It is now widely recognized that colorectal cancer (CRC) rates have been increasing in the U.S. in persons under age 50, while simultaneously decreasing in older persons. The latter effect seems largely the result of screening.
In a study of high-quality incidence data of CRC from 42 countries on 5 continents, there were 8 other countries (Germany, Australia, Canada, New Zealand, UK, Denmark, Slovenia, and Sweden) besides the U.S. with a similar pattern of rising rates in young people and declining rates in older people. Similarly, there were 3 countries (Cypress, Netherlands, and Norway) with rising rates in older adults but faster rising rates in young people.
A variety of potential explanations for increasing incidence rates in people under age 50 have been discussed, including the introduction of processed foods in the 1970s, the obesity epidemic, and early-life antibiotic exposure (altered fecal microbiota). The reasons remain unclear, but all of the countries in this study with patterns similar to the U.S. are high income, with potentially similar etiologic factors.
Siegel RL, Torre LA, Soerjomataram I, et al. Global patterns and trends in colorectal cancer incidence in young adults. Gut
2019 Sep 5. (Epub ahead of print) (http://dx.doi.org/10.1136/gutjnl-2019-319511