Colon cancer is rising in young Americans. It’s not clear why

omething strange is happening in the United States in the long war on cancer. Although progress has been substantial in lowering the overall death rate from cancer, deaths due to some types of cancer have increased among people younger than 50.

Colorectal cancer is one of the drivers of this trend. In the past three decades, incidence of the disease has risen significantly among people younger than 50, many of whom have no obvious risk factors, such as having a genetic predisposition. No one knows why.

American life expectancy trails that of similarly developed nations, and the gap is widening. The dismaying reality is that multiple factors are taking the lives of people who have not yet reached a ripe old age. Colorectal cancer is a tiny element in that complex story, but the recent rise in the disease among seemingly healthy young people is a reminder that the health landscape is constantly evolving in ways not readily understood by medical science.

A report released early this year by the American Cancer Society found that people younger than 55 went from accounting for 11 percent of all colorectal cancer in 1995 to 20 percent in 2019. About 3,750 people younger than 50 will die of colorectal cancer in 2023, according to the report.

The rise in early-onset colorectal cancer is driven primarily by cancer forming on the patient’s left side, in the lowest portion of the colon or the adjacent rectum, said Rebecca Siegel, senior scientific director of surveillance research for the American Cancer Society. These cases tend to be more advanced than cancers detected in older people.

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