Purpose Obesity and circadian rhythm disruption are risk factors for various cancers, including advanced prostate cancer. Given the differences in obesity rates by racial groups, the emerging evidence suggesting racial differences in circadian function, and the well-known racial disparities in cancer incidence and mortality, we aimed to explore the association between obesity and melatonin levels within a diverse population and assess how this association differed by racial/ethnic group. Methods This study leveraged 2,786 male controls from a nested case-control study within the Multiethnic Cohort Study. Melatonin was measured by its primary metabolite, 6-sulfatoxymelatonin, in first-morning void and overnight urine samples collected between 2001 and 2006 prior to cancer diagnosis. We categorized men based on body mass index (BMI) as underweight:<18.5 kg/m2; normal: 18.5 kg/m2 to <25 kg/m2; overweight: 25 kg/m2 to <30 kg/m2; and obese: 30 kg/m2. We used linear regression models to evaluate the association between obesity and melatonin levels, adjusted for urinary creatinine levels, age, race/ethnicity, occupation, years of schooling, month of urine collection, sleep duration, physical activity, smoking, and alcohol use. We further examined associations between obesity and melatonin within racial/ethnic groups. Results The prevalence of obesity differed by race/ethnicity, with 34.3% of Native Hawaiian (NH), 25.0% of African American (AA), 22.1% of Latino, 14.8% of White, and 10.6% of Japanese men categorized as obese (chisq p-value < 0.001). Latino and White men had the highest levels of melatonin with a median of 25.1 ng melatonin/mg creatinine; the median levels in Japanese (23.4 ng/mg), AA (23.0 ng/mg), and NH men (22.3 ng/mg) were lower (Kruskal-Wallis p-value=0.03). These differences remained after adjusting for BMI, with AAs having the lowest and Latinos the highest melatonin levels. In multivariable models, men who were obese had melatonin levels that were 16.9% (95% CI: 9.8%, 23.5%) lower than normal weight (NW) men. When stratified by race/ethnicity, we found that obese white men had 16.5% (95% CI: 3.0%, 28.1%) lower levels than NW white men; obese NH men had 23.7% (95% CI: 2.9%, 40.0%) lower levels than NW NH men; and obese Japanese men had 18.4% (95% CI: 7.5%, 28.1%) lower levels than NW Japanese men. Although not statistically significant, obesity was associated with lower melatonin levels among AA (13.9% lower, 95% CI: 29.6% lower, 5.2 % higher) and Latino (8.6% lower, 95% CI: 28.5% lower, 17.0% higher) men. Conclusion To our knowledge, this is the first study looking at the association between obesity and melatonin levels in a diverse population of men. We found that obese men had lower melatonin levels and that melatonin levels and obesity rates differed by racial groups. These findings will be expanded on to investigate the interplay between melatonin, obesity and race/ethnicity on risk of prostate cancer and potentially point to an underlying reason for racial disparities in cancer.

Citation Format: Ilkania M Chowdhury-Paulino, Iona Cheng, Unnur A Valdimarsdottir, Loic Le Marchand, Christopher A Haiman, Lynne Wilkens, Lorelei A Mucci, Sarah C Markt. Racial and ethnic differences in the association of body mass index (BMI) and melatonin levels among men in the Multiethnic Cohort Study [abstract]. In: Proceedings of the Twelfth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2019 Sep 20-23; San Francisco, CA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(6 Suppl_2):Abstract nr C046.