Colorectal cancer is rising in younger adults. Here's who is most at risk and symptoms to watch for

WASHINGTON — Colorectal cancer is a threat not just to older adults but increasingly to young men and women, too. It's now the top cancer killer of Americans younger than 50.

The deaths of ''Dawson's Creek'' actor James Van Der Beek at 48 this week, and a few years ago ''Black Panther'' star Chadwick Boseman at 43, highlight the risk for younger adults.

''We're now starting to see more and more people in the 20-, 30- and 40-year-old range developing colon cancer. At the beginning of my career, nobody that age had colorectal cancer,'' said Dr. John Marshall of Georgetown University's Lombardi Comprehensive Cancer Center, who has been a cancer doctor for more than three decades.

That trend ''is shaking us all, to be blunt,'' said Marshall, who is also medical consultant to the Colorectal Cancer Alliance.

Here's what to know about colorectal cancer — at any age — and how to protect yourself.

How common is colorectal cancer?

More than 158,000 cases of colorectal cancer will be diagnosed in the U.S. this year, according to the American Cancer Society. Among all ages, it's the nation's second leading cancer killer, behind lung cancer — expected to claim more than 55,000 lives this year.

Who's most at risk?

The vast majority of colorectal cancer cases and deaths still are in people 50 and older. That older age group has seen the most progress, with deaths dropping by about 1.5% a year over the past decade, according to cancer society statistics.

But while it's still relatively rare in the under-50 crowd, their colorectal cancer diagnoses have been rising since the early 2000s.

And last month, cancer society researchers reported that colorectal cancer mortality in Americans under 50 had increased by 1.1% a year since 2005, becoming the deadliest cancer in that age group. This year, the society estimates 3,890 people under age 50 will die of it.

Risk factors at any age include obesity, lack of physical activity, a diet high in red or processed meat and low in fruits and vegetables, smoking, heavy alcohol use, having inflammatory bowel disease or a family history of colorectal cancer.

And a recent study found that a three-year exercise program improved survival in colon cancer patients and reduced cancer recurrence.

What are the symptoms of colorectal cancer?

Symptoms include blood in stool or rectal bleeding; changes in bowel habits such as diarrhea, constipation or narrowing of stool that lasts more than a few days; unintended weight loss; and cramps or abdominal pain.

''Don't ignore symptoms. Get it checked,'' Marshall stressed. Survival is far more likely when colorectal cancer is diagnosed early, before it spreads.

When to get screened for colorectal cancer

People known to be at higher risk are supposed to talk with their doctors about whether to start screening even earlier.

How often people need to get checked depends on the type of screening they choose. There are a variety of options, including yearly stool-based tests or colonoscopies that may be done every 10 years as long as no problems were found. There's also a newer blood test for adults 45 or older.

What's causing the colorectal cancer rise in younger adults?

No one knows what's causing the rise in cases in younger adults. But Georgetown's Marshall said many young patients lack common risk factors. He wonders if changes in young people's gut bacteria — the microbiome — might play a role.

Also, where cancer occurs along the question mark-shaped colon — it starts on one side of the abdomen and swoops to the other before ending with the rectum — influences how aggressive it is and how it's treated. Marshall said there's a marked difference in where younger and older people's tumors tend to strike, another clue being explored.