Globally, the number of cancer cases diagnosed in adults under 50 has skyrocketed in recent decades, and a new study suggests diagnosis rates are rising too rapidly to be fully explained by better screening or earlier detection.
The incidence of several early-stage cancers, including tumors of the breast, colon, esophagus, kidney, liver and pancreas, has increased dramatically worldwide since 1990, researchers report in Nature Reviews Clinical Oncology.
“Based on our data, we have observed what is known as the birth cohort effect. This effect shows that each successive group of people born later—say, a decade later—have a higher risk of developing cancer later in life, likely due to risk factors they were exposed to when they were young ‘ senior study author Shuji Ogino, MD, PhD, a professor of pathology at Harvard Medical School and Brigham and Women’s Hospital in Boston, said in a statement.
That risk increases with each generation, added Dr. Ogino added. “For example, people born in 1960 had a higher risk of cancer before they turned 50 than people born in 1950, and we expect this level of risk to continue to increase in subsequent generations.”
For the study, the researchers first examined data on the global incidence of 14 different types of cancer, which were more common in adults under the age of 50 between 2000 and 2012. Then they reviewed previous studies of cancer risk factors to assess what factors might be contributing to increases in cancer rates in younger and middle-aged adults.
One clear trend emerged in their review: that dramatic lifestyle changes coincided with a steep rise in cancer cases among adults under 50. The researchers found that several risk factors that are modifiable early in life — such as what people eat, how much they exercise, and whether they maintain a healthy weight — can all play a role in cancer risk.
A so-called Western diet — lots of meat, potatoes, and processed foods — and a sedentary lifestyle could both explain why cancer is becoming so much more common in adults below retirement age, the study team concluded.
Increased screening rates and improved techniques for diagnosing and treating many types of cancer when tumors are less advanced and easier to treat have also contributed to rising cancer incidence in adults under 50, the study team wrote. However, cancer rates are increasing too rapidly to be fully explained by advances in early detection, they concluded.
Another possible explanation for rising cancer rates at a younger age is that the microbiome — the bacteria and microorganisms that live in our gut — has changed over time in a way that encourages tumor growth. Several risk factors for an unhealthy microbiome — such as eating lots of processed foods, drinking lots of alcohol and sugary drinks, inactivity, obesity and type 2 diabetes — have all increased since the 1950s, the study team points out.
“Of the 14 types of cancer we looked at, eight were linked to the digestive system. The food we eat feeds the microorganisms in our gut,” said the study’s lead author, Tomotaka Ugai, MD, PhD, also of Brigham’s Department of Pathology, in the statement.
“Diet directly affects the composition of the microbiome, and ultimately these changes can affect disease risk and outcomes,” added Dr. Ugai added.
The study was not a controlled experiment to prove whether or how certain risk factors could directly lead to an increase in cancer cases in younger adults.
A limitation of the analysis is that the researchers did not have enough data on cancer cases over time in low- and middle-income countries, making it impossible to assess truly global trends in cancer incidence over time. In addition, they did not have enough data on children to fully assess which specific life circumstances at a young age might be most responsible for the rising incidence of cancer among adults under 50 years of age.
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