Kate Middleton says she is cancer-free. But why are she and so many young people getting sick?

Kate Middleton — now Catherine, the Princess of Wales — announced Monday in a video message that she had completed chemotherapy for the cancer diagnosis she first revealed in March. She said she would resume some public events, though engagements are expected to remain limited through the end of the year.

“Doing what I can to stay cancer free is now my focus,” the princess wrote in a post shared on X. “Although I have finished chemotherapy, my path to healing and full recovery is long and I must continue to take each day as it comes.”

She was diagnosed with cancer after doctors performed abdominal surgery on her and discovered evidence of the disease.

Kensington Palace has not released details on what type of cancer the princess has, nor did it specify what stage the cancer was when it was found. But as a 42-year-old woman, Middleton is far from alone in her diagnosis: Worldwide, more and more people under 50 are developing cancers.

Cancer typically strikes those who are in their 50s, 60s, or even older. Yet, in recent decades, early-onset cancers — which are usually defined as occurring in patients younger than 50 — are occurring at a higher rate, particularly in wealthy countries. As illustrated in the chart below, a 2023 BMJ analysis found that the early onset of 29 different cancers had risen nearly 80 percent globally between 1990 and 2019.

Separately, a study published in JAMA Network Open the same year concluded that the occurrence of a wide range of cancers among people under 50 had increased between 2010 to 2019 among American adults, particularly among women.

Much of the increase is attributable to colon and rectal cancers: In 2019, there were about 5.7 cases of colorectal cancer among 100,000 people ages 14 to 49. That’s up 63 percent since 1990, when there were approximately only 3.5 cases per 100,000 people. Breast, cervical, and skin cancers are still the most likely cancers to develop in adults under 50, but individual cases of colorectal cancers taking the lives of well-known celebrities, like actor Chadwick Boseman in 2020, have made that trend more visible in recent years.

Cancers of the bladder, kidney, ovaries, pancreas, prostate, thyroid, and uterus also became significantly more common in the nearly three decades covered by the global investigation published in BMJ.

John Marshall, director of the Ruesch Center for the Cure of Gastrointestinal Cancers at Georgetown University, told me recently that, early in his career, he rarely saw a patient under 50. Today, half of his patients are in that age range, many of them appearing to be healthy and fit. At first, these young patients usually came in with colorectal cancer, but Marshall has increasingly seen younger people with cancers attacking other parts of the gastrointestinal tract.

No single factor is to blame for this widespread uptick in numerous cancers among young people, particularly the rise in the gastrointestinal system, but scientists are starting to put together a picture of the reality behind one of the most important medical mysteries of our time.

One review published in Frontiers in Nutrition in 2022 found several dietary factors were associated with early-onset colorectal cancer. Eating a lot of deep-fried and processed foods, foods high in fat, and sugary drinks and desserts was a significant risk factor, as was having a diet low in fiber. Other studies have found higher alcohol consumption is associated with an increased risk of developing cancer early.

Toxins in our environment, such as microplastics, could also be a contributing factor. These tiny particles can be found in everything from food containers to synthetic clothing, before making their way into our bodies and our GI tracts.

One New Zealand research team concluded the upticks in cancers among young adults matched the timeline that we would expect from the multiplication of microplastics in the environment. Cellular and rodent models have suggested that microplastics could promote tumor growth. Though more research is needed, these materials contain chemicals that can disrupt hormones and pose a risk to our health.

But these are still theories. Scientists still don’t fully understand what is primarily driving earlier-in-life cancer cases. What we do know is that stories like Middleton’s are becoming all too common.

Update, September 9, 12:55 pm ET: This story was originally published on March 22 and has been updated to reflect new information about Kate Middleton’s health status.