Interrelationship between Vitamin D and Calcium in Obesity and Its Comorbid Conditions

Obesity has been linked to vitamin D (VD) deficiency and low calcium (CAL) status. In the last decade, dietary supplementation of vitamin D and calcium (VD–CAL) have been extensively studied in animal experiments and human studies. However, the physiological mechanisms remain unknown as to whether the VD–CAL axis improves homeostasis and reduces biomarkers in regulating obesity and other metabolic diseases directly or indirectly. This review sought to investigate their connections. This topic was examined in scientific databases such as Web of Science, Scopus, and PubMed from 2011 to 2021, and 87 articles were generated for interpretation. Mechanistically, VD–CAL regulates from the organs to the blood, influencing insulin, lipids, hormone, cell, and inflammatory functions in obesity and its comorbidities, such as non-alcoholic fatty liver disease, cardiovascular disease, and type-2 diabetes mellitus. Nevertheless, previous research has not consistently shown that simultaneous VD–CAL supplementation affects weight loss or reduces fat content. This discrepancy may be influenced by population age and diversity, ethnicity, and geographical location, and also by degree of obesity and applied doses. Therefore, a larger prospective cohort and randomised trials are needed to determine the exact role of VD–CAL and their interrelationship.


Introduction
Due to inappropriate diets and nutrient intake, obesity has become more common in all age groups, such as children [1], adolescents [2], and the elderly [3]. Obesity is increasing in prevalence in Asia-Pacific [4], Europe [5], Africa [6], and America [7] and has become a worldwide epidemic [8]. Taking this into account, the member states of the United Nations have developed a global platform, the Sustainable Development Goals, addressing noncommunicable diseases as core priorities [9].
Obesity is caused by micronutrient deficiency [10], inadequate intake of vitamins, such as cobalamin (vitamin B12), ascorbic acid (vitamin C), fat-soluble vitamins, and folic acid [11], a deficiency of vitamin D (VD) [12], poor mineral status [13], and low calcium (CAL) diet [14]. The average plasma concentration of 25(OH)D is used to determine VD level. A less than 50 nmol/L of 25(OH)D serum concentration indicates VD deficiency. [15]. Furthermore, obesity is characterised by accumulation of >70% of body fat mass [16]. VD 3 , and its metabolites 25(OH)D 3 and 1,25(OH) 2 D 3 , are deposited in adipocyte lipid droplets [17]. 25(OH)D is a fat-soluble metabolite disseminated to fat, muscle, liver, and in smaller amounts to other tissues. The stomach's ability to absorb CAL depends on the presence of active VD. VD deficiency causes osteomalacia and rickets in adults and children, respectively, whereas moderate deficiency results in an upsurge in the risk of bone turnover and bone fractures [12].
On the other hand, CAL ion (Ca 2+ ) is available in abundance in the body. Ca 2+ is deposited in bones, mostly as CaPO 3 (hydroxyapatite), and plays a structural role. It is required for muscle contraction and pancreatic secretion of insulin and glucagon in response to blood glucose fluctuations. Therefore, alterations in Ca 2+ homeostasis can have a strong impact on tissues, including the heart and skeletal muscle, contributing to obesity and diabetes [18]. The adipocyte metabolism pathway is regulated by intracellular Ca 2+ . In human adipocytes, intracellular Ca 2+ stimulates energy and fat storage through de novo lipogenesis (DNL) promotion and lipolysis prevention [19]. Hypocalcemia is characterized by CAL concentrations under 8 mg/dL or ionized CAL levels under 4.4 mg/dL [20].
Numerous experiments have established a link between vitamin D and calcium (VD-CAL) supplementation and the occurrence of metabolic diseases [21] such as chronic liver disease [22], osteoporosis [23], diabetes mellitus [24], and high blood pressure [25]. However, there are some discrepancies in the results, and the role of VD-CAL supplementation remains unclear [26]. Contradictory outcomes have been reported for VD levels [27] and CAL status [28] in obesity. Furthermore, an expert panel concluded that no confirmation supports the extraskeletal role of CAL or VD [29].
In light of current knowledge, this study assessed the interrelationship of VD-CAL in the onset of obesity and its comorbid conditions using scientific evidence from recent animal studies and human clinical trials. Besides that, this study also collected the relationship of VD-CAL in the metabolic functions of obesity, such as in the gut, bone, blood lipid and glucose levels, kidney, pancreas, liver, adipose tissue, and the immunoregulatory system. This review could aid in developing dietary recommendations for obese people and future studies into the influences of VD-CAL simulataneus supplementation in this group.

