Relationship and Effects of Vitamin D on Metabolic Syndrome: A Systematic Review

Metabolic syndrome (MetS) is a persistent public health problem in the United States (U.S.) due to its increasing prevalence and its positive correlation with type-2 diabetes (T2DM) and cardiovascular disease (CVD). According to National Cholesterol Education Program's Adult Treatment Panel III (NCEP-ATP III) criteria, MetS has six main components, which are obesity, dyslipidemia, raised blood pressure (BP), insulin resistance (IR) or glucose intolerance, pro-inflammatory state, and prothrombotic state. Vitamin D (Vit D) regulates the absorption of calcium and phosphorus and thus, is universally accepted as an essential vitamin for bone strength as well as a facilitator of immune system function. Vit D was also shown to reduce the risks of CVD, multiple sclerosis, and developing seasonal flu. We conducted a systematic review to identify the general association between Vit D level and MetS, to highlight specific associations between Vit D level and individual components of MetS, and finally, to explore the effects of Vit D supplementation on each component of MetS. In this paper, we reviewed 14 recent studies investigating the relationships between Vit D, MetS, and components of MetS. From the review of seven studies, we confirmed a significant association between Vit D and MetS as a whole. Four out of the five observational studies we reviewed support that Vit D level is significantly associated with the following components of MetS: obesity and BMI, dyslipidemia, BP, and insulin and glucose metabolism. We did not discover any significant relationship between Vit D level and other MetS components. The review of seven additional randomized clinical trials (RCT)-based studies suggest that Vit D supplementation has significant effects on BP, abdominal obesity, and insulin and glucose metabolism.


Introduction And Background
Metabolic syndrome (MetS) in the United States (U.S.) adults 18 years or older increased from 25% in 1988 to 34% in 2012 [1]. A more recent study by Hirode and Wong found that the prevalence of MetS among the U.S. population increased slightly to 36.9% as of 2016 [2]. MetS is known as one of the most important risk factors of type-2 diabetes (T2DM) and cardiovascular disease (CVD), which in turn can increase the risk of myocardial infarction and stroke two-fold [3,4]. According to the Centers for Disease Control and Prevention (CDC), patients with some form of MetS are also likely to be more affected by COVID-19 [5].
According to the National Cholesterol Education Program's Adult Treatment Panel III report (NCEP-ATP III), there are six components of MetS related to CVD. They are abdominal obesity, atherogenic dyslipidemia, raised blood pressure (BP), insulin resistance (IR) or glucose intolerance, pro-inflammatory state, and prothrombotic state [6,7]. Despite being known as a serious public health issue, MetS does not have any straightforward, definitive treatment due to its multifaceted nature [8]. The most common clinical management of MetS is to reduce the risk factors; for example, maintaining low-density lipoprotein (LDL) cholesterol levels and BP under a certain threshold, mitigating the risk of T2DM for those who belong to atrisk populations, and encouraging patients to adopt lifestyle changes including eating a healthy diet and doing regular exercise [8,9].
Vitamin D (Vit D) is a fat-soluble, essential vitamin that facilitates calcium absorption. Vit D is produced in the human body when the skin is exposed to ultraviolet rays from sunlight. Vit D is also available from certain foods such as oily fish, egg yolks, red meat, liver, and in certain vitamin supplements [10]. Inadequate Vit D level commonly leads to osteomalacia in adults and rickets in infants [11]. The daily dietary allowance of Vit D is 400 -800 IU in humans depending on age and sex [12]. However, physicians sometimes prescribe more than 4000 IU to compensate for the Vit D deficiency [13]. 1 1 1 1 1 Vitamin D deficiency is generally accepted as below 50ng/mL of 25-Hydroxyvitamin D concentration in blood [11]. Forrest and Stuhldreher found that Vit D deficiency is 41.6% in the general U.S. population but can be as high as 82.1% among African Americans and 69.2% among Hispanics [14].
Researchers have conducted several studies to establish the relationship between Vit D and specific components of MetS. For example, some studies investigated the effects of Vit D on BP, triglycerides (TG), and glucose intolerance [15][16][17][18][19][20][21]. Some studies explore the association between Vit D and MetS as a whole [22,23]. Some of the studies deployed randomized clinical trials (RCTs), whereas some relied on observational studies. The population recruited for these studies vary by sex, age, and country of origin [19,[23][24][25]. A few studies reported a non-significant association between Vit D and different components of MetS [24][25][26]. By organizing these disparate studies into one systematic review, we hope to highlight the commonalities and differences in these studies, providing a big-picture analysis and understanding of the correlation between Vit D and MetS that applies to the general adult population around the world. This systematic review aims to illustrate the relationship between Vit D level and different components of MetS and the effects of Vit D supplementation on components of MetS. Based on the findings in this systematic review, we hope to accomplish two goals in our future studies: first, to identify the threshold of Vit D levels that can be used as a risk indicator of MetS for specific demographics, and second, to identify the specific dosage of Vit D that can reduce the risk of CVD.

