Gen Xers and millennials—that is, people born between 1965 and 1996—are now more likely than older generations to be diagnosed with 17 different types of cancer, according to new research published Wednesday in Lancet Public Health.

The study, which was conducted by researchers from the American Cancer Society and the University of Calgary in Canada, analyzed data from millions of people and supports related evidence of the concerning trend.

Here, an expert in cancer research and prevention explains what could be behind the rising numbers.

Which types of cancers are being diagnosed more in younger generations?

Researchers studied diagnosis trends of 34 types of cancers, and 17 of them had increasing rates in young adults. Notably, cancer risk was two to three times higher in people born in 1990 for pancreatic, kidney, and small intestine cancers compared to people born in 1955.

There are also several cancers that have been declining for decades in older generations, but are now on the rise again in younger adults, including:

• Anal cancer
• Breast cancer
• Colorectal cancer
• Endometrial cancer
• Gallbladder cancer
• HIV-linked cancer
• Ovarian cancer
• Stomach cancer
• Testicular cancer

Likely due to the advanced technology and treatments available for these diseases now, the study found that mortality rates declined or were at least stable in younger generations for most cancers—with a few exceptions. The mortality rate increased in younger age groups for certain types of cancer, including endometrial, liver, gallbladder, colon, and testicular cancers.

What are the possible causes?

“Reasons for the rise in cancer cases among young adults are complex, multi-factored, and not well understood,” Tracy E. Crane, PhD, RDN, co-lead of the Cancer Control Program at Sylvester Comprehensive Cancer Center, part of University of Miami Health System, tells Well+Good.

“The increases are likely due to lifestyle factors such as excess body weight [and] exercise and diet [changes], as well as environmental factors such as [changes in] air, food, and water quality,” Dr. Crane explains.

According to the American Cancer Society, about 20 percent of cancer diagnoses in the U.S. are linked to obesity. And more than 40 percent of American adults now have obesity, per the Centers for Disease Control and Prevention (compared to about 13 percent in 1960).

It’s worth noting, though, that this link doesn’t necessarily mean that simply having a body mass index (BMI) of 30 or higher (the definition of obesity) causes cancer. BMI is a flawed measurement, for one, especially for BIPOC communities. And there’s also some research that argues that weight stigma in the medical field (AKA, medical weight bias) contributes to poor health outcomes for people in larger bodies.

Crane notes other cancer risk factors include:

• Lack of physical activity
• Excess calories
• High intake of red and processed meats and added sugar
• Low intake of vegetables, fruits, and whole grains
• Increases in alcohol consumption
• Environmental factors including air and water pollution as well as additives in food

“The COVID-19 pandemic also impacted rates of cancer screening, vaccinations for things such as HPV and hepatitis [B] (viruses known to be associated with cancer), and we saw increases in alcohol and rates of obesity,” Dr. Crane says.

“This has set us up for potentially even higher rates of cancer in the coming decade. Now more than ever we need to focus on helping people change these modifiable risk factors to decrease cancer risk.”

What can people do to lower their risk?

There are several ways to lower your risk for cancer by modifying lifestyle choices, Dr. Crane says.

“My number one recommendation if people are using tobacco products is to get help to quit,” she says. “We know that working with a tobacco treatment specialist can increase an individual’s success of quitting by up to 50 percent when used in combination with nicotine replacement.”

A great place to start is smokefree.gov or 1-800-QUIT-NOW. Every state in the U.S. has free resources to help people quit tobacco.

Beyond that, small behavior changes can make a big difference in your cancer risk. You can start by making adjustments in one area, then adding more changes over time. Dr. Crane suggests:

• Choose to add to your lifestyle versus focusing on removing. For example, add a salad, a serving of whole grains, a few extra steps by parking farther away, or a flight of stairs by taking the stairs instead of the elevator.
• Focus on a predominantly plant-based diet (not only fruits and vegetables but beans, legumes, nuts, seeds, and whole grains).
• Limit your intake of red and processed meats (e.g., bacon, sausage, deli meats) as well as alcohol and added sugars.
• Aim to get 150 minutes of moderate activity each week and include a variety of activities, ranging from balance exercises to resistance training and aerobic activities.
• Use sunscreen regularly (find the best SPF for you).
• Complete cancer screenings at the recommended ages.
• Stay up to date on all vaccinations, and talk with your doctor about testing for potential cancer-causing infections.

“When you focus on small changes throughout the day, these start to add up until you look back and realize you are eating better and moving your body more with simple changes,” Dr. Crane says.

“It is not an all-or-nothing approach, and if you have a bad day, all is not lost, because you have the opportunity to start again the next day.”

—reviewed by Jennifer Logan, MD, MPH