Search Strategy and Method
Scientific databases, such as Web of Science, Scopus, and PubMed, were utilised to conduct a methodical search for relevant articles by employing the following key phrases: "correlation", "relationship", "link", "association", "vitamin-D", "calcium", "obesity", "rats", "mice", and "human clinical trial". The search generated 87 articles published between 2011 and 2021, which investigated the relationship of VD-CAL in obesity ( Figure 1). Table 1 shows the inclusion and exclusion criteria for this literature search strategy. Duplicates, review articles, protocol studies, second analysis reports, articles not related to VD-CAL, and articles not published in English were excluded. The interrelationship findings were compiled by summarising them in tables.   Figure 2 depicts the role of the VD-CAL axis in regulating the physiological mechanisms involved in obesity. VD-CAL status determines adipose tissue functions, body fatness, bone density, inflammatory, insulin, hormone, and cell functions.

Physiological Association of VD-CAL
VD exists naturally in a limited number of food sources and is endogenously formed when solar ultraviolet rays (290-315 nm) stimulate VD synthesis in the body's outermost layer and is then primarily kept in fat in the body [30].
The available evidence indicates that VD-CAL is involved in physiological interac-   Figure 2 depicts the role of the VD-CAL axis in regulating the physiological mechanisms involved in obesity. VD-CAL status determines adipose tissue functions, body fatness, bone density, inflammatory, insulin, hormone, and cell functions.  [30,35]. CAL in the extracellular fluid stimulates CaSR on the parathyroid cells, which raises intracellular CAL levels and suppresses PTH release. In the kidneys, PTH regulates the conversion of 25(OH)D to 1,25(OH)2D, which augments intestinal CAL absorption. PTH has the ability to administer circulating IL-6 and TNF-α concentrations, which promote the production of high-sensitivity C-reactive protein [32,36].  VD exists naturally in a limited number of food sources and is endogenously formed when solar ultraviolet rays (290-315 nm) stimulate VD synthesis in the body's outermost layer and is then primarily kept in fat in the body [30].

Physiological Association of VD-CAL
The available evidence indicates that VD-CAL is involved in physiological interactions; 1,25(OH) 2 D, an active VD metabolite, regulates CAL transport across the intestinal wall and binds to the VD receptor (VDR) in the intestinal epithelial cells, inducing the synthesis of CAL-binding protein CaBP-9K and activating TRPV6 and TRPV5 CAL channels. CAL enters the cell through the intestinal lumen and is transported by CAL-binding protein throughout the cell and to the interstitium via an ATP-dependent mechanism [31]. In the intestinal epithelium, 1,25(OH) 2 D modulates paracellular CAL channels in the transepithelial electrochemical gradient by regulating claudin-2 and claudin-12 [32]. In adipose tissue, it stimulates voltage-dependent and voltage-insensitive Ca 2+ access pathways and controls Ca 2+ release from endoplasmic reticulum stores via inositol 1,4,5-triphosphate and ryanodine receptors [16]. In bone regulation, 1,25(OH) 2 D enters cells by diffusion by binding to complex VDRs and regulates mineralised bone mass by controlling intestinal CAL absorption and supplying adequate CAL to the bone matrix [33,34]. In beta cells, activated VD metabolites increase the sensitivity of insulin receptors, glucose homeostasis transcription factors, and/or CAL regulation in peripheral insulin-target cells [30,35]. CAL in the extracellular fluid stimulates CaSR on the parathyroid cells, which raises intracellular CAL levels and suppresses PTH release. In the kidneys, PTH regulates the conversion of 25(OH)D to 1,25(OH) 2 D, which augments intestinal CAL absorption. PTH has the ability to administer circulating IL-6 and TNF-α concentrations, which promote the production of high-sensitivity C-reactive protein [32,36].

