Association of Serum Calcium with Carotid Atherosclerosis: A Meta-Analysis

Carotid artery stenosis, a prevalent cerebrovascular disease, significantly impacts brain blood supply and, in severe cases, can lead to serious cerebrovascular events such as ischemic stroke [1]. It is often a result of carotid atherosclerosis and plaque formation. Various factors influence carotid atherosclerosis and artery stenosis, including non-modifiable risk factors like age, gender, race, and genetics, as well as modifiable factors such as smoking, alcohol consumption, obesity, hypertension, dyslipidemia, diabetes, physical activity, and emotional stress [2‒4]. Literature reviews reveal ongoing research into the relationship between serum calcium levels and carotid atherosclerosis. The findings to date have been mixed; some studies indicate a positive correlation between serum calcium levels and the severity of carotid stenosis [5], while others refute the notion that increased calcium intake or serum levels exacerbate carotid intima-media thickness or carotid atherosclerosis [6]. Therefore, we conducted a meta-analysis and systematic review to clarify the association between serum calcium and carotid atherosclerosis, aiming to contribute new insights for its prevention.

This study is registered with PROSPERO (registration number: CRD42023482551).

Our search strategy involved a combination of subject terms and free words. We explored PubMed, Embase, Cochrane Library, Scopus, Web of Science, China National Knowledge Infrastructure (CNKI), Chinese Biomedical Literature Database (CBM), Weipu (VIP), and Wanfang databases for studies published from inception to October 2023 on the relationship between calcium and carotid atherosclerosis The English search strategy included (1) various terms related to carotid stenosis, such as “Carotid Stenosis,” “Carotid Artery Narrowing,” “Internal/Common/External Carotid Artery Stenosis,” and “Carotid Artery Plaque”; (2) keywords pertaining to serum calcium, including “calcium supplements,” “calcium supplementation,” “calcium intake,” and “serum calcium.” The search was a combination of (1) and (2). In the Chinese search strategy, the terms were (1) various forms of “stenosis,” “atherosclerosis,” “plaque,” and “sclerosis;” (2) different iterations of “carotid” including “internal,” “external,” and “common carotid artery;” (3) “serum calcium,” “calcium intake,” “dietary calcium,” and “calcium supplementation.” The search combined (1), (2), and (3).

Studies included met the following criteria: (1) observational in nature; (2) inclusion of data on participants’ serum calcium levels or calcium supplementation; (3) outcomes related to carotid atherosclerosis, such as carotid intima-media thickening, stenosis, and plaque. Two researchers independently reviewed the literature based on these criteria, extracted relevant data, and cross-checked the findings. Any discrepancies were resolved by a third reviewer. The extracted data encompassed the first author’s name, publication year, country, study design, sample size, gender and age of participants, prevalence of carotid atherosclerosis, diagnostic criteria for stenosis, and primary study results.

Two researchers independently assessed the quality of the selected studies, with any ambiguities resolved by a third researcher. The quality evaluation criteria from the Agency for Healthcare Research and Quality (AHRQ) for observational studies were applied [7]. This assessment included 11 items such as definition of information sources, inclusion and exclusion criteria, patient continuity, blinding, quality assurance assessments, confounding factors, missing data, response rates, and patient completeness. Each item was rated as “yes,” “no,” or “unclear,” with “yes” scoring 1 point and the others 0. Item 5 was reverse-scored. The maximum score was 11, with ≥8 indicating high quality, 6–7 medium quality, and ≤5 low quality [8].

Review Manager 5.3 software was utilized for the analysis. The I² statistic assessed heterogeneity, and a forest plot was generated. A fixed-effect model was applied p > 0.1 and I² < 50%, indicating good homogeneity. Conversely, p < 0.1 and I² > 50%, indicating substantial heterogeneity, a random-effects model was used. Publication bias was examined using Egger’s test. Standardized mean difference (SMD), 95% CI, and mean or median of serum calcium levels were analyzed using the control group as a reference. Means not reported were estimated using medians and quartiles.

