Influence of vitamin D status on respiratory infection incidence and immune function during 4 months of winter training in endurance sport athletes [conference abstract] HeCheng-Shiun HandzlikMichal K. FraserWilliam D. MuhamadAyu S. PrestonH. RichardsonA. GleesonMichael 2014 Influence of vitamin D status on respiratory infection incidence and immune function during 4 months of winter training in endurance sport athletes [conference abstract]