In addition, millennials (anyone born between 1981 and 1996) these days are twice more likely to develop colorectal, endometrial, pancreatic and gallbladder cancers than baby boomers (people born worldwide between 1946 and 1964) when they were the same age. Patients were divided into five-year age groups from 25-29 to 80-84 years old. In contrast, for all but two of the 18 non-obesity related cancers, rates either stabilized or declined in successive younger birth cohorts.
The researchers noted that young adults still have an overall lower risk of developing these cancers, compared with older adults.
"Most cancers occur in older adults, which means that as the young people in our study age, the burden of obesity-related cancer cases and deaths are likely to increase even more".
In the last few decades, there has been mounting evidence that certain cancers can be linked to obesity. In some cancers, excess bodyweight during early adulthood could be a more important influence on cancer risk than weight gain in later life .
Overall, the number of people in the United States who succumb to cancer has declined.
In the new study, researchers from the American Cancer Society accessed a database provided by the North American Association of Central Cancer Registries covering 67 percent of the population of the United States.
For instance, pancreatic cancer is traditionally most common in people over the age of 65.
During the period examined, the incidence of pancreatic cancer, for example, increased by about one percent per year for adults aged 45 to 49.
Those between the ages of 30 and 34 saw a 2.47 percent increase, while those between the ages of 35 and 39 saw a 1.31 percent increase. In 2016, the International Agency for Research on Cancer (a branch of the World Health Organization) published a report linking obesity to a higher risk of 12 cancers: Colorectal, esophageal, gallbladder, gastric cardia (a type of stomach cancer), kidney, liver and bile duct, multiple myeloma (a type of bone marrow cancer), pancreatic and thyroid cancer; and, in women, endometrial, breast and ovarian cancer.
The researchers found incidence rates increasing for six of 12 cancers related to obesity in younger USA adults, with steeper increases in successively younger generations.
But of course, obesity is only one factor - the environment, genetics and other issues can also come into play.
Researchers stressed that it remains the case that cancer is far more common in older age groups.
According to the charity Cancer Research UK, obesity is the second biggest preventable cause of cancer in the UK, after smoking.
That upward trajectory has experts concerned about associated medical conditions such as heart disease, diabetes and up to 13 types of cancer. The quality of the American diet also has worsened in recent decades. Such a discovery could negate our own recent advances in treating cancers but until the NHS seriously begins to screen for obesity, as recommended by the study's authors, we may not know.
"Less than half of primary care physicians regularly assess body mass index despite national screening recommendations", he told NBC News. Not everyone who gets these cancers is overweight either, and not everyone who is obese will necessarily get these cancers. The authors suggest that restrictions on advertising calorie-dense food and drinks, taxes on sugary drinks and urban planning that promotes physical activity could be effective strategies to stem the emerging trend.