Several recent papers have discussed whether the health benefits of sun exposure are due to vitamin D production or other factors. This question has been raised since randomized controlled trials (RCTs) of vitamin D do not always support the findings from observational studies that report reduced risk of disease for those with higher vitamin D levels. This issue was famously proposed by Philippe Autier and colleagues in a 2014 paper, in which he suggested that “reverse causality”, i.e., the disease state could cause low vitamin D levels, could explain the findings from observational studies.
“The discrepancy between observational and intervention studies suggests that low 25(OH)D is a marker of ill health. Inflammatory processes involved in disease occurrence and clinical course would reduce 25(OH)D, which would explain why low vitamin D status is reported in a wide range of disorders.”
In my opinion, there is little evidence to support that hypothesis. In fact, John Cannell, Michael Holick and I reviewed papers reporting changes in biomarkers of inflammation in vitamin D RCTs. We found that for those studies where the baseline vitamin D level was >20 ng/mL (50 nmol/L), there was only a 26% chance that a beneficial effect would be found. However, if the baseline level was <20 ng/mL, there was a 50% chance. Thus, the discrepancies between observational studies and RCTs may simply be due to the fact that many vitamin D RCTs have been poorly designed, conducted and analyzed.
Additional evidence that “reverse causality” does not explain the discrepancy between observational studies related to vitamin D and RCTs was provided on January 17, 2017. An observational study of children who developed respiratory tract infections in Norway found inflammatory markers did not predict 25(OH)D status. Despite increased inflammation, vitamin D levels did not change during illness and after recovery.
Read the full article: “Health Benefits of Sun Exposure: Vitamin D and Beyond“