I wonder whether the researchers used the wrong vitamin D metabolites, when testing to see whether vitamin D supplementation was as good as UVB, they dosed them with 25OHD3 or 1,25OH2D3. Neither of these are now thought to be relevant metabolites for vitamin D's effect on the immune system. All the cells apart from those involved with calcium metabolism (bone, gut, kidney) are believed to utilise plain vitamin D3, not 25OHD3 and certainly not 1,25OHD3. If they had simply dosed the animals daily with vitamin D3, maybe the effects would have been different? see: Hollis, B.W. and Wagner, C.L., 2013. The role of the parent compound vitamin D with respect to metabolism and function: why clinical dose intervals can affect clinical outcomes. The Journal of Clinical Endocrinology & Metabolism, 98(12), pp.4619-4628.
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If they had simply dosed the animals daily with vitamin D3, maybe the effects would have been different?
see: Hollis, B.W. and Wagner, C.L., 2013. The role of the parent compound vitamin D with respect to metabolism and function: why clinical dose intervals can affect clinical outcomes. The Journal of Clinical Endocrinology & Metabolism, 98(12), pp.4619-4628.