In addition, importantly, no significant association was found with total vitamin D levels, suggesting that vitamin D2 levels were not changed in patients with PD (they may even have been higher). The authors offer no explanation for these seemingly conflicting results - in fact, they do not mention this in their discussion at all.
This failure to discuss important negative findings, combined with the borderline statistical significance and the lack of correction for multiple comparisons mean this study should be treated with a degree of scepticism.
- Confounding - some additional, unmeasured factors may account for the association
- Generalisability - patients and controls were drawn entirely from two hospitals in a small area of Massachusetts and are therefore not representative of all patients with Parkinson's disease. Further studies in minority populations that may be at even higher risk of vitamin D deficiency are necessary.
- Correlation does not imply causation - the association found in this study could be explained if vitamin D deficiency accelerates PD pathology or if PD predisposes to vitamin D deficiency, because of a reduced tendency to go outdoors (sunlight is needed for the body to synthesise vitamin D).