Why is colorectal cancer rising and what does it mean for Black, Brown communities?

Colorectal cancer is a leading cause of cancer death among people under 50 in the United States, with rates of new diagnoses still climbing in this age group. The staff at the National Cancer Institute shared their logic and reasoning for this phenomenon.

Rising Rates around the World

Although relatively rare, nearly 18,000 people under the age of 50 will be diagnosed with colorectal cancer this year in the United States, said Rebecca Siegel, M.P.H., of the American Cancer Society.

Black people are more likely to get colorectal cancer at a young age than White people, even though the gap is shrinking, said Nathan Ellis, Ph.D., of the University of Arizona Cancer Center. The recent spike has been seen among Alaska Native, American Indian, and White people.

According to Jeffrey K. Lee, M.D., of Kaiser Permanente Northern California cases of colorectal cancer in younger adults have been documented in the United States, Canada, New Zealand, Australia, and some parts of Europe and Asia. In most of these places, the number started trending upward around 1995.

Causes of Colorectal Cancer in Young Adults

Experts don’t know exactly what’s causing the jump in colorectal cancer among young adults. But some factors that they believe raise the risk of colorectal cancer in older adults, including obesity, physical inactivity, and smoking.

“Some of those [risk factors] have become more common over the last 45 years, along with this rise in early-onset cases,” said Phil Daschner, a program director in NCI’s Division of Cancer Biology. So, it’s possible that some of the same factors are responsible for the rise of early-onset disease, he noted.

On the other hand, there may be a set of unique risk factors for colorectal cancer in younger adults that researchers haven’t yet identified, he added.

Only 10% to 20% of early-onset colorectal cancers are caused by inherited factors, explained Kimmie Ng, M.D., of Dana-Farber Cancer Institute. Certain genetic conditions—like Lynch syndrome and familial adenomatous polyposis—have historically raised the likelihood.

When the incidence of a disease changes by generation, that suggests the culprit is something in the environment, rather than something biological, Dr. Ng added.

Diet, Gut Bacteria, and Inflammation

Most of the discussions about the possible causes of early-onset disease focused on three factors: diet, bacteria in the gut, and inflammation.

There’s evidence linking an unhealthy diet—particularly one high in processed meat and fat, and low in fruits and vegetables—to early-onset colorectal cancer.

Several studies have found that being overweight or obese may raise someone’s chance of getting early-onset colorectal cancer. Using data from electronic health records, Nathan Berger, M.D., of Case Comprehensive Cancer Center, found that half of younger adults with colorectal cancer were overweight and 17% were obese.

Some scientists have turned their focus to bacteria that live in the gut, also called the gut microbiome. Certain types of bacteria have been marked as accomplices in the growth and spread of colorectal cancer, and some may affect how well certain cancer treatments work.

In lab studies, toxins from several types of bacteria that are normally found in the human gut caused cancer in the intestines of mice, explained Cynthia Sears, M.D., an infectious disease expert from Johns Hopkins University.

Unhealthy diets and gut bacteria are connected in another way, too, they can lead to inflammation. In one study of mice, a high-fat diet triggered gut inflammation and accelerated the growth of tumors in the intestines.

As for gut bacteria, some bacterial toxins intensify inflammation, Dr. Sears noted. Studies have also shown that certain gut bacteria can recruit immune cells that help cancer grow, as well as block immune cells that fight cancer. Inflammation can also generate harmful chemicals that can mutate DNA and promote cancer, explained Dr. Ng.

Chemicals in the Environment

Scientists are also examining factors in the environment as potential causes of early-onset colorectal cancer. Factors include things like air and water pollution, chemicals in soil and food, and pesticide use.

The National Toxicology Program, led by NIEHS, has identified 18 chemicals that cause cancer in the intestines of mice or rats, said NIEHS Director Rick Woychik, Ph.D., who also heads the National Toxicology Program. Some of these chemicals might damage DNA, potentially leading to harmful mutations in cells of the colon and rectum.

Other chemicals may have more indirect effects, pointed out Barbara Cohn, Ph.D., M.P.H. of the Public Health Institute. Mixtures of certain environmental chemicals (sometimes called endocrine disrupters and obesogens) can disrupt the body’s metabolism, leading to obesity, she said. Even though some of those chemicals are now banned, their prior use could have effects later in life for people who were born back then, Dr. Cohn explained.

In addition, some environmental chemicals may have harmful effects on the complex assortment of bacteria in the gut, Dr. Woychik noted.

People are exposed to many chemicals at the same time, some of which may interact in different ways, he added. So, it’s important to consider all of an individual’s environmental exposures over the course of their life, including exposures in the womb, said Dr. Woychik.

Informing Approaches for Prevention and Treatment

Defining the causes and risk factors for early-onset colorectal cancer will likely help inform approaches for prevention, screening, and treatment, Daschner said.

For instance, health care professionals could recommend lifestyle changes or more frequent screening tests to people who are at higher risk of developing colorectal cancer at a young age, due to their exposure.

A few medical organizations have lowered the recommended age to start colorectal cancer screening from 50 to 45. For anyone younger than 45, tailoring colorectal cancer screening approaches to each person based on their risk factors (otherwise known as precision screening) may improve the efficiency and cost-effectiveness of screening, said Dr. Lee.

Understanding the causes and risk factors will also help scientists uncover the underlying biology of early-onset colorectal cancer. Specifically, it can help scientists pinpoint specific molecules that drive the growth of colorectal cancer in young people. Which, in turn, could discover new ideas for colorectal cancer screening and treatment.

For example, some screening tests check for specific molecules made by colorectal cancer or polyps (growths that could turn into cancer). Knowing which molecules are key to the growth of early-onset tumors could help researchers design screening or diagnostic tests that are designed for younger adults. It could also help them develop treatments that target those key molecules (an approach known as targeted therapy).

Read on for More at the National Cancer Institute website.