Impact of VD-CAL on Adipose Tissue
The accumulation of a large amount of fat in adipose tissue characterises obesity. The relationship between VD-CAL and obesity, both with and without supplementation, is shown in Table 2. Table 2. VD-CAL in adipose tissues in animal and human studies.

Reference Experimental Design Diet Intervention Comorbidities Highlight Result
Animal study [37] Male C57BL/6J wild-type mice aged eight weeks old A high-fat diet with 500 IU/kg diet followed by a 10,000 IU/kg diet of VD 3

Obesity
Gut and adipose tissue cross-talked in VDRs signalling associated with lipid homeostasis. [38] C57BL/6 mice aged six weeks old ↓ hepatic lipid; ↑ adipose tissue expression and inflammation. Adipose tissue stores most of VD, which influences CAL homeostasis and energy metabolism. A link has been shown between VD levels and CAL in adipose tissue. In obese individuals, supplementation with VD causes a significant accumulation of this vitamin in the liver and adipose tissue [38], increased leptin-to-adiponectin ratio [46], and decreased inflammation in adipose tissue [37] and hepatic steatosis by downregulating the gene expression associated with hepatic de novo lipogenesis (DNL) and oxidation of fatty acid [39]. It was also shown that administration of a high-fat diet supplemented with 1,25(OH) 2 D 3 resulted in a reduction in macrophage infiltration in adipose tissue [40].
Besides VD intervention, supplementation of CAL in obese people was found to confer a prebiotic-like effect on the gut microbiota and decrease the stress marker expression in adipose tissue and liver inflammation [41], reduce body mass and modify glucocorticoid receptors expression and Vdr in the visceral adipose tissue [42], increase the expression of Cyp27b1/1α-hydroxylase and adipogenesis [43], decrease adipocyte hypertrophy and adipokines levels (TNF-α, IL-6, MCP-1, leptin), and rise adiponectin levels [45].
Supplementation of VD and CAL may be a useful and cost-efficient strategy for preventing and treating obesity. Rosenblum et al. [47] reported that a serving of 350 mg CAL and 100 IU VD per 240 mL glasses of orange juice resulted in significant depletion of visceral adipose tissue. Similarly, Sergeev and Song [44] observed that a high intake of VD-CAL influenced the adipose tissue Ca 2+ -mediated apoptotic pathway. The authors noted that VD 3 -CAL supplementation was more potent than either of these alone in decreasing adiposity in male C57BL/6J mice aged four weeks old.

Impact of VD-CAL on Body Fatness
Obesity is commonly linked to a lack of VD-CAL, which are nutrients that regulate body fat. The relationship between these nutrients and obesity is summarised in Table 3. Table 3. VD-CAL in body fatness.

