OBJECTIVES : Obesity is a risk factor for many malignancies. Among patients already diagnosed with primary cancer, we aim to study whether an elevated body mass index (BMI) and change in BMI (ΔBMI) increase the risk of developing a second primary obesity-related cancer.

METHODS : Data from the National Health and Nutrition Examination Survey, 1999-2016, were used. We included individuals who reported any cancer diagnosis of any type (first primary cancer diagnosis). The outcome was whether an individual had a diagnosis of a second primary obesity-related cancer, defined as obesity-related cancer diagnosed in the year after the year of the first primary cancer diagnosis. Obesity-related cancer included brain, breast, colon, esophagus, gallbladder, kidney, liver, lymphoma/hodgkin's disease, ovary, pancreas, prostate, rectum, stomach, thyroid, uterus, as identified by the International Agency for Research on Cancer. We used multivariable logistic regression with the following covariates: age at first cancer diagnosis, race/ethnicity, marital status, family income, smoking status (never, former, current), alcohol consumption (never drinker, below limit, above limit), whether first cancer was obesity-related, recalled max BMI, ΔBMI, defined as (measured BMI-max BMI)/(age at measured BMI-age at max BMI), and year of survey. Complex sampling design was adjusted for based on the analytic guidelines.

RESULTS : A total of 3,236 individuals (population size = 16,536,827) had a cancer diagnosis of any type. Among them, 64 (population size = 351,308, 2.12% of the population with a primary cancer diagnosis) had a diagnosis of a second primary obesity-related cancer. In the multivariable-adjusted analysis, a 1 kg/m²/year increase in BMI was positively associated with a diagnosis of a second primary obesity-related cancer (odds ratio 1.22, p-value 0.001). The other covariates were not statistically significant.

CONCLUSIONS : This study suggests that an increasing BMI overtime is associated with an elevated risk of developing a second primary obesity-related cancer among individuals diagnosed with primary cancer.