**7. Role of vitamin D in antiphospholipid syndrome**

Although APS represents acquired, autoimmune condition, its pathophysiology and, espe‐ cially pathophysiology of thrombosis in APS is highly heterogeneous, involving different genes and acquired factors [86], VitD insufficiency/deficiency being among them.

Same as for relationship between MetS and APS, much more is known about the impact of VitD status on antiphospholipid syndrome, associated with autoimmune rheumatic diseases, than on primary antiphospholipid syndrome. Patients with PAPS represent the population of particular interest for the investigation of interrelations with components of MetS and/or VitD status since these patients, unlike those with sAPS, were not treated with drugs (i.e. cortico‐ steroids, immunosuppressants) which may affect expression of most of the MetS characteris‐ tics as well as VitD level.

One of the first announcements on the prevalence of VitD insufficiency or deficiency in PAPS and their impact on its manifestations dated from 2010 [87]. This letter to the editor presented the results of research conducted by Brazilian investigators in the group of forty‐six PAPS patients, younger than 60 years, in whom the VitD levels were assessed in the autumn, when it was expected to be highest. VitD deficiency was found in 11% and insufficiency in 74% of these PAPS patients, resembling the findings of Italian authors [88] which have reported the prevalence of VitD deficiency in 17% and insufficiency in 60% of PAPS patients. It is interesting that Brazilian authors have noticed that VitD insufficient PAPS patients tended to be more overweighed than those with adequate VitD level [87]. Also, it seems that thrombotic APS is characterized with significantly lower concentrations of VitD than purely obstetric clinical syndrome (20.8 vs. 33.3 ng/ml, p<0.01) [88] highlighting once again the role of this vitamin in thrombosis. High prevalence of VitD deficiency among patients with APS (49.5%) and its significant correlation with thrombotic events were confirmed by Israeli authors [68]. In vitro, they have also demonstrated VitD ability to inhibit anti‐β2‐glycoprotein I autoantibody (anti‐ β2‐GPI Ab)‐mediated TF expression [89].

(r=‐0.044, p=0.461) [74], low density lipoprotein (LDL)‐ (r=‐0.005, p=0.932) and high density lipoprotein (HDL)‐cholesterol (r=0.065, p=0.276) [74]. Interestingly, hypothesis was proposed ten years ago stating the possibility that statins could be the analogues of VitD, acting via same receptors, particularly when we are talking about their mutual effect of enhancement of immune competence [78]. So, it seems that this absence of association between VitD status and

Another component of MetS for which association with VitD status has not been unequivocally confirmed is hypertension. Variability of blood pressure driven by the seasons or latitude speaks for the existence of this association, as well as the results of experimental studies pointing to VitD as an inhibitor of the renin‐angiotensin‐aldosterone axis [79, 80]. Negative correlation between VitD concentrations and blood pressure was demonstrated in most but not all of the surveys. This negative association was stronger in subjects younger than 50 years [81–83], while the absence of any relationship between VitD status and hypertension was also registered in some of the trials [74, 76, 84, 85], particularly those conducted in older subjects [84, 85]. However, in postmenopausal women with the VitD concentrations in the lowest quartiles corresponding to its severe deficiency, odds ratio (OR) for hypertension was 1.81 (95%

Although APS represents acquired, autoimmune condition, its pathophysiology and, espe‐ cially pathophysiology of thrombosis in APS is highly heterogeneous, involving different

Same as for relationship between MetS and APS, much more is known about the impact of VitD status on antiphospholipid syndrome, associated with autoimmune rheumatic diseases, than on primary antiphospholipid syndrome. Patients with PAPS represent the population of particular interest for the investigation of interrelations with components of MetS and/or VitD status since these patients, unlike those with sAPS, were not treated with drugs (i.e. cortico‐ steroids, immunosuppressants) which may affect expression of most of the MetS characteris‐

One of the first announcements on the prevalence of VitD insufficiency or deficiency in PAPS and their impact on its manifestations dated from 2010 [87]. This letter to the editor presented the results of research conducted by Brazilian investigators in the group of forty‐six PAPS patients, younger than 60 years, in whom the VitD levels were assessed in the autumn, when it was expected to be highest. VitD deficiency was found in 11% and insufficiency in 74% of these PAPS patients, resembling the findings of Italian authors [88] which have reported the prevalence of VitD deficiency in 17% and insufficiency in 60% of PAPS patients. It is interesting that Brazilian authors have noticed that VitD insufficient PAPS patients tended to be more overweighed than those with adequate VitD level [87]. Also, it seems that thrombotic APS is characterized with significantly lower concentrations of VitD than purely obstetric clinical syndrome (20.8 vs. 33.3 ng/ml, p<0.01) [88] highlighting once again the role of this vitamin in

genes and acquired factors [86], VitD insufficiency/deficiency being among them.

parameters of cholesterol metabolism made this hypothesis shaky to some extent.

