Why Early-Onset Colon Cancer Is Rising — and What Researchers Still Can’t Explain

Early-onset colorectal cancer is rising sharply among younger adults in high-income countries, with incidence rates increasing most dramatically for people in their 20s and 30s. While researchers have theories — from changes in diet and lifestyle to early-life environmental exposures — the root cause remains elusive. This trend signals a broader shift in the timing and prevalence of chronic disease, with wide-ranging implications for healthcare systems, public awareness, and resource allocation.

Rates of early-onset colorectal cancer are climbing at a pace that has caught experts off guard. Between 2015 and 2019, the disease became one of the leading causes of cancer deaths among people under 50 in the United States, according to Yin Cao, M.Sc., from the Washington University Siteman Cancer Center. Nearly ten percent of new colorectal cancer cases globally now occur in adults under age 50, according to data from the National Cancer Institute (NCI). The surge is most pronounced in the youngest age brackets: adults in their 20s and 30s, population studies show.

“It’s the million-dollar question everybody is asking," said Dr. Y. Nancy You of MD Anderson Cancer Center in Texas. "But it’s a long-horizon answer.” When Dr. You was completing her medical fellowship in 2009, the average age of colorectal cancer diagnoses in the U.S. was 72, according to her team’s data. By 2025, that average had dropped to 67.

The trend is not confined to the United States. A global analysis published by the American Cancer Society in The Lancet Oncology shows incidence rates increasing in 27 of 50 countries examined. Countries like Australia, New Zealand, Puerto Rico, the U.S., and South Korea now report some of the highest rates of early-onset cases — reaching as high as 17 per 100,000 people. In New Zealand alone, early-onset incidence rates rose by four percent annually.

Scientists agree on some contributing factors: obesity, dietary shifts toward processed foods, increased alcohol consumption, and sedentary lifestyles have all been linked to higher risks of colorectal cancer. However, these do not fully explain the dramatic rise among young adults or the stark geographic differences. “For many of these [factors], there's no strong epidemiological evidence that they're [individually] linked to early-onset cancers,” said Ulrike Peters, Ph.D., of Fred Hutch Cancer Center.

One hypothesis gaining attention centers on early-life exposures. Emerging molecular research suggests that some of the DNA damage patterns seen in tumors of younger patients align with risks that may arise during childhood or even infancy. These exposures could include shifts in gut microbiome composition, dietary additives, or other subtle environmental toxins. For example, recent studies have identified DNA-damaging bacterial toxins — produced by certain strains of gut bacteria like E. coli — as potential contributors.

Microplastics, now pervasive in human environments and bodies, have also surfaced as a possible factor. Younger generations, growing up in a more industrialized and chemically saturated world, may face unique risks that earlier cohorts did not encounter. “The global scope of this concerning trend highlights the need for innovative tools to prevent and control cancers linked to dietary habits, physical inactivity, and excess body weight,” said Dr. Hyuna Sung of the American Cancer Society.

Public health responses have begun to adapt to this new reality. In 2021, the U.S. Preventive Services Task Force lowered the recommended screening age for colorectal cancer from 50 to 45. However, awareness efforts have been slower to keep pace. Young adults often overlook symptoms because they perceive colorectal cancer as an issue for older generations. "Raising awareness of the trend and the distinct symptoms of early-onset colorectal cancer (e.g., rectal bleeding, abdominal pain, altered bowel habits, and unexplained weight loss) among young people and primary care providers can help reduce delays in diagnosis and decrease mortality," Dr. Sung said.

The rise of early-onset colorectal cancer also fits into a broader pattern of younger people experiencing diseases previously associated with later life. Across high-income countries, researchers have noted earlier diagnoses for conditions ranging from obesity-linked type 2 diabetes to hypertension. This shift raises critical questions about how economic development, urbanization, and the accumulation of stressors over a lifetime may be reshaping disease dynamics.

The stakes are not just medical but also economic. Chronic conditions diagnosed earlier require longer-term care, putting added pressure on health systems already grappling with aging populations. Beyond healthcare costs, these trends could create ripple effects across workplace productivity, insurance markets, and even urban planning as policymakers adapt to higher disease burdens across generational cohorts.

The underlying causes of this trend will likely remain unclear for years. As Dr. You observed, there is unlikely to be a single "smoking gun." The answers will emerge slowly, potentially revealing complex interactions between genetics, environment, and lifestyle. For now, researchers like Dr. Yin Cao are focusing on large-scale epidemiological studies like PROSPECT, which aims to investigate cancer risks in younger adults through extensive data collection and multidisciplinary analysis.

But while researchers search for answers, the increase in early-onset colorectal cancer demands action now. "These rising rates are both an urgent warning and a call to prepare for a healthcare future that looks very different from the past," Dr. Peters said. The reality may force a reconsideration not just of specific screening guidelines but also of how public health approaches prevention in the context of a rapidly changing industrial and environmental landscape.