A new study published in Nature Medicine offers a striking glimpse into how lifetime exposures — from diet and tobacco to agricultural chemicals — might be leaving molecular footprints that help explain the troubling rise of colorectal cancer in younger adults.

Using patterns of DNA methylation, scientists have reconstructed exposure histories embedded in tumour tissue and identified links between known risk factors and early-onset disease.

They also report an unexpected association with a widely used herbicide, raising fresh questions about environmental contributors to cancer risk.

Colorectal cancer traditionally has been considered a disease of later life. That picture is changing. Incidence in adults under 50 has climbed steadily over recent decades, and in many regions the increase in younger adults is outpacing that seen in older populations.

In some countries colorectal cancer is now the leading cause of cancer death for men and women under 50. In Malaysia, colorectal cancer is ranked as the top cancer for men and second for women. It is the third leading cause of cancer death. Affecting 1 in 44 men and 1 in 62 women, it is frequently diagnosed at late stages, with the local Chinese population having a higher incidence.

Researchers in this study searching for explanations have turned to the exposome — the totality of environmental, lifestyle and occupational exposures accumulated across a lifetime — for answers.

Rather than attempting the near-impossible task of measuring every exposure directly, investigators took another route.

“The most novel and provocative result was an association between early-onset colorectal cancer and exposure to picloram, an agricultural herbicide”

They examined epigenetic marks in tumour DNA, specifically DNA methylation. These chemical tags do not alter genetic code. They modify how genes are read. Think of them as annotations in a book, they do not change the words but they change how those words are interpreted. Because methylation patterns change in predictable ways in response to particular exposures, they can serve as molecular records of past influences.

Scientists compared methylation patterns in colorectal tumours from people diagnosed before its usual age threshold and those diagnosed later in life. From those patterns, they developed epigenetic risk scores that reflect exposure histories. These scores captured signatures linked to dietary patterns, tobacco use and several environmental chemicals.

The results confirmed long-established associations, poor diet and smoking were apparent in the molecular profiles of early-onset tumours. Those findings bolster the idea that known lifestyle risks also drive cancers in younger people.

The most novel and provocative result was an association between early-onset colorectal cancer and exposure to picloram, an agricultural herbicide.

Using county-level data from the United States, researchers observed that regions with higher reported picloram use had higher rates of early-onset colorectal cancer. That correlation persisted after adjustment for socioeconomic factors and the presence of other pesticides.

Picloram is also commonly used in Malaysia, particularly as a herbicide in agricultural sectors such as oil palm, rubber, and cocoa plantations. It is utilised for controlling a wide range of broadleaf and grass weeds.

Tumours from areas of higher picloram use also showed distinct molecular features. For example, they had a lower frequency of mutations in the APC gene, a canonical driver of colorectal tumour formation. That pattern suggests the herbicide-exposed tumours may follow different developmental pathways from those driven predominantly by classic genetic mutations.

Caution is essential. Observational studies can reveal associations but cannot prove cause and effect. Epigenetic patterns imply exposure; they do not alone establish that a chemical caused the cancer. County-level pesticide usage is an imperfect proxy for individual exposure. Confounding factors may persist despite statistical adjustment. Researchers explicitly describe these findings as hypothesis-generating rather than definitive.

Even so, the study advances several important themes. First, it demonstrates the utility of epigenetic signatures as proxies for lifetime exposures. Methylation patterns can be mined to reconstruct individual histories of diet, smoking and chemical contact. That approach offers a pragmatic way to study the exposome without decades of prospective exposure measurement.

Second, the findings highlight heterogeneity in the molecular genesis of early-onset colorectal cancer. Not all tumours arise through the same pathways; environmental drivers could be responsible for distinct molecular subtypes.

Third, the results underscore the preventive potential of modifiable behaviours. The epigenetic evidence that both unhealthy diets and tobacco use are linked to early-onset disease strengthens public-health messages on diet quality and smoking cessation.

How convincing are the data linking picloram to cancer risk?

The association was the strongest among the pesticides examined. In methylation analyses, signatures corresponding to picloram exposure ranked highly when distinguishing early-onset tumours from later-onset cases.