Method and results
We used the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and principles in designing this systematic review and reporting its results [27].

Search strategy
Using the major research literature database and search engines such as PubMed, PubMed Central (MEDLINE), Google Scholar and, ResearchGate, we searched for appropriate keywords and medical subject headings (MeSH) thesaurus to uncover potentially relevant articles demonstrating the relationship between Vit D and MetS [28]. The keywords used in our literature search include "low vitamin D", "hypovitaminosis D", "rickets", "metabolic syndrome", "obesity", "glucose insensitivity", "insulin resistance", "dyslipidemia" and "high blood pressure". We searched for their corresponding MeSH terms using PubMed Central [29].

Inclusion and exclusion criteria
We selected the studies published within the past five years (2017 to 2021) in English. We filtered for systematic reviews, analytical studies, experimental studies with RCTs, and observational studies. In addition, we only selected studies with human subjects. We ensured that most patients recruited in these studies were between the ages of 13 and 75 years.

Analysis of study quality
The RCTs were critically evaluated with the Cochrane bias assessment tool, systematic reviews were assessed using the assessment of the multiple systematic reviews (AMSTAR) tool, and observational studies were analyzed with the Newcastle-Ottawa assessment scale (NOS). Each research work was scored as either a high, medium, low or unclear quality, and we selected the ones with medium and high-quality scores in our final analysis. The overall quality and scores for each study are provided in Table 1, Table 2, and Table 3.  Our evaluation 6/9 (Medium quality) 6/9 (Medium quality) 6/9 (Medium quality) 6/9 (Medium quality) 7/9 (Medium quality) 6/9 (Medium quality)

Results
A total of 1598 articles were identified in our initial search of the PubMed Central (MEDLINE) database. Out of these articles, 1581 were discarded either due to duplication or not being directly relevant to our research focus, leaving us with 17 articles to analyze. In these 17 research papers, we were able to access the full text of seven papers; the other 10 could only be obtained via paid subscription, and thus, we excluded them. To compensate for the discarded articles, we used Google Scholar and ResearchGate to obtain seven more articles that met our inclusion and exclusion criteria. A total of 14 articles were reviewed in this paper. We provide the complete PRISMA flow diagram in Figure 1 below. Following these findings, several more papers revealed the association between Vit D and MetS and the effects of the former on the latter. Among them, we selected 14 papers published since 2017, which summarize the relationship between Vit D level and MetS. In addition, these papers reported the effects, or lack thereof, of Vit D level on each of the six major components of MetS. To be generalizable for most adult populations, we selected different research types consisting of experimental and observational studies, a systematic review, and an analytical study.