Reference Experimental Design Diet Intervention Comorbidities Highlight Result
Animal Study [48] Obese male C57BL/6J mice aged twenty four weeks old 162 IU, 1282 IU and 5169 IU of VD 3 /kg Obesity ↓ serum calcitriol; ↑ serum parathyroid hormone. [49] Male C57BL/6 mice aged ten weeks old A high-fat diet with VD-enriched mushrooms extract Obesity ↓ total body fat; ↓ hepatic fat content; ↑ immunomodulatory effect (CD4/CD8 lymphocyte ratio). [50] Wistar and genetically predisposed obese IIMb/b rats A low dose of 0.2% CaCO 3 . Obesity ↑ body fat, ↑ liver weight, ↑ perigonadal and retroperitoneal fat in low CAL intake; ↓ body ashes and ↓ total skeleton bone mineral in low CAL intake. [51] Male C57BL/6J mice aged five weeks old A high-fat diet Obesity ↓ CYP2R1 and CYP27A1, CYP27B1, VDR; ↑ CYP24A1 (24-hydroxylase); ↑ DNA methylation, Dnmt activity, and 5-methylcytosine; ↓ Tet activity and 5-hydroxymethylcytosine. [ The prevalence of obesity may be decreased by a dose of VD or CAL. Evidence suggests that VD-CAL intake can decline body fat. The higher the concentration of 25(OH)D, the lower the body fat mass [54]. Similar to VD, a low CAL intake can negatively impact the levels of various lipid metabolic markers (glucose, triglyceride, and insulin) and increase body fat [50]. BMI and body fat levels strongly correlate with 25(OH)D concentration and the CAL-phosphorus product [56]. A high-fat diet has been revealed to promote hypermethylation and hypomethylation of Cyp24a1, resulting in glutathione deficiency and an altered VD biosynthesis pathway [51]. Additionally, there was a correlation between decreased Cyp2r1 gene expression and decreased circulating 25(OH)D concentrations [52].
However, VD-CAL levels in obese people have an unsatisfactory correlation. In obese individuals, neither BMI nor body fat percentage were significantly related to VD levels [48]. After treatment, no impact of VD supplementation was obtained on body fat, subcutaneous and visceral adipose tissue, and intrahepatic and intramyocellular lipids in this population [55]. Additionally, VD supplementation had no discernible effect on total body fat mass [57]. These results support the most recent finding that weekly VD 3 administration at a high dose (3750 IU) had no impact on sarcopenia or adiposity indices [60].
VD-CAL have been gaining increasing interest in obesity management. These nutrients have been tested in combination and also formulated in food products. A VD-enriched Lentinula edodes preparation reduced total body fat accumulation and hepatic fat content in obese C57BL/6 mice [49]. Similarly, a study showed that consumption of VD-fortified yoghurt drink (containing 170 mg CAL and 12.5 µg VD 3 /250 mL) for twelve weeks led to a decrease in waist circumference, body fat mass, and truncal fat in people with type-2 diabetes aged 30-60 years old [53]. In addition, supplementation of CAL and VD 3 caused a significant reduction in weight, BMI, waist circumference, and body fat percentage in obese women aged 18-48 years old [58]. In another study on overweight or obese males aged 18-25 years old, a 12-week supplementation of CAL and VD 3 (600 mg of CAL and 125 IU of VD 3 ) resulted in body fat loss and visceral fat loss [59].

Impact of VD on Bone Density
Patients and healthcare practitioners frequently overlook the importance of VD in bone health. Table 4 presents the findings of studies on the advantageous effects of VD.  ↓ phenotypes related to dendritic cells function expression; ↓ production of IL-12p70 by bone marrow-derived dendritic cells; ↑ LC3 II/I and VPS34 protein concentrations; ↓ p62 expression; ↓ Vdr mRNA levels.
Human study [67] Healthy overweight and obese women VD 3 doses (100,000 IU; 3420 IU; and 3420 IU) Obesity ↓ BMD; ↑ bone turnover markers; normal parathyroid hormone concentrations; No association between weight loss and changes in BMD; Positive association between low levels of bone loss and high levels of 25(OH)D. [68] Healthy males and females aged 18-50 years old 7000 IU cholecalciferol Obesity ↑ plasma 25OHD, ↓ PTH, ↓ CTX; Inverse correlation between alterations in plasma 25(OH)D, bone-specific alkaline phosphatase, and CTX; Inverse correlation between alterations in CTX and in spine BMD; Association between ↑ levels of 25OHD with ↓ PTH, ↓ bone turnover, and ↑ BMD at the forearm. Diet-induced obesity has been found to decrease the volume, number, and thickness of trabecular bone [63]. Diets containing saturated fatty acids and adequate amount of VD altered the metabolism of VD and bone changes, which suggests the critical role of dietary fat composition [65]. Supplementation of VD increased intestinal permeability and systemic lipopolysaccharide levels, resulting in stronger trabecular bone [61,62], and inhibited differentiation and maturation [64], as well as elevated autophagy flux of bone marrow-derived dendritic cells [66]. Furthermore, VD levels have been linked with bone loss [67], bone turnover [68], and areal bone mineral density [69].

