October 23, 2018, 05:38 pm Chris Crawford – New cases of colorectal cancer (CRC) have been increasing in recent years among people younger than age 50, and the cause of this upturn is largely unknown.
However, there's been speculation that the drivers of the increase in incidence of early-onset CRC could be tied to risk factors that include obesity, sedentary lifestyle, Western diet and diabetes.
A study published early online Oct. 11 in JAMA Oncology may have shed some light on the issue, finding a connection between obesity and an increased risk of early-onset CRC among women younger than age 50.
Researchers used data from the Nurses' Health Study II (NHSII), a prospective, ongoing cohort study of U.S. female nurses ages 25-42 at study enrollment in 1989.
This study included detailed assessments of body weight, family and endoscopy histories, lifestyle factors and other potential CRC risk factors to examine the association between obesity and CRC diagnosed before age 50.
More than 85,250 women who were free of cancer and inflammatory bowel disease at enrollment were included in this analysis, with follow-up through Dec. 31, 2011. Validated anthropomorphic measures and lifestyle information were self-reported biennially.
Statistical analysis was performed from June 12, 2017, to June 28, 2018.
Researchers found 114 cases of early-onset CRC (median age 45 at diagnosis) among the women studied.
Higher current body mass index (BMI), BMI at age 18 and weight gain since early adulthood were associated with increased risk of early-onset CRC. Overall, obesity (BMI of 30 or higher) was associated with the highest risk.
Compared with women who had had a current BMI of 18.5 to 22.9, the multivariable relative risk (RR) was 1.37 for overweight women (BMI of 25-29.9) and 1.93 for obese women (BMI of 30 or higher). The RR for each five-unit increment in BMI was 1.20.
Similar associations were observed among women without a family history of CRC and without lower endoscopy within the past 10 years.
In addition, compared with women with a BMI of 18.5 to 20.9 at 18 years of age, the RR of early-onset CRC was 1.32 for women with a BMI of 21 to 22.9 and 1.63 for women with a BMI of 23 or greater at 18 years of age. Compared with women who had gained less than 5 kg or had lost weight, the RR of early-onset CRC was 1.65 for women who gained 20 to 39.9 kg and 2.15 for women who gained 40 kg or more.
Family physician Richard Wender, M.D., chief cancer control officer for the American Cancer Society (ACS) and chair of the National Colorectal Cancer Roundtable, told AAFP News these findings didn't shock him.
"The link between being overweight and obesity and CRC is well established -- and roughly at the risk ratios reported in this study," he said. "The fact that this association is seen in individuals under age 50 is not surprising."
Rates of overweight and obesity started to increase in the 1970s, Wender said. Prior to that, he said, obesity rates were around 12 percent; today, they're around 35 percent.
"If you think about it, it actually would be surprising to find that rising obesity rates were not leading to some increase in CRC rates," he said. "This study shows that a higher risk (for CRC) begins before age 50."
When asked if his expectations would change if men were the focus of this study instead of women, Wender said he'd predict a similar finding.
Wender added that although it cannot be assumed that this early-onset CRC study would translate to other groups, he suspects there would be similar findings with all subgroups and ethnicities.
These findings also bolster the argument that patients who are overweight or obese should eat healthier and exercise more to avoid increased risk for early-onset CRC, he said.
However, Wender pointed out that although this is an important finding, people shouldn't jump to the conclusion that diet, exercise and obesity fully explain the spike in CRC in younger people.
"In fact, I think this is highly unlikely," Wender said. "This will explain some of the increase (in early-onset CRC), but it's very unlikely to explain all of the increase, which has been rapid and significant. There are very likely other factors, and we need to keep looking."
Wender said it's very likely this higher risk for CRC will continue after age 50, but there's another factor to consider.
"Screening is preventive -- finding and removing polyps," he said. "So, someone might be at higher risk, but screening might negate that risk."
The ACS updated its CRC screening guideline on May 30 to recommend screening begin at age 45 for patients at average risk.
The U.S. Preventive Services Task Force (USPSTF) currently recommends screening for CRC beginning at age 50 and continuing through age 75. This recommendation for regular screening in adults ages 50 and older is designated as a "strong recommendation" based on the quality of the evidence supporting it.
The AAFP's own CRC screening recommendation agreed with that of the USPSTF in recommending that patients ages 50-75 be screened. But the Academy graded its recommendation a "B" and differed from the task force by offering a preferential recommendation for three screening tests supported by the best evidence available: fecal immunochemical tests, flexible sigmoidoscopy and colonoscopy.
When the ACS updated its screening recommendation, Jennifer Frost, M.D., medical director for the AAFP's Health of the Public and Science Division, told AAFP News it was important to note that the ACS' recommendation to screen for CRC starting at age 45 was not based on clinical trials.
"The ACS made this recommendation based on the increasing incidence of colon cancer in younger adults, with the assumption that screening would have similar benefits and harms in this age group as it does in older adults," she said at the time. "They assumed that colon cancer in younger adults is similar to cancer that occurs in older adults, and that screening would have the same benefits and no additional harms."
Also, the USPSTF pointed out that if the recommended age for screening were lowered to 45, there would be an increase in the lifetime number of colonoscopies patients would receive, with the associated risk of harms.
"Because of this concern, and the lack of empirical evidence in younger populations, the USPSTF recommended starting screening at 50," Frost said.
Frost recommended individual family physicians have a conversation with their patients about whether earlier screening was appropriate.
"In the meantime, we will focus our efforts on increasing screening rates in individuals ages 50-75 and helping patients reduce their modifiable risks for colon cancer, which include tobacco and alcohol use, excess body weight and unhealthy dietary choices," she concluded.
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