Relationship of Vit D with each of the six major components of MetS
From 2017 to 2020, we selected four observational studies investigating the association between Vit D and the components of MetS. These studies have the same objective i.e., to investigate the association between Vit D and MetS. However, they differ in sample size, locale, and the demographic of the trial participants, as well as case-control criteria.
In a study by Mirhoseini et al.,192 obese patients with body mass index (BMI) > 25 from two hospitals in Iran's third-largest city were enrolled [18]. Patients were divided into healthy obese, which means obese without MetS, and obese with MetS. NCEP-ATP III criteria of TG level > 150mg/dL, high-density lipoprotein C (HDL-C) < 40mg/dL in men and < 50mg/dL in women, and fasting blood sugar (FBS) > 100mg/dL were applied in determining the participants of MetS group [6,36]. After 12 hours of fasting, Vit D level was measured for each group. Based on the laboratory report, a Vit D level of < 20ng/mL is defined as low, a level of 20-30ng/mL as insufficient, and > 100ng/mL is defined as toxic. The components of MetS in focus for this study were TG, HDL-C, FBS, BP, waist circumference (WC), and BMI. Among these components, TG, HDL-C, FBS, and BP were significantly associated with Vit D levels. As mentioned, Vit D is a fat-soluble vitamin.
Although it was shown that Vit D levels become more diluted as lipid volume increases, the study did not find any significant association between Vit D and obesity-related indicators such as WC and BMI [37].
In a cross-sectional study by Krishnaswamy et al.,80 Indian patients aged between 18 and 60 years were divided into MetS (case) and healthy (control) groups [23]. The Joint Interim Statement (JIS), which was issued by world-renowned organizations such as the International Diabetes Federation and the American Heart Association, was applied when selecting patients in the MetS group [38]. The study found that Vit D level in the case group was significantly lower than that in the control group. The relationships between Vit D and each component of MetS, however, were not reported in detail. Although the study reported that low Vit D level is significantly associated with MetS, the overall Vit D levels in both case and control groups were lower than the normal range (20 to 40ng/mL). This could be explained by the fact that Vit D deficiency (VDD) is prevalent in India [39]. The study was also limited not just by its small sample size but by its participants only being recruited from the hospital's outpatient department.
In a case-control study by Shamy et al., 92 patients from an Egyptian hospital's outpatient department, who are older than 18 years, were divided into MetS (case) and healthy (control) groups [32]. JIS criteria were used to determine the patients in the MetS group. The parameters of MetS in focus for this study were TG, high-density lipoprotein (HDL), lipid accumulation product (LAP), FBS, systolic blood pressure (SBP), diastolic blood pressure (DBP), BMI, WC, alanine aminotransferase (ALT), and aspartate aminotransferase (AST). Contrary to the studies discussed above, no significant difference in Vit D level between the case and control groups was found. In particular, the researchers found no significant correlation between Vit D vs. Compared to other studies discussed before, this study has the largest participant population and was able to detect the significant association between Vit D and some components of MetS for specific gender and BMI levels.
Although each of the studies above observed significant relationships between Vit D and some components of MetS, none of them observed a significant association between Vit D and all of the components of MetS.
The studies differ greatly in the number of participants and their demographics, as well as the study types. Thus, we must be careful not to draw generalizable conclusions regarding the association between Vit D and all MetS components. In other words, differences in participant demographics, varying size of participant groups, and other disparities such as diet and the climate of each population could have significantly affected the outcomes and findings of these studies. In our quest to find a more generalizable finding, we examined a systematic review and a paper that analyzed the data from the 2005-2006 National Health and Nutrition Examination Survey (NHANES). Both studies have large sample sizes covering more than one demographic or locale.
The first of these studies was a systematic review with the meta-analysis by Hajhashemy et al., which reviewed 38 cross-sectional studies, four cohort studies, and one case-control study [22]. A total of 309,206 adult participants were included in this systematic review. The participants represented 24 geographically diverse countries such as the U.S., China, Korea, Thailand, Qatar, Australia, Taiwan, Iran, Northern Finland, India, South Africa, Portugal, Japan, United Kingdom, Germany, Netherland, Italy, Belgium, Poland, Sweden, Spain, Hungary, and Estonia. The studies included in this systematic review used one of the two different criteria -NCEP-ATP III and JIS.
In the systematic review of the 38 cross-sectional studies (N=298,187) by Hajhashemy et al., the researchers found that the highest Vit D level can be translated into 43% and 40% reduction MetS in developed and developing countries, respectively. Due to the sheer number of studies reviewed in the paper and the high heterogeneity of the data, the researchers also conducted a subgroup analysis. The meta-analysis outcome showed a significant relationship between Vit D and MetS for all 38 cross-sectional studies, but not in the other five studies (N=11,019), four cohort studies, and one case-control study. The strength of the systematic review by Hajhashemy et al. is that it analyzed the data from several studies covering the adult population from around the world, which made its findings more generalizable than studies focused on specific locale and demographics. The brief descriptions of each study, including the year of publication, author name, number of patients and country of origin, type of study, significant and non-significant findings, are provided in Table 4.

