Cancer is a horrible diagnosis to receive at any age, but an apparent rise in the rate of cancers among young adults uncovered by a recent study is posing a concerning mystery epidemiologists are especially keen to solve.

Researchers have observed this worrying trend for some time, although they need to keep abreast of data to see how things might have changed, for better or for worse.

Last year, a review of three decades of global cancer data found adults under the age of 50 have increasingly developed cancer from the 1990s onwards.

Benjamin Koh, a physician-scientist at the National University of Singapore, and colleagues wanted to understand what has been happening more recently, specifically in the United States. The results of their new analysis echo the changes seen abroad.

Research shows young adult cancers are distinct from the kinds of cancers affecting the same organs in older adults – differences that influence treatment options – so we need to understand which cancers are affecting who, and how.

Aside from global trends, data on specific populations is useful to inform public health policies and research funding priorities. Overall, age remains the biggest risk factor for cancer, a disease of accumulating genetic mutations.

But now something is happening in younger age groups, and health experts aren't sure what.

Across many countries, but particularly the US, the rising rates of young adult cancers could be due to a range of factors: changing diets, lifestyles, and sleep patterns; increasing obesity, antibiotic use, and air pollution.

Teasing out trends is complicated by the fact that cancer screening programs are finding more cancers, hopefully earlier, while vaccination programs are preventing them too.

However, the 2022 international review suggests the rise in early-onset cancers has emerged over and above increased screening programs – which rarely include people under 50 anyhow.

Although this new study didn't account for the impact of these programs, in scouring existing data sources it has provided a comprehensive, updated overview of cancer rates in under-50s between 2010 and 2019 for the US.

Koh and colleagues identified a total of 562,145 young adults in 17 linked data registries that record new cancer diagnoses across different parts of the US, and used these records to estimate population-wide incidence rates over the decade to 2019.

Incidence refers to new cases diagnosed in a population over a period of time.

Overall, the incidence of cancers in under-50s rose such that an extra 3 cases were diagnosed per 100,000 people in 2019 compared to 2010.

Looking at specific age groups and the incidence of different cancer types is more revealing, and hints at underlying risk factors that might be contributing to the growing cancer burden.

"Gastrointestinal cancers had the fastest-growing incidence rates among all early-onset cancers," Koh and colleagues write in their published paper.

This included bowel cancer, the most common form of gastrointestinal cancer in young adults in 2019, and cancers of the appendix, bile duct and pancreas, rates of which surged in the decade studied.

People who you would think are in the prime of their lives, aged 30 to 39 years old, are actually increasingly affected by cancer too. The incidence of all cancers, but especially gastrointestinal cancers, rose in this age group when cancer incidence either remained stable or declined in older adults.

This growing burden of gastrointestinal cancers reflects the results of the 2022 international review, recent data from Australia, and other studies spanning multiple continents. Diets high in ultra-processed foods might have something to do with it.

Of course, these types of studies are only as good as the data available to them, so the results might not paint the whole picture.

"There could have been underreporting or underdiagnosis among underserved populations, such as Black individuals; hence, these results require cautious interpretation," Koh and colleagues conclude.

The study has been published in JAMA Network Open.