Encyclopedia
Please note this is a comparison between Version 1 by Laura Sabatino and Version 2 by Lindsay Dong.
Beyond its key role in calcium homeostasis, vitamin D has been found to significantly affect the cardiovascular (CV) system. In fact, low vitamin D levels have been associated with increased CV risk, as well as increased CV morbidity and mortality. The majority of effects of this molecule are related directly or indirectly to its antioxidative and anti-inflammatory properties. Generally, vitamin D insufficiency is considered for 25-hydroxyvitamin D (25(OH)D) levels between 21–29 ng/mL (corresponding to 52.5–72.5 nmol/L), deficiency as 25(OH)D levels less than 20 ng/mL (<50 nmol/L), and extreme deficiency as 25(OH)D less than 10 ng/mL (<25 nmol/L). However, the definition of an optimal vitamin D status, as defined by 25(OH)D, remains controversial for many extra-bone conditions, including CV disease.
- vitamin D
- 25(OH)D
- antioxidant
- determinants
- vitamin D status
- cardiovascular risk
- cardiovascular disease
1. Introduction
It is well known that the role of vitamin D extends well beyond the traditional effect on muscle and bone health, now including a number of extra-bone conditions, such as cancer, diabetes, and cardiovascular disease (CVD), many of them related to the antioxidant and anti-inflammatory in which this molecule is involved [1]. Thus, an increase in requests for serum total 25(OH)D, widely recognized as the most reliable marker of vitamin D status, has exponentially grown in the last few years as a consequence of its clinical value, in parallel to the improvement of methodological and standardization-related aspects. However, reference values of vitamin D are still not clearly defined. The Endocrine Society advised reaching serum 25(OH)D levels of at least 30 ng/mL (>75 nmol/L), preferentially to maintain levels in the range of 40–60 ng/mL (100–150 nmol/L) [2]. Accordingly, vitamin D insufficiency is generally considered for 25(OH)D levels between 21–29 ng/mL (corresponding to 52.5–72.5 nmol/L), deficiency as 25(OH)D levels less than 20 ng/mL (<50 nmol/L), and extreme deficiency as 25(OH)D less than 10 ng/mL (<25 nmol/L) [2]. Nonetheless, the Institute of Medicine (IOM) recommended 20 ng/mL as the threshold for physiologically adequate levels of 25(OH)D [3]. In any case, according to these categories, a large percentage of the entire world population can show inadequate serum 25(OH)D levels, especially when considering obese subjects, people with dark skin, and those insufficiently exposed to sunlight, making it necessary to assess the significance of these findings in terms of disease risk [4]. In addition, it has been estimated that a considerable number of children and adolescents are at high risk for vitamin D deficiency and insufficiency worldwide [5].
Thus, due to seasonal changes based on sunlight exposure, it would be preferable to evaluate the annual variation to adequately estimate the vitamin D status in each subject [6][22][23][24][25][14,44,45,46,47]. Indeed, approximately 85% of the world population lives at latitudes between the 40th parallel north and south and, consequently, these individuals are routinely exposed to sunlight [26][48]. The remainder population (15%) lives at higher latitudes, receiving relatively lower amounts of sunshine, and in late winter/early spring their vitamin D status typically declines and reaches its nadir [27][28][29][49,50,51].
