Phthalates and Incident Diabetes in Midlife Women: The Study of Women’s Health Across the Nation (SWAN)

Phthalates are hypothesized to contribute to diabetes, but longitudinal evidence in humans is limited.

We examined whether phthalate exposure was associated with a higher incidence of diabetes in a racially/ethnically diverse cohort of midlife women.

In the Study of Women’s Health Across the Nation Multipollutant Study, we followed 1308 women without diabetes in 1999-2000 for 6 years. Eleven phthalate metabolites were measured in spot urine samples in 1999-2000 and 2002-2003. Incident diabetes was ascertained between 1999-2000 and 2005-2006. Cox proportional hazards models with time-varying exposure were used to estimate the hazard ratio (HR) of diabetes associated with each phthalate metabolite, adjusting for demographic, lifestyle, and health-related factors. Effect modification by race/ethnicity was examined with interaction terms.

Sixty-one women developed diabetes over 6 years (cumulative incidence = 4.7%). Among all women, several high-molecular-weight phthalate metabolites were associated with a higher incidence of diabetes, but none were statistically significant. There was effect modification by race/ethnicity. Among White women, each doubling of the concentrations of mono-isobutyl phthalate (MiBP), monobenzyl phthalate, mono-carboxyoctyl phthalate, mono-carboxyisononyl phthalate (MCNP), and mono(3-carboxypropyl) phthalate was associated with a 30% to 63% higher incidence of diabetes (HR = 1.30, 95% CI, 1.03-1.65 for MCNP; HR = 1.63, 95% CI, 1.18-2.25 for MiBP). In contrast, phthalates were not associated with diabetes incidence in Black or Asian women.

Some phthalate metabolites were associated with a higher incidence of diabetes over 6 years, but the associations were inconsistent across racial/ethnic groups. Whether phthalates cause diabetes requires further investigation.