Cancer diagnosis rates among older adults have declined, but there has been a considerable spike in certain types of cancers among young adults in the U.S., especially in young women, a new study has found.

In the latest government-funded study, which involved 17 National Cancer Institute registries, researchers observed a concerning trend of increasing cases of gastrointestinal, endocrine and breast cancers in people younger than 50.

The study evaluated data from around 560,000 patients in the U.S. who were diagnosed with early-onset cancer (cancer before 50 years) between 2010 and 2019. The findings of the study were published in the Jama Network journal.

Researchers found that overall early-onset cancer diagnoses spiked by around 1% during the period, while the rate was around 19% for people in the 30-39 age group.

"This is a population that has had less focus in cancer research and their numbers are getting bigger, so it's important to do more research to understand why this is happening," Paul Oberstein, director of the Gastrointestinal Medical Oncology Program at NYU Langone's Perlmutter Cancer Center, told Washington Post. Oberstein was not involved in the study.

Breast cancer accounted for the highest number of cases, while the rate of increase was highest (15%) for gastrointestinal cancers, including the colon, appendix, and bile duct cancers.

"Although breast cancer had the highest number of incident cases, gastrointestinal cancers had the fastest-growing incidence rates among all early-onset cancers. These data may be useful for the development of surveillance strategies and funding priorities," the researchers wrote.

Early-onset cancers in women increased by 4.4%, while the rate declined among men by nearly 5%. Researchers also found that the increase in early-onset cancers was highest among American Indians or Alaska Natives, Asians and Hispanics, while the growth rates remained stable in white people and decreased among Black people.

Although the exact reason behind the spike in cancer cases among young adults is not known, experts believe factors such as obesity, alcohol consumption, smoking, improper sleep and a sedentary lifestyle play a role. Exposure to pollutants and carcinogenic chemicals also contributes to the increase in cases.