Studies have repeatedly shown an association between low vitamin D levels and increased incidence of several different autoimmune diseases, including autoimmune thyroid diseases (ATD), type 1 diabetes, and Addison’s disease. Additionally, in certain autoimmune diseases, increased disease activity is associated with lower vitamin D levels.
While vitamin D has been studied in relationship to various autoimmune diseases individually, no study has examined the association between vitamin D and patients with multiple autoimmune diseases.
Autoimmune polyendocrine syndromes (APS) are a group of rare diseases that are characterized by coexistence of at least two endocrine autoimmune diseases.
Italian researchers recently decided to evaluate whether the existence of several autoimmune diseases further decreases vitamin D levels.
To determine this, they enrolled 35 patients with type 1 diabetes (T1D) and 60 patients with APS including T1D, from the University Hospital of the Second University of Naples. Ten of the APS patients were classified as type 2, which means they had Addison’s disease, an adrenal disease that is can be caused by autoimmunity, and T1D. The other 50 patients had type 3 APS, which means they had autoimmune thyroid disease (ATD) and T1D. They also selected 72 healthy participants to serve as controls.
The researchers then took blood samples and measured the vitamin D status of each participant.
When they ran analyses of the data, they discovered:
- Compared to control subjects, who had levels of 34.3 ng/ml, both T1D patients and APS patients had significantly lower vitamin D levels, with average levels of 26.3 ng/ml and 25.9 ng/l, respectively (p < 0.001).
- There was no significant difference between the vitamin D levels of T1D patients and APS patients.
- Among the patients with APS, all 8 cases with vitamin D deficiency (less than 20 ng/ml) belonged to the APS type 3 subgroup.
The research team concluded,
“To the best of our knowledge, this is the first study comparing vitamin D concentrations between patients with a single autoimmune disease, such as T1DM, and patients with APS including T1DM. We found that both groups of patients had circulating 25-OHD levels significantly reduced as compared to control subjects, but without significant differences in 25-OHD levels between the two groups of patients. These results suggest that the kind of autoimmune disease rather than the association of several autoimmune diseases, as it happens in APS, may influence negatively vitamin D status of affected patients.”
The researchers speculate that the reason why all 8 APS patients were vitamin D deficient was due to the presence of other autoimmune diseases associated with thyroid diseases and T1D, which could impair the absorption and activation of vitamin D in the skin, liver, and kidney.
The researchers also noted that vitamin D levels were surprisingly low in some of the control participants, stating,
“This seems to indicate that an impairment of vitamin D status is more frequent than that so far thought and that further studies should be promoted to better investigate the factors interfering negatively with vitamin D metabolism in subjects without diseases and with apparently normal sun exposure.”
This is an observational study, and therefore, the results do not show causality.
Further prospective studies are needed to determine if low vitamin D levels play a causal role in the pathogenesis of autoimmune diseases.