Association between lithium in tap water and suicide mortality rates in Miyazaki Prefecture

BACKGROUND: Most studies have reported that suicide mortality rates are negatively associated with lithium levels in tap water; however, a few studies showed either no association or a positive association. Thus, the association between suicide mortality and lithium levels in tap water remains controversial. To clarify the association, our study evaluated the association between lithium levels in tap water and suicide mortality rates in Miyazaki Prefecture of Japan, after adjusting for confounding factors. METHODS: We measured lithium levels in tap water across the 26 municipalities of Miyazaki Prefecture in Japan. We examined the standardized mortality ratio (SMR) for suicide in each municipality and used the data as the average suicide SMRs over 5 years (2009–2013). Weighted least-squares regression analysis, adjusted for the size of each municipality’s population, was used to investigate the association between lithium levels in tap water and suicide SMRs. In addition to a crude model, in an adjusted model, potential confounding factors (proportion of elderly people, proportion of one-person households, annual marriage rate, annual mean income, unemployment rate, the density of medical doctors per 100,000 people, annual total rainfall, and proportion of people with a college education or higher) were added as covariates. RESULTS: We showed that male and female suicide SMRs were not associated with lithium levels in tap water in Miyazaki Prefecture. After adjusting for confounders, male suicide SMRs were significantly and positively associated with the proportion of elderly people in the population and annual total rainfall, and female suicide SMRs were associated with the proportion of elderly people in the population. CONCLUSIONS: No association between lithium levels in tap water and suicide mortality rates was found in Miyazaki Prefecture.