From an initial tally of 5,555 articles identified through a search of nine databases and manual examination of relevant bibliographies, 9 cross-sectional studies encompassing 9,720 individuals were selected for the final analysis (Fig. 1).

The analysis included 9 cross-sectional studies published between 2002 and 2023, comprising 7 articles in English and 2 in Chinese. The specific characteristics of these studies are detailed in Table 1. The quality of the included articles was appraised using AHRQ’s recommended standards for cross-sectional studies, categorizing all as medium or high quality. Specifically, 3 articles received a score of 8, 4 articles scored 7, and 2 articles scored 6. The details of this evaluation are presented in Table 2.

The serum calcium levels assessment in the included studies was compiled and is summarized in Table 3. Carotid artery stenosis, atherosclerosis, plaque, and carotid intima-media thickness thickening groups were the case group, and the group with normal calcium levels was the control group. These 9 cross-sectional studies used serum calcium levels as a key evaluation metric. In the subgroup analysis, notable differences in serum calcium levels were observed between the control group and the case group. Specifically, mean or median blood calcium levels were higher in the case group than in the control group, except for Yan et al. [17].

Significant heterogeneity was observed among the 9 articles included in this study (I² = 93%, p < 0.001), necessitating the use of a random-effects model. There was a large difference in serum calcium levels between the control group and the case group (p = 0.03), and overall the case group had higher serum calcium levels than the control group (Fig. 2a).

Subgroup analysis was conducted based on study region (Asia and Occident) and publication year (2002–2012, 2013–2023). The results were as follows: (1) study region subgroup – Asia [9, 10, 15‒17]: I² = 96%, p = 0.37, SMD = 0.19 (95% CI: −0.23 to 0.61); Occident [11‒14, 16]: I² = 0%, p < 0.01, SMD = 0.24 (95% CI: 0.16–0.32), with no significant difference between the groups (p = 0.82) (Fig. 2b). (2) Publication year subgroup – 2002–2012 [10‒12]: I² = 0%, p < 0.01, SMD = 0.16 (95% CI: 0.11–0.21); 2013–2023 [9, 13‒17]: I² = 95%, p = 0.29, SMD = 0.23 (95% CI: −0.19 to 0.65), with no significant difference between the periods (p = 0.76). (Fig. 2c).

With fewer than 10 articles included, the power to assess funnel plot symmetry was insufficient. Therefore, Egger’s test was conducted using R4.3.2 software, yielding a result of p = 0.744 > 0.05, indicating no potential significant bias.

Upon sequentially excluding individual studies, the I² varied between 83% and 93%, and the Hedges effect size varied from 0.15 to 0.33, aligning closely with the combined Hedges effect size of 0.21. These findings affirm the reliability of the meta-analysis results.

We found higher serum calcium levels in patients with carotid atherosclerosis than in controls, a meta-analysis of 9,720 participants (specifically obese/overweight individuals, postmenopausal women, and type 2 diabetes patients) from nine cross-sectional studies. This suggests a positive correlation between elevated serum calcium levels and carotid atherosclerosis, identifying high serum calcium as a potential risk factor, although one study showed a nonlinear negative correlation between serum calcium levels and the risk of carotid plaque. The association of serum calcium with carotid atherosclerosis is controversial.

The relationship between serum calcium and carotid atherosclerosis remains contentious. Variability in study populations, designs, methods, and evaluation approaches of research indicators contribute to these conflicting results. For instance, Montalcini et al. [13] studied 413 postmenopausal women aged 49–65, using carotid ultrasound and serum calcium measurements. They established a positive correlation between serum calcium levels, carotid atherosclerosis, and general risk factors such as age and systolic blood pressure through logistic regression analysis. Some studies have noted either a negative correlation or no correlation between serum calcium levels and carotid atherosclerosis. For example, a cross-sectional survey [17] observed a nonlinear decrease in carotid plaque risk with increasing serum calcium levels among postmenopausal women. Despite these varying findings, the consensus of our study leans toward a possible positive association between serum calcium levels and carotid atherosclerosis.

