1. Introduction
Vitamin D deficiency is a widespread pandemic worldwide, regardless of age, gender, and country of origin [
1]. Low 25(OH)D levels have been shown to be associated with many potential adverse health consequences, such as cancers, autoimmune disorders, infectious diseases, type 1 and 2 diabetes, neurological disorders, and cardiovascular illnesses [
1,
2,
3,
4,
5,
6,
7,
8]. Several studies have suggested an inverse association between 25(OH)D and biomarkers of cardiovascular risk, including an atherogenic lipid profile [
9,
10,
11,
12,
13,
14,
15]. The cardiovascular system is a target for vitamin D. VDR and 1-α-hydroxylase are expressed by cardiomyocytes, smooth muscle cells, and vascular endothelial cells, mainly fibroblasts[
16]. Local vitamin D activation by the VDR has various potential cardiovascular benefits, including reduced renin production [
17,
18], relaxation of vascular smooth muscle cells, and reduced output of atherosclerosis-forming foam cells [
19,
20]. Vitamin D can also reduce inflammation, which is integral to the development of cardiovascular disease (CVD)[
19,
21,
22].
Cardiovascular disease is the leading cause of mortality and morbidity worldwide, representing 32% of all deaths in 2019 [
23,
24]. In Morocco, 2 out of 5 deaths are attributable to CVD (38%), making it the leading cause of death nationwide [
25]. CVD is also the leading cause of death in women worldwide and was responsible for 35% of all deaths in 2019 [
26]. Contrary to popular belief, cardiovascular disease does not just affect older men and women. This type of problem also increasingly affects young women, with other alarming trends, such as the stagnation in the incidence and mortality of coronary heart disease and the increase in acute myocardial infarction, particularly in younger women (<55 years) [
27,
28,
29]. The national survey (Stepwise) on the risk factors (RF) of non-communicable diseases (NCDs) conducted by the Ministry of Health in 2018 showed that 94.3% of Moroccans aged 18 and over present at least an RF for NCDs, and the prevalence of cardiovascular risk is 4.9%[
30].
Dyslipidemia, which refers to elevated levels of total cholesterol (TC), triglycerides (TG), and low-density lipoprotein cholesterol (LDL-C), and decreased levels of high-density lipoprotein cholesterol (HDL-C) has been identified as significant risk factors for atherosclerosis and cardiovascular disease [
31,
32]. Several Cross-sectional studies have reported an association of vitamin D with lipid levels and CVD risk [
33,
34,
35,
36]. Nevertheless, the association between 25-hydroxyvitamin D concentrations and serum lipids is inconsistent in various studies [
37,
38]. It has been argued that different combines of these lipid profile parameters, such as the Castelli Risk Index-I (CRI-I), the CRI-II, the Atherogenic Coefficient (AC), the CHOLIndex and the Atherogenic Index of Plasma (AIP), are diagnostic alternatives for predicting the risk of developing cardiovascular events and identifying high-risk individuals[
39,
40,
41,
42,
43,
44,
45,
46,
47]. A few studies have assessed the relationship between vitamin D and atherogenic biomarkers. Moreover, most of these studies have been conducted on elderly patients, people with diabetes, or patients with metabolic syndrome[
48,
49].
Despite the increase in cardiovascular disease risk factors among young people in Morocco and the elevated vitamin D deficiency, particularly among adult women under 50 years of age, estimated at 78.8% in 2019 [
50], no study has addressed the association between vitamin D and the biomarkers of cardiovascular disease in this population. Thus, this study aimed to assess the relationship between vitamin D insufficiency, lipid profile and atherogenic indices in apparently healthy women aged 18-50 years, and to estimate the optimal 25 (OH) D threshold for maintaining lipid parameters and atherogenic indices at least at limit normal values.
