Why do young people get cancer even if they’re healthy?

While early-onset cancer rates are rising for many kinds of cancers — including breast, uterine, colorectal and prostate — they’re still relatively infrequent. In 2019, early-onset colorectal cancer occurred at a rate of 5.7 per 100,000 people (up from 3.5 in 1990) and breast cancer occurred at a rate or 13.7 per 100,000 people (up from 9.6 in 1990) worldwide.

This is why asking family members of their health history is crucial — it will affect when your physician recommends you get your first mammography or colonoscopy and whether you may need more genetic testing. About a quarter of patients with early-onset colorectal cancer, for instance, have a family history that would have warranted screening earlier than age 45 — a missed opportunity to catch or even prevent those cancers early.

When my patients ask why people get cancer, I often tell them to think of a tower of blocks. Let’s say it takes 100 blocks for a person to get cancer. Genetics, environment and lifestyle habits can all add blocks to the tower.

Some of us will have genetic mutations we inherit from our parents or will have been exposed to things in the womb that add blocks to that tower before we can crawl. Some of us will grow up in environments where we’re exposed to pollutants in the air, soil or water; live in areas where ultra-processed foods are most plentiful and what our families can afford; or become infected with viruses that stimulate cancer formation — none of which are factors we made a conscious choice about.

And then our age and numerous other exposures and risk factors — many of which scientists have yet to uncover, but include alcohol intake, exposure to UV rays, smoking and obesity — all add more blocks to that tower.

Behind the scenes, our immune system is constantly scrambling to knock blocks off — with variable success rates. No two people are going to acquire the exact same constellation of 100 blocks in the same amount of time, and many will live their whole lives without reaching 100 blocks.

The factors that determine when and how someone’s tower tips over to cancer are unique, not always known and too often beyond individual control. It’s why some people who smoke never get cancer and others who are in peak physical shape still do.

“One of the biggest challenges in dissecting the causes of early-onset cancers is that there is not a single explanation,” said Bilal Siddiqui, an assistant professor and medical oncologist at the University of Texas MD Anderson Cancer Center. “There are multiple environmental factors, and we don’t yet fully understand all of them.”

Why doesn’t the immune system fight off cancer?

Cancers appear in all of us every day — but fortunately, they are typically detected and removed by our immune system.

Several important factors that aid cancer to evade our immune system are modulated by the environment. Smoking, alcohol and genetics are well-known risk factors. Other risk factors that were less common in earlier generations may also explain the rise of young cancers.

Maternal obesity and a high birth weight are associated with an increased risk of colorectal cancer. A rise in sedentary behaviors such as prolonged TV viewing and consuming sugary beverages — all of which are linked to high fasting blood sugar and obesity — have been implicated in early-onset cancers. Younger age at the start of a woman’s first period, older age at first birth and use of birth-control pills all increase the risk of getting breast cancer before menopause. And for early-onset colorectal cancer, consumption of red meat appears to play a detrimental role.

It’s important to remember that almost half of all cancers are preventable, Siddiqui added.

A 2022 study published in the Lancet found that about 44 percent of cancer deaths involved a modifiable risk factor such as tobacco use or air pollution.

Early-life exposures that could lead to cancer are hard to study — imagine being asked today to recall how many minutes you played in the sun 15 years ago. But scientists are working on a variety of approaches to overcome these obstacles — such as scouring electronic health records for objective evidence and analyzing biospecimens such as saliva and stool collected in childhood for clues.

What I want my patients to know

Besides mitigating any known risk factors, we can do a better job in identifying cancer early.

After learning your family history, go get the screening recommended by your doctor. Fewer than 20 percent of people ages 45 to 49 are up to date with colorectal cancer screening, and only about 66 percent of women age 40 and over were up to date with breast cancer screening.