In the past two decades, doctors have observed a troubling trend: colorectal cancer (traditionally associated with older adults) is steadily increasing in people under 50 years of age. This phenomenon has puzzled oncologists, epidemiologists, and scientists alike, as it occurs in individuals who do not present typical risk factors. What is changing in the bodies of young adults? A new line of research points to a silent suspect: the gut microbiome.

The gut microbiome is the collection of trillions of microorganisms (bacteria, viruses, and fungi) that inhabit the digestive tract. This ecosystem fulfills essential functions for human health: it helps digest food, synthesizes vitamins, educates the immune system, and even influences our mood. But in recent years, scientists have discovered that an imbalance in this microbiome, known as dysbiosis, can also contribute to the development of chronic diseases, including cancer.

Colorectal cancer in young people is not only increasing but often appears in more aggressive forms and at more advanced stages. Researchers have identified a possible connection between certain intestinal bacteria and the early onset of this cancer. A clear example is Escherichia coli that produces colibactin, a toxin that directly damages the DNA of intestinal cells. These genetic lesions can pave the way for cancer-causing mutations.

Other microorganisms such as Fusobacterium nucleatum have also been linked to colorectal cancer by promoting a chronic inflammatory environment and suppressing the antitumor immune response. These bacteria are not only present in greater quantities in cancer patients but have also been detected in metastases, suggesting an active role in tumor progression.

What is causing dysbiosis?

The modern lifestyle seems to have a direct impact on the balance of the gut microbiome. Diets low in fiber and high in ultra-processed foods, excessive consumption of refined sugars, indiscriminate use of antibiotics, and low levels of physical activity are all factors that alter the microbial diversity of the intestine. Added to this is early exposure to environmental pollutants, food additives, and increasing levels of chronic stress.

All of this not only affects digestive health but can also induce a state of persistent inflammation, weakening of the intestinal barrier, and activation of molecular pathways that promote carcinogenesis.

Figure 1. An imbalance in gut microbes (exacerbated by poor diet, antibiotics, and sedentary habits) promotes the presence of pro-carcinogenic bacteria like E. coli and F. nucleatum, which may contribute to early CRC through DNA damage and inflammation.

Additionally, one of the biggest obstacles in the fight against colorectal cancer in young people is the lack of clinical suspicion. Since it is not considered a common disease in this group, early symptoms (such as rectal bleeding, changes in bowel habits, or abdominal pain) are often ignored or attributed to less serious causes. As a result, many cases are diagnosed at advanced stages, when treatment is more complex and the chances of cure are lower.

For this reason, current recommendations have begun to change. In the United States, the starting age for screening tests (such as colonoscopy) has dropped from 50 to 45, and even earlier in people with a family history or persistent symptoms.

Although the exact causes are still being studied, the role of the microbiome offers a window of opportunity: it is modifiable. Unlike DNA, which we cannot change at will, the microbiome can be influenced by what we eat, how we live, and the decisions we make every day.

Some science-backed strategies include:

• Increasing fiber intake: fosters the growth of beneficial bacteria and reduces inflammation.

• Reducing ultra-processed foods: these foods negatively alter the diversity of the microbiome.

• Avoiding unnecessary use of antibiotics: helps preserve microbial balance.

• Engaging in regular exercise and managing stress: both have proven positive effects on the microbiota.

The rise of colorectal cancer in young adults is a complex phenomenon, but science is beginning to unravel its causes. The gut microbiome, once ignored, is emerging as a central piece of this puzzle. Taking care of our digestive health is not just a trend, it is a long-term investment in our digestive, immune, and, as we now understand, oncological health.

Main Reference: 

(S/f). Ascopubs.org. Recuperado el 4 de junio de 2025, de https://dailynews.ascopubs.org/do/more-than-gut-feeling-answer-rising-incidence-crc-young-adults-lies-within-microbiome?cid=DM21075&bid=498434827

Other References: 

Sinicrope F. A. (2022). Increasing Incidence of Early-Onset Colorectal Cancer. The New England journal of medicine, 386(16), 1547–1558. https://doi.org/10.1056/NEJMra2200869

Bray, F., Laversanne, M., Sung, H., Ferlay, J., Siegel, R. L., Soerjomataram, I., & Jemal, A. (2024). Global cancer statistics 2022: GLOBOCAN estimates of incidence and mortality worldwide for 36 cancers in 185 countries. CA: a cancer journal for clinicians, 74(3), 229–263. https://doi.org/10.3322/caac.21834

Sung, H., Siegel, R. L., Laversanne, M., Jiang, C., Morgan, E., Zahwe, M., Cao, Y., Bray, F., & Jemal, A. (2025). Colorectal cancer incidence trends in younger versus older adults: an analysis of population-based cancer registry data. The Lancet. Oncology, 26(1), 51–63. https://doi.org/10.1016/S1470-2045(24)00600-4