4. Discussion
Vitamin D deficiency and insufficiency is an epidemic health problem worldwide. This study showed that 67.6% of women aged 18-50 suffered from vitamin D insufficiency below 20 ng/ml. This prevalence is roughly in line with that reported in other studies. In Morocco, the national nutrition survey [
50] revealed vitamin D insufficiency in 78.8% of women aged 18 to 49. In the United Arab Emirates, a study of migrant women from Arab and South Asian countries aged 18 and over revealed an overall prevalence of vitamin D deficiency < 20ng/ml of 67% [
64]. In a cross-sectional study in China, Wang et al. [
33] reported vitamin D deficiency in 63.7% of adult women. The causes of vitamin D deficiency in adult women could be explained by conservative clothing, lack of sun exposure, relatively high obesity, and low vitamin D dietary intake [
54,
65,
66,
67,
68,
69].
Hypovitaminosis D has recently been implicated in several health conditions, such as cardiovascular disease. Crowe et al. [
70], in a large prospective UK study of 180,263 patients aged 18 and over with no history of CVD, suggested that circulating 25(OH)D concentrations below 15 ng/ml are associated with an increased risk of CVD. A meta-analysis of 34 publications involving 180,667 participants found an inverse association between serum 25(OH)D levels and total cardiovascular events, and the pooled RRs per 10-ng/mL increment were 0.90 (95% CI: 0.86, 0.94) for total CVD events [
71].
Various explanations for the role of vitamin D in the development of cardiovascular disease have been suggested. Previous studies have supported that vitamin D may reduce the risk of CVD by inhibiting the renin-angiotensin system [
17,
18], decreasing parathyroid hormone levels, reducing inflammation, lowering coagulation, subsequently reducing atherosclerosis, and increasing insulin production[
19,
21,
22]. Vitamin D may also prevent cardiovascular disease by regulating several genes involved in cell differentiation, proliferation, apoptosis, and angiogenesis [
72]. In fact, another proposed mechanism for the protective effect of vitamin D on CVD is the control of lipid parameters [
11,
12,
33,
73,
74,
75,
76]
In the present study, lipid profile assessment revealed dyslipidemia in 33.3% of all participants. Non-parametric regression showed that 25(OH)D concentration was inversely associated with TC, TG, and LDL-C levels and positively associated with HDL-C. Per of 1ng/ml in 25(OH)D serum was associated with decreases of 1.53 mg/dl in TG, 0.55 mg/dl in TC, 0.49 mg/dl in LDL-C, and an increase of 0.22 mg/L in HDL-C. These results are consistent with previous research on the association between 25(OH) D and lipid parameters. In Southern Thailand, Jeenduang and Sangkaew [
35] showed that serum 25(OH)D was negatively correlated with TC, TG, and LDL-C levels in women. Wang et al. [
33] in a study of 646 Chinese adult women, showed that serum 25(OH)D levels were inversely associated with LDL-C and positively associated with TC after adjustment for age and BMI. However, there was no significant association with HDL-C and TG. The authors reported that each 10 nmol/L increase in 25(OH) D concentration was associated with a 0.25 mmol/L decrease in LDL-C and a 0.39 mmol/L increase in TC in women [
33]. In the Very Large Database of Lipids (VLDL) included 20,360 adults (≥18 years of age) in the United States, Lupton et al. [
77] showed that deficient serum 25(OH)D below 20 ng/ml was associated with significantly lower serum HDL-C (-5.1%) and higher TC (+9.4%), LDL-C (+13.5%), and TG (+26.4%) when compared with the optimal group.
Vitamin D insufficiency was associated with triglyceride dyslipidemia, with an OR (95% CI) adjusted for BMI, physical activity, and sun exposure of 3.876 (1.427-10.525), p=0.008. In contrast, there was no significant association of vitamin D status with TC, LDL, and HDL dyslipidemia. These results are consistent with those of Alquaiz et al.[
11], in their study of apparently healthy men and women, showed a significant excess odds ratio for elevated TG levels in association with 25(OH) D deficiency in women (AOR = 3.0; 95% CI = 1.1, 7.9), but not in men. Similarly, Jeenduang and Sangkaew [
35] showed that elevated serum 25(OH)D levels were associated with a reduced risk of hypertriglyceridemia and reduced HDL-C levels, particularly in women. Furthermore, our study revealed that a serum vitamin D level ≤ 15.15 ng/ml may predicts the likelihood of hypertriglyceridemia with a sensitivity of 77.19% and a specificity of 52.26%.
