## 1 Introduction  
The essential fat-soluble prohormone vitamin D, also known as the \"sunshine vitamin,\" plays a fundamental biological role in the human body [1]. It is vital for maintaining normal growth, controlling calcium and phosphorus levels, and promoting the mineralization of bones and other calcified tissues, such as teeth [2; 3; 4]. Vitamin D deficiency is a common issue in many communities because of limited dietary sources and fortified foods with vitamin D [5]. Epidemiological studies provide compelling evidence that vitamin D deficiency represents a pervasive global epidemic, posing a substantial risk to approximately 1 billion of the world's population [6; 7]. Vitamin D deficiency can cause musculoskeletal disorders or non-communicable diseases [8]. Since 25-hydroxyvitamin D (25[OH] D) plays a role in tooth formation, this deficit makes teeth susceptible to caries because of enamel defects [3; 9]. Therefore, a lack of vitamin D has been linked to enamel abnormalities and a higher chance of developing dental caries [10].  
Dental caries, also known as tooth decay, is a multifaceted infectious condition characterized by the adherence of bacteria to teeth and their metabolic conversion of sugars into acid [11; 12]. This acid attack on dental tissue leads to discomfort, pain, chewing difficulties, and having a negative effect on overall quality of life [13; 14; 15]. The available evidence substantiates that individuals across all age groups are susceptible to dental caries [16; 17; 18; 12]. Moreover, as indicated by reports from the World Health Organization (WHO), dental caries ranks as the fourth most costly chronic disease to manage [19]. Dental caries prevalence, according to the researchers, is linked to a number of variables, including low socioeconomic position, restricted dental access, sugar consumption, parental education, frequency of brushing, regular dental visits, and fluoride intake [20; 21; 22; 23; 24]. Because dental caries and weakened oral conditions impede people's ability to chew and bite, inadequate 25[OH] D levels can also induce vitamin D insufficiency and nutritional issues [25].  
Numerous epidemiological studies have suggested a possible connection between vitamin D levels in the general population and the development of dental caries [9; 26; 27; 28]. Nevertheless, current research evidence remains inconclusive in establishing a definitive relationship [29; 30]. A systematic review and meta-analysis of studies that involved controlled clinical trials revealed that vitamin D supplementation in children may have the potential to lessen caries experience [31]. Low levels of 25[OH] D and childhood dental caries were linked, according to a recent systematic review [32]. In addition, a Chinese meta-analysis discovered that children's dental caries risk may be increased by vitamin D insufficiency [33].  
The existing literature indicates an association between serum vitamin D levels and dental caries. However, previous review studies are limited as they often lack meta-analyses [32], fail to evaluate the certainty of the evidence [32; 33], and their study population consisted of only children [31; 32; 33]. Therefore, this study aims to address these limitations by (1) including new studies not covered in previous systematic reviews and meta-analyses [16; 18; 34; 35; 36; 37], (2) conducting a meta-analysis of cross-sectional studies, (3) assessing the certainty of the evidence, and (4) including all age groups. Hence, the aim of this study was to provide a systematic review of the available cross-sectional data on measurements of dental caries status and serum vitamin level in all age groups.