Impact of VD-CAL on Inflammatory, Insulin, Hormone, and Cell Functions
Obesity and its associated health problems are linked to low levels of VD-CAL in the blood. VD-CAL contributes to obesity and its comorbidities by influencing inflammation, insulin sensitivity, hormone production, and cell functions (Table 5). Table 5. VD-CAL in inflammatory, insulin, hormone, and cell functions.

Reference Experimental Design Diet Intervention Comorbidities Highlight Result
Animal study [70] Male BALB/C mice A high-fat diet Obesity ↑ cytokine levels and IL-1b mRNA expression; Negative correlation between VD levels, inflammatory cells, IL-1b and IL-17 levels. [71] Male Wistar rats A high-fat diet Obesity ↓ intracellular CAL transient rates, ↓ rates and amplitude of salivary acinar cells Table 5. Cont.
No intervention of VD Obesity ↓ blood levels of VD; ↑ US-CRP, IL-6, HOMA [93] Obese Apart from its vital role in CAL homeostasis, VD-CAL is involved in modulating inflammatory, insulin, hormone, and cell functions in obesity. Administration of VD was shown to reduce intestinal inflammation [77] and the level of inflammatory markers [82,83,94], and also regulate the concentrations of IL-1β, NF-Kβ, acetylcholine, brainderived neurotropic factor [73], blood-brain barrier permeability [75], and pro-inflammatory cytokine levels [70]. However, previous studies suggest that VD-CAL supplementation does not affect inflammatory regulation in obesity. VD intervention did not cause any change in the concentrations of us-CRP and IL-6 [92], sICAM-1, hs-CRP, AGP [90], and inflammatory cytokines [89,91]. Similarly, a study showed that a CAL-rich diet had no beneficial impact on inflammation, fibrinolysis, and endothelial function [85].
VD has been linked to cell regulation in obesity. The intake of this vitamin was shown to inhibit the expression of NF-κB [78] and regulate HIF1α in T cells [79]. However, VD intake had no significant effect on IFN-γ intracellular expression, NKG2D, and CD107a surface expression in NK cells, Vdup1 and Vdr mRNA levels [76], and T cell autophagy [72]. Apart from its role in inflammatory, insulin, and cell functions, a connection has been found between VD and hormone regulation, with VD exerting a suppressing effect on PTH levels in obese individuals [84,86,88,97].

Potential Applications of VD-CAL in Obesity Prevention
Low levels of VD-CAL have been connected with an improved risk of obesity. Supplementation of VD-CAL can be beneficial in the treatment of obesity, as summarised in Table 6. Table 6. The evidence of VD-CAL supplementation in obesity.

