PROJECT SUMMARY The parent grant, entitled “Low-level Arsenic Exposure and Cardiovascular Disease in Multi-Ethnic Adults (MESA As)”, aims to investigate the cardiovascular effects of low-level arsenic (As) exposure in US adults of multiple racial/ethnic groups. The Multi-Ethnic Study of Atherosclerosis (MESA) is a population-based study of nearly 7,000 participants from 6 US cities, with known health outcomes over two decades. During the measurement of urinary As species in the parent grant, we noticed that we could identify selenium (Se) species during the same analytical run in which we measured As species. With support from a P30 pilot study, we are validating those species in a subset of MESA urine samples. We propose to expand this pilot, and the scope of the parent grant, to assess the extent to which low-level Se exposure is associated with cardiovascular disease, by adding new data on Se species in urine to the parent grant. Selenium is an essential element with a narrow safety range, meaning that both low and high intake can increase disease risk. Humans mainly excrete Se through urine and urinary Se speciation is of particular interest for understanding human bio-metabolism and Se-related health outcomes. Human urinary Se speciation can be influenced by dietary Se intake and genetic variability and is dominated by selenosugars, trimethylselenonium ion, and inorganic Se oxyanions. However, the individual influence of Se species on human health and disease remains largely unexplored as it is rare for epidemiological studies to measure Se species in the urine. We plan to augment our current analytical method in MESA, originally established for speciation of As in urine, for the simultaneous determination of As and Se species in a single measurement. The availability of Se species in urine will provide key data to examine the association of individual Se species and gene-Se species interactions, rather than total Se only, on human health outcomes.