The gut might tell the story of rising colorectal cancer in young adults
The gut might tell the story of rising colorectal cancer in young adults
By Zehra Esra Ilhan
**Many news outlets have a reported a huge rise in early-onset colorectal cancer among Millenials and Gen Z. Maybe we can provide some context to this; bacteria that causes this, best practices to prevent. **
Please note: This article is not meant to be health advise but rather a microbiology perspective on a growing health concern* *
For decades, colorectal cancer (CRC) was considered a disease of aging. But that narrative is changing fast. Over the past twenty years, CRC has been showing up in Millennials and Gen Z. Despite this alarming trend, most screening programs still don’t begin until the age of 45 or 50. Even more troubling, young patients are often diagnosed at later stages, and their tumors are primed for malignancy.
Some of these patients fit the ‘classic’ risk profile: obesity, family history, diets heavy intake of red and processed-meats, alcohol, smoking, and sedentary lifestyles. But many don’t and still face the disease. That has scientists looking beyond the usual suspects and toward an overlooked but powerful player: the gut microbiome.
The idea that microbes might influence CRC isn’t new. Back in 1974, by studying CRC in germ-free and conventionally raised rats, scientists provided the first hint that gut microbes mattered in the development of CRC. Decades later, the sequencing revolution turned that hint into a revelation. Profiling stool samples revealed clear microbial shifts along the progression from precancerous adenomas to carcinomas. These observations sparked a bold question that whether gut microbes could serve as biomarkers of CRC.
Several microbial suspects have since been tied to CRC. Among those, Fusobacterium nucleatum kept stealing the spotlight. Normally, an oral microbe linked to gum disease, it somehow migrates to the colon and nests inside tumors. Once there, it activates tumor growth pathways and also suppresses immune defenses. Other suspects include Bacteroides fragilis, Escherichia coli, and several Streptococcus species. Not every strain is harmful but the ones found in CRC are either producing enterotoxins, genotoxins such as colibactin, or inflammation triggering molecules that all create fertile ground for cancer. What we know about CRC microbiome is not just about the ‘bad guys’. CRC patients also tend to be depleted of beneficial microbes like Eubacterium rectale. These species are known butyrate producers that nourishes colon cells and help maintain a healthy gut environment. The loss of these protective species leaves the colon more vulnerable to pathogens.
Here's where things get even more interesting: the gut microbiome in younger people with CRC doesn’t look exactly like that of older patients. Sure; both groups show reduced within microbial community diversity, but younger patients seem to carry their own unique microbial fingerprints. One striking example is Flavonifractor plautii that is more often popping up in younger patients (Kaur et al. 2020; Kong et al. 2023) while F. nucleatum tends to be linked with older age CRC. What makes F. plautii intriguing is that it is well-known for its flavonoid degradation properties. These plant-based compounds have been studied for their anticancer potential due to their antioxidant, anti-inflammatory, and anti-angiogenesis effects. In fact, Gupta and colleagues pondered on the idea that whether having more F. plautii in CRC patients could mean losing some of the protective effects of flavonoids, in particularly in populations that have traditionally flavonoid rich diets as found in India.
These microbial fingerprints extend into metabolism too. Early-onset CRC was associated with higher tryptophan, bile acid, and choline metabolism, whereas late-onset CRC showed a depletion of colon health promoting short chain fatty acids and GABA biosynthesis. In short, younger patients are not only developing CRC differently but also harboring fundamentally different microbial and subsequently metabolic landscape.
So why now? Why this surge among younger generations? The leading hypothesis points to the way our modern world reshapes the microbiome. Diets have shifted from fiber-rich whole foods toward ultra-processed, high sugar alternatives. Overuse of antimicrobials, antibiotics, and other drugs and their accumulation in the environment directly and unintentionally increased our exposure levels. Together, these pressures can push the microbiome into a pro-inflammatory, barrier-weakening, or carcinogen/mutagen-producing state. That shift may be setting the stage for CRC decades earlier than expected.
Colorectal cancer is no longer just an ‘older person’s disease’. While the story is still unfolding, the microbiome is emerging as a key player. And that means the answers to prevention, early detection, and treatment may come from listening to the gut. Because in the end, microbes may be pulling more strings in our health than we ever imagined.
About the author:
Zehra Esra Ilhan is a microbiome researcher at the Micalis Institute, National Research Institute for Agriculture, Food, and Environment (INRAE) in France. She explores how diet, medications, and environmental exposures reshape our gut microbes and influence health. Her work aims to turn microbiome insights into real-world solutions for prevention and precision medicine. She is also interested in science communication and has been serving as social media editor for FEMS Microbiology Reviews journal since 2022.