Cancers among younger adults are a growing global problem and are likely related to factors like poor diet, obesity and inactivity, a new research review finds.
Since the 1990s, researchers say, rates of various cancers have been rising in many countries among people under 50. And while the reasons are not fully clear, it's likely that changes in lifestyle and environment — starting early in life — are playing a role.
The review found that in recent decades, rates of 14 cancers have been inching up annually among younger adults in a diverse range of countries — from the United States and Canada, to Sweden and England and to Ecuador, Uganda and South Korea.
The cancers, similarly, run the gamut, and include those of the breast, colon, esophagus, kidneys, liver, stomach and pancreas.
For certain cancers, increased screening may partly explain the rising incidence, according to senior researcher Dr. Shuji Ogino, of Brigham and Women's Hospital in Boston. Screening tests for diseases like breast and colon cancers can detect more cases, at an earlier point in time.
But for the most part, he said, the growing cancer incidence among younger adults is beyond what would be expected from heightened detection.
And many of the cancers that are increasing arise along the digestive tract — "anywhere from the mouth to the anus," Ogino said. That, he added, points to a potential role for the microbiome.
The microbiome refers to the vast array of bacteria that normally dwell in the body, largely the digestive tract. Research in recent years has been revealing how important the microbiome is to overall health, playing a role in immunity, fighting chronic inflammation and other vital functions.
For any one person, makeup of the microbiome depends partly on genes. But Ogino noted that environmental factors are critical, too — including diet, alcohol intake, smoking, exercise and antibiotic use.
And many of those environmental exposures have shifted substantially in recent decades.
The spread of the "westernized" diet is a clear example, Ogino said. It's high in heavily processed foods, added sugar and red meat, but low in fruits, vegetables, fiber and "good" fats — qualities that have been linked to increased risks of certain cancers, like colon cancer.
The rise in colon cancers among younger adults has been gaining particular attention. According to the U.S. National Cancer Institute, the incidence of that disease among Americans younger than 50 has more than doubled since the 1990s — in sharp contrast to a decrease among people older than 65.
In fact, the trend spurred experts to lower the recommended starting age for colon cancer screening. It's now age 45 for people at average risk of the disease.
Dr. Benjamin Weinberg, an associate professor at MedStar Georgetown University Hospital in Washington, D.C., is studying the potential role of the gut microbiome in earlier-onset colon cancer. He also treats patients with the disease.
When a younger adult develops colon cancer, Weinberg said it suggests that something in the immune system response to early tumors has gone awry.
There is some evidence, he said, that greater diversity in the gut microbiome may support that immune response.
On the other hand, certain bacteria might stymie it. A bug normally tied to gum disease — Fusobacterium nucleatum — is a case in point. Research suggests F. nuc may promote cancerous growths by suppressing the immune response in the colon. And Weinberg and his colleagues have found that colon tumors from younger adults have a high presence of the bacteria.
Obesity among children and younger adults has, of course, skyrocketed in recent years. And on the population level, Weinberg said, there is a relationship between obesity and colon cancer risk. But many younger people who are diagnosed with the disease are not obese, and the reasons behind the rising incidence would appear to go beyond a single factor.
Much more research is needed to understand what's driving the rise in various early-onset cancers, both doctors said.
But, Weinberg said, it would be wise to do what experts have long advised: Eat a healthy diet full of nutrient-rich whole foods (which may promote a diverse gut microbiome, among many other benefits); exercise regularly; don't smoke; limit alcohol; and take antibiotics only when necessary.
And, Ogino said, healthy lifestyle habits should be cultivated early in life.
"I think the most important message is: Your kids' cancer risk in the future depends on what you do now," Ogino said.
He added, though, that in a world of easily accessed junk food and sedentary screen time, parents need help. It's up to society, Ogino said, to prioritize healthy eating, regular exercise, healthy sleep patterns and more.
The review was recently published online in the journal Nature Reviews Clinical Oncology.