Obesity in the children and adolescents aged 5 to 19 year represents a global health issue with an ever-increasing prevalence, as WHO has estimates that it has nearly tripled since 1975 to 2018 [1,2]. In China, the children obesity similarly increases rapidly [3]. As we all known, obesity is associated with a strong predisposition towards metabolic diseases and even cancer among the childhood and even adulthood [3,4]. Thus, early prevention of children obesity is of great significance for minimizing the adverse consequences through the entire life cycle. Notably, the main characteristics of obesity is the abnormal expansion of adipose depots, which can be driven by the imbalance of hypertrophy and hyperplasia [5]. Generally, the hyperplasia of adipocytes plays an important role on the early stages of obesity to be considered healthy and adaptive by maintaining proper vascularization, increasing many metabolism modulatory adipokines and so on [5,6]. However, with the further progresses of obesity, the hypertrophy of adipocytes with massively expanded expansion posits the most important outcomes, which is associated with the limitations of oxygen diffusion and angiogenesis to elevate the lipolysis, increase the secretion of inflammatory cytokines and reduce the adipokines to persistently contribute the occurrence of metabolic diseases [7,8]. Here, in this situation, it is becoming more crucial to explore the different grades of childhood obesity as the degree I, II and III obesity clearly for their clinical interventions using more detailed researches.

As previously described, the causation of childhood obesity are complex and multifaceted, presenting the clinicians with myriad challenges. Moreover, a key contributor of micro-nutrients is an significant element to prevent the later chronic diseases of children obesity by arising the energy metabolism, adipogenesis process, inflammation status and etc in the adipocytes. Hence, it is necessary to study the relationships between the micro-nutrients intake and obesity [9,10], in which vitamin D (VD) deficiency has been reported commonly among the Chinese children and adolescents [11,12]. Recently, many researches have confirmed that both VD insufficient intake and its decreased bio-availability have been associated with features of metabolic syndrome using the animal experiments by affecting the insulin resistance, endocannabinoid tone, glucose and lipid metabolism, gut dysbiosis, chronic inflammation, differentiation of adipocytes and etc [13], [14], [15], However, it is still unknown about the explanations for the increasing risks of VD deficiency in the development of obesity. Of note, more than 50% patients of metabolic diseases have low circulating levels of total and free 25-hydroxyvitamin D [25(OH)D], including the 25(OH)D2 and 25(OH)D3, and 1,25 dihydroxyvitamin D [1,25(OH)2D][16]. However, different cohorts have put forward the inconsistent effectiveness on the cutoff points for VD deficiency on the progress of obesity, incompatible VD metabolites (25(OH)D, 1,25(OH)2D) due to no consensus of cause-effect relationships, different ages and different grades of obesity and so on [17], [18], [19]. Meanwhile, there is few researches on the dose-responses and causal relationships between the concentrations of VD metabolites and different grades of obesity. To go further in determining the impacts of VD status on the progression of obesity and its related metabolic disorders, we implemented a longitudinal study of school-aged children aged 7-12 years with degree I, II and III obesity to explore the VD bio-availability (total VD, 25(OH)D and 1,25(OH)2D) and its correlations with the clinical characteristics, which could provide the appropriate basis for personalizing the VD interventions among the school-aged children with different grades of obesity.

Notably, there are two hydroxylation steps on catalyzing the VD into their biologically active metabolites (25(OH)D and 1,25(OH)2D) [17], [18], [19]. On the first step, the endogenous or dietary VD is enzymatically hydroxylated to 25(OH)D, which is catalyzed by CYP2R1 (vitamin D 25-hydroxylase) and CYP27A1 (Cholesterol 27α hydroxylase) as the key enzymes of 25 hydroxylation. On the second step, the VDBP-bound 25(OH)D is then transported to the kidneys and various other organs to be metabolized into 1,25(OH)2D by CYP27B1 (25-hydroxyvitamin D-1-α hydroxylase). Then 1,25(OH)2D is attached to the vitamin D receptor (VDR), as a transcription factor in the steroid hormone receptor super-family in the cytoplasm, which is transported into the nucleus and acts as the ligand-dependent transcription factor to regulate the target genes to exert their corresponding biological functions [18]. Therefore, the disorders of these VD metabolic pathway genes (CYP27A1, CYP2R1, CYP27B1, VDR and VDBP) in the two hydroxylation steps could be related with the decreases of 25(OH)D and 1,25(OH)2D during the occurrence of many diseases such as Rheumatoid Arthritis, tuberculosis, type 2 diabetes, cancer and so on [20], [21], [22], [23]. Moreover, it suggests that DNA methylation is one of the most common epigenetic mechanisms for the changes in gene expression. High methylation levels in the promoter region of VD metabolic pathway genes may cause gene silencing to affect vitamin D response variation [20], [21], [22], [23]. However, the relationships between the altered DNA methylation on these VD metabolic pathway genes and progress of obesity still need to be further verified. Therefore, the aim of this study was to test the hypothesis that obesity could disrupt the VD homeostasis via modulating the related gene expressions by DNA methylation to conversely aggravate the metabolic disorders among the school-aged children under the degree I, II and III obesity.