The results from a cross-sectional survey [17] reveal a nonlinear decrease in carotid plaque risk corresponding to elevated serum calcium levels, suggesting complexities in the influence of serum calcium on carotid plaque. Atherosclerosis develops through calcium mineral absorption and deposition, with increased calcium ion concentration in arterial wall cells being a contributing factor [18, 19]. Prior research identifies high serum calcium as an independent risk factor for arterial vascular calcification severity [20]. Calcium supplementation significantly raises serum calcium levels, potentially accelerating vascular calcification [21, 22] and favoring plaque formation [11, 23]. Additionally, serum calcium concentration correlates with increased carotid plaque thickness in the general population [11], and arterial calcification score is a predictor of atherosclerosis [24].

Our study only found differences in serum calcium levels between the carotid atherosclerosis group and the control group and did not conclude a causal relationship between serum calcium levels and the development of carotid atherosclerosis. Studies have shown that elevated serum calcium can increase or decrease the risk of carotid atherosclerosis [5, 6, 25], but whether the severity of carotid atherosclerosis will increase or decrease the serum calcium level is still unknown. In addition, the dose-response relationship between serum calcium level and the incidence and severity of carotid atherosclerosis has not been investigated. Therefore, further investigation into the mechanisms, causality, and dose-response relationship between serum calcium levels and carotid atherosclerosis is necessary, necessitating more prospective studies with larger, multicenter, and diverse populations.

Our study has certain limitations. First, it solely comprises cross-sectional studies, lacking diversity in research methodologies to robustly support the results. The small number of included studies also diminishes the reliability of the outcomes, rendering the verification of the relationship between serum calcium levels and carotid atherosclerosis less convincing. Second, the majority of the included literature focuses on menopausal women or patients with type 2 diabetes mellitus, thus lacking representation of the broader population. Furthermore, the findings might be confounded by factors such as age, sex, and health status, potentially obscuring the actual relationship between serum calcium levels and carotid atherosclerosis. Third, most of the literature included in our study focused on menopausal women or patients with type 2 diabetes mellitus, which lacked representation of the overall population. In addition, the findings may be confounded by other factors, such as age, sex, health status, and so may not fully reflect the true relationship between serum calcium levels and carotid atherosclerosis.

Despite the limitations, our study holds significant research value and implications. By synthesizing data from multiple studies, we observed that serum calcium levels in individuals with carotid atherosclerosis were higher compared to those without stenosis, and the incidence of carotid atherosclerosis was elevated in populations with higher serum calcium levels. These observations suggest a possible positive correlation between serum calcium levels and carotid atherosclerosis. Such findings provide a novel perspective and direction for future research and also offer a reference for clinical practice and the primary prevention of carotid atherosclerosis.

We thank the institutions and individuals who have supported our work. The views expressed in this publication are those of the author(s) and not necessarily those of their institution.

Ethical approval and consent were not required as this study was based on publicly available data.

All authors have no potential conflicts of interest to disclose.

This study was supported by the Natural Science Foundation of Shandong Province (Grant No. ZR2020MH140), Shandong Province Medical Health Science and Technology Development Plan Project (Grant No. 202012050480), and Shandong First Medical University (Shandong Academy of Medical Sciences) Youth Science Foundation Cultivation Grant Program (Grant No. 202201-029).

Huan Li: conceptualization and writing – original draft; Ruicai Xu: data curation and methodology; Yizhi Liu: formal analysis; Yanli Dong: validation; Dongyue He: investigation; Xiaohui Liu: funding acquisition and writing – review and editing; Qinjian Sun: methodology and writing – review and editing; and Xuena Liu: funding acquisition and writing – review and editing.

Huan Li and Ruicai Xu contributed equally to this work.

All data generated or analyzed during this study are included in this article. Further inquiries can be directed to the corresponding author.