This trial was the first study, to our knowledge, specifically designed to assess the effects of vitamin D supplementation on key measures of cognitive and emotional functioning. Our findings indicate that vitamin D supplementation or increasing serum concentrations of 25OHD3 had no beneficial effects on (i) core executive functions of working memory, response inhibition or cognitive flexibility, (ii) psychotic-like experiences and hallucination proneness, or (iii) ratings of depression, anxiety or anger. This trial utilised an adequate dose of vitamin D, was placebo-controlled, was adequately powered for primary analyses and displayed high retention rates. As such, our findings appear to represent a true non-effect of vitamin D supplementation on cognitive and emotional functioning in healthy young adults.
These findings should be viewed in the context of an increasing number of studies associating low vitamin D status with impairments in mood and cognitive function and subsequent recommendations of widespread supplementation 
. The Institute of Medicine issued a recent report on vitamin D 
. This report drew attention to the fact that biological claims linking vitamin D status to brain-related outcomes have some biological plausibility, but observational studies related to these outcomes have been difficult to interpret due to residual confounding and/or reverse causality. In particular, individuals with depression and impaired cognition are prone to reduced outdoor activity, which in turn could lead to reduced vitamin D status. Even if low serum vitamin D was found to be causally contributing to cognitive and emotional impairments, it cannot be assumed that supplementation will ameliorate the impairment. The Institute of Medicine called for well-conducted, randomised controlled trials to examine whether changing vitamin D status can improve outcomes.
Our cognition findings support other studies which have examined the relationship between vitamin D status and cognitive functioning in young adults. For example, the NHANES study 
reported no association between vitamin D status and neurocognitive functioning in adolescents and adults. Fewer clinical studies have examined the relationship between vitamin D status and psychotic-like experiences. Our findings indicate that vitamin D supplementation is unlikely to influence psychotic-like experiences or delusion ratings in healthy volunteers. The relationship between vitamin D supplementation and mood is more complex. A number of controlled trials report no effect of vitamin D on mood 
. Two double-blind controlled trials do report some positive effects of vitamin D supplementation on mood 
. However, these studies have a number of limitations, such as using analyses that do not incorporate the effects of both treatment group and time 
, or reporting only minor reductions on depression measures that are unlikely to be clinically significant 
. As such, no well-conducted trials to date suggest that vitamin D supplementation is associated with clinically significant changes in mood. No previous studies have examined the relationship between vitamin D and other emotional states such as anger and anxiety.
Our study has a number of limitations. Our sample consisted of healthy young adults, who were free of major psychiatric illness and cognitive impairment. As such, our findings may not generalise to clinical populations exhibiting cognitive impairment or emotional disorders. Further controlled trials need to be conducted in key populations of interest, including those with established deficiency. Existing studies have not identified a putative ‘effective dose’ or ‘necessary threshold’ for vitamin D to improve brain function. The dose used in this study was higher than that used in many other studies, and thus, our negative findings are unlikely to have resulted from inadequate doses. The putative mechanism of action of vitamin D on the adult brain is not established. It is unclear whether vitamin D is active only in individuals who are deficient, or whether it also exerts specific pharmacological effects in those with adequate concentrations of vitamin D. Whilst our sample was adequately powered for primary analyses and the first stage of secondary analyses, it is possible that too few participants exhibited low baseline concentrations of 25OHD3 to conduct well-powered analysis in this potentially important subgroup. Analyses were not adjusted for multiple outcome analyses; however, this strengthens the likelihood that our negative findings represent a true lack of effect in this group. It is also feasible that low vitamin D status operates over many years, and that brain-related outcomes are ‘long latency’ disorders 
. Just as hip fracture and osteoporosis only emerge as adverse health outcomes associated with decades of vitamin D insufficiency, perhaps a similar latency is required for brain outcomes. There is robust evidence from in vitro and animal studies indicating that the active form of vitamin D has neuroprotective properties 
. Thus, it is possible that chronic hypovitaminosis D could leave individuals more vulnerable to subsequent neurobiological insults.
In conclusion, our findings indicate that vitamin D supplementation does not influence cognitive or emotional functioning in healthy young adults. Despite promising clues from observational studies, there are currently no clinical data that supports the use of vitamin D supplementation as a treatment for cognitive or emotional impairments. Although detection and treatment of vitamin D insufficiency remains important for a range of health outcomes (e.g. bone health), future controlled trials in targeted populations of interest are required to elucidate the causal contribution of vitamin D status to brain-related outcomes and determine whether supplementation can improve functioning in these domains.