Only five studies met eligibility criteria for this review,18–22 three of which were published within the past 3 years.20–22 The PRISMA flow chart summarising the data collection process is shown in figure 1. Reports from four separate cohorts, comprising three reports from two US cohorts,19–21 and two reports from two cohorts in Norway18 22 were included. Sample sizes ranged from 447 19 to 1311.22 The cohorts were either school18 20–22 or community based.19 Study design, methods and results are summarised in table 1.
