Bipolar disorder (BD) is a chronic mental disease related to recurrent episodes along with cognitive or functional decline. Comparing efficacy of pharmacological interventions targeting to treat cognitive deficit in BD is unexplored. The majority of the observational studies demonstrate a significant association between low serum vitamin D and compromised cognition in a wide range of neuropsychiatry diseases. Recent survey further revealed the high prevalence of hypovitamin D in BD. The effect of supplement and association between vitamin D and cognition in BD and the underlying mechanism is unclear.~The investigators will collect vitamin D levels and assess their association with cognition in a euthymic bipolar cohort (N=200) for screen. Then, the investigators estimate to recruit 120 of participants with cognitive deficits and combined vitamin D deficiency, to conduct a RCT. Participants will keep their usual treatment and will be randomized to receive either vitamin D supplement or a placebo for eight weeks. Cognitive test, adverse side effect, mood status and blood 25(OH)D levels and biomarkers will be measured before and after intervention.
Conduct randomized double-blind controlled trial (RCT), to examine the effects of vitamin D on cognitive performance in euthymic BD.