Rising Cancer Incidence Trends in Millennials Partially Attributed to Climbing Obesity Rates

The incidence of several obesity-related cancers showed a temporal increase in young adults aged 25 to 49, according to study findings published in The Lancet Public Health.

Overall, the obesity rates have doubled between 1980 and 2014, and evidence suggested the epidemic may be linked to rising cancer incidences among younger adults.

Future incidences of obesity-related cancers may continue to grow as Millennials age, due to increasing rates of obesity among younger adults, “potentially halting or reversing the progress achieved in reducing cancer mortality over the past several decades,” the researchers wrote.

The population-based study examined contemporary incidence trends for 30 cancer types, including the 20 most common cancers and 12 cancer types considered to be obesity-related. Researchers found a significantly higher incidence in adults aged 25 to 49 for six of the 12 cancers considered to be obesity-related — including multiple myeloma, colorectal, uterine corpus, gallbladder, kidney and pancreatic cancer — and reported even higher rates in consecutively younger generations.

For further insight, CURE spoke with Ahmedin Jemal, Ph.D., DVM, co-author of the study and scientific vice president of surveillance and health services research at the American Cancer Society.

CURE: Can you share a bit of background on this study?Jemal: In 2017, we published a paper in Journal of the National Cancer Institute (JNCI). We reported an increase in colorectal cancer incidence rates in young adults but observed a decrease in older adults. That study received a lot of media attention because we also compared risk by generation. The risk (of colorectal cancer) in millennials (born in 1985) compared to Baby Boomers (those born in 1950) of the same age was more than twice as high. The findings informed the American Cancer Society guidelines for colorectal cancer screening. Based on the study, the American Cancer Society lowered the age guidelines for beginning screening from age 50 to age 45. One factor that may have contributed to the increase in colorectal cancer rates in young adults is the obesity epidemic. Other factors include bad diet, increased incidence of diabetes, environmental issues and other unknown causes.

In the recent paper, we extended our research to determine whether we see a similar increase in rising incidence rates in young adults for other obesity-related cancers. There are 13 obesity-related cancers, including colorectal cancer. We wanted to see if there was a similar rapid increase in incidence rates among young adults for other obesity-related cancers. We did not find a link between obesity and cancer, rather a rapid increase in obesity-related cancers in young adults. We looked at the pattern and changes over time by age and generation.

Why do only some obesity-related cancers show temporal trends of markedly rising younger adult incidence?

For all of these cancers, there are other risk factors that play a major role in deciding the direction of the trend. When we look at these six out of 12 obesity-related cancers, the increase is much faster in younger adults than in older adults. Still, rates are increasing (across the board). For example, thyroid cancer is one of the obesity-related diseases that is increasing rapidly in both younger and older adults. Another cancer that was not included is liver cancer. Both cancers had rates that were increasing, but not significantly for most generations.

Obesity is one of many factors that causes each of the cancers studied. For example, we say colorectal cancer is obesity-related, but we know colorectal cancer is also caused by other factors, such as bad diet, consumption of red meat and physical inactivity. There are other competing factors that affect the trend.

For ovarian cancer, obesity is a risk factor, but taking hormones or pills for reproductive reasons is a protective factor. Use of oral contraceptives has increased over the years. Obesity is pushing ovarian cancer incidence rates up, but the increase in use of oral contraceptives is simultaneously pushing it down.

In the case of breast cancer, we didn't expect to see an increase in incidence rates in young adults because obesity is a risk factor only for post-menopausal women. Obesity is actually a protective factor against breast cancer in pre-menopausal women under age 50. Therefore, we didn't expect an increase in incidence rates for breast cancer due to obesity, rather we expected a decrease.

Why do some obesity-related cancers appear to have declined incidence in the older age groups?

Many of the cancers that are increasing in younger adults are also increasing in older adults, but the rate of increase is much smaller compared to younger adults. There are also other factors besides obesity, as I've mentioned. Colorectal cancer is a good example. For colorectal cancer, rates are decreasing in older adults primarily because of the use of screening and colonoscopy. Colonoscopy not only detects cancer at an early stage, it actually removes precancerous lesions before they become cancer. The large decrease that we see in age 55 and older for colorectal cancer is because of the increased use of colonoscopies in the last 20 to 25 years.

Do your findings suggest that those with obesity-related cancers can improve their chance of survival or remission by maintaining a healthy body mass index (BMI)?

Definitely. They can reduce their risk of cancer in the future, too, by maintaining a healthy body weight, keeping a healthy diet and staying physically active. These are important actions that can be taken at the individual level. Also, actions can and should be taken at the provider level and community/policy level.