Effects of Vit D supplementation on components of MetS
Seven out of 14 articles, which we selected for review, used RCTs to detect the effects if any, of Vit D on components of MetS such as BP, insulin, and glucose metabolism. The articles reviewed are different in the length of study, the components of MetS studied, and the findings. In an RCT by Lerchbaum et al.,192 male patients aged between 18 and 70 years with a low Vit D level of < 75nmol/L (< 30ng/mL) were recruited [19].
The RCT was a twelve-week long trial, and the progression of metabolic parameters was regularly monitored while providing Vit D supplements to the participants. In this study, Vit D supplementation was significantly associated with improving fasting glucose to insulin ratio. Other components of MetS, such as dyslipidemia and body composition, were not significantly associated with Vit D supplementation. Since this study was solely conducted on the male population, the significant findings in this study might not be attributable to the female population. In this study, the baseline Vit D level of the selected participants was higher than the commonly-used baseline of 20ng/mL.
In an RCT by Cefalo et al., a significant association between Vit D supplementation on insulin metabolism was observed [20]. RCT by Cefalo et al. lasted 13 weeks and was performed with just 18 patients who had low Vit D < 75nmol/L (< 30ng/mL) and BMI > 25. Insulin sensitivity and total trunk fat mass measured by X-ray absorptiometry were significantly improved with Vit D supplementation. Other components of MetS such as TG, HDL, LDL, and BMI were not significantly associated with Vit D supplementation. In a 2011 study by Mheid et al., researchers proved the effect of Vit D in suppressing the proliferation of vascular smooth muscle, which in turn affects BP by regulating the renin-angiotensin-aldosterone (RAA) system [43]. Two of the seven RCT-based papers we selected investigated the effects of Vit D supplementation in BP and arterial stiffness. An RCT by Raed et al. recruited 70 African American patients between 13 to 45 years of age who have a Vit D level < 20ng/mL [15]. The trial lasted 18 weeks, and carotidfemoral pulse wave velocity (PWV) and carotid-radial PWV were measured to check the participants' arterial stiffness. The researchers found that Vit D supplementation is significantly associated with a reduction of arterial wall stiffness.  [24]. Like in the trial by Karefykalis et al., Vit D supplementation had no significant effect on the components of MetS studied, including the lipid profile, C-reactive protein (CRP), and CVD risks.
Based on the review of all seven RCTs selected, we found that the researchers demonstrated the effects of Vit D on three main components of MetS, which are BP, abdominal obesity, and insulin and glucose metabolism. The studies did not find Vit D to significantly affect other components of MetS such as dyslipidemia, pro-inflammatory state, and prothrombotic state. A notable limitation of our RCT papers is the small number of participants in each of the trials -six out of seven studies in these papers were performed with less than a hundred participants.
The brief descriptions of each study, including the year of publication, author name, number of patients and country of origin, duration of the study, significant and non-significant findings, are provided in Table 5. As a result of these differences, we could only compare these studies on a higher level. Another limitation is that we do not have access to the detailed analyses and datasets used in these studies; therefore, we could not verify and critique the analytic approaches, nor could we perform a statistical meta-analysis. Last but not least, individual studies have their limitations, such as small sample size and insufficient study length.

Conclusions
In our systematic review, we aimed to verify the overall relationship between Vit D and MetS, the associations between Vit D and each of the six major components of MetS, and the effects of Vit D supplementation on MetS components. Our review of three observational studies, one systematic review, and one analytical study verified the significant overall relationship between Vit D and MetS. Four out of five observational studies established significant associations between Vit D level and four main components of MetS, namely dyslipidemia, insulin, glucose metabolism, obesity and BMI, and BP. In addition, four out of seven RCT studies revealed significant effects of Vit D on BP, abdominal obesity, and insulin and glucose metabolism, which are the three core components of MetS. Although proinflammatory state and prothrombotic state are important components of MetS, the studies reviewed did not discover any significant association between Vit D level and each of these two components, nor did the studies establish any significant effects of Vit D supplementation on them.
Two of the RCT studies, namely the ones conducted by Cefalo et al. and Karefykalis et al., studied BMI and body fat content as the focal MetS components concerning Vit D level. These studies recruited relatively few participants (N=18 and N=40) and could be statistically underpowered in their findings. More RCT studies that explore the relationship between Vit D and obesity, as measured by BMI and body fat content, are needed. In addition, there is a need for future studies that perform subgroup analyses to account for the differences in sex, age group, the underlying VDD prevalence of each population; specific medical conditions of the participants such as pre-existing renal or liver issues that could affect their Vit D levels and MetS diagnoses; and other factors such as Vit D in daily dietary intake, passive vs. active lifestyle, and current medications.

Conflicts of interest:
In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.