Why allergy and vitamin D trials failed so far

The following figure highlights the exposure scheme in the three published vitamin D / allergy clinical trials so far: Grant 2016, Litonjua 2016 and Chawes 2016.

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None of them found anything important, maybe due to the design flaw in all these studies? All three studies have been designed with the assumption of a beneficial vitamin D effect. They are dose finding studies that do not allow to prove or disprove any vitamin D effect.