• A new study suggests the rise of young colon cancer may be linked to a common weed killer.
• More evidence is needed to tie the herbicide definitively to young colon cancer.
• It's a first clue for what may be behind this trend. Scientists are studying bacterial infections, too.

A new study suggests a common weed killer may be linked to the mysterious global rise of young colorectal cancer.

The first-of-its kind study published Tuesday in the journal Nature Medicine, suggests that picloram — a herbicide used globally to kill woody plants and shrubs while keeping grasses intact — could explain the rising incidence of colon and rectal cancer cases in people under 50.

Picloram did not appear to be significantly linked to cases of colorectal cancer in adults over 70 years old, which are most common.

Jose Seoane, a computational biologist and the senior author of the study, told Business Insider his team set out to look for unique DNA methylation "signatures" in colorectal cancer tumors, which can be imprinted by the things we are exposed to over a lifetime.

Those exposures can leave a tell-tale "fingerprint" on our DNA methylation, the mechanism that controls how and when our genes turn on and off.

Young colon cancer cases tend to have DNA "fingerprints" linked to certain exposures

Seoane's team found that certain "fingerprints" appeared in the DNA of young colorectal cancer tumors they studied, and those fingerprints were linked back to exposures, including:

• Smoking
• Poor diets, lacking fresh vegetables, beans, nuts and other "Mediterranean" staples
• Obesity
• Educational attainment (which is also linked to poorer diets)
• and finally, the weed killer picloram

"When we saw picloram, we were like, okay, this has to be a mistake," Seoane, a principal investigator at the Vall d'Hebron Institute of Oncology in Barcelona, said.

So, his team set out to look for any other explanation for their results, but they "haven't seen anything" yet, he said.

Then, his team checked to see if this same pattern persisted across populations, comparing the incidence of young colorectal cancer in seven US states, including California, Connecticut, Georgia, Iowa, New Mexico, Utah, and Washington, to the level of county-wide pesticide use. The strongest pesticide signal of all tied to higher rates of young colon cancer was for picloram. (In second place was glyphosate.)

Because this study is observational, it can't prove that picloram is causing the cancers, and more work will need to be done to collect tumor samples from young cancer patients who lived in areas where picloram was in high use.

"It could be that where you live or what you work with could be more associated with the risk," Seoane said.

Picloram, which was developed in the 1960s, was one of many herbicides used in the "agents" the US Military used to clear forest during the Vietnam War. It works by disrupting the way plant hormones normally function, and can persist in the soil for years.

Other scientists are investigating if gut bacteria is linked to young colon cancer

This new research on picloram is simply a first clue, and much more research is needed to confirm that it's contributing to young colon cancer.

Cancer researchers have been racing to find environmental drivers of young colon cancer, which has been on the rise for more than three decades. In 2021, the US lowered the initial screening recommendation for colonoscopies from 50 to 45 years old, due to the mysterious trend, which experts agree is likely caused by a combination of modern lifestyle factors.

Last April, researchers discovered another potential link between young colon cancer and colibactin bacteria infections in early life. It's possible that specially designed probiotics could, one day, help combat that potential threat.

"Everything is on the table right now," cancer epidemiologist Rebecca Siegel, the senior scientific director of surveillance research at the American Cancer Society, told Business Insider.

Siegel, who was not involved in the picloram study, emphasized that more work will be needed to corroborate the finding. "I think this is an important step forward in the exploration of newer exposures that may be contributing to the rise," she said.

Robin Mesnage, a visiting research fellow at King's College London who studies the gut microbiome, said in a statement that the study is "fascinating" and "well done," but also cautioned it doesn't prove a definitive link between picloram and young colon cancer, since exposure to the herbicide wasn't directly measured in any of the patients studied.

It could be, he said, that other substances that were historically mixed in with picloram during the manufacturing process, like the known carcinogen hexachlorobenzene, could also be at play.

"It is always important to remember that real-world exposures involve not just the active ingredient, but also co-formulants and potential contaminants," he added.