Association of 25-hydroxy Vitamin D levels with indexes of general and abdominal obesity in Iranian adolescents: The CASPIAN-III study.

BACKGROUND: This study aimed to determine the association of serum 25-hydroxy Vitamin D (25(OH)D) levels with measures of general and abdominal obesity in Iranian adolescents.
MATERIALS AND METHODS: This nationwide cross-sectional study was conducted among 1090 students, aged 10-18 years, living in 27 provinces in Iran. Serum concentration of 25(OH)D was analyzed quantitatively by direct competitive immunoassay chemiluminescence method. Body mass index (BMI) and waist-to-height ratio (WHtR) were considered as measures of generalized and abdominal obesity, respectively.
RESULTS: Study participants consisted of 1090 adolescents (51.9% boy and 67.1% urban residents) with mean age, BMI, and waist circumference of 14.7 (2.6) years, 19.3 (4.2) kg/m(2), and 67.82 (12.23) cm, respectively. The median serum 25(OH)D was 13.0 ng/mL (interquartile range: 20.6). Overall, 40% of participants were Vitamin D deficient, and 39% were Vitamin D insufficient. Serum 25(OH)D level was not associated with BMI and WHtR.
CONCLUSION: We did not document any significant association between serum 25(OH)D level and anthropometric measures in adolescents. This finding may be because of considerably high prevalence of hypovitaminosis D in the study population.

INTRODUCTION

It is well-documented that overweight and obesity are increasing in both developed and developing countries.[123] It is estimated that in 2030, >2 billion individuals would be overweight, and 1 billion would be obese.[4] Globally, high amounts of expenditures are allocated for obesity due to its indispensable role in chronic and complicated diseases such as cardiovascular diseases, cancers, hypertension, and type 2 diabetes.[56789] Moreover, obesity is associated with inflammation[10] that may be because of the entrance of macrophages in adipose tissues and their cytokine production,[11] which in turn would contribute in the development of type II diabetes and other metabolic disorders.[12]

Different studies show that Vitamin D, as an endocrine hormone, has not only crucial roles in metabolic pathways including glucose metabolism, and insulin secretion and functions, but this hormone might also modify the immune responses.[1314] For instance, the level of 25-hydroxy Vitamin D (25(OH)D), which is the most important form of Vitamin D, inversely correlates with glucose intolerance and type II diabetes;[1516] some studies showed direct correlation between 25(OH) D levels and obesity.[17] The latter collaboration occurs when lipogenesis and inhibition of lipolysis are induced by Vitamin D in vivo and in vitro, respectively.[17] Nonetheless, some studies did not document any evidence about the correlation of Vitamin D and obesity.[1819]

Different factors might significantly affect Vitamin D level, e.g., demographic variables, gender, aging, ethnicity, dietary habits, sunlight exposure, and season. Vitamin D deficiency is a pandemic problem, even in sunny regions.[2021222324]

Conflicting results exist on the correlations of 25(OH)D with weight status.[1718192021222324252627282930] This study aims to determine the association of serum 25(OH)D) levels with measures of general and abdominal obesity in a nationally-representative sample of Iranian adolescents.

MATERIALS AND METHODS

Study design and participants

This nationwide cross-sectional study was conducted as part of the third national survey of a school-based surveillance system[31] entitled the Childhood and Adolescence Surveillance and PreventIon of Adult Noncommunicable disease-III study. National regulatory organizations and institutional ethical and scientific review boards approved the study. After explanation of the study aims and methods, verbal assent was obtained from students and written informed consent from their parents.

This sub-study was conducted in 2009-2010 among 1090 students, aged from 10 to 18 years. They were selected by random cluster sampling from urban and rural areas of 27 provinces in Iran. Detailed methodology is published previously.[32]

Variables assessment

Trained health professionals conducted the physical examination under standard protocols and by using calibrated instruments. Height (Ht) and weight (Wt) were measured, by trained research assistants, according to standardized protocols, without shoes and with light clothing to the nearest 0.1 unit of measure (cm for height and kg for weight). Body mass index (BMI) was calculated as weight (kg) divided by height squared (m2). BMI categories were defined according to the reference curves of the World Health Organization.[33] Waist circumference (WC) was measured over the skin, midway between the lower border of the rib margin and the iliac crest at the end of the normal expiration, to the nearest 0.1 cm. Both WC and Ht were measured using nonelastic tapes. Waist-to-height ratio was calculated by dividing WC (cm) to Ht (cm), and levels greater than 0.5 were considered as abdominal obesity.

