TARA HAELLE / National Geographic:

Colorectal cancer is often thought to affect older people, but one in five cases diagnosed today occurs in people younger than age 55, compared to one in 10 cases in 1995, according to a recent study published by the American Cancer Society. There’s no clear explanation for this trend, but a new paper just published in Science suggests a number of possible reasons, including environmental and genetic factors. Low screening rates and misdiagnosis in people who don’t suspect cancer likely play a role as well. “We’re coming to a point where we shouldn’t consider colorectal cancer a disease of only older adults,” said Andrew Chan, a professor of medicine at Harvard Medical School and vice chair of gastroenterology at Massachusetts General Hospital.
The findings also revealed an increase in diagnoses of advanced disease, which is particularly concerning because colonoscopies are “a great tool for prevention and early detection of colorectal cancer in terms of screening that can actually detect and remove precancerous lesions,” said lead author Rebecca Siegel, senior scientific director of cancer surveillance research at the American Cancer Society. Survival rates are 90 percent if detected early enough. The rising rates in younger adults led the U.S. Preventive Services Task Force to change its recommendation in May 2021 to begin screenings at age 45 instead of 50, but those with risk factors may need to start even earlier, said Siegel, who noted that nearly a third of colorectal cancers are associated with a family history of the disease. “Until we see these trends start to reverse, we’re going to have to continue to consider what appropriate strategies we need to take to really stem this increase in early onset disease,” Chan said. Identifying colorectal risk factors
Genetic risk scores may be helpful for identifying those who may be more likely to develop colorectal cancer at an early age but could be more effective if they took interaction with environmental factors into account, suggested Marios Giannakis, an oncologist at Dana-Farber Cancer Institute who coauthored the Science paper. The question is which environmental factors? Finding out requires the kind of long-term studies of large populations that are expensive and difficult to conduct, especially since they would be most useful if they included stool, blood, and tissue samples collected over time. Lifestyle factors seem an easy culprit for early onset disease at first, but the reality is more complicated. Excess body weight increases the risk of colorectal cancer, Siegel said, but only about 5 percent of colorectal cancers are attributed to excess body weight. Excess weight is also predominantly linked to tumors on the right side of the colon, not the left colon, which is where the cancer society found that the increases are occurring. Excess weight is also a bigger risk factor for men than women, yet the trend in younger adults is similar for all people. “Diet, obesity, and physical inactivity may be driving some of this increase, but it’s not the complete story,” Chan said.
“There are other contributors that remain to be uncovered, and I think it’s those factors on which we need to really focus our attention because they’re going to be things that may potentially have a greater impact in reducing incidence.” Giannakis’s paper notes that higher consumption of sugar-sweetened beverages, as well as red and processed meats are possible factors. Others include “antibiotics, more ubiquitous environmental toxins, and higher rates of Cesarean sections and other surgical procedures.” What all those factors have in common is an effect on the microbiome, the population of bacteria and other microorganisms that populate the human digestive system. Mark A. Lewis, director of gastrointestinal oncology at Intermountain Health in Utah, said early onset disease is at least “partly explained by antibiotic usage in childhood and young adulthood, as shown most convincingly,” in 2019 study from the United Kingdom. Don’t dismiss troublesome symptoms
It’s challenging to tease out how much increased mortality is due to greater risk factors versus low screening rates, particularly in rural or low-income areas, but it’s likely both, said Rishi Naik, an assistant professor of medicine in gastroenterology, hepatology, and nutrition at Vanderbilt University Medical Center. Screening gaps are evident in the fact that 27 percent of younger adults are diagnosed with advanced disease compared to 20 percent of older adults. Survival rates are similar across ages despite younger patients typically receiving more aggressive treatment and having fewer other conditions. “We fear this may also indicate a more aggressive biology for reasons that we need to understand,” Giannakis said, but it’s still not clear whether disease in younger people is more aggressive or just getting caught too late or both. Siegel’s paper noted that symptomatic patients under age 50 took 40 percent longer to receive a diagnosis compared to older patients.