A major health crisis may be rooted in an unlikely source.

That’s according to new research out of Spain, which suggests a widely used weed killer could be fueling the alarming rise in early-onset colorectal cancer worldwide.

In the US, roughly one in five diagnoses now occurs in Americans under 55 — and the disease has become a leading cause of cancer-related death in young people.

Past research has pointed to lifestyle and environmental factors like smoking, diet and gut health as possible drivers of the trend, but tracking those exposures over a lifetime has proven difficult.

The new study takes a different approach, looking instead for clues embedded deep in our biology.

The researchers focused specifically on epigenetic marks, or chemical modifications added to DNA that alter gene expression without changing the underlying genetic sequence.

“If we imagine the genome as a book, epigenetic marks don’t change the text but function like post-its or markers that indicate which chapters should be read and which should be skipped,” José A. Seoane, lead study author, said in a press release. 

“These post-its can be added or removed depending on the environment and lifestyle — diet, stress, or exposure to toxins — influencing how the same book is interpreted over time.”

To explore possible links to cancer, researchers analyzed DNA samples from both younger and older patients, searching for distinct epigenetic patterns.

They found expected signals tied to known risk factors like tobacco use and diet, particularly in younger patients.

But they also looked at pesticides and uncovered a new association between early-onset colorectal cancer and exposure to the agricultural herbicide picloram.

Picloram has been in use since the mid-1960s, primarily to control woody plants and broadleaf weeds in pastures, forests and along industrial sites, like railways and roadsides.

It works by mimicking plant growth hormones, triggering uncontrolled growth that eventually kills plants from the roots up. The chemical can linger in soil for extended periods, staying active anywhere from several months to more than three years.

Because it has only been around for the past several decades, older patients diagnosed with colorectal cancer today would have had little to no exposure to it during childhood — unlike younger patients, who have likely been exposed for a larger share of their lives.

Researchers say that difference in long-term exposure could help explain why the disease struck at different ages in younger and older patients.

“Given such a clear signal, we decided to analyze the molecular characteristics of the tumors exposed to picloram in more detail,” Seoane said.

When the team took a closer look, they found that tumors linked to higher pesticide exposure had fewer mutations in the APC gene — a key gene that normally acts like a brake on cell growth in the colon.

When APC is damaged, cells begin to grow out of control and can form polyps that may later turn cancerous. This is often one of the earliest steps in colorectal cancer.

The fact that early-onset tumors with higher picloram exposure showed fewer APC mutations suggests the pesticide may be helping drive cancer through a different pathway.

The study is observational, meaning it can’t prove picloram is causing the rise in early-onset colorectal cancer. But it does give researchers a new lead to investigate.

If confirmed, the findings could point to another risk factor people may be able to reduce to lower their chances of disease.

“People are aware that diet is associated with cardiovascular diseases and smoking is associated with lung cancer,” Seoane told Medical News Today.

“Picloram is more complex, because if causal analysis confirms our results, very likely this should be regulated by governments.”

At present, picloram is not approved for residential use in the US.

Still, when researchers compared regions, they found that counties with higher picloram use also tended to have more colorectal cancer cases.

Across the country, the American Cancer Society estimates more than 158,000 new colorectal cancer cases will be diagnosed in the US this year, and over 55,000 people will die from the disease.

Nearly half of those diagnoses are expected to be in people younger than 65, a sharp shift from the 1990s, when the disease was largely confined to older adults.

To stay ahead of the trend, the American Cancer Society recommends starting routine screening at age 45.