Cancer trends in young adults, often under 50 years, reflect recent changes in carcinogenic exposures, which could foreshadow the future overall disease burden. Previous studies reported an increase in early onset colorectal cancer, which could partly reflect the obesity epidemic. We examined age-specific contemporary incidence trends in the USA for 30 common cancers, including 12 obesity-related cancers.
We obtained incidence data for invasive cancers among people aged 25–84 years diagnosed from Jan 1, 1995, to Dec 31, 2014, for 25 population-based state registries in the USA. All patients in the registry were included in the analyses. We considered the 20 most common cancer types and 12 obesity-related cancers (30 cancer types in total). We used age-period-cohort modelling to estimate average annual percentage change in incidence rates by 5-year age group (25–29 years to 80–84 years in 5-year increments) and incidence rate ratios (IRR) by birth cohort (10-year overlapping birth cohorts from 1910–19 to 1980–89 in 5-year increments). No exclusion criteria were applied after including all invasive cancer cases based on age group and diagnosis year.
From 1995 to 2014 there were 14 672 409 incident cases for 30 types of cancer. Incidence significantly increased for six of 12 obesity-related cancers (multiple myeloma, colorectal, uterine corpus, gallbladder, kidney, and pancreatic cancer) in young adults (25–49 years) with steeper rises in successively younger generations. Annual increases ranged from 1·44% (95% CI −0·60 to 3·53) for multiple myeloma to 6·23% (5·32–7·14) for kidney cancer at age 25–29 years, and ranged from 0·37% (0·03–0·72) for uterine corpus cancer to 2·95% (2·74–3·16) for kidney cancer at age 45–49 years. Compared with people born around 1950, IRRs for those born around 1985 ranged from 1·59 (95% CI 1·14–2·21) for multiple myeloma to 4·91 (4·27–5·65) for kidney cancer. Conversely, incidence in young adults increased in successively younger generations for only two cancers (gastric non-cardia cancer and leukaemia), and decreased for eight of the 18 additional cancers, including smoking and HIV infection-associated cancers.
The risk of developing an obesity-related cancer seems to be increasing in a stepwise manner in successively younger birth cohorts in the USA. Further studies are needed to elucidate exposures responsible for these emerging trends, including excess bodyweight and other risk factors.