A new study has found that six obesity-related cancers are on the rise in young adults, another sign of the worldwide epidemic of excess weight.

The individual risk of cancer remains small in young adults, and unrecognized factors may also be at play. Still, the study found that for millennials and Generation Z — people in their 30s and even their 20s — incidence rates are rising for endometrial, gallbladder, kidney, multiple myeloma, pancreatic, and colorectal cancers, all of which have been linked to obesity.

To put it in perspective, millennials (those born between the early 1980s and mid-1990s) are twice as likely to develop four of those cancers — colorectal, endometrial, pancreatic and gall bladder — as baby boomers (born from 1946 to 1964) were at the same age.

The researchers, from the American Cancer Society and the National Cancer Institute, first reported the increase in colorectal cancer among young adults — a trend that defies an overall steady decline in colon cancer — several years ago. They extended that analysis by using cancer registries to examine age-specific trends over the last two decades for 30 types of cancers, including 12 known to be linked to obesity.

Although the public is generally not aware, scientists have found that cancer can be fueled by the same metabolic abnormalities that occur in obesity and a related disease, diabetes. The abnormalities include resistance to insulin, the hormone that enables cells to absorb blood sugar and turn it into energy, and chronic inflammation.

The new study, published Monday in Lancet Public Health, also looked at 18 cancers that are not related to obesity, including malignancies caused by smoking and infections. Only two of these cancers (gastric and leukemia) were increasing in young adults, while the rest were stable or decreasing.

Cancer remains a disease primarily driven by aging. Colon cancer is now diagnosed in about one in 100,000 people in their 20s, compared with 50 in 100,000 people in their early 60s. Pancreatic cancer strikes two in 100,000 people ages 25 to 49, compared with 37 in 100,000 adults ages 50 to 84.

However, more Americans are being exposed to a carcinogen — namely, excess weight — for most or all of their lives. Between 1980 and 2014, the prevalence of overweight and obesity among children and adolescents grew by 100 percent. More than a third are now overweight, defined as a body mass index of 25 to 29, or obese, defined as a BMI of 30 or more. The prevalence during those years grew by 60 percent among adults, of whom 78 percent are now too heavy, according to federal data.

Ahmedin Jemal, American Cancer Society researcher
unknown / American Cancer Society
Ahmedin Jemal, American Cancer Society researcher

“The future burden of these cancers could worsen as younger cohorts age, potentially halting or reversing the progress achieved in reducing cancer mortality over the past several decades,” said senior author Ahmedin Jemal, scientific vice president of surveillance at the American Cancer Society.

That concern was echoed by Pamela Goodwin, an oncologist at Mount Sinai Hospital and a professor at the University of Toronto who has studied the role of diet, weight, and exercise in breast cancer.

“The study provides a cautionary tale — that recent improvements in cancer mortality, reflecting factors such as smoking cessation, early diagnosis, and improved treatment, may not continue,” she said. “The improvements may actually be reversed by increased rates of obesity-associated cancers in those exposed to obesity in childhood or early adulthood.”

The public health implications are clear, experts said.

“This study, and others like it, just highlight the need to continue to inform people, particularly children and young adults, about the importance of a healthy diet and lifestyle,” said Ryan Dowling, a biochemist at Princess Margaret Cancer Center in Toronto who studies how obesity disrupts metabolism.