Why is the incidence of colon cancer rising in young adults?

In about 20% of people with early-onset colon cancer, a genetic condition is the underlying cause. However, most people diagnosed with early-onset colon cancer have no such condition.

Certain risk factors are common across all age groups, but early-onset colon cancer appears to have its own distinct risk profile, especially among people without genetic risk factors. These unique factors include:

• Gut microbiome changes. Studies show that people with colorectal cancer often have less variety in their gut bacteria compared with that of healthy people. Certain bacteria are found more often in people with cancer. These bacteria can make substances that damage DNA and cause inflammation.
• Antibiotic use. Using antibiotics, especially early in life, may disrupt the gut microbiome, increasing long-term cancer risk.
• Western diet early in life. High intake of sugary drinks and processed foods in adolescence and young adulthood has been linked to greater early-onset colon cancer risk.
• Sedentary behavior in youth. Sitting for long hours during adolescence and early adulthood — watching TV or working at a desk, for example — is strongly linked to early-onset colon cancer and rectal cancer.
• Certain genetic variants combined with environmental factors. Some people may have small changes in their genes that don't cause problems on their own. But when combined with factors such as a high-fat diet, these gene changes might lead to colon cancer. For example, one gene called HNF1A may only cause cancer if someone also eats a lot of foods that are not healthy.
• Lack of screening. Most people don't get checked for colon cancer until they turn 45, which is when doctors recommend starting screening for those at average risk. This means younger people usually aren't screened unless they have symptoms. But signs such as stomach pain or bleeding are often mistaken for something minor, so testing might be delayed. When cancer isn't found early, it can be harder to treat.