A wide array of studies has identified several factors causing vitamin D deficiency and thus, leading to childhood obesity such as genetic factors, environmental factors, lifestyle, dietary and supplementation intake, comorbid diseases, etc.13, 17–19. In the current study, we have investigated the association of vitamin D levels and VDR gene polymorphism with childhood obesity. To determine the connection between obesity and levels of vitamin D, children’s BMI, lifestyle, and dietary intake were taken into consideration and studied. Further to reveal the association between obesity and VDR gene polymorphisms four single nucleotide polymorphisms of the VDR gene including ApaI, FokI, TaqI, and BsmI were selected and investigated.
1. Association Of Vitamin D Levels And Bmi
Defining and determining obesity in children is challenging since there is no standard threshold set for them like adults. Therefore, obesity diagnosis in children is normally determined by BMI calculation20. Studies have reported a major impact of BMI on vitamin D levels. An inverse relationship between vitamin D deficiency and BMI has been established by multiple studies conducted previously21.
In this study, a total of 164 school-going Bangladeshi children between 6–13 years old (with a mean age of 10.85 years) were included and of them, male participants were predominant (51.2%) and the female participants were 48.8%. Participants were categorized into 4 groups according to their BMI including underweight (< 17.9), normal body weight (18-24.9), overweight (25-29.9), and obese (> 30). BMI measurement confirmed that 31 (18.9%) and 35 (21.3%) children were obese and overweight, respectively. The findings of serum vitamin D level analysis showed that the lowest vitamin D level (37.54 ± 14.39 ng/mL) was found in obese participants, whereas, the highest vitamin D level (50.46 ± 19.25) was determined in underweight children. Overweight children had a lower vitamin D concentration (44.08 ± 15.57) than normal-weight children (46.37 ± 16.58). From the ANOVA test, it is noteworthy to mention that a highly significant (F = 3.710; p = 0.013) correlation between low levels of vitamin D with obesity in children was found compared to underweight children; normal body weight, and overweight children. Furthermore, significant (p = 0.012, p = 0.081) associations of vitamin D deficiency with obese children were observed compared to under-weight (p = 0.002), normal body weight (p = 0.012) and overweight (p = 0.081), respectively. A previously conducted study to reveal the association of 25(OH)D concentration with BMI suggested that each 10% increase in body weight resulted in a 4.2% reduction in vitamin D concentration 22. In 278 children and adolescents, a homeostatic model analysis revealed a significant correlation between BMI and 25(OH)D deficiency (p < 0.001)23. A study on 9–13 years old Spanish children found an association between BMI and abdominal obesity and vit-D insufficiency24.
2. Association Of Vitamin D Levels With Lifestyle
A wide range of research suggests the association of vitamin D levels with lifestyle25. Since vitamin D is synthesized on exposure to sunlight spending much time indoors and using sun protection are reported as two vital causes of vitamin D deficiency26. In our study among 164 children 7 participants had low levels of vitamin D ((> 20 ng/mL), 28 participants had insufficient levels of vitamin D (20.1–29.9 ng/mL) and 129 had normal levels of vit-D (< 30 ng/mL). Our findings showed a significant (p = 0.003) correlation was found between sunlight exposure time and vitamin D levels. In our study, increased/sufficient/normal vit-D levels were determined in children who had regular sunlight exposure (~ 1.4 hr) and spent more time in outdoor activities (~ 1.3 hr). Our study findings are scientifically supported by the research performed by Das et al., on Bangladeshi urban and rural 12–24 months old children27. Similarly, a large multiethnic cohort study on 6-year-old children in the Netherland revealed that 30% of children were found with vit-D deficiency and the causative factors identified were spending much time indoors for example more television watching, less biking to school, and playing less outside26. The association of vitamin D status with physical activity, participants’ habits, and sedentary life was studied on 5–11 years old school-going children (a total of 200 participants) in Northern Ireland, and the study outcomes demonstrated the higher prevalence of lower vit-D levels in children who spent much time indoor and sedentary life. In contrast, vit-D sufficient children had spent significantly higher hours outdoors and less inactive time28. A higher prevalence of vit-D deficiency was found in US children (aged 1–21 years) with a more indoor and sedentary lifestyle29. A cross-sectional study on Belgian children to determine the association of dietary and lifestyle-induced health effects with vit-D status demonstrated the weekly number of hours playing outdoors as a potential determinant of vitamin D status24. Similar findings were also reported by another research group where 5–11 years old Saudi Arabian school-going children were studied to find the link between vit-D levels and lifestyle (Exposure to sunlight and more physical activity). Children with no sunlight exposure had significantly lower vit-D levels which were increased with increasing exposure time to sunlight24. A cross-sectional study was conducted on a total of 150 urban and rural Bangladeshi children where children with inadequate sunlight exposure had 2.5 times higher vit-D deficiency compared to children with adequate sunlight exposure30. Another study was conducted on children of the South-East region of Bangladesh and less sun exposure was identified as one of the risk factors for hypovitaminosis D among the children31. Similar study findings have been reported by other research groups globally32–34.
