Since 1990, the number of adults under the age of 50 developing cancer has increased dramatically around the world.
What's concerning is that the increase in early-onset cancers doesn't seem to be slowing down – and improvements in screening alone don't seem to be able to fully explain the trend.
"We found that this risk is increasing with each generation," says one of the researchers, Shuji Ogino, a pathologist and epidemiologist at Brigham and Women's Hospital in Boston.
"For instance, people born in 1960 experienced higher cancer risk before they turn 50 than people born in 1950 and we predict that this risk level will continue to climb in successive generations."
It's not a groundbreaking notion that cancers are on the rise in modern society.
Researchers are already aware that since the 1940s and 1950s, there's been an increase in people getting late-onset cancer, which means developing cancer after the age of 50.
But what the team wanted to find out was whether early-onset cancer – or the rate of cancer in people under the age of 50 – was increasing too.
To do this, they needed to look at people born in the 1950s and 1960s but study their rate of cancer from the 1990s onwards.
The review looked at data across 14 cancer types: breast, colorectal (CRC), endometrial, esophageal, extrahepatic bile duct, gallbladder, head and neck, kidney, liver, bone marrow, pancreas, prostate, stomach, and thyroid cancer.
All of these cancers had been shown by global cancer data to be on the rise in adults under the age of 50 between the years 2000 and 2012.
But the researchers took things one step further and reviewed any available studies that could shed light on possible risk factors for these cancers.
They also looked for clues in the literature describing any unique clinical and biological characteristics of tumors of early-onset cancers, compared to those of late-onset cancers that are diagnosed after 50.
According to their results, the answer is yes. At least, this seems to be the case since the 1990s.
"The incidence of later-onset CRC (in those born in the late 19th and early 20th centuries) started to increase in the 1950s whereas that of early-onset CRC (in those born in the mid-20th century) did not start to increase until the early 1990s," the researchers write in the paper.
So what has changed to make those turning 50 after the 1990s more at risk of early-onset cancer?
One of the biggest changes is increased screening, which has undoubtedly contributed to the increased detection rates of early-onset cancers.
But the team notes that this on its own doesn't seem to be able to fully explain the change – particularly as some early-onset cancers are on the rise even in countries that don't have screening programs.
"A genuine increase in the incidence of early-onset forms of several cancer types also seems to have emerged," the team writes in the paper.
On top of simply being better at finding early-onset cancers nowadays, the evidence suggests that the 'shift' in cancer rates actually happened earlier, when those now in their middle ages were children, around the middle of the last century.
It's no secret that our lives changed a lot since then – particularly since the rise of highly processed foods – and the clues suggest that some combination of diet, lifestyle, weight, environmental exposures, and microbiome, are involved.
"Among the 14 cancer types on the rise that we studied, eight were related to the digestive system," explains epidemiologist Tomotaka Ugai from Harvard Medical School.
"The food we eat feeds the microorganisms in our gut. Diet directly affects microbiome composition and eventually, these changes can influence disease risk and outcomes."
Other risk factors include sugary beverages, type 2 diabetes, obesity, a sedentary lifestyle, and alcohol consumption, all of which have significantly increased since the 1950s.
Interestingly, while adult sleep duration hasn't drastically changed over the past few decades, children are getting a lot less sleep than they were decades ago, the team notes.
Of course, this study is far from conclusive. It's a review of existing studies. So the team wasn't able to make any changes here and directly measure the impacts.
They also didn't have much data from low- and middle-income countries to go on, but suggest that "the rise of early-onset cancers is likely to be increasingly prominent in those countries, potentially leading to a global early-onset cancer pandemic".
The team will now continue their work and hope to be able to set up longitudinal cohort studies going forward, which will involve young children being followed up over several decades.
"Without such studies, it's difficult to identify what someone having cancer now did decades ago or when one was a child," says Ugai.
The long-term hope is that we can educate people to lead healthier lifestyles in their early years, to reduce the risk of early-onset cancers.
But there's still a lot more work to be done to fully understand just how we got here, and where to go next.
The research has been published in Nature Reviews Clinical Oncology.