Effect of fasting status on association between exposure to phthalate and phenolics, and thyroid hormones in adult women of reproductive age.

Diet is one of the important exposure sources for many urinary chemicals that have been investigated in association studies for thyroid hormone outcomes. For these chemicals, the fasting status can substantially affect the results of urinary biomonitoring of the study population. Such variability presents challenges for cross-sectional association studies, particularly when the substances of concern have short excretion half-lives. This study aims to investigate how fasting influences the association of urinary levels of phthalate metabolites and phenolics, with thyroid hormones among Korean women of childbearing age. In the total population, comprising both fasting and non-fasting groups, monobenzyl phthalate (MBzP) and di(2-ethylhexyl)phthalate (DEHP) metabolites showed negative associations with total and free T4, and positive associations with total T3 and the estimates of peripheral deiodinase activity (GD). In the non-fasting group, while the overall direction of associations remained consistent, the association of phthalate metabolites became more pronounced. In contrast, the significant associations disappeared in the fasting group. The non-fasting group showed higher levels of most phthalate metabolites than their fasting counterparts. For parabens, similar negative associations with free T4 and total T4 were observed in both fasting and non-fasting groups. These observations suggest that diets are more important exposure sources of phthalates, and hence, fasting may bias the exposure profile of these chemicals and the results of association studies. This study underscores the need to consider fasting status in association studies of urinary chemicals with short half-lives to prevent the potentially misleading influence of fasting on these associations.