The study is the first to examine incidence trends for obesity-related cancers in young adults in the United States. Millennials — people born in the 1980s or ‘90s — have double the risk of some types of cancers compared with the rates baby boomers experienced at that age, according to the American Cancer Society. “Cancer trends among young people can be often considered as a bellwether for future disease burden," says Hyuna Sung, PhD, a cancer epidemiologist and principal scientist of the Surveillance and Health Services Research Program at the American Cancer Society.
Obesity — as Risky as Smoking?
The link between obesity and cancer is of growing concern. Almost 40 percent of U.S. adults are obese, according to the Centers for Disease Control and Statistics (CDC). CDC surveys show 18.5 percent of children ages 2 to 19 are obese. Overall, cancer-related deaths in the United States have fallen 27 percent in the past 25 years, mostly because fewer people are smoking and developing lung cancer, according to the most recent cancer statistics, published January 8, 2018, in the journal CA: A Cancer Journal for Clinicians (from the American Cancer Society). But the new research hints at a potential reversal of that progress. 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. They looked at 30 types of cancers, including 12 that are considered obesity-related. They then constructed statistical age-cohort models to estimate the average annual percentage change in the incidence of various cancers and incidence rates by birth cohorts from 1910–19 to 1980–89. The analysis showed that cancer incidence is rising in young adults for 6 of the 12 obesity-related cancers: colorectal, endometrial, gallbladder, kidney, multiple myeloma, and pancreas. The risk has increased more for the younger birth cohorts. The risk of colorectal, uterine, pancreas, and gallbladder cancer in millennials is about double the risk that baby boomers had at that age. “Our results showed, compared with persons born circa 1950, persons born circa 1985 had from 1.6 to 4.9 times higher risks, depending on cancer types,” Dr. Sung says. “The younger, the faster. The steepest increases were in the youngest age group, those aged 25 to 34 years.” In contrast, rates of cancers that are linked to infection or smoking (nonobesity related) declined or were stable in all but 2 of 18 types of cancers.
Could Other Factors Play Into the Risk?
Excess body weight is considered a known carcinogen and is linked with more than a dozen cancers. Still, the ways in which obesity may increase the risk of cancer is unclear, says Stephen Schwartz, PhD, a professor of epidemiology in the Fred Hutchinson Cancer Research Center at the University of Washington in Seattle, who was not involved in the study. Previous research, such as studies in breast cancer, have linked excess body fat to changes in certain hormones in the body that can spur the development of cancer. But, Dr. Schwartz says, other studies show “how obesity contributes to a chronic state of inflammation in our bodies, and that chronic state is known to promote carcinogenesis in animal models. There may be other biological mechanisms, too.” The current study did not include data on obesity and can only infer a link between obesity and rising cancer rates, says Schwartz. “It’s an assessment of the trends in cancer risk over time in cancers believed to be strongly related to obesity,” he says. “But I do think it’s an important piece in understanding what obesity is doing at the population level.” It’s important to remember that cancer usually develops due to multiple factors, he says, such as genetics, age, lifestyle, and environmental factors. In addition, cancer rates sometimes rise due to better screening, such as the recent increase in thyroid cancer rates that is attributed, in part, to more people having ultrasound examinations that detect the disease. “There are these noncausal aspects, too, that have to be paid attention to,” Schwartz says. “But I think the overall picture is that most of these obesity-related cancers are increasing.” Sung agrees that more evidence is needed to link rising rates of some types of cancers to obesity. “Although the obesity epidemic is a quite plausible scenario, we need more knowledge to explain the trends with additional known and unknown cancer risk factors that may have contributed to these trends,” she says. The bacteria in the intestinal tract, called the gut microbiome, may also influence cancer risk, Sung says. Others studies show the location of fat — such as fat in the abdomen — may impact cancer risk.
Addressing the Problem
More effort is needed to combat the obesity epidemic, experts say. “The lesson we’ve learned from other cancers is if steps aren’t taken to stop the things causing those trends, then they will continue to get higher,” Schwartz says. “The next generation of kids born will also experience higher rates. We need, as a society, to figure out ways to prevent, much more effectively than we do now, individuals from developing obesity.” “Health professionals, especially primary care physician, should increase obesity screening and counseling,” Sung says. “Less than one-half of primary care physicians regularly assess for body mass index (BMI) — overweight or obese or normal weight — despite national screening recommendations.”
