**6. Role of vitamin D in metabolic syndrome**

Net result of numerous effects of this vitamin on different haemostatic factors is its antith‐ rombotic role. Prothrombotic state that exists in VitD receptor knockout animal models proves the importance of these extra‐skeletal effects of VitD as well as the observation that all of them

However, there are still relatively few indirect and even less direct clinical evidences for the association between VitD status and thrombotic events in humans. First of them came from the epidemiological studies in which have been observed that cardiovascular morbidity and mortality depended on season of the year and latitude [47, 57, 58]. Seasonal variations were also demonstrated for tissue plasminogen activator (tPA) antigen, fibrinogen, D‐dimer and von Willebrand factor (vWF) concentrations in 6538 British subjects without significant cardiovascular disorders, aged 45 years [59]. In this population, negative correlation between VitD level and tPA, fibrinogen and D‐dimer concentrations was observed indicating that at least some of the seasonal variations of these thrombotic markers could be attributed to the VitD status. More direct proof for the association between VitD status and thrombosis came from the research conducted in huge population of 18 791 subjects from general population of Copenhagen [60]. Authors have observed that every quartile of a decrease in VitD concentra‐ tions was accompanied by an increase in risk of venous thromboembolism (VTE), resulting in a 37% increased VTE risk between subjects with the VitD concentrations, in the lowest quartile

Recent publication which retested the seasonality of different cardiovascular events in regard to VitD levels, in the Scottish Heart Health Extended Cohort (SHHEC), brings a dose of confusion in previously proposed relations. Namely, it failed to prove that seasonal appearance of cardiovascular events resembled seasonal variations in serum VitD concentrations nor that these events expressed more pronounced seasonality in those with lower VitD concentrations, compared with those with its higher concentrations [61]. But, during follow‐up, significant correlations were observed between lower baseline concentrations of VitD and subsequent incident cardiovascular morbidity and incident cardiovascular and all‐cause mortality [61].

Results of recent trials assessing the effects of VitD supplementation on the risk of throm‐ boembolism were inconclusive [62–64]. In the Multiple Environmental and Genetic Assess‐ ment (MEGA) case‐control study which included 2506 patients with venous thrombosis, thrombotic risk was 37% lower in those supplemented with various vitamins including VitD [62]. However, in a large cohort of postmenopausal women (n=36282) from the Women's Health Initiative, daily supplementation with calcium and VitD failed to reduce the overall risk of thromboembolism [63]. Even when high doses (300,000 IU) of VitD were given intra‐ muscularly, in a small group of patients with proven deep vein thrombosis and pulmonary embolism, observed reduction in plasma concentrations of P‐selectin and high‐sensitive C‐ reactive protein (hs‐CRP) did not reached statistical significance [64]. Additional information could be expected from the ongoing Vitamin D and OmegA‐3 Trial (VITAL) and that is why

the results of this investigation are eagerly awaited [65].

are VitD receptor‐mediated [47, 56].

190 A Critical Evaluation of Vitamin D - Clinical Overview

and those in highest quartile.

Currently, increasing prevalence and co‐existence of obesity, MetS and hypovitaminosis D represent serious public health concern [66, 67]. New data have considerably changed hierarchy of MetS components, with the shift of the focus from obesity and insulin resistance, firstly toward fatty liver and now toward VitD deficiency [68].

It is still questionable if relationship between VitD status and obesity is unidirectional or bidirectional, with the accumulating evidence favouring the influence of VitD on body composition and not vice versa. Namely, few years ago tempting hypothesis on essential role of VitD in evolvement of obesity has been postulated [68]. It started from a situation that is completely opposite to the "thrifty genotype hypothesis" proposed long ago [69] and gave the feasible explanation not only for obesity and MetS epidemic in adults but also for their growing prevalence among children [70]. According to this hypothesis, we are living in "obesogenic" environment, loaded with energy resources and unsuitable for efficient metabolism. It has been proposed that VitD as an ultraviolet (UV)‐B radiation sensor (i.e. decline in its concentrations) could induce shift toward "winter metabolism", characteristic for MetS [68]. If this is true, then it could be expected that VitD supplementation may be efficient in prevention and treatment of obesity and MetS. Significant decrease in body fat mass after 12 week of VitD repletion (25 μg of cholecalciferol daily), compared to placebo (−2.7+2.0 kg vs. −0.4+2.0 kg, p<0.001), could be the proof for this presumption [71]. It was also speculated that VitD deficiency during pregnancy could lead to the epigenetic changes predisposing, in that manner, new‐born children to obesity and MetS later in life [68, 70]. Experimental support for these assumptions is the expression of VitD receptors on adipocytes and its involvement in adipogenesis which is regulated by the intracellular concentrations of VitD [72], as well as inhibition of lipid accumulation in adipocytes and their atrophy achieved by the knock‐down of VitD receptors [72, 73].

Nowadays, VitD deficiency is common even in general population (49%), but significantly more prevalent (p=0.006) and quite similar in overweight/obese patients with MetS (72%) and without MetS (69%) [74]. Premise that exaggerated adiposity could lead to VitD insufficiency or deficiency by its seclusion within adipose tissue could not be confirmed. It has been shown that VitD concentrations varied considerably (range 4–2470 ng/g) in the subcutaneous ab‐ dominal fat of 17 severely obese patients (BMI=48.7+8.1 kg/m2) undergoing bariatric surgery [75]. In spite of the average weight loss of 54.8 kg after one year and continuous VitD supple‐ mentation with more than 2500 IU/day, mean serum VitD concentrations did not change significantly during this period (23.1+12.6 vs. 26.2+5.36, p=0.58) [75].

Most of the studies have confirmed that serum VitD concentrations significantly inversely correlated with obesity parameters, BMI (r=‐0.159, p=0.007) [74], or waist circumference (p<0.001) [76] as well as with serum triglycerides (r=‐0.149, p=0.012) [76]. In the lowest quartiles of VitD concentrations corresponding to its severe deficiency, odds ratio (OR) for hypertrigly‐ ceridemia was 2.74 (95% CI: 1.64–4.57) [77]. This association between serum concentrations of VitD and triglycerides could be explained by the activation of lipoprotein lipase by VitD in adipocytes [76]. No significant relation could be demonstrated between VitD status and total‐ (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 parameters of cholesterol metabolism made this hypothesis shaky to some extent.

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% CI: 1.15–2.85) [77].
