SLE an autoimmune systemic inflammatory disease predominantly affecting women. Mortality in SLE has a bimodal distribution, with a second peak due to cardiovascular disease (CVD) [¹]. The excess cardiovascular risk, which may be up to 52 times in SLE, is not explained by traditional cardiovascular risk factors [^2–4]. The identification of these novel risk factors is important to ensure development of targeted therapies to reduce cardiovascular morbidity and mortality in patients with SLE.

Recently, the roles of vitamin D beyond calcium homeostasis have been widely studied. The vitamin D receptor has been identified in both cells of the immune system (monocytes, antigen-presenting cells, macrophages, T cells) and the cardiovascular system (endothelial cells and vascular smooth muscle cells) [⁵, ⁶]. Epidemiological studies within the general population have demonstrated that vitamin D deficiency is an independent risk factor for adverse cardiovascular events [^7–9]. Furthermore, lower serum 25(OH)D is associated with the presence of subclinical CVD, including carotid IMT, coronary artery calcification and endothelial dysfunction [^10–15].

Vitamin D deficiency is more common in patients with SLE than in age- and gender-matched controls [^16–19]. This is perhaps unsurprising given that 90% of serum 25(OH)D is obtained from sunlight, and that photosensitivity is a common feature of lupus. Other studies have also suggested contributions from renal disease and the use of CSs [¹⁷, ^20–23].

The importance of vitamin D deficiency beyond its role in bone health in SLE is unclear and is the subject of current debate [²⁴]. Vitamin D has important immunomodulatory roles in vitro raising the possibility of disease-specific roles in SLE. Indeed, vitamin D deficiency has been associated with increased lupus disease activity in some [¹⁸, ²⁰, ²⁵, ²⁶] but not all studies [^27–31].

If vitamin D deficiency were to contribute to CVD in SLE, it would represent a readily modifiable novel risk factor. In this study, we aimed to determine whether vitamin D deficiency was associated with markers of subclinical CVD in patients with SLE.

We recruited SLE patients aged 18–70 years from regional outpatient clinics between January 2007 and January 2009. All patients fulfilled ≥4 ACR Revised 1997 Classification criteria for SLE. A full clinical history and physical examination was undertaken in all patients. Lupus disease activity was measured using the SLEDAI-2000 scale [³²]. Fasting blood samples were taken for measurement of lipid profiles and glucose, serum insulin, serum C3 and C4 complement, biochemical profile, full blood count and anti-dsDNA titre. Insulin resistance was calculated using the homeostatic model assessment (HOMA-IR) method [³³]. Vascular cell adhesion molecules (VCAM-1, E-selectin) were measured using DuoSet ELISA development kits (R&D Systems, Abingdon, UK). Serum 25(OH)D and PTH were measured in randomly selected patients using ELISA. Serum vitamin D deficiency was defined as 25(OH)D ≤20 ng/ml according to regional guidelines (J. Berry, personal communication). The estimated glomerular filtration rate (eGFR) was calculated using the Modification of Diet in Renal Disease (MDRD) equation and normal renal function was defined as an eGFR > 90 ml/min/1.73 m² [³⁴].

Aortic pulse wave velocity (aPWV) was measured using the automated PulseTrace 3600 (Micro Medical, UK) by sequentially recording ECG-gated carotid and femoral artery waveforms as described by Asmar et al. [³⁵]. Briefly, the distance from the sternal notch to the femoral artery was measured as a straight line between the points on the body surface using a tape measure. A 4 Mhz Doppler pencil probe was placed at 45° to the skin surface of the artery and measured with the probe facing towards the flow direction (towards the heart). The median (IQR) aPWV in a group of healthy female controls was 4.2 (2.9–9.2) m/s (S. Haque, personal communication). Carotid intima–media thickness (cIMT) and carotid plaque (CP) were measured using B-mode Doppler US by an experienced vascular sonographer. The mean carotid IMT was determined from three separate measurements on the far wall of the common carotid artery along a section 1 cm proximal to the carotid bulb on the left and right as previously described [³⁶]. CP was defined as present if two of the three following conditions were met: (i) a distinct area of protrusion >50% into the lumen compared with the surrounding wall; (ii) increased echogenicity; and (iii) IMT > 0.15 cm [³⁷]. Neither assessor was aware of the vitamin D status of the patient.

Statistical analysis was conducted using STATA v10.0 (StataCorp LP, TX, USA). Differences between vitamin D-deficient and non-deficient groups were determined using non-parametric tests (Mann–Whitney U-test). Linear and logistic regression models were used to investigate linear associations between 25(OH)D and markers of subclinical CVD. For linear regression, aPWV was log transformed to satisfy assumptions of normality. Logistic regression was used to analyse binomial variables.

Ethical approval for this study was obtained from the local research ethics committee (Northwest 5 REC, reference number 05/MRE08/62) and written consent was obtained in accordance with the Declaration of Helsinki.

This is the first study to demonstrate an association between 25(OH)D and subclinical CVD in patients with SLE. Serum vitamin D was inversely associated with aortic stiffness in a linear regression model, but not with either CP or IMT. This association persisted after adjustment for traditional CVD risk factors and BMI. Our results confirm those of Wu et al. [³⁸], who also found no association between vitamin D and CP or IMT.

Increased arterial stiffness, as measured by aPWV, predicts cardiovascular outcomes in the general population. A recent meta-analysis of 17 studies revealed that a 1 m/s increase in aPWV can increase the risk of a cardiovascular event by 14% [³⁹]. In our study, the difference in median aPWV between the vitamin D-deficient and -replete groups was 2.8 m/s. This suggests that vitamin D deficiency may significantly increase the risk of future CVD within the lupus population.

Vascular stiffness can be partly driven by inflammation, and better disease control in patients with inflammatory arthritis results in a reduction in pulse wave velocity [⁴⁰]. Selzer et al. [⁴¹] noted that inflammatory biomarkers in SLE were particularly associated with aPWV. In our study, the association between 25(OH)D and stiffness was at least in part accounted for by disease activity since in a regression model that includes SLEDAI score the association was no longer significant.

Patients with more active disease (SLEDAI score in the upper quartile of the range) had significantly lower 25(OH)D. There was no association, however, between 25(OH)D and serum C3 and C4 complement or anti-dsDNA titre. Furthermore, there was no association between 25(OH)D and use of immunosuppressants, anti-malarials or CSs. The precise mechanism by which this association may be mediated requires further study.

Our results suggest that the association between 25(OH)D and disease activity is strongest in those patients with the most active disease/lowest vitamin D. Given that 90% of vitamin D is synthesised in ultraviolet (UV) light-exposed skin, sunlight avoidance may contribute significantly to the association between D and disease activity. Photosensitivity is a common feature of SLE and the use of high-factor sun block is a mainstay of conservative management. Patients with active disease may also be less inclined to spend time outdoors due to feeling unwell, an example of reverse causality.

Vitamin D has, however, been shown to have important roles in the regulation of the inflammatory response. In vitro studies have shown that the active metabolite of vitamin D can modulate the inflammatory response by driving a Th1 cell response towards a Th2 response via inhibition of Th1 cell proliferation, reducing the production of inflammatory cytokines (IL-2 and IFN-γ), and the induction of regulatory T cells [^42–44]. Furthermore, autoantibody production from peripheral blood mononuclear cells isolated from patients with SLE is inhibited by vitamin D [⁴⁵]. Vitamin D deficiency may therefore augment the inflammatory response in SLE, underpinning both increased disease activity and vascular stiffness.

Insulin resistance (elevated serum insulin and HOMA-IR) and obesity were the only traditional cardiovascular risk factors to be significantly associated with vitamin D status. Increased BMI and insulin resistance have been associated with 25(OH)D in other studies of both adults and adolescents with SLE [³⁷, ³⁸] and within the general population [^46–48]. We have shown, however, that aPWV was associated with vitamin D independently of either BMI or serum insulin levels.

Vitamin D deficiency was common in our population of patients with established SLE [52% of patients had serum 25(OH)D <20 ng/ml]. This confirms the findings of others, although a direct comparison of serum 25(OH)D between studies is difficult due to variation in the study populations, study location and perhaps more importantly, in the methods used to measure vitamin D. In patients with SLE, mean levels of 25(OH)D have been reported from 11.5 to 44.6 ng/ml between different studies. Regardless of the absolute serum values reported, levels of vitamin D are invariably lower in SLE patients than in controls [^16–19].

Our study is limited by a relatively small study population and may therefore lack power to identify other associations between vitamin D and risk factors for CVD. Patients with vitamin D deficiency had a trend towards increased diastolic blood pressure, central obesity, lower HDL cholesterol and renal impairment, although none of these parameters reached statistical significance. Although an association was seen between aPWV and 25(OH)D following adjustment for season and traditional risk factors, we did not adjust for ethnicity in this study. Indo-Asians and Afro-Caribbeans have more severe SLE [⁴⁹, ⁵⁰], and in the general population, these ethnicities are also associated with lower vitamin D concentrations [⁵¹, ⁵²]. Whether the relationship between vitamin D and vascular disease differs in these subgroups beyond its influence on lupus disease activity cannot be answered in this study. A larger study will be needed to examine whether the relationships we have noted varies in different ethnic subgroups. Furthermore, due to the cross-sectional design of the study, only the 25(OH)D status at the time of assessment was measured.

Others have found no association between vitamin D and subclinical CVD in patients with SLE and our results are in apparent contradiction to this [³⁸]. In this context, it is, however, important to consider temporal differences in the development of vascular stiffness compared with carotid IMT/plaque. Whereas atherosclerosis develops slowly over a period of many years, vascular stiffness is a dynamic process that can change in response to therapy over a relative short period of time [⁴⁰]. A single measurement of 25(OH)D is unlikely to reflect its contribution to the development of increased IMT, which occurs over many years, resulting in an apparent lack of association. Furthermore, different risk factors appear to be associated with the development and progression of aPWV, CP and cIMT [⁴¹, ⁵³, ⁵⁴], suggesting different pathological processes. It could be concluded that our observations of aPWV reflect the study of an earlier and more dynamic pathological process than either CP or cIMT, and that aPWV may therefore be more responsive to current vitamin D status.

Vitamin D deficiency is common in SLE and is associated with vascular stiffness independently of traditional cardiovascular risk factors and insulin resistance. There is a strong inverse association between vitamin D deficiency and lupus disease activity which may, in part, drive this process in vitamin D deficient patients (Fig. 2). Prospective interventional studies and randomized controlled trials should focus on the effect of treating vitamin D deficiency on both SLE disease activity and vascular stiffness.

I.N.B. is supported by Arthritis Research UK, the Manchester Academic Health Sciences Centre and the NIHR Manchester Biomedical Research Centre.

Funding: This work was supported by an Arthritis Research UK Clinical Research Fellowship (grant ID 17574, to S.H.) and NIHR Manchester Biomedical Research Centre Clinical Fellowship (to J.A.R.).

Disclosure statement: The authors have declared no conflicts of interest.