2. Vitamin D Metabolism
UVB sun rays are the main source of vitamin D, whereas less than 10% derives from dietary intake (e.g., salmon, mackerel and herring, mushrooms, eggs, and fish liver oil), but may be also added to other foods or available as a dietary supplement. Skin exposure to solar UV irradiation induces photolysis of a derivative of cholesterol (7-dehydrocholesterol) into pre-vitamin D3, then isomerized to vitamin D3 (cholecalciferol) [1]. As a liposoluble/hydrophobic molecule, vitamin D3 requires the binding with a transporter protein (vitamin D-binding protein, VDBP) to circulate in the blood. Then, it is hydroxylated in the liver to form 25(OH)D, the major circulating metabolite [1]. In the kidney, hydroxylation catalyzed by the 1alfa-hydroxylase enzyme produces the active hormone, 1,25-dihydroxy vitamin D (1,25(OH)2D), while 24-hydroxylase (CYP24) promotes the production of inactive forms [1]. When released by its binding with DBP to the tissues, 1,25(OH)2D mediates a number of actions through its intracellular vitamin D receptor (VDR). The main objectives are control of calcium and phosphorus homeostasis (kidney and intestine as principal target tissues) and bone health and turnover. Although 1,25(OH)2D represents the active form, there is general agreement on the measure of 25(OH)D as the best index of vitamin D status [2]. Notably, 25(OH)D has a higher concentration in the bloodstream with respect to the active form 1,25(OH)2D and shows a longer circulating half-life if compared to 1,25(OH)2D (3 weeks vs. 4 h, respectively), thus providing more representative information about the vitamin D status. Moreover, 1,25(OH)2D values are regulated through PTH (upregulation) and higher serum calcium and phosphate levels (downregulation). Therefore, because vitamin D deficiency may induce secondary hyperparathyroidism, 1,25(OH)2D may result in reduced, normal or even elevated despite evidence of vitamin deficiency.
3. Methodological Determinants
3.1. Preanalytical Issues
25(OH)D is generally measured in plasma or serum samples, although serum is the most used. In some cases, 25(OH)D levels were found to be higher in heparinized plasma than in serum samples or in ethylenediamine tetraacetic acid (EDTA) plasma [6][7][8][14,15,16]. When tubes with gel are used, no impediment for immunoassay evaluation was registered, whereas interferences have been observed with high-performance liquid chromatography (HPLC) or mass spectrometry approaches [6][9][14,17]. Different storage conditions (fresh samples vs. up to 24 h at room temperature, different centrifuging times/temperature, multiple freeze–thaw cycles) did not significantly affect 25(OH)D values [6][8][9][10][11][12][7,14,16,17,18,19]. Long-term stability of 25(OH)D at −20 °C and −80 °C is generally acceptable, although a 15% variation after two months [13][20] or significant loss after four months at −20 °C was observed [6][14]. In addition, a variation of 25(OH)D levels after five years of storage also at low temperature (−80 °C) has been reported, which may be taken into consideration for studies involving sample long-term storage [14][21].3.2. Analytical Issues
There are several different analytical assays available for the determination of 25(OH)D, which include immunoassays (e.g., chemiluminescence immunoassay-CLIA and radioimmunoassay-RIA, high-performance liquid chromatography-HPLC with UV/fluorescence detection, liquid chromatography-mass spectrometry-LC-MS or tandem mass spectrometry-LC-MS/MS) [15][16][22,23]. Although LC-MS/MS maintains high analytical performance, aspects related to the high cost of instruments, time-consuming, limited throughput, and complexity of methodological problems requiring skilled professional staff, greatly limit the inclusion of this technique in clinical practice. Thus, the introduction of automated immunoassays has enabled rapid uptake of testing and the ability to respond to an ever-increasing demand for vitamin D testing (with the exception of RIA, due to available alternatives that avoid radiolabeled compounds). Nonetheless, these assays still present a highly variable analytical performance, and many cross-reactivity problems with several vitamin D metabolites as well as matrix effects (e.g., heterophilic antibodies) [14][21].4. Environmental Determinants, Lifestyle Habits, and Skin Pigmentation Affecting 25(OH)D Status
A variety of biological and environmental factors can influence vitamin D status in humans [17][39]. (Table 12). Synthesis of vitamin D in human skin occurs under ultraviolet exposure, thus any factor (e.g., season, latitude, time of day, cloud, ozone) influencing ultraviolet radiation levels, may significantly affect vitamin D production [18][19][40,41]. The day length, which is the period between sunrise and sunset, is dependent on latitude and time of the year, thereby peak levels of 25(OH)D are recorded following the summer months [20][21][42,43].Table 12.