Obesity
↓ weight and BMI in all interventions Obesity and its related conditions have been studied in relation to VD's potential role as a treatment. Supplementing with VD lowered the rate of weight gain [99] and body mass [104], increased glucose homeostasis, and regulated energy expenditure in obese people [100]. Additionally, VD intake prevented histological damage of the colon [101], reduced hepatic steatosis, improved mitochondrial dysfunction in the liver, decreased stress oxidation, and triggered phase II enzymes [102].
Clinical studies, on the other hand, have found no link between 25(OH)D concentrations and hepatic enzymes, indicating a lack of interaction between VD deficiency and obesity [110] and no impact of VD on post-load glucose or other glycemic indices [112]. Supplementing with VD had no significant effect on weight, fat mass, or waist circumference [115]. Lack of VD increased the activity of neutral sphingomyelinases in obese people with cardiovascular disease, which had implications for chylomicron and lowdensity lipoprotein (LDL) and very-low-density lipoprotein clearance, as well as decreased postprandial inflammation and macrophage adhesion to endothelia [111].
On the other hand, in obese patients with VD deficiency, VD supplementation significantly enhanced serum 25(OH)D levels [117], reduced the size of the left atrium [113], reduced hs-CRP and TNF-α concentrations [114], and increased insulin sensitivity [116].
Furthermore, VD intake combined with physical exercise led to a rise in peak power and VD status, reduced waist-to-hip ratio [104], and improved insulin resistance and hepatic disease [98]. However, this combination did not influence inflammatory biomarkers such as CRP, TNF-α, and IL-6 [107].
The role of diets enriched with VD-CAL has also been investigated. Faghih et al. [106] reported that diet of a daily 500 kcal deficit with daily 500-600 mg CAL (1), a daily 500 kcal deficit with daily 800 mg CAL (2), a daily 500 kcal deficit with three servings of low-fat milk (3), and a daily 500 kcal deficit with three portions of CAL-fortified soy milk (4), resulted in decreased body weight and BMI in all groups of healthy, overweight, or obese premenopausal women aged 20-50 years. Valle et al. [103] showed that supplementation of salmon peptide fraction (25% of protein) and VD (15,000 IU/kg of diet) aided in maintaining metabolic syndrome via gut-liver axis by controlling hepatic and gut inflammation (increasing Mogibacterium and Muribaculaceae) in mice fed with high-fat and high-sucrose diets containing VD (25 IU/kg of diet). Sharifan et al. [108] observed a correlation between fortified dairy products containing VD 3 (1500 IU) and anthropometric indices, glucose homeostasis, and lipid profiles in adults (30-50 years old) with abdominal obesity. Similarly, Vinet et al. [109] observed that consumption of 200 mL fruit juice supplemented with VD 3 (4000 IU) every morning led to the weakening of microvascular dysfunction without any impact on macrovascular function by increasing 25(OH)D levels, decreasing HOMA-IR and CRP, and improving endothelium-dependent microvascular reactivity in obese adolescents aged 12-17 years old and with a BMI of 31.2-36.2 kg/m 2 .
Unfortunately, prior studies could not identify a compensatory mechanism and did not exhibit the impact of VD-CAL channels, namely TRPV5 and TRPV6, which can influence the CAL transport process. According to Veldurthy et al. [118], 1,25(OH) 2 D 3 regulated transcellular transport by influencing the epithelial CAL channel TRPV5. TRPV5 enables the entry of apical CAL entry by inducing calbindins (calbindin-D 9k and calbindin-D 28k ). Moreover, calbindin-D 9k and TRPV6 expression are administered by 1,25(OH) 2 D 3 . TRPV6 overexpression results in hypercalciuria, hypercalcemia, and calcification of soft tissues, indicating that this channel plays a key role in the intestinal absorption of CAL.

Inadequate Evidence for the Effect of VD-CAL on Obesity
Although the benefits of VD-CAL supplementation have been documented, previous reports have established that these two nutrients are incompatible. The inconsistent results about the impacts of VD-CAL observed in experiments on obese populations are summarised in Table 7.
Thomas et al. [119] demonstrated insufficient evidence for the influence of high dietary CAL (CAL carbonate) on obesity-related phenotypes, including impaired weight gain and hyperphagia, in diet-induced obese 4-week-old male C57BL/6J mice. Wood et al. [120] found no effect on physical function after VD 3 supplementation (400 or 1000 IU) for one year in postmenopausal women aged 60-70 years old with a BMI of 18-45 kg/m 2 . Salehpour et al. [121] presented inadequate data on the impact of VD 3 intake (25 µg as cholecalciferol) on glucose homeostasis in overweight or obese women with an average age of 38 ± 8 years old and a BMI of 29.9 ± 4.2 kg/m 2 . Brzezinski et al. [122] observed an insignificant impact of 26-week VD (1200 IU) intake on body weight reduction in overweight and obese children aged 6-15 years old who had VD insufficiency (<30 ng/mL). Additionally, Jones et al. [123] showed that supplementation of dairy (~700 mg per day of CAL with 500 kcal per day) and CAL-rich dairy (~1400 mg per day of CAL with 500 kcal per day) had no effect in increasing weight loss. However, CAL-rich dairy improved plasma levels of peptide tyrosine tyrosine in men and women aged 20-60 years old and with a BMI of 27-37 kg/m 2 . Kerksick et al. [124] demonstrated that the supplementation with VD-CAL did not significantly affect the alteration of body composition in overweight postmenopausal women.

