Do studies reporting 'U'-shaped serum 25-hydroxyvitamin D-health outcome relationships reflect adverse effects?
Level 5 - mechanism / opinion, no new human data
Narrative review synthesizing mechanisms and potential confounding across observational literature
PubMed 27489574 · doi:10.1080/19381980.2016.1187349
What was done
The author reviewed published literature examining U-shaped relationships between serum 25-hydroxyvitamin D [25(OH)D] concentrations and various health outcomes—including falls, fractures, cancers, cardiovascular disease, cognitive function, all-cause mortality, birth outcomes, allergic reactions, and frailty—evaluating whether these patterns reflect genuine biological mechanisms or confounding.
What was found
The abstract reports no numerical data. Qualitatively, it states that while some U-shaped associations appear biologically plausible (such as allergic reactions potentially driven by immune phenotype shifts toward Th2 balance), many likely stem from confounding factors, including lifestyle and late self-supplementation started after disease onset. For prostate cancer, 25(OH)D concentration showed little effect on incidence but was inversely correlated with mortality rates.
Why it matters
This review cautions researchers and clinicians against assuming high normal 25(OH)D levels are inherently harmful, emphasizing the need to adjust for baseline self-supplementation and solar UVB exposure in future studies.
Limits
The abstract describes a non-systematic narrative review with no defined search protocol, study count, sample size, or quantitative effect estimates. It relies largely on observational data prone to residual confounding and reverse causation.
Cited by
- contradicts Studies demonstrate that serum 25-hydroxyvitamin D levels around 80 ng/mL are associated with lower all-cause mortality.