The pathophysiology of vitamin D deficiency leading to lipid profile is unclear. Multiple mechanisms could explain the relationship between vitamin D and lipid and lipoprotein concentrations. Previous data suggest that increased intestinal calcium absorption may reduce fatty acids in the gut due to the formation of insoluble calcium fatty acid soaps complexes, increase fat absorption and reduce hepatic triglyceride levels [
78]. Serum LDL-C levels are thought to be reduced by decreased fat absorption, particularly of saturated fatty acids [
79,
80]. Moreover, calcium may favor the translation of cholesterol into bile acids, thus reducing cholesterol levels [
81]. Vitamin D has also been shown to play a role in cholesterol transport by regulating apolipoprotein A-1 levels, and may act to reduce LDL-C uptake, decrease foam cell formation and increase HDL-C production [
82]. In addition, previous studies have provided strong evidence that vitamin D deficiency could be linked to altered B-cell function and insulin resistance, which may affect lipoprotein metabolism and eventually lead to elevated TG levels and decreased HDL-C levels [
83,
84]. Similarly, vitamin D can affect lipoprotein metabolism, with Larrick et al.[
85], demonstrating that by regulating the expression of genes involving lipid metabolism, vitamin D enhances HDL-C levels, decreases fatty acid synthesis and improves fatty acid β-oxidation, thereby lowering triglyceride levels.
An alternative explanation of this association may be related to confounding factors affecting both 25(OH)D and lipid levels. For example, obesity, lower amounts of physical activity and sun exposure. Our results showed that vitamin D deficiency and insufficiency was associated with high BMI, and low physical activity and sun exposure scores. These results are similar to other studies [
65,
86,
87,
88,
89,
90] . In addition, this study showed that BMI has positively significant association with TC, TG and LDL-C, which could increased risk of dyslipidemia and obesity-related cardiovascular events [
91,
92,
93]. In Morocco, there has been an alarming increase in the number of overweight and obese women (29.2% and 28.4%, respectively) [
50]. Associated with these changes in BMI, other pathologies may emerge, such as metabolic syndrome and diabetes, which increase the risk of developing CVD. Excess adipose tissue appears to be directly responsible for the dyslipidemia associated with abdominal obesity, by inducing an increase in the flow of free fatty acids to the liver and contributing to insulin resistance[
94].
Moreover, sun exposure and physical activity scores were found positively associated with HDL-C and negatively associated with TG levels. Weller [
95] demonstrated that exposure to sunlight has health benefits independently of Vitamin D. The skin contains significant stores of nitrogen oxides, which can be converted to NO by UV radiation and exported to the systemic circulation, thus causing arterial vasodilation, decreased blood pressure. Other studies suggest that enhancing the bioavailability of nitric oxide could have beneficial effects on obesity and the lowering of triglyceride levels [
96,
97]. Additionally, regular physical activity and fitness have shown inverse associations with cardiovascular disease in many observational studies [
98,
99,
100]. It has been shown that physical activity can lead to significant positive changes in lipid profile [
101,
102].
Other than these risk factors, the identification and validity of new measures of cardiovascular disease risk have attracted considerable recent interest. In fact, atherogenic indices, including AIP, AC, CRI-I, Non HDL-C and others, have been proposed as alternative biomarkers to diagnose individuals at CVD risk beyond the usual risk factors [
40,
45,
103]. The AIP was suggested by the National Cholesterol Education Program [
60], as a powerful predictor of atherosclerosis and coronary heart disease could be used as an effective mass screening method to identify patients at high risk of cardiovascular events [
103,
104]. Non-HDL-C was recommended by the 2019 ESC/EAS guidelines for the management of dyslipidemia as an effective biomarker for the estimation of cardiovascular risk in subjects with elevated TG levels, T2DM, obesity or a very high risk of cardiovascular events. low LDL levels [
105]. For CRI-I it has been particularly shown to reflect coronary plaque formation and intima-media thickness in the carotid arteries of young adults [
39].