“Smoking cessation and improved dietary patterns are likely to reduce colorectal cancer risk at any age. Those remain the simplest, most evidence-based steps individuals can take today”

County-level use data corroborated that signal, places where picloram was applied more frequently also tended to report higher early-onset incidence. Moreover, the tumours in those counties showed molecular distinctions. Taken together, the converging lines of evidence make a compelling case for further investigation.

Still, several limitations merit emphasis. The epigenetic signatures are derived from tumour tissue. Tumour methylation is shaped by both the initiating exposures and by processes that accompany tumour growth. Some methylation changes may be consequences of the cancer rather than antecedent causes.

Use of regional pesticide data cannot capture personal exposure pathways, protective measures, or indoor versus outdoor contact. Temporal relationships are also crucial. To support a causal role, exposure must precede disease by an appropriate latency period. Ecological analyses at the county level struggle to establish that temporality.

Finally, while picloram was the standout chemical in this dataset, other unmeasured exposures may co-vary with its use and confound apparent associations.

The study prompts clear next steps. Laboratory work should test whether picloram or its breakdown products alter colorectal epithelial cells in ways that promote tumour initiation or progression. Animal models could examine dose–response relationships and identify mechanisms. Human studies are also needed. Prospective cohorts with detailed exposure assessment and stored biospecimens could validate whether pre-diagnostic methylation changes tied to picloram predict future cancer risk.

“For now, widely accepted measures remain vital, awareness of symptoms, prompt medical evaluation for concerning signs such as rectal bleeding or unexplained weight loss, and adherence to age-based screening guidelines where applicable”

Geospatial analyses that incorporate finer-scale exposure measures, such as proximity to treated fields or biomonitoring data, would strengthen causal inference. Finally, regulatory reviews could reassess safety data for picloram in light of new mechanistic and epidemiological evidence.

Beyond picloram, the study reinforces a broader public-health message. Lifestyle choices matter. The molecular traces of smoking and poor diet in early-onset tumours align with decades of epidemiology linking these behaviours to colorectal cancer.

Interventions that promote healthy eating, reduce processed-meat intake, increase fibre consumption, and eliminate tobacco use remain cornerstone prevention strategies. For policymakers, the findings add impetus to actions that reduce harmful environmental exposures where feasible, and to strengthen surveillance of emerging cancer trends.

The study also carries implications for clinical practice and screening policy. If environmental exposures create distinct molecular subtypes, clinicians might in the future personalise risk stratification based on an individual’s epigenetic exposure profile.

That could refine decisions about when to start screening in people under 50 or which screening modalities to favour. For now, widely accepted measures remain vital, awareness of symptoms, prompt medical evaluation for concerning signs such as rectal bleeding or unexplained weight loss, and adherence to age-based screening guidelines where applicable.

Communication about the findings should avoid alarmism while taking concerns seriously. An association is not a verdict of causation. Individuals exposed to picloram should not assume imminent danger, but they should press for more robust research and stronger protections if future studies corroborate the risk.

Agricultural workers and communities near treated land might benefit from improved protective measures, better monitoring, and, where warranted, regulatory review of application practices.

This study is a call to action on multiple fronts. Scientists should pursue mechanistic confirmation of suspected environmental hazards. Public-health agencies should monitor young-onset cancer trends closely and consider environmental determinants alongside lifestyle and screening policies.

Clinicians should maintain a high index of suspicion for colorectal cancer in younger patients with relevant symptoms and continue to advocate for modifiable-risk reduction. Regulators should be prepared to review pesticide effects in light of new evidence.

One clear takeaway for the public, established risks remain relevant. Smoking cessation and improved dietary patterns are likely to reduce colorectal cancer risk at any age. Those remain the simplest, most evidence-based steps individuals can take today. The discovery of a novel association with an herbicide underscores the need for vigilance beyond personal behaviour. It highlights how the places we live and the products we use can shape cancer risk in ways that are only now becoming apparent.

Science moves in steps. This work does not deliver definitive answers. It points. It signals where more rigorous tests must follow. Those tests will determine whether a widely used herbicide really plays a role in early-onset colorectal cancer, and whether policy changes are needed to protect public health.

Meanwhile, the study illuminates the hidden history of exposures that may be shaping the modern cancer landscape.