## 6 Critical Appraisal and Considerations for Future Trials
Much like we would not include slim individuals in a trial examining obesity reduction, if we are to optimize the detection of potential benefits from vitamin D supplements, participants with sufficient vitamin D levels should not be included in trials examining the efficacy of vitamin D supplementation. This is even more pertinent in vitamin D trials as baseline 25(OH)D can influence the half-life of ingested vitamin D, since higher concentrations can lead to faster catabolism of vitamin D to maintain equilibrium [88]. Therefore, despite a lack of consensus regarding the inflection points or optimal concentrations required to achieve health gains in pregnancy, at the very least, RCTs should aim to recruit participants with vitamin D deficiency (<50 nmol/L) and provide supplementation regimens adequate for achieving sufficiency (>75 nmol/L). A detailed discussion of this issue is provided by [PERSON] [89], where, in addition to recruiting participants who are vitamin D-deficient, it is recommended that trials provide a sufficiently large dose of vitamin D to shift their status into the range where disease risk decreases (or benefit increases), and assess temporal changes in vitamin D status to verify the adequacy of the dose and adherence. As noted above, the exact 'point' at which disease risk shifts can vary depending on the disease in question and is generally not known, especially in pregnancy. Moreover, the exact dose and regimen required to achieve sufficiency also varies and can heavily influence results. For instance, bolus doses may lead to short-lived peaks in intracellular vitamin D, followed by longer periods of insufficient vitamin D levels, resulting in an overall absence of physiological effects [90]. Similarly, a daily dose of 400 IU for several months will achieve little or no detectable effect on circulating vitamin D, whereas extended periods of 2000-6000 IU daily can maintain stable concentrations in the range of 25-100 nmol/L in a linear fashion [91; 92]. These factors have clear implications in vitamin D trials but remain to be fully understood within the pregnancy context, emphasising the need for further research to clarify the optimal 25(OH)D concentrations for pregnancy-related benefits and the doses needed to achieve these. An important caveat to note with regard to 25(OH)D levels is the ethical dilemma introduced by providing placebo in the presence of deficiency. Ethics review committees may require that participants in the control arm also be given vitamin D, particularly in the case of pregnancy where deficiency (especially severe deficiency) can pose undue risk to the mother or fetus. While the vitamin D doses in prenatal vitamins are usually low (e.g., 200-400 IU daily), they are still high enough to shift control participants out of severe vitamin D deficiency, hence impacting the power to detect differences as noted above. An alternative would be to store blood samples at baseline for measurement of 25(OH)D later once the trial is completed. However, while this can be justified on the basis of equipoise about the effect of vitamin D supplementation on most outcomes, this is not possible during pregnancy as treatment of severe deficiency is critical for preventing infantile complications such as rickets. The use of multivitamins or low dose vitamin D supplements in the control group is therefore inevitable and will always be an issue for vitamin D trials in pregnancy, one which requires appropriate planning and consideration in the design phase of these trials. Closely linked to the point above regarding baseline 25(OH)D status and target levels, the third issue with vitamin D trials, both in and outside of pregnancy, is that they do not follow the traditional model for conducting clinical trials, where the evaluation of interventions is entirely under the control of clinicians and researchers. Unlike drug trials for example, participants in vitamin D trials have easy access to vitamin D supplements, widely available from pharmacies, supermarkets and online.