Sung and her colleagues used cancer diagnosis and mortality data from two databases –– the North American Association of Central Cancer Registries and the U.S. National Center for Health Statistics –– to analyze cancer trends in people born between 1920 and 1990, who were diagnosed with cancer between 2000 and 2019.
The data included 34 types of cancer, nearly 24 million diagnoses and more than 7 million deaths. To get a better view of how cancer diagnoses and mortality rates changed in groups of people born around the same year — called a birth cohort — the researchers grouped people by birth year in five-year intervals. For example, people born in 1920 through 1924 were all one birth cohort.
Seventeen of the 34 cancers had increasing incidence in younger people. The risk was two to three times higher in people born in 1990 for pancreatic, kidney and small intestine cancers, compared to people born in 1955. Liver cancer diagnoses in women followed the same pattern.
“The most important thing it tells us is there is something that changed for the group of individuals born after this period of time. They have been exposed to some environmental or lifestyle factor that is leading to this shift,” said Dr. Andrea Cercek, a gastrointestinal medical oncologist at the Memorial Sloan Kettering Cancer Center, who was not involved with the research.
After declining for decades, these types of cancers have begun to climb again:
- Colorectal
- Endometrial
- Non-cardia gastric
- Gallbladder
- Ovarian
- Testicular
- Anal
- Estrogen-receptor positive breast cancers
- HIV-linked cancer called Kaposi sarcoma
While the study found that mortality declined or was stable in younger generations for most cancers, mortality rate increased among younger age groups for endometrial, intrahepatic bile duct, gallbladder, colorectal and testicular cancers, as well as liver cancer among women.
Endometrial cancer was the fastest growing for both diagnoses and mortality.
“That was a sobering finding,” said Sung. “Although many cancer rates are rising, we don’t necessarily see this increase in mortality because we are treating them a lot better than before.”
Many of the cancers found to be on the rise are still rare in young people and, while rates have increased, the overall number of cases is comparatively low.
“It is clearly happening. Almost all the oncologists I know of can say they see it,” Brawley said, adding that despite the alarming increases, it’s important to keep in mind that most cancer diagnoses still happen in people older than 50 years.
“In the 1990s, 10% of people diagnosed with colon cancer were under age 50. Now it’s 20%, but we should not forget the 80% that are still over the age of 50,” he said.
Looking at people born within a specific time period can give important clues into why certain types of cancer are rising among younger generations.
“All of these cancers are linked to the obesity epidemic. We know that’s the second-leading cause of cancer right now, behind tobacco use,” said Dr. Otis Brawley, Bloomberg Distinguished Professor of Oncology and Epidemiology Johns Hopkins University, who was not involved with the new study.
About 20% of cancer diagnoses in the U.S. are linked to excess body weight, according to the American Cancer Society. Obesity rates in the nation changed little in the 1960s and 1970s but increased sharply after that. About 13% of adults had obesity in 1980, compared to 34% in 2008, according to data from the Surgeon General.
Among children, obesity rates grew from 5% to 17% in the same period. Today, more than 40% of American adults and about 20% of children and adolescents are obese, the Centers for Disease Control and Prevention reports.
If obesity is a culprit, it’s likely one of several lifestyle and environmental factors that is leading to the rise. Other factors could be more sedentary behavior or something in the food or the water, common medications or chemical exposures or chemical agents, the experts said.
The overuse of antibiotics is another possible link under scrutiny. Antibiotics are known to change the gut microbiome, which has been linked to colorectal cancer. While antibiotics are needed to treat many bacterial infections, they’re often misused and taken for issues that are not bacterial, or that do not require antibiotics.
“The list of things we are potentially investigating is very long,” Cercek said. “Antibiotics are one of the top culprits on the list.”
Researchers still don’t understand what’s behind the rise in certain types of cancers among younger generations. Although obesity and antibiotics are primary suspects, “we can’t rule out other chemical exposures or chemical agents,” Brawley said.
