Participants

This study was a double-blind, placebo-controlled, randomised clinical trial conducted from February 2013 to October 2013 in Dunedin, New Zealand (45° 52′0 S, similar to the latitude of Montreal, Canada; or Lyon, France in the northern hemisphere). Young adult women aged 18–40 years were recruited from February 2013 to April 2013 through posters, leaflets, Facebook posts and direct emails to staff of major employers in the city. Women who were not currently pregnant or breastfeeding, had access to the Internet, and were willing to provide a repeated blood sample were eligible for the study. Specific exclusion criteria included current/planned vitamin D supplementation (including as part of a multivitamin supplement); chronic liver and kidney disease; arteriosclerosis or cardiac function impairment; sarcoidosis and other possible granulomatous diseases; medication, including anticonvulsants, glucocorticoids and barbiturates that might affect vitamin D metabolism; and, overseas travel during the study period (February–October 2013). Based on a power analysis, we determined that a sample size of seventy-six persons per group would provide an 80 % chance of detecting a difference of five points in the average total Center for Epidemiologic Studies Depression Scale (CES-D) depression score between the groups at a two-sided significance (P) level of 0·05, with an assumed standard deviation of 9·67 (based on Crawford et al. 2011⁽,41⁾), and a loss to follow-up of 20 %. A total of 152 participants were randomised into the study and randomly assigned to consume vitamin D supplements (n 76) or placebo supplements (n 76) for 6 months. The study was conducted according to the guidelines outlined by the Declaration of Helsinki. The University of Otago Human Ethics Committee (no. 13/031) approved the study, and informed written consent was obtained from all participants. The trial was registered with the Australian New Zealand Clinical Trials Registry (registration number: ACTRN12613000540718).