Observational cohort

All methods and materials related to the human sample have been previously described [8]. This study is based on the population of Oita prefecture in the year 2006, at that time totaling 1,206,174 individuals that are distributed over 18 municipalities with differing individual population sizes.

Standardized mortality ratios

Taking the difference in gender and age distribution of individual municipality populations into account, the standardized mortality ratio (SMR) was calculated for each individual municipality. SMR is an indirect method of adjusting a mortality rate, defined as the number of observed deaths in an individual municipality population divided by the number of expected deaths, compared with the gender- and age-matched general population [8].

Tap water lithium levels

Lithium levels in the tap water suppliers of each municipality were measured using ion chromatography at Oita City Waterworks Bureau or by using mass spectroscopy at Oita Yakuzaishi Kensa Center [8]. Both methods have a sensitivity of 0.1 mg/L. Lithium levels of drinking water were measured at multiple water suppliers in the same municipality, and the median value was calculated for each municipality. Although lithium levels were typically measured only once per supplier, we observed a very small time-dependent fluctuation in concentrations: the correlation coefficient between the lithium levels and those re-quantified after one year in the same places was 0.998.

The distribution of median lithium levels was considerably skewed (skewness = 3.39; kurtosis = 12.80). We thus had to employ logarithmic transformation (thereafter: skewness = 0.002; kurtosis = 0.075) before applying parametric statistical procedures. Because of considerable differences in population size across the 18 municipalities, weighted least squares regression analysis, adjusted for the size of each population, was used to investigate the association of lithium levels in drinking water and the SMRs [8].

C. elegans life span analysis

The C. elegans strain used was Bristol N2. All experiments were performed exactly as previously described [9] except that streptomycin and 5-fluoro-2′-deoxyuridine were omitted, and synchronization was performed using an overnight egg-laying period (i.e. without applying bleach to eggs), as previously described elsewhere [10]. Living E.coli OP50 bacteria were used as the only food source. Analytical grade lithium chloride was purchased from Sigma–Aldrich (Munich, Germany).