Vitamin D Deficiency, Childhood Allergic Disease and Asthma
Vitamin D Deficiency, Childhood Allergic Disease and Asthma
In the 2 years since the last review of this topic in this journal, the number of studies that has investigated the relationship between vitamin D and asthma and allergies has increased tremendously. An increasing number of studies have measured circulating 25OHD as a determinant of vitamin D status. However, several issues regarding the determination of vitamin D status need to be elucidated. Firstly, the level of 25OHD that determines optimal vitamin D status for asthma and allergies, and for overall health, remains elusive. Whereas the Institute of Medicine (IOM) has recently recommended that a 25OHD level of 50 nmol/l (20 ng/ml) should be considered sufficient, this was based mainly on studies of bone health and the IOM acknowledged that studies in other disease states are sorely lacking. There is dissenting opinion to the recommendations. Furthermore, there are data that suggest that optimal circulating levels may be much higher than the current IOM recommendations. Future studies will need to attempt to define appropriate levels rather than just using predetermined cut-offs in the analyses. Next, most studies have only measured one vitamin D level at one point in time. It is known that vitamin D levels vary over seasons, and likely over time. In an unpublished analysis in CAMP, where we measured 25OHD levels 4 years apart, the correlation between measurements is only r = 0.3, even when restricted to those whose blood draws were in the same season of the year. Thus, it is likely that levels at birth stay stable over time as the child grows. Future studies will need to measure 25OHD at multiple time points in relation to the outcome of interest.
Small trials of short duration have been conducted for asthma exacerbation and eczema. Larger trials, using adequate dosing, and of sufficient duration are needed to determine whether vitamin D supplementation decreases the risk for these exacerbations and more severe disease. Finally, results of trials of primary prevention of asthma and allergies are eagerly awaited.
Conclusion and Future Directions
In the 2 years since the last review of this topic in this journal, the number of studies that has investigated the relationship between vitamin D and asthma and allergies has increased tremendously. An increasing number of studies have measured circulating 25OHD as a determinant of vitamin D status. However, several issues regarding the determination of vitamin D status need to be elucidated. Firstly, the level of 25OHD that determines optimal vitamin D status for asthma and allergies, and for overall health, remains elusive. Whereas the Institute of Medicine (IOM) has recently recommended that a 25OHD level of 50 nmol/l (20 ng/ml) should be considered sufficient, this was based mainly on studies of bone health and the IOM acknowledged that studies in other disease states are sorely lacking. There is dissenting opinion to the recommendations. Furthermore, there are data that suggest that optimal circulating levels may be much higher than the current IOM recommendations. Future studies will need to attempt to define appropriate levels rather than just using predetermined cut-offs in the analyses. Next, most studies have only measured one vitamin D level at one point in time. It is known that vitamin D levels vary over seasons, and likely over time. In an unpublished analysis in CAMP, where we measured 25OHD levels 4 years apart, the correlation between measurements is only r = 0.3, even when restricted to those whose blood draws were in the same season of the year. Thus, it is likely that levels at birth stay stable over time as the child grows. Future studies will need to measure 25OHD at multiple time points in relation to the outcome of interest.
Small trials of short duration have been conducted for asthma exacerbation and eczema. Larger trials, using adequate dosing, and of sufficient duration are needed to determine whether vitamin D supplementation decreases the risk for these exacerbations and more severe disease. Finally, results of trials of primary prevention of asthma and allergies are eagerly awaited.
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