Environmental, anthropometric, and lifestyle determinants affecting 25(OH)D levels.
| Environmental Determinants | Anthropometric Determinants | Life-Style Determinants |
|---|---|---|
sunlight exposure: intensity and duration season latitude length of day presence of clouds air pollution/ozone |
aging race/phototype gender body mass index/obesity genetic asset: presence of specific polymorphisms hepatic/renal dysfunction pregnancy |
dietary intake supplementation/fortified foods sunscreen/clothes time spent outdoor/outdoor sports |
5. Anthropometric Characteristics
Differences in age together with other factors, including vitamin D receptor gene polymorphisms, and constitutive skin pigmentation are responsible for a not negligible part (up to 15%) of the interpersonal variation in the UVB-induced 25(OH)D synthesis in the skin [30][73]. It is known that the ability to produce vitamin D3 is impaired in the elderly, with an aging-related progressive reduction in skin levels of 7-dehydrocholesterol [31][55]. This effect can be further exacerbated by reduced nutritional intake of vitamin D, increasing adiposity, less exposure to sunlight due to immobility, and staying indoors, all common factors in adult aging [31][32][55,74]. Additionally, as the kidney plays a central role in the regulation of vitamin D metabolism and circulating levels, a reduced renal function can lead to the inhibition of renal 1α-hydroxylase expression, upregulation of 24-hydroxylase and 1,25(OH2)D degradation, and the resulting vitamin D deficiency observed among patients with chronic kidney disease or undergoing dialysis [33][34][75,76]. Vitamin D status could also vary according to sex, although there is contrasting evidence as regards this association [35][77]. In this context, Muscogiuri et al. (2019) found that 25(OH)D levels were lower in females than males across all body mass index (BMI) categories [35][77]. 25(OH)D is lipophilic and it has been estimated that about 17% of orally-administered vitamin D dose is stored in adipose tissue and the rest is consumed or metabolized, indicating that adipose tissue acts both as a storage and buffering site of vitamin D [36][37][80,81]. Accordingly, clinical studies have found that obese individuals have a greater risk (35–40%) of vitamin D deficiency, regardless of age and latitude [38][39][40][82,83,84]. Vitamin D has been shown to affect adipocyte development, although results from in vitro and in vivo studies assessing the effect of vitamin D in adipogenesis are conflicting [37][41][81,85].6. Vitamin D and Genetic Determinants
Besides environmental and nutritional aspects, circulating levels of vitamin D are also influenced by genetic patterns. Many investigations on the causal role between vitamin D and several diseased conditions relied on studies on identical and non-identical twin pairs, which normally have similar trait-relevant environments [42][87] and allow better esteem taking into account genetics and/or environmental effects. These studies have shown that vitamin D concentrations are highly heritable, between 29% and 86% [42][43][44][87,88,89]. In general, the considerable variance in the evaluation of vitamin D heritability depends on many different reasons, such as age, gender, seasonality, and comorbidities [45][90]. Therefore, well-powered twin studies with reliable controls are fundamental in the determination of genetic contribution to the vitamin D circulating component. Over time, the studies of genetic determinants associated with the modification of vitamin D circulating levels in humans proceeded through several stages. The first classical approach was the linkage analysis of specific genetic intervals on the chromosome and their relation to the disease [46][47][91,92]. In general, linkage analysis conducted on affected subjects of the same family unveils which locus segregates with certain disease/phenotype; therefore, this approach may help to identify which chromosome region(s) may be associated with vitamin D variability in many diseases [46][91]. Differently from the linkage analysis-based studies, the candidate gene approach and genome-wide association study (GWAS) allowed the identification of reliable and reproducible associations with vitamin D circulating concentration. By candidate gene studies, it is possible to define if the frequency of a specific variant (single nucleotide polymorphism or gene) is associated with the variation of vitamin D levels, usually in the context of