For the study, published in The Lancet Public Health, researchers analyzed 20 years of U.S. incidence data (1995-2014) for 30 different types of cancers, including 12 linked to obesity.
They found that six of the obesity-related cancers — colorectal, endometrial, gallbladder, kidney, multiple myeloma and pancreatic — were occurring at higher rates among adults aged 25 to 49 than in previous generations.
How much higher? Millennials (people born between the early 1980s and mid-1990s) were found to be twice as likely to develop four of those cancers — colorectal, endometrial, gallbladder and pancreatic — as baby boomers (born from 1946 through 1964) were at the same age.
Of the 18 cancers not known to be associated with obesity, only two — stomach cancer and leukemia — were not increasing among younger adults.
An unsettling pace
Obesity-related cancers still strike older adults more frequently than younger ones, and the incidence rate for such cancers (except for colorectal cancer) are rising among older adults too.
But the biggest increases were found in the younger age groups.
The incidence of pancreatic cancer, for example, rose between 1995 and 2014 by an average annual rate of 0.77 percent among adults age 45 to 49, by 2.47 percent among those aged 30 to 34, and by 4.34 percent among those aged 25 to 29.
Kidney cancer had the sharpest jump in incidence among young adults. Between 1995 and 2014, it rose by an average annual rate of 6.23 percent in the 25-to-29 age group.
Heavier at younger ages
As the authors of the current study point out, the obesity epidemic of the past 40 years has meant that younger generations are experiencing earlier and longer exposure to excess body fat than their parents and grandparents.
The proportion of children and adolescents in the United States who are overweight or obese has increased by more than 100 percent since 1980, the researchers note.
Growing evidence has linked obesity early in life to several types of cancer, they add.
In a study published last year, for example, researchers found that excess body weight accounted for 60 percent of endometrial cancers, 36 percent of gallbladder cancers, 33 percent of kidney cancers, 17 percent of pancreatic cancers and 11 percent of multiple myelomas diagnosed in American adults aged 30 and older.
Although the precise mechanisms behind the obesity-cancer link are unknown, some studies have found that excess body fat produces hormones that can accelerate the transition of normal cells into cancerous ones.
‘A sentinel’ for the future
The absolute risk for any particular young adult with obesity of developing cancer is still low, the authors of the current study stress. Indeed, only 4 percent of all cancers diagnosed in the U.S. occur among young adults age 20 to 39.
Furthermore, the study was observational, so it can’t prove that obesity is causing the rising cancer incidence rates among young adults. Other factors may be involved.
But the findings are troubling nevertheless.
“Given the large increase in the prevalence of overweight and obesity among young people and increasing risks of obesity-related cancers in contemporary birth cohorts, 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 Ahmedin Jemal, the study’s senior author and scientific vice president of surveillance at the American Cancer Society, in a released statement.
“Cancer trends in young adults often serve as a sentinel for the future disease burden in older adults, among whom most cancer occurs,” he added.
FMI: You can read the study in full on The Lancet Public Health website.