**2. Basic definitions**

Metabolic syndrome (MetS) and antiphospholipid syndrome (APS) are among most prevalent and still highly controversial syndromes. While clinical relevance of antiphospholipid antibodies (aPL) was recognized more than 30 years ago, definite classification criteria for antiphospholipid syndrome were given at the International Workshop in Sapporo, Japan 1998 [4] and revised 2006 in Sidney, Australia [5]. Very interesting proposal of APS criteria based on biological mechanisms is presented lately aiming at simplicity and greater accuracy and, at the same time, avoiding non‐specific formulations [6] (**Table 1**). Recent investigations have also shown that, beside characteristic thrombotic or obstetric symptoms, there is growing number of systemic non‐criteria manifestations (for example, thrombocytopenia, livedo reticularis, skin ulcerations, pseudovasculitis, migraine and epilepsy) correlating with certain type of aPL and with important predictive role [7, 8]. It is likely that a prominent place among these manifestations belongs to components of MetS, but it is still to be proved. The prevalence of APS in the general population is estimated to be around 2–4%.

Initial Reaven's postulate in 1988, which draw attention to the causal association between insulin resistance with ensuing hyperinsulinemia and cardiovascular diseases [9], was followed by numerous definitions of MetS. Three of them, i.e. definitions given by World Health Organization (WHO) [10], the National Cholesterol Education Program Adult Treatment Panel III (NCEP ATP III) [11] and International Diabetes Federation (IDF) [12], were most frequently used and still neither of them is obsolete. While all three definitions share central obesity, atherogenic dyslipidaemia and arterial hypertension as common criteria,WHO definition put the insulin resistance in focus of metabolic syndrome while an obligatory criterion requested by IDF definition is elevated waist circumference (WC) with population‐ and country‐specific cut‐offs (**Table 2**). All of these three definitions are very similar but different enough, especially when used forthe assessment of prevalence of MetS in some other entities, in this case, among patients with APS. Even the latest joint attempt of several major professional organizations (the IDF Task Force on Epidemiology and Prevention, National Heart, Lung and Blood Institute, American Heart Association, World Heart Federation, International Atherosclerosis Society and International Association for the Study of Obesity) to unify interconnected cardio‐metabolic risk factors into a universal definition of metabolic syndrome did not seem to be final [13].


**Table 1.** Antiphospholipid syndrome definitions.

**1. Introduction**

182 A Critical Evaluation of Vitamin D - Clinical Overview

patients.

**2. Basic definitions**

The antiphospholipid syndrome (APS), primary or associated with certain autoimmune rheumatic diseases, especially systemic lupus erythematosus, represents prothrombotic state. Coexistence of metabolic syndrome (MetS) and autoimmune rheumatic diseases is already recognized [1, 2], while clinical significance of MetS in patients with APS has not been system‐ atically studied [3]. Recent recognition of certain pleiotropic functions of vitamin D (VitD) has enabled us to hypothesize on its role in the pathogenesis of obesity, MetS, APS, autoimmunity and thrombosis. Therefore, the aim of this review will be: (1) to clarify the possible linking role of VitD between APS and MetS, (2) to critically assess the need for estimation of VitD status in APS patients, depending on the coexistence of MetS and (3) to explore the potential therapeutic role which VitD, as an immunomodulator and anti‐thrombotic agent, could have in these

Metabolic syndrome (MetS) and antiphospholipid syndrome (APS) are among most prevalent and still highly controversial syndromes. While clinical relevance of antiphospholipid antibodies (aPL) was recognized more than 30 years ago, definite classification criteria for antiphospholipid syndrome were given at the International Workshop in Sapporo, Japan 1998 [4] and revised 2006 in Sidney, Australia [5]. Very interesting proposal of APS criteria based on biological mechanisms is presented lately aiming at simplicity and greater accuracy and, at the same time, avoiding non‐specific formulations [6] (**Table 1**). Recent investigations have also shown that, beside characteristic thrombotic or obstetric symptoms, there is growing number of systemic non‐criteria manifestations (for example, thrombocytopenia, livedo reticularis, skin ulcerations, pseudovasculitis, migraine and epilepsy) correlating with certain type of aPL and with important predictive role [7, 8]. It is likely that a prominent place among these manifestations belongs to components of MetS, but it is still to be proved. The prevalence

Initial Reaven's postulate in 1988, which draw attention to the causal association between insulin resistance with ensuing hyperinsulinemia and cardiovascular diseases [9], was followed by numerous definitions of MetS. Three of them, i.e. definitions given by World Health Organization (WHO) [10], the National Cholesterol Education Program Adult Treatment Panel III (NCEP ATP III) [11] and International Diabetes Federation (IDF) [12], were most frequently used and still neither of them is obsolete. While all three definitions share central obesity, atherogenic dyslipidaemia and arterial hypertension as common criteria,WHO definition put the insulin resistance in focus of metabolic syndrome while an obligatory criterion requested by IDF definition is elevated waist circumference (WC) with population‐ and country‐specific cut‐offs (**Table 2**). All of these three definitions are very similar but different enough, especially when used forthe assessment of prevalence of MetS in some other entities, in this case, among patients with APS. Even the latest joint attempt of several major

of APS in the general population is estimated to be around 2–4%.

Similar ambiguity exists concerning the definition of adequate circulating VitD level, as well as of its deficiency and insufficiency. Earlier definition of VitD insufficiency by its blood level of <20 ng/mL (50 nmol/L), given by the World Health Organization (WHO) [14], has been recently accepted by most researchers as a definition of the deficiency of this vitamin [15, 16]. Its insufficiency is defined as a VitD concentration between 20 and 30 ng/mL (50 and 75 nmol/ L), while its concentrations >30 ng/mL (75 nmol/L) are regarded as sufficient [17, 18].


**Table 2.** Metabolic syndrome definitions—similar but different enough.
