I’m frequently reminded that science doesn’t work that way. The latest case in point is an article published Tuesday in the Journal of the National Cancer Institute that sets out to explore a trend in colorectal cancer among younger Americans.
More than a decade ago, scientists noticed an odd quirk in the data: While overall rates of colorectal cancer have been falling dramatically since the mid-1980s, there’s been a steady uptick of this disease among people younger than 50.
The numbers are small. Cancer incidence is creeping up by 1 or 2 cases per 100,000 people under 50. By way of comparison, the disease rate among older Americans has plummeted by more than 100 cases per 100,000 people.
And the vast majority of colorectal cancer cases are among people over 50: These older Americans are 16 times more likely to get colon cancer, compared with adults who are younger. That’s why a small trend in younger adults is far outweighed by the dramatic decrease of disease among people over 50.
Still, the under-50s will eventually grow older. What will happen to their risk then?
Will the trends that started in their 20s and 30s continue? If that’s the case, overall colorectal cancer rates might ultimately end their steady decline, and could start to rise.
Another possibility is that, once people turn 50, they will follow the current medical guidance and get colonoscopies or other recommended screening tests, which can actually prevent colorectal cancer by finding and removing precancerous polyps. And their risk profile could end up looking much like it does today.
Here’s where even Poirot would be stumped. There simply isn’t enough information to know what will happen.
Epidemiologist Rebecca Siegel and her colleagues at the American Cancer Society have published their take in the JNCI. In their study, they break down the population into generational cohorts, focusing on millennials and members of Generation X.
By breaking down the cases by age group, Siegel says, it’s easier to disentangle generational changes such as differences in diet from trends in medical diagnosis and treatment, which vary less by age. Still, she and her colleagues can only say so much.
“[T]he results do not provide any direct evidence about the role of specific exposures or interventions,” they note in the study. Even so, the researchers say, because trends in the young “could be a bellwether of the future disease burden, our results are sobering.”
Siegel tells Shots, “It appears that under the surface, the underlying risk for this disease is actually increasing in the population.”
What’s driving that is hard to say. Obesity is more common among younger than it used to be, so perhaps it’s partly to blame.
Or it may not be obesity itself; it could be that poor diet and lack of exercise, which contribute to obesity, are also influencing colon cancer rates.