In a sweeping study covering two-thirds of the U.S. population, they showed that half a dozen cancers for which obesity is a known risk factor became more frequent from 1995 to 2015 among women and men under 50.
The younger the age bracket, the more quickly these cancers gained ground, they reported in The Lancet, a medical journal.
During the period examined, the incidence of pancreatic cancer, for example, increased by about one percent per year for adults aged 45 to 49. Among 30 to 34-year-olds, the average annual per cent increase was more than twice that high.
And among 25 to 29-year-olds, the rate jumped by 4.4 per cent per year.
Comparing five-year age brackets from 25 to 80, the annual hike was similarly highest among the 25 to 29 cohort for four other obesity-linked cancers: kidney (6.23 per cent), gallbladder (3.71 per cent), uterine (3.35 per cent), and colon (2.41 per cent).
“Our findings expose a recent change that could serve as a warning of an increased burden of obesity-related cancers to come in older adults,” said co-author Ahmedin Jemal of the American Cancer Society.
Obesity has more than doubled in the United States over the last four decades.
It has also risen sharply in other rich nations and, more recently, the developing world. Today, some two billion people are overweight or obese.
With few exceptions, cancer has been seen as a disease of aging.
Indeed, the researchers note that the number of new cancer cases reported remains much higher in older age brackets, even if the rate of increase is now highest among young adults.
Two pancreatic cancer cases, for example, were diagnosed among every 100,000 24 to 49-year-olds from 2010 to 2014, compared to 37 cases for every 100,000 people aged 50 to 84.
Overall, the number of people in the United States who succumb to cancer has declined.
From 1980 to 2014 — when cancer claimed some 20 million lives — mortality dropped by 20 per cent, from 240 to 192 deaths per 100,000 people, due in part to reduced tobacco use.
“But in the future, obesity could reverse that progress,” Jemal cautioned.
“Obesity is now one of the most preventable causes of cancer in the U.S. and U.K. — around 1 in 12 cases in the U.S. are caused by excess weight, and more than 1 in 20 in the U.K.”
Building on earlier research suggesting a link between obesity and more frequent colon cancers in young adults, Jamel and colleagues analysed all cancer cases from 1995 to 2015 in 25 U.S. states home to 67 per cent of the population.
The data covered 30 types of cancer, 12 of which had previously been linked to obesity.
For five of the 12, the rate of increase for new cases was highest in the youngest age group, and for a sixth — a form of bone marrow cancer called multiple myeloma — the biggest jump was among adults in their early 30s.
Of the other 18 types of cancer, only two showed a similar trend, with the others either stable or — for those related to smoking and infection — in decline.
“The investigators speculate that these findings are driven in part by the obesity epidemic, a hypothesis that is both provocative and plausible,” Catherine Marinac from the Dana-Farber Cancer Institute and Harvard University’s Brenda Birmann commented, also in The Lancet.
Still unexplained, however, is why the six other forms of cancer classified by the UN’s International Agency for Research on Cancer (IARC) as related to obesity did not also show similar rates of increase among younger adults.
The authors called for more aggressive screening for obesity by front-line doctors, and called on them to warn patients about the cancer risk of being seriously overweight.
Currently, less than half of primary care physicians in the U.S. regularly measure the body-mass index (BMI) of their patients.
“The quality of the American diet has worsened in recent decades,” said lead author Hyuna Sung, also of the American Cancer Society.
More than half of 20 to 49-year-olds eat for too little fruits, vegetables and whole grains, and too much salt, fast food and sugary drinks, she said.