3. Association Of Vitamin D Levels With Dietary, And Supplementation Intake
Various research suggests the association of vitamin D levels with dietary, and supplementation intake25,26. In our study, although not statistically significant, a higher percentage of children who had milk, milk beverages, vit-D rich foods, and vit-D rich fish weekly showed sufficient vit-D levels. A previous study reported a significant association between higher dietary vit-D (p = 0.021) and vit-D supplements (p = 0.028) daily intake and vit-D sufficient participants28. Some studies revealed that dietary intake of certain vit-D-rich foods has a significant influence on vit-D deficiency reduction19. Another research demonstrated that regular consumption of vit-D-rich foods, vit-D fortified foods, and vit-D supplementation can contribute significantly to improving vit-D status in children28. Among Canadian children who consumed vit-D fortified milk daily (77%) and vit-D-containing supplements (9%) were reported to have sufficient levels of vit-D35. Giving critical importance to vit-D levels in children the International Health Organizations, namely the Institute of Medicine (IOM) has placed guidelines and recommendations to surmount widespread vit-D deficiency challenges. Also, the website of the US Department of Agriculture’s Nutrient Database has a list of foods and their vit-D content for general use36.
4. Association Of Vitamin D Levels With Medical Histories
Epidemiological studies have demonstrated an association of vit-D deficiency with various chronic diseases including metabolic disorders, cancer, cardiovascular diseases, diabetes, neuropsychiatric disorders, autoimmune diseases, and infectious diseases37. Thus, in our study, clinical conditions such as inflammatory bowel disease (IBD), diarrheal history, autoimmune diseases such as rheumatoid arthritis, osteoarthritis, muscle weakness, kidney disease, etc., were considered, and data were collected and analyzed. Among all these a significant (p = 0.08) correlation was found between children suffering from IBD and vit-D deficiency. The findings of our study are in line with the results of the previous studies that reported a high prevalence of vit-D deficiency in children with IBD. For instance, 77 (80.2%) Korean children and adolescents out of 96 were identified with vit-D deficiency and a positive correlation was found between vit-D deficiency and IBD38. Similarly, another study reported that hypovitaminosis D was most prevalent (44%; P = 0.05) in children and adolescents suffering from IDB (Crohn’s disease)39. In a randomized controlled trial on 120 children with IBD and vit-D deficiency supplementation of vit-D significantly improved the condition compared to the placebo group40. Suboptimal vit-D concentration was most prevalent in children with Crohn’s disease (69.1% followed by ulcerative colitis (46.4%) in a previous study conducted by Jasielska and Grzybowska-Chlebowczyk41. In addition to that, children with IBD are more likely to be at an increased risk of developing vit-D deficiency due to malabsorption of bile salts, impaired absorption of nutrients, restricted dietary intake, and physician’s advice to avoid sunlight exposure while taking immunosuppressants42.
Vitamin D signaling plays an important role in the maintenance of epithelial barrier integrity by regulating epithelial cell gap junction proteins and protects against infection and inflammation by enhancing its resistance to irritants, increasing epithelial cell repair, and reducing epithelial cell apoptosis. On the contrary, hypovitaminosis D disrupts gut barrier integrity and immune functions, increasing translocation of the gut microbiome and dysbiosis, and thus, leading to initiation and progression of IBD42,43.
5. The association of vitamin D levels and vitamin D receptor gene polymorphism with obesity and overweight in children
Obesity and overweight have become an epidemic problem in different areas, and its rate is increasing across the world. Since vitamin D deficiency is still a silent problem, we must take the necessary precautions to avoid it.44. Different studies in different countries identified an association between obesity and low vitamin D level10. In 2015, researchers in China who were studying obese people discovered a link between vitamin D deficiency and obesity45.
The VDR gene is located at the 12q13 chromosome and many polymorphisms are indicated at this region. The VDR genes are the FokI, BsmI, TaqI, and ApaI46. Among these genes polymorphisms have an important role in the function of the VDR receptor. The FokI polymorphism which is located in exon 2 is linked with a second methionine start site, forming a shorter protein receptor47. If we compare, we would see that this receptor has greater transcriptional activity than the wild type receptor. The BsmI polymorphism can be linked to a variable length polyadenylate sequence within the 3′-untranslated region. Bsml genotype frequencies for the obese child and control groups had a statistically significant difference with a P value of 0.02. The BB and Bb genotypes were significantly associated with obesity.
