Young people appear to be increasingly vulnerable to gastrointestinal cancers, but researchers aren’t entirely sure what is driving the surge in disease.

In a new study out this week, scientists led by the Dana-Farber Cancer Institute found that rates of early-onset GI cancers—those occurring in people younger than 50—are rising more rapidly than other types of cancer. In particular, colorectal cancer cases have significantly increased, while rates of other forms of the disease, like stomach cancer, are creeping up, too. While the paper doesn’t come to definitive conclusions as to why these conditions are becoming more common, the researchers point to risk factors like obesity, a poor diet, and sedentary behavior.

“Colorectal cancer is the most common early-onset GI cancer worldwide, accounting for more than half of the cases, but it is not the only GI cancer that is rising in younger adults,” said senior study author Kimmie Ng, director of the Young-Onset Colorectal Cancer Center at Dana-Farber, in a statement.

“Unfortunately, pancreatic, gastric, and esophageal cancers are also increasing in young people,” she added.

Ng and her colleagues reviewed data collected from 100 studies, as well as past scientific reviews, and cancer-related guidelines. They also analyzed three publicly available cancer statistic databases.

Since 2010, the incidence of early-onset GI cancers has been increasing globally, they found, but especially in high-income countries like the U.S. Between 2010 and 2019, the age-adjusted incidence rate in the U.S. increased by 2.16% annually—the greatest increase of any early-onset cancer during that time. In 2022, there were at least 25,000 Americans under 50 who developed GI cancer. The team’s findings were published Thursday in JAMA.

An earlier, separate review of data—by some of the same authors—over a similar time period concluded that the rate of GI cancer has tripled in young people aged between 15 and 19, and almost doubled in those aged between 20 and 24.

“The rising incidence of early-onset GI cancers is alarming and underscores the need for enhanced prevention strategies and early detection methods,” said Ng.

As the rates of these cancers have risen, so have some of their risk factors, including rates of obesity and time spent sitting; ultraprocessed foods are also more common now, especially in Western diets. Still, the researchers estimate that as much as 30% of early-onset GI cancer may be linked to genetic mutations. Other scientists have argued that other issues, like common stomach infections, could also be driving the increase in cancer rates.

Even as the causes remain unclear, what is clear is that more work needs to be done to help catch these cancers early enough, before they become impossible to treat, the researchers said. In 2020, public health experts recommended routine screening for colorectal cancer in people 45 and up, but the researchers noted that a year later, fewer than 20% of people ages 45 to 49 went for a screening. Another outstanding question is whether early-onset GI cancers are different from those caught later in life, and if so, should they be treated differently.

“We need to be thinking not only about the risk factors for these diseases but also how to screen, diagnose, and treat young people with these cancers,” said Ng.