**7. Role of vitamin D in antiphospholipid syndrome**

CI: 1.15–2.85) [77].

192 A Critical Evaluation of Vitamin D - Clinical Overview

tics as well as VitD level.

Seasonal variations in VitD concentrations were demonstrated in PAPS patients same as in healthy controls, with preserved differences in its level between these two groups through all seasons [88, 90]. These differences were most pronounced during summer, while they were not statistically significant only during the spring. This observation was somewhat surprising, given the lack of banning from sun exposure in these patients. That sun avoidance is not a cause of highly prevalent VitD deficiency and insufficiency in PAPS patients was indirectly demonstrated in previous Italian studies [88, 90] by observed absence of any difference in VitD levels between antinuclear antibodies (ANA)‐positive and negative PAPS patients.

Until now, there is no valid explanation for the probable cause‐and‐effect relationship between insufficient VitD level, on one side, and PAPS or sAPS, on the other. There are only assump‐ tions, and even they are much better clarified for sAPS [91–93], especially that accompanying SLE [91, 94, 95]. It is obvious that low levels of vitamin D in sAPS could not be attributed purely to banning of sun exposure or the use of certain medication in these patients. In an Israeli and European cohort of patients with SLE, significant negative correlation (r=‐0.12, p=0.018) was demonstrated between the serum VitD concentrations and disease activity, assessed by SLE disease activity‐2000 (SLEDAI‐2K) and European Consensus Lupus Activity Measurement (ECLAM), which were converted into standardized z‐value [94]. Severe VitD deficiency was found in 17.89% of 123 SLE patients with short disease duration, while the presence of renal disease (OR 13.3, 95% CI 2.3–76.7, p<0.01) and photosensitivity (OR 12.9, 95% CI 2.2–75.5, p<0.01) were its strongest predictors [95]. Investigation conducted in a small group of young women with newly diagnosed SLE, from one of the sunny places in Iran, gave very interesting results. VitD deficiency was highly prevalent among these patients, mild in 12.5%, moderate in 62.5% and severe in 17.5% of them [96]. It was much more pronounced in them than in general population of the similar age in that region, in whom mild VitD was present in 15.5%, moderate in 47.1% and severe in 7.1%. Very interesting was also an observation that serum VitD concentrations showed significant negative correlation with another disease activity score, the British Isles Lupus Assessment Group (BILAG) (r=‐0.486, p=0.001), in spite of the short duration of disease [97]. Hypothetical explanation for the low serum concentrations of VitD in SLE patients by the existence of inhibiting anti‐VitD antibodies in circulation could not be confirmed by the literature data [97, 98]. Their existence could be proven in 4–6% of patients with SLE and 3.5% of APS patients. Its association with anti‐dsDNA (p=0.0004) could point to its potential role in this condition, but being only one of 116 different antibodies present in SLE patients characterized by the polyclonal B lymphocyte activation, it is still uncertain if it is pathogenic [97]. It seems that their presence did not affect VitD levels in these patients [97, 98], and it was speculated that they could be the consequence of high‐dose VitD consumption rather than the cause of this vitamin deficiency [99].

Once again, it is important to emphasize that VitD deficiency is more pronounced in more severe APS phenotypes, i.e. thrombotic APS [88]. It could be speculated that supplementation of this vitamin in these very patients may have certain beneficial effects [88, 99], but there is still no prospective studies proving them. Hypothesis of statins as VitD analogues has not still been tested in well‐designed, randomized prospective trials [78]. However, since its proposal, there have been many experimental and small clinical studies confirming statins therapeutic value in APS patients, particularly in those with its thrombotic form [99–103]. So, future studies are badly needed to determine all the aspects of VitD repletion in APS prevention/therapy (choice between VitD precursors, its active form or VitD analogues, their dosage and treatment goals).

## **8. Key messages**