Reference Experimental Design Diet Intervention Comorbidities Highlight Result
Animal study [119] Obese male C57BL/6J mice aged four weeks old A high-fat diet, high-CAL, and high-CAL + non-fat dry milk Obesity ↓ body weight and adiposity; ↑ adiposity in high-CAL diet; ↑ feed efficiency and hyperphagia develop obesity in high-Ca mice; A strong correlation between mRNA markers of macrophages.
Human study [120] Healthy post-menopausal women aged 60-70 years old 400 and 1000 IU of VD 3 Obesity No improvement in physical function (grip strength or falls) [121] Overweight and obese women aged 38  Based on the literature review, it can be concluded that the role of CAL and VD in metabolic disorders and obesity is well understood. However, the results on the impact of the deficit or the supplementation used on body weight, fat content, or biochemical parameters obtained especially in human studies are often ambiguous. It seems that many factors, including population diversity, may lead to the inconsistency of results of VD-CAL in the obese. The cited studies were conducted on a variety of populations, geographies, and races. For instance, low VD status affects the occurrence of a VD deficiency in Western European residents during the wintertime periods, South Africa, Oceania, and Asian countries (Middle East, China, Mongolia, and India) [125]. In the same vein as the global VD status, many countries have low average dietary CAL ranges between 175 and 1233 mg across half the world's nations, such as Asia, Africa, and Latin America, and only a handful of European countries [126]. Furthermore, low VD concentrations were discovered to be a significant difference between native residents with immigrant residents [127] and multi-ethnic residents [128]. Besides that, there was an association between VD levels and elevation in a population [129]. A difference of one or two degrees in latitude enormously influenced 25(OH)D serum levels and bone density amid a low VD-sufficiency population [130]. Two different continents in the same population also influenced the CAL concentrations [131]. Therefore, these variables may predominance the inadequate evidence of VD-CAL supplementation in obese populations [120][121][122][123][124].
Using a comprehensive literature strategy, this article has presented results collected from the studies conducted in the last ten years on the interrelationship of VD-CAL in obesity. However, the present article also has some limitations, including the lack of discussion about the side effects of VD-CAL intake, including its toxicity, which can be helpful for designing an obesity management therapy. It should be mentioned that CAL intervention studies have provided minimal evidence [133,134] supporting the application of CAL supplements in treating obesity and its comorbidities.

Conclusions and Perspectives
There is little research into the effects of the concomitant use of VD and CAL on obesity and metabolic disorders. The results of the current research, where VD and CAL were used separately and in combination, are inconclusive. Mechanistically, the VD-CAL axis affects lipid status, insulin, hormones, cells, and inflammatory functions in obesity and its comorbidities from the organs to the blood. Previous research on animals and humans has not consistently supported the hypothesis that VD-CAL supplementation can accelerate weight loss or fat loss in obesity. Many factors may affect the obtained results, including the age of subjects, degree of obesity, applied doses, population diversity, ethnicity, and geographical location. Therefore, more large-scale prospective cohort studies and randomised trials are required to fully clarify the simultaneous administration of VD-CAL as well as their interrelationship for the safe use of these nutrients in the management of the global obesity epidemic.

Conflicts of Interest:
The authors state no conflict of interest.