Vitamin D (VD) is a fat-soluble secosteroid hormone, having both autocrine and endocrine roles [1
]. While the main roles of VD include calcium homeostasis and bone metabolism [2
], the presence of vitamin D receptors (VDR) in major cell types of the body gives it multiple extra-skeletal functions, one of which is modulation of inflammatory pathways [4
The anti-inflammatory and immune-modulating properties of vitamin D (VD) are well-established [6
]. Multiple studies consistently reveal the beneficial effects of VD supplementation in terms of increasing levels of anti-inflammatory markers and decreasing the production of inflammatory cytokines [7
]. In a recent systematic review and meta-analysis involving 10 clinical trials and 924 participants, Chen and colleagues concluded that supplementation with VD can decrease C-reactive protein (CRP) levels, a well-known acute-phase inflammatory marker predictive of cardiovascular events, by as much as 2.21 mg/L [10
]. Other inflammatory markers have been investigated, such as (IL)-10, IL-6 and TNF-α, all of which have been observed to be significantly associated with varying levels of 25(OH)D status [8
Another acute-phase inflammation-induced protein is the serum amyloid P component (SAP), not to be confused with serum amyloid protein. Together, SAP and CRP are the short pentraxins chiefly produced by hepatocytes [13
]. In humans, SAP contributes to host defense, either via opsonins or through complement activation. In a calcium-dependent way, SAP binds to several lipoprotein ligands, which suggests that this process could have significant inferences in amyloidosis and atherosclerosis in humans. Moreover, many studies support the fact that SAP has a significant role in inflammatory regulation. [14
]. Significantly, SAP and CRP share structural characteristics (being organized in five identical subunits arranged in a pentameric radial symmetry) and biological functions, including activation of the complement system and pathogen recognition [13
]. In calcium-free conditions, SAP pentamers physically interact with CRP pentamers to form very stable mixed decamers [15
], which could have functional consequences on inflammation activation [16
]. In a nested case-control proteomic analysis study, sera from 60 obese women with gestational diabetes mellitus (GDM) identified three candidate predictors of GDM: SAP, afamin, and vitronectin [17
]. Lastly, for cardiovascular disease (CVD), SAP is considered as a valuable biomarker, as it contributes to CVD pathogenesis through modulating innate immunity and inflammation [18
We hypothesize that improving VD status can favorably regulate SAP activity. In this single-arm trial, we aim to evaluate for the first time the effects of vitamin D supplementation on serum SAP levels in Saudi adults with VD deficiency.
A total of 210 (75 males and 135 females) Saudi adults deficient in vitamin D were included in this 6-month interventional study. Table 1
shows the clinical characteristics of participants before and after intervention for responders and non-responders to VD supplementation. At baseline and using the Bonferroni-corrected p
-value, responders were significantly older than non-responders (p
= 0.007). Similarly, WHR measures were significantly higher in responders than non-responders (p
< 0.001). Baseline BMI, blood pressure, and other parameters were not significantly different between groups.
Post-intervention, 25(OH)D and HDL-cholesterol levels significantly increased after 6 months (p
-values < 0.001 and 0.007, respectively) in the responders group. In contrast, SAP levels significantly decreased post-intervention (p
= 0.002), as well as CRP levels (p
= 0.014). No significant changes were observed in other parameters. Among non-responders, no changes in 25(OH)D levels were observed post-intervention. The same non-significance was observed for SAP and CRP levels. Total cholesterol, HDL-cholesterol, and triglycerides all significantly increased after intervention (p
-values = 0.006, 0.003 and 0.02, respectively). For the rest of the other parameters, no significant differences were observed (Table 1
shows the between-group comparisons of both responders and non-responders. Serum 25(OH)D increased over time, and this was clinically significant in favor of the responders, even after adjusting for age and BMI (p
< 0.001). A clinically significant decrease in SAP levels was observed over time, again in favor of responders, and this effect remained significant even after adjusting for age and BMI (p
shows comparisons of responders’ characteristics pre- and post-intervention according to sex. Levels of 25(OH)D significantly increased over time in both sexes (p
< 0.001). Similarly, HDL was significantly increased in both sexes. SAP was significantly decreased over time in both sexes; remarkably, this reduction in SAP was more significant in males [55.7 (31.2–78.4) vs. 57.3 (27.7–100.9), p
= 0.01] than in females [28.9 (1.4–62.4) vs. 38.4 (1.3–74.1), p
= 0.046]. Conversely, CRP was significantly reduced post-intervention in females [7.8 (4.4–32.4) vs. 22.2 (3.9–61.6), p
= 0.036], but no significant difference was observed in males. No reduction in glucose levels was observed in both sexes; contrary to what was expected, glucose had a significant increase in males [5.87 ± 0.9 vs. 5.61 ± 0.9, p
= 0.029], with no significant difference observed in females.
shows the bivariate correlation coefficients of SAP with other study parameters in responder participants at baseline, where SAP had a significantly positive relationship with systolic BP (r = 0.20, p
< 0.05) and diastolic BP (r = 0.33, p
< 0.01) in our study participants overall. This relationship was observed for diastolic BP only in males (r = 0.30, p
< 0.05) after stratification according to sex. At baseline, SAP also had a significant inverse correlation with HDL-cholesterol (r = −0.30, p
< 0.01). Overall, this clinically significant inverse correlation persisted in females (r = −0.37, p
< 0.01) but not in males after stratification according to sex. In addition, SAP had a significantly positive correlation with glucose (r = 0.32, p
< 0.05) in males at baseline, as well as with CRP overall and in both sexes (p
Post-intervention, SAP was inversely correlated with VD overall (r = −0.17, p < 0.05) and only in males (r = −0.27, p < 0.05) after stratification according to sex, whereas such a relationship was not observed at baseline. Triglycerides had a significant positive correlation with SAP only in females post-intervention (r = 0.23, p < 0.05) but not in males.
shows the delta change correlation analysis between SAP and other parameters. Overall, there was a significant inverse relationship between Δ SAP and Δ HDL (r = −0.30, p
< 0.01), and it was positively correlated with Δ CRP (r = 0.28; p
< 0.01) in our study population. After stratification according to sex, Δ SAP was inversely correlated with Δ HDL (r= −0.31; p
< 0.05) and Δ triglycerides (r = −0.27; p
< 0.05) only in males.
shows the responders’ characteristics pre- and post-intervention using the SAP cut-off values, a normal reference interval for serum SAP concentration, for both sexes (males; 32 mg/L and females; 24 mg/L) [23
]. Of the participants, 98 (42 males and 56 females) had high values of serum SAP above referenced normal levels. Over time, 25(OH)D significantly increased in both sexes (p
< 0.001). Remarkably, post-supplementation with VD, the reduction in SAP serum levels was more significant in this sub-group compared to the main group in both sexes (in males, −9.5 (−33.9–7.1), p
= 0.007 vs. −1.75 (−21.7–7.4), p
= 0.011; in females, −13.9 (−33.3–2.2), p
< 0.001 vs. −0.57 (−16.5–1.2), p
The present interventional study is, to our knowledge, the first to show a clinically significant reduction in serum SAP levels after 6 months of VD correction. Remarkably, the post-intervention reduction in serum SAP levels was even more significant than without applying the cut-off values. At baseline, SAP levels were inversely correlated with cardiometabolic factors, such as BMI and HDL-cholesterol, and positively correlated with blood pressure, with no association between VD and SAP. However, at post-intervention, our results showed a significant inverse correlation between SAP and VD among responders, and this significant correlation persisted in males after stratification for sex.
The link between SAD and VD based on the present results is most likely tied to their associations with cardiometabolic factors. SAP has a key role in innate immunity and cardiometabolism [24
]. Furthermore, like VD, it is also directly influenced by calcium [25
]. In a calcium-dependent manner, SAP binds to many different lipoprotein ligands, and this can have a significant contribution in the progression of amyloidosis and atherosclerosis [26
]. In fact, it has been found in the plaques of advanced human atherosclerosis and is proposed to have an active role in atherogenesis [28
]. Previous studies indicated a significant increase in SAP levels in the early phase of post-acute myocardial infarction [29
]. Furthermore, SAP deficiency prevents the atherosclerotic process [30
] and other pathological processes, such as fibrosis, hypercoagulation, and inflammation [24
]. Lastly, pentraxins including SAP have been demonstrated to be involved in obesity and other states of a chronic low-grade inflammatory [32
]. Hence, VD supplementation can reduce the cardiovascular risk associated to overweight and obesity by reducing the pro-inflammatory pentraxin SAP.
Another highlight of the present study is the significant inverse correlation post-supplementation between SAP and VD only among male responders, which highlights sex-specific extra-skeletal properties of VD correction. Previously, we found that VD deficiency and its association with cardio-metabolic risk factors were mostly limited to males, in a study which involved more than 3000 Saudi adolescents and adults. This led us to believe that correction of VD status could prove more beneficial to men than women, at least in terms of extra-skeletal benefits [20
]. One explanation that we have also recently documented at the proteomic level is that the conversion of 25(OH)D to its active form, 1,25(OH)D2, is higher in men than women, and this can be linked to the sex hormone metabolism [33
Lastly, it is worthy to discuss that the primary grouping variable used in the present study to elicit differences between circulating SAP was the participants’ response to vitamin D supplementation. Despite monitoring all participants for compliance and adherence, it was anticipated that some will not be able to achieve full vitamin D sufficiency despite large boluses of vitamin D. The failure to achieve full vitamin D correction despite above-average supplementation has been a consistent dilemma in Saudi Arabia and the rest of the region, and this has been fully acknowledged by national and regional experts, prompting vitamin D guidelines unique to the Middle-Eastern region and the Gulf Cooperation Council (GCC) countries in particular (21, 22). A recent genetic study within the Saudi community that could partially explain the non-responsiveness to exogenous vitamin D sources are the variants in vitamin D binding proteins (rs7041 and rs4588), carriers of which are three to 12 times more likely to be non-responders to vitamin D treatment [34
The authors acknowledge some limitations. First, we used the non-responders as our comparator group, since we wanted to clearly delineate that the modest but significant changes in circulating SAP was associated with acute changes in vitamin D status brought about by a favorable response to vitamin D supplementation. Furthermore, since VD deficiency is very common in Saudi Arabia, the use of a true control group (without supplementation) is inappropriate, given that the inclusion criteria are participants with VD deficiency. Whether the present results will be the same using a control group remains to be investigated. Second, important factors influencing VD status were not measured in the current study, such as sunlight exposure, season, and outdoor physical activity, and as such, essential adjustments were not carried out. Nevertheless, this is the first study of its kind to investigate the effects of VD supplementation on SAP levels.