The incidence of several early-onset cancers, including tumors of the breast, colon, esophagus, kidney, liver, and pancreas, has dramatically increased around the world since 1990, researchers report in Nature Reviews Clinical Oncology. “From our data, we observed something called the birth cohort effect. This effect shows that each successive group of people born at a later time — for example, a decade-later — have a higher risk of developing cancer later in life, likely due to risk factors they were exposed to at a young age,” 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. This risk is increasing with each generation, Dr. Ogino added. “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.” For the study, researchers first examined data on the global incidence of 14 different cancer types that became more common among adults under 50 between 2000 and 2012. Then, they reviewed previous studies of cancer risk factors to assess what factors might be contributing to surging cancer rates in younger and middle-aged adults. One clear trend emerged in their review: that dramatic changes in lifestyle have coincided with steep spikes in cancer cases among adults under 50. Researchers found that several modifiable risk factors early in life — like 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 — heavy on the meat, potatoes, and processed foods — and a sedentary lifestyle may both help explain why cancer is becoming so much more common in adults that haven’t yet reached 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 combat have also contributed to rising cancer incidence in adults under 50, the study team wrote. But cancer rates are rising too fast to be completely explained by advances in screening, they concluded. Another possible explanation for surging cancer rates at younger ages is that the microbiome — the bacteria and microorganisms living in our gut — has changed over time in ways that fuel tumor growth. Several risk factors for an unhealthy microbiome — like eating lots of processed foods, drinking lots of alcohol and sugary beverages, inactivity, obesity, and type 2 diabetes — have all increased since the 1950s, the study team points out. “Among the 14 cancer types on the rise that we studied, eight were related to the digestive system. The food we eat feeds the microorganisms in our gut,” lead study author Tomotaka Ugai, MD, PhD, also of the pathology department at Brigham, said in the statement. “Diet directly affects microbiome composition and eventually these changes can influence disease risk and outcomes,” Dr. Ugai added. The study wasn’t a controlled experiment designed to prove whether or how any specific risk factors might directly cause a spike in cancer cases among younger adults. One limitation of the analysis is that researchers lacked sufficient data on cancer cases over time in low-income and middle-income countries, making it impossible to assess truly global trends in cancer incidence over time. Beyond this, they didn’t have enough data on children to fully assess what specific lifestyle circumstances early in life might be most responsible for rising cancer cases among adults under 50.