Blood samples were collected in the morning after 12 h overnight fasting. Serum concentration of 25(OH)D was analyzed quantitatively by direct competitive immunoassay chemiluminescence method using LIASON® 25 OH Vitamin D assay TOTAL (DiaSorin, Inc.), with a coefficient of variation of 9.8%. Serum 25(OH)D level of <10 ng/mL was considered as Vitamin D deficiency and levels between 10 and 30 ng/mL as Vitamin D insufficiency.[34]

Statistical analysis

Continuous variables are expressed as mean (standard deviation) and categorical variables are presented as number (percentage). The values of Vitamin D are presented as median and interquartile range (IQR). Association between continuous and categorical variables with sex was assessed using t-test and Chi-square test, respectively. The median value of Vitamin D between sexes was compared using Mann-Whitney U-test. Association between Vitamin D status with BMI levels and abdominal obesity was assessed using Chi-square test. Multivariate linear regression was used to assess association of Vitamin D status (as categorical variable) and Vitamin D concentration (as continuous variable) with anthropometric measures (BMI and WC) after adjusting for age, sex, and living area. Multivariate logistic regression was used to examine the association of Vitamin D status (as categorical variable) and Vitamin D concentration (as continuous variable) with generalized and abdominal obesity after adjusting for age, sex, and living area. Linear and logistic analysis also was performed according to sex groups. Data were analyzed by SPSS software (version 18:0, SPSS Inc., Chicago, Illinois). P < 0.05 was considered as statistically significant.

RESULTS

Study participants consisted of 1090 adolescents with a mean age, BMI, and WC of 14.7 (2.6) years, 19.3 (4.2) kg/m2 and 67.82 (12.23) cm, respectively. They consisted of 51.9% boys and 67.1% urban residents. The prevalence of general and abdominal obesity was 9.5% and 13.8%, respectively. The median serum 25(OH)D was 13.0 ng/mL (IQR: (20.6). Overall, 40% of participants were Vitamin D deficient and 38.9% were Vitamin D insufficient. The demographic characteristics, anthropometric measures, and Vitamin D status of study participants according to sex are presented in Table 1. The mean value of WC in girls was statistically higher than boys. Overweight and generalized obesity were significantly more prevalent in boys compare to girls, while no association was found between sex and abdominal obesity. The association of BMI levels and abdominal obesity with Vitamin D status is presented in Table 2. It shows that no association was observed between BMI level and abdominal obesity with Vitamin D status. Table 3 shows β-coefficients (SE) for WC and BMI by Vitamin D status and Vitamin D concentration. As presented in this table, there was neither not significant association between the Vitamin D concentration (as a continuous variable) nor the Vitamin D status (as categorical variable) with WC and BMI. The association of Vitamin D status and Vitamin D concentration with generalized and abdominal obesity in multivariate logistic regression is presented in Table 4. It shows that the association between Vitamin D with generalized and abdominal obesity in both sexes and whole population was not statistically significant.

Table 1

Distribution of demographic characteristics, anthropometric measures, and Vitamin D status by sex: The CASPIAN-III study

Table 2

Relationship of categorical BMI and abdominal obesity with serum 25(OH)D

Table 3

Association between Vitamin D concentration and Vitamin D status with BMI and WC according to sex and in the whole population: The CASPIAN-III study

Table 4

Association between Vitamin D concentration and Vitamin D status with generalized and abdominal obesity according to sex: The CASPIAN-III study

DISCUSSION

We did not document significant association between serum 25(OH)D levels and measures of general and abdominal obesity. Previous studies showed inconsistent results.[1718192021222324252627282930] Our findings are consistent with the study of Nesby-O’Dell et al. and Kamycheva et al.[1819]

However, it is demonstrated that Vitamin D could significantly induce both in vitro and in vivo lipogenesis. It is documented that obese individuals have significantly higher Vitamin D levels than their other counterparts. Furthermore, 25(OH)D and intracellular Ca are considered as causative factors for lipogenesis and inhibition of lipolysis.[17]

On the other hand, some other studies showed an inverse correlation between the levels of 25(OH)D and obesity.[262728] For instance, Parikh et al. found that Vitamin D levels in obese individuals were 23% lower than others.[25] Therefore, it is presumed that Vitamin D would be probably diminished due to its entrapment in fatty masses.[35363738] Furthermore, Wong et al. found that the alteration of gene expression encoding Vitamin D regulation in mice induces both basic metabolism and beta oxidation reduction.[39]

The discrepancies between the results of different studies are based on the assumption that different methods for Vitamin D measurement (such as radio receptor assay, radioimmunoassay, and chemiluminescence) would result in various findings. Moreover, other factors as sample size, climate circumstances, nutritional patterns, environmental conditions, and lifestyle habits might contribute in the diverse results of the previous studies.[40]

Study limitations and strengths

The main limitation of this study is its cross-sectional design, which limits the causal relationship of variables. Furthermore, single blood sampling for every individual was another limitation. The strengths of this study were its nationwide coverage, the large sample size, and including participants from different parts of the country with various lifestyle habits and environmental conditions.

CONCLUSION

We did not document significant association between serum 25(