The study showed the decreased level of VDR mRNA with the VDR B allele compared to those not bearing the B allele48. A previous study on a French population, found an association between BsmI genotypes and obesity. For French Caucasians with type 2 diabetes mellitus, VDR is not a significant gene. However, in people with early-onset type 2 diabetes mellitus, polymorphisms in the VDR gene are linked to an increased risk of obesity. The pathophysiological processes behind these correlations are yet unknown, however they may be connected to either a direct role for vitamin D in the development and metabolism of adypocytes or a secondary role for vitamin D in modulating insulin production. They also found that a bb genotype is more susceptible to obesity than BB and Bb genotypes49. To the contrary, in our study there was no contribution of the VDR polymorphism FokI, TaqI and APaI to obese children with a P value of 0.47, 0.75 and 0.15, respectively. In contrast the polymorphic alleles (TT) of ApaI seems to be a risk factor for asthma in children with normal weight, while that of BsmI seems to be a risk factor for asthma in overweight conditions. In a study on Brazilian school children, it has been found that children with asthma who are eutrophic have a greater frequency of the TT allele for the ApaI gene, while children with asthma who are overweight have a higher frequency of the TT allele for the BsmI gene. That means weight might be a factor10. In a population-based case–control comprehensive study, the severity of vitamin D gene variants including FokI, BsmI, TaqI, and ApaI has been accomplished in the Iranian population. Their results indicated that the VDR ApaI polymorphism and obesity susceptibility, whereas the ApaI A allele and AA genotype, were related to obesity phenotypes. The higher serum levels of FBS and BMI in genotype AA carriers provided clear evidence of the associations between the VDR gene polymorphisms and the anthropometric and biochemical characteristics of obesity5. In our recent study, positive findings in polymorphisms of vitamin D-related genes were obtained. Among the four kinds of VDR SNPs, the FokI/Ff allele, BsmI/bb allele, TaqI/Tt allele, and ApaI/Aa genotypes showed an association with vitamin D3 level in obese children. On the other hand, a study done on Indian women did not confirm this association50. According to TaqI genotypes, in 2011, a study on a Vietnamese population confirmed the association between TaqI genotypes and development of obesity51. Also, a recent study confirmed this association on an Egyptian population. Here a recent cross-over experiment has been carried out that was randomized, double-blinded, and controlled for placebo, 39 type 1 diabetes patients got 4000 IU of cholecalciferol per day for three months before receiving placebo or the opposite treatment. As a result, the vitamin D status also improved, and the regulatory T cells (Treg) showed a distinct response to vitamin D after three months of treatment based on VDR SNPs. Additionally, under vitamin D administration, this experiment demonstrated an improvement in glycemic parameters. The number of Treg cells could be increased in patients with the genotypes aa, TT, and bb. In case of our results, we only found that tt genotype is not associated with obesity may be due to the genetic variation in Egyptian region and Asian region52. Regarding our study limitations, it can be said that this study was conducted on school going obese children in Bangladesh, which represents a small size sample, so a large sample study is recommended to confirm the association of VDR polymorphism and obesity. We have also investigated the VDR polymorphism study in overweight children as well. However, we found that there was no significant association of VDR polymorphism with Vitamin D serum level in overweight children of Bangladesh whereas a study that has been done in Brazil in overweight children found out that there BsmI might be a risk factor in overweight children to develop asthma disease10. It has been shown that children and adolescents who are overweight or obese have a significant prevalence of vitamin D deficiency, which increases as obesity becomes more severe. A study highlights that the necessity for larger cholecalciferol dosages to attain serum calcifediol objectives in overweight and obese children and adolescents as well as the most recent recommendations for treating vitamin D deficiency, while in our study it does not reflect any significance which might be variable due to genetic variation, ethnicity, area etc20. In children, low serum 25(OH)-D is positively correlated with obesity or high BMI. Genetic factors at least partially affect the levels of vitamin D in the blood. The adipogenesis process and the level of inflammation in adipocytes and adipose tissue are both significantly influenced by vitamin D53. The GGT haplotype and the BsmI, ApaI, and TaqI wild variations of the VDR gene were connected to reduced vitamin D levels, indicating that VDR gene polymorphisms may increase a subpopulation of children's susceptibility to vitamin D insufficiency. It has been claimed that, this is the first report in South Brazil relating VDR gene polymorphisms and haplotypes to 25(OH)D levels in healthy children and adolescent girls from the general population. Other studies that focused on a particular condition, such as the overweight Insulin Resistance Atherosclerosis family study, in which only the BsmI SNP was examined, and the Metabolites in Multiple Sclerosis study, which covered the ApaI and TaqI SNPs, did not discover a relationship between the VDR polymorphism and 25(OH)D levels. These investigations, however, involved adult populations with disease, whereas the patients in our results were younger and healthier. BsmI, ApaI, and TaqI SNPs of the VDR gene were not linked to 25(OH)D levels in investigations including older, overweight patients.54