unrelated subjects. Several genes, closely related to vitamin D metabolism, have been studied: CYP2R1 and CYP27B1, involved in vitamin D hydroxylation [48][93]; GC, encoding for a vitamin D carrier protein [49][94]; VDR, coding for Vitamin D receptor [49][94]; CYP24A1, a cytochrome P450 gene [50][95]. GWAS constituted the major advancement in the identification of novel links between specific diseases and their biological determinants. Instead of focusing on a limited number of specific gene variations as in candidate gene studies, this innovative approach, which relies on the haplotype map of the genome and array-based advanced technology, rapidly explores the whole genome. The first GWAS of vitamin D consisted of 1012 related subjects from the Framingham Heart Study and genotyped 70,987 SNPs. However, because of the limited power and coverage of the analysis, none of the SNPs has passed the genome-wide significant p threshold at 5 × 10−8, obtained from Bonferroni’s correction for multiple testing [51][96]. Ultimately, important key findings on vitamin D genetics were obtained though large-scale international efforts to perform GWAS meta-analyses, involving large numbers of individuals from different cohorts, so as to obtain sufficient statistical power to reliably identify an association between circulating vitamin D levels and genetic determinants of several health outcomes [52][97].7. Vitamin D Mechanisms Related to Its Antioxidant/Antiinflammatory Action and Vascular Health
If a causal role of vitamin D for bone health is widely recognized, with vitamin D deficiency associated with most cases of rickets and osteomalacia, a number of genetic, molecular, cellular, and animal studies strongly suggest that vitamin D signaling has many extraskeletal effects, including regulation of cell proliferation, immune and muscle function, skin differentiation, and reproduction, as well as vascular and metabolic properties, which are not strictly related to calcium homeostasis [53][54][102,103]. On the other hand, these extraskeletal effects are characterized by some controversies due to conflicting results between observational and interventional studies [55][104]. Nonetheless, it is well established that vitamin D is able to modulate immune response (Th1/Th2 reduction) [56][105], while vitamin D deficiency has been associated with numerous conditions (e.g., multiple sclerosis, type 1 diabetes, rheumatoid arthritis, systemic lupus erythematosus, hepatitis, asthma, respiratory infections) and with an increased risk of any type of cancer and a reduced survival rate [55][57][104,106]. The biological effects of vitamin D are mediated by VDR, a member of the transcription factor superfamily of nuclear receptors which, upon activation by its binding to the active form of vitamin D and to a retinoid X receptor, translocate to the nucleus where it may regulate the transcription of vitamin D-sensitive target genes within hours or days [58][107]. Nitric oxide (NO), produced in the endothelium by endothelial NO synthase (eNOS), in addition to its potent vasodilatory effect, protects the vessels from developing atherosclerosis [59][115]. Experimental studies reported the ability of vitamin D to stimulate NO production via a direct increase of eNOS gene expression and activation of eNOS in intracellular calcium-dependent pathways (Figure 1) [60][61][116,117]. Vitamin D elicits a vasoprotective effect also through a decrease of oxidative stress (a major indicator of NO bioavailability and cause of damage to protein, lipids, and DNA), by upregulating expression of antioxidative enzymes and activating the nuclear factor erythroid 2-related factor 2 antioxidant pathway (Figure 1) [62][63][64][118,119,120].Figure 1. Main Vitamin D mechanisms of actions related to oxidative stress and inflammatory processes and vascular health. ↑ increase, ↓ decrease.
The chronic inflammation process, which is mediated by several factors, including cell-derived proinflammatory cytokines such as tumor necrosis factor-alpha (TNFα) and interleukin (IL)-1 and IL-6, contributes to the development of endothelial dysfunction, atherosclerosis, and CVD (Figure 1) [59][115]. The active metabolite of vitamin D has an anti-inflammatory effect through negative regulation of nuclear factor κB (NF-κB) and STAT1/5-mediated signaling, which leads to the downregulation of expression and production of several pro-inflammatory cytokines (TNF-α, IL-1, IL-2β, monocyte chemoattractant protein-1) [65][121].
Renin–angiotensin–aldosterone system (RAAS), a regulatory cascade having a crucial impact on the cardiovascular system tonus through the production of angiotensin II, increases vasoconstriction, extracellular volume, and cardiac output, and represents a major target of vitamin D (Figure 1) [66][67][123,124]. In mice, vitamin D was shown to suppress renin transcription by a VDR-mediated mechanism independent of extracellular calcium or phosphorus, which could block the cyclic AMP signaling pathway, a signaling pathway that plays a critical role in renin transcription and release in response to various physiological factors [68][69][70][125,126,127]. The fundamental role of vitamin D in regulating RAAS has been reported in animal models [69][70][71][126,127,128] but, with inconsistent results, in humans [72][73][74][129,130,131]. In fact, although most of the observational studies reported an inverse association between vitamin D and the incidence of hypertension [75][76][132,133], pooled results of randomized controlled trials (RCTs) showed that there was no significant reduction in systolic blood pressure or diastolic blood pressure following vitamin D supplementation in the general population [76][77][133,134] but may slightly decrease peripheral blood pressure in vitamin D-deficient patients [78][135]. Notably, the role of vitamin D deficiency in arterial hypertension could be also explained by decreased bioavailability of NO and atherosclerosis, and not exclusively by RAAS hyperactivation [58][61][79][107,117,136]. As for arterial stiffness, a strong predictor of CV events and all-cause mortality, in the meta-analysis of 18 RCTs by Rodriguez et al. (2016), no evidence of significant associations was found between vitamin D supplementation and reductions in pulse wave velocity (PWV) (Figure 1) [80][137].
8. Observational Studies
An inverse relationship between circulating vitamin D levels and different biomarkers related to oxidative stress and inflammation has been found in subjects with cardiometabolic risk or patients with CV disease. In particular, obese subjects (children and adolescents or adults) or T2D patients with hypovitaminosis D presented elevated levels of oxidative stress and inflammatory biomarkers, and an inverse correlation is also found between 25(OH)D and levels of different oxidative stress and inflammatory biomarkers [81][82][83][84][85][86][146,147,148,149,150,151]. In coronary artery disease (CAD) patients, an inverse relationship between vitamin D and homocystine (Hcy) was observed. Moreover, the association of Hcy with CAD severity was significant only among patients with hypovitaminosis D, suggesting that an adequate vitamin D status can prevent the adverse consequences of hyperhomocysteinemia on coronary atherosclerosis [87][152]. Always in CAD patients, an inverse association between gamma-glutamyltransferase (GGT, another oxidative-related biomarker) and 25(OH)D levels was found [88][153]. In acute myocardial infarction (AMI), vitamin D was inversely related to metalloproteinases (MMP-2) and leptin, biomarkers known as involved in CAD and AMI [89][154]. Accordingly, overall, most observational studies have reported an inverse association between vitamin D levels and CVD [54][103].
In order to assess the relevance of plasma concentrations of 25(OH)D for vascular mortality, a meta-analysis including 12 prospective studies (published up to January 2012) with 4632 vascular deaths, showed that subjects with 25(OH)D in the highest vs. the lowest quarter of distribution, had on average, 21% (95% CI: 13–28%) lower vascular mortality [90][157]. These results were supported by a following meta-analysis that used data from eight independent prospective cohort studies from Norway, Germany, Iceland, Denmark, and the Netherlands, for a total of 26,916 participants. After adjustment for age, sex, season of blood drawing, BMI, active smoker status, history of CVD, the authors reported that, compared to subjects with 25(OH)D concentrations of 75 to 99.99 nmol/L, the adjusted hazard ratios (HRs) for CV mortality in the 25(OH)D groups with 40 to 49.99, 30 to 39.99, and <30 nmol/L, were 1.65 (95% CI 1.39–1.97), 1.61 (95% CI 1.46–1.77), and 2.21 (1.50–3.26), respectively [91][158].
These findings are consistent with the increased likelihood of CVD risk factors in older adults, however, the heterogeneity of measurement assays of vitamin D across studies may represent a non-negligible confounding factor [92][159]. Conversely, within a previous dose-response meta-analysis of 34 prospective studies for a total of 180,667 participants, Zhang and co-workers found that for 10 ng/mL increment of serum 25(OH)D, pooled RRs were 0.90 (95% CI: 0.86–0.94) and 0.88 (95% CI: 0.80–0.96) for total CVD events and CVD mortality, respectively, although it should be evidenced that the number of participants with high concentrations of serum 25(OH)D was small [93][160].
