Atlanta, Georgia - New research shows obesity may be contributing to increasing rates of colorectal cancer among younger Americans. A study published October 11, 2018 in JAMA Oncology tracked the health of more than 85,000 women for 22 years and found the higher a woman’s body mass index (BMI), the greater her risk of developing colorectal cancer before age 50.

Last February, a major analysis led by American Cancer Society researchers found that new cases of colorectal cancer and deaths from colorectal cancer in the US had occurred at a growing rate among adults under the age of 55. This trend is expected to continue, and the analysis prompted the ACS to update its guideline for colorectal cancer screening. The new guideline recommends screening begin at age 45 for people at average risk. Previously, the guideline recommended screening begin at age 50 for people of average risk.

The new obesity-related study included data from women enrolled in the Nurses’ Health Study II, who were ages 25 to 42 years in 1989. This was an observational study, designed to show an association, but not intended to prove cause and effect.

Health experts have long known that obesity is a risk factor for colorectal cancer, but previous studies looked at ages 50 and older. The new research is among the first to analyze possible causes of early-onset colorectal cancer that occurs among younger patients – those below age 50. “We had hypothesized that obesity was associated with early-onset colorectal cancer, but we were struck by the strength of the association,” said co-senior author, Yin Cao, an assistant professor at Washington University Medical School.

The study found that women ages 20 to 49 who were considered overweight or obese based on BMI had up to twice the risk of developing early onset colorectal cancer before age 50, compared with women who reported the lowest BMIs. The study defined normal weight as BMI between 18.5 and 24.9, overweight as BMI between 25 and 29.9, and obese as BMI over 30.

In addition to current BMI, the amount of weight gained since age 18 was also analyzed and was found to be linked to a higher risk of colorectal cancer before age 50. Women with a BMI of 23 or higher at age 18 who reported a weight gain of 44 pounds or more had the highest risk of early-onset colorectal cancer compared with other women in the study.

Take-home message

Cao says the overall take-home message from the study is that weight control in early adulthood can be a factor in preventing early-onset colorectal cancer.

According to the researchers, an estimated 22% of early-onset colorectal cancers reported in the study could have been prevented if the participants had maintained a healthy weight with BMI in the normal range. However, Cao says despite the rise in colorectal cancer among people under 50, it is still not a common diagnosis, at about 8 cases per 100,000 people.

The risk of early onset colon cancer for overweight and obese women was the same regardless of whether or not the woman had a family history of the disease.

One limitation of the study is that it included mostly white women. Cao said she and her team are looking for additional databases to look into men and other ethnic populations.

Signs and symptoms; risk factors

Before and during the time of the study, early-onset colorectal cancer was more likely to be diagnosed through signs and symptoms than through preventive screening. That’s because, until recently, colorectal cancer screening guidelines called for screening beginning at age 50 for those of average risk.

Health experts say young people should be educated about when to get screened and learn about all the screening options available. They should talk to their health care providers about traditional colorectal cancer risk factors, signs and symptoms to recognize, and healthy lifestyle behaviors.

The most common signs and symptoms of colorectal cancer include:

• A change in bowel habits, such as diarrhea, constipation, or narrowing of the stool, that lasts for more than a few days
• A feeling that you need to have a bowel movement that is not relieved by doing so
• Rectal bleeding
• Dark stools, or blood in the stool
• Cramping or abdominal (belly) pain
• Weakness and fatigue
• Unintended weight loss

You may be able to lower your risk by:

• Eating lots of vegetables, fruits, and whole grains and less red meat (beef, pork, or lamb) and processed meats (hot dogs and some luncheon meats).
• Getting regular exercise.
• Watching your weight.
• Avoiding tobacco.
• Limiting alcohol. The American Cancer Society recommends no more than 2 drinks a day for men and 1 drink a day for women.