The enigma of colorectal cancer: Why is it rising among young people and falling among the elderly?

Experts from half the planet have sounded the alarm about an unsettling phenomenon. Cases of colorectal cancer are decreasing in elderly people, but they are increasing worldwide in people under 50. The rate has increased at an “alarming” speed of 3% per year in many countries, with even more rapid increases in people under 30, according to two top specialists, Kimmie Ng and Marios Giannakis, from the Dana-Farber Cancer Institute in Boston. The causes of this increase can be intuited but are yet to be confirmed.

Oncologist Kimmie Ng recounts what she observes each day in the hospital. “For many years, we’ve been seeing more and more very young patients arrive with metastasized colorectal cancer, many of them without genetic predisposition or obvious risk factors,” she says. They are men and women in their twenties, thirties or forties who have had vague symptoms —like constipation or diarrhea— for months, without anyone suspecting cancer at their age. The doctor considers the situation “extremely concerning” and urges the scientific community to come together to understand what is happening. Her call to action was published this Thursday in Science.

Colorectal cancer causes almost a million deaths every year. It is the second most lethal cancer in the world, behind lung tumors. The team of Rebecca Siegal, Senior Scientific Director of Surveillance Research at the American Cancer Society, alerted in 2019 that the cancer was increasing in people under 50 in at least 19 countries. In a dozen of them, the tumors decreased in people over 50, but increased rapidly in young adults, as in New Zealand (a 4% annual increase), the United Kingdom (3.3%), Canada (2.8%), Australia (2.8%), the United States (2.2%), Sweden (1.6%) and Germany (1.3%).

The two researchers from the Dana-Farber Cancer Institute recognized that the exact reasons behind the phenomenon “are unknown,” but they list their main hypotheses. “We suspect that environmental risk factors, like diet and lifestyle, contribute,” Kimmie Ng explains. “We have identified obesity, sedentary lifestyles, the increase in consumption of sugary beverages and the deficiency of Vitamin D as risk factors,” she adds.

The biotechnologist Cayetano Pleguezuelos mentions another possible cause: gut bacteria. “From my perspective, the most suspicious factor is the change in the intestinal microbiota,” says the researcher at the Hubrecht Institute in the Netherlands. His team was the first to demonstrate a direct connection between those bacteria and DNA damage in human cells that leads to cancer.

Pleguezuelos explains that fast food changes the composition of the bacteria in the intestine, but he recognizes that scientific understanding of the topic is just beginning. “We still don’t know exactly what factors contribute to these changes in the microbiota, nor what bacterias are linked to cancer,” he admits. The biotechnologist collaborates with Kimmie Ng in an international consortium, called Optimisticc, to investigate the impact of intestinal bacteria on colorectal cancer. In October, a group from Yale University indicated that the bacteria Morganella morganii, common in the human intestine, could be a cause of these tumors.

Oncologists Kimmie Ng and Marios Giannakis predict that early-onset colorectal cancer will become the main cause of cancer deaths in people between 20 and 50 years old by 2030. The authors recommend reinforcing early-detection programs for young adults, as has already been done in the US, where it is recommended to test for blood in the feces of patients over 45.

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