The current study showed elevated cardiovascular risk according to atherogenic indices values, in particular AIP, CRI-I, non-HDL-C, and AC (60.7%, 59%, 43%, and 22.3% respectively). Molani et al. [
62], in adults healthy women reported that the prevalence of elevated AIP, AC, CI, CRI-I, CRI-II and non-HDL-C indices were 64.5%, 36.2%, 20.4%, 77%, 7.2% and 44.7%, respectively. According to AIP levels, Fernández-Macías et al.[
46], showed that around 55% of healthy Mexican women aged 18 and over were at moderate or high risk of cardiovascular disease. Wang et al. found a lower incidence among Chinese women, at 25% [
33].
25(OH)D values were negatively and independently correlated with AIP, AC, Non-HDL-C and CRI-I levels. Logistic regression showed that vitamin D insufficiency compared with the sufficient group increased significantly the risk of elevated AIP, AC, Non-HDL-C and CRI-I values with an OR (95% CI) of 3.637(2.149-6.158), p<0.0001; 3.589(1.673-7.700), p=0.001; 2.074(1.215-3.540), p=0.007, and 2.481(1.481-4.123), respectively, after adjustment for BMI. These findings corroborate those presented in other studies. Lupton et al. [
77] showed that serum 25(OH) D deficiency (<20 ng/ml) was associated with higher levels of non-HDL-C (+15.4%, +20.3 mg/dL). Barbalho et al. reported that patients with impaired vitamin D values had significantly higher values of CRI-I, CRI-II and non-HDL-C [
8]. Similarly, Pokhrel et al. [
107], demonstrated that 25 (OH) D was negatively associated with AIP, AC and non-HDL-C. In addition, in a longitudinal community study of 13,039 participants in the ARIC (Atherosclerosis Risk in Communities) study, Faridi et al. [
108], reported that deficient 25(OH)D was prospectively associated with greater CRI-I ratio after considering factors such as diabetes and adiposity.
A gender difference was reported by Wang et al. who showed that vitamin D levels were inversely significantly associated with AIP levels in men, but not in women [
33]. Huang et al. confirmed a relatively stronger association of AIP and vitamin D in men [
36], while, Izadi et al. showed a significant negative association of vitamin D and AIP in both sexes [
106]. Mahmoodi et al. [
49], in a case-control study among participants with T2DM found that all atherogenic indices including AIP, CRII, and AC decreased significantly with improving serum vitamin D status in men in the control group only. Although, the trends of the levels of atherogenic indices were irregular in case and control females with improving vitamin D status. Furthermore, studies in children and young adolescents revealed an inverse association between vitamin D levels and atherogenic indices [
109,
110,
111,
112]. Thus, maintaining adequate vitamin D status contributes to the early prevention of cardiovascular disease in individuals with or without risk factors.
ROC curve analysis showed that a decrease in vitamin D concentration independently predicted an increase in atherogenic index values. Thresholds predicting elevated AIP, AC, non-HDL-C, and CRI-I values were serum 25(OH) D ≤ 17.5 ng/ml, ≤ 19.8 ng/ml , ≤ 20.1 ng/ml, and ≤ 19.5 ng/ml respectively, all p<0.001. These results attest that serum 25(OH) D concentrations below 20 ng/ml imply an increased risk of short- or long-term cardiovascular disease. In contrast, Barbalho et al. [
8] showed that for an individual to maintain triglycerides, CRI-I , CRI-II, and non-HDL-C metabolic parameters, at least at borderline values, the levels of VD should be 37.64; which is above the reference values; implying that further in-depth studies are needed to determine the 25(OH)D threshold value predictive of dyslipidemia and high levels of atherogenic indices.
Our study has several limitations. Firstly, the generalizability of the results of our study to the general population is limited; the study sample was based solely on women from the city of Meknes. Due to the cross-sectional nature of this study, the results should not be considered in terms of cause and effect relationships. Another limitation is the absence of baseline data affecting lipid and vitamin D status, such as dietary intake, which can provide information on participants' daily lipid and vitamin D intake. Finally, although the sample size was sufficient to achieve the required significance, a larger sample could have strengthened the results.
Despite these limitations, the study is well founded. This is the first study to examine healthy adult women and evaluate the association of vitamin D with lipid and atherogenic profiles in Morocco. This could open up new avenues of research and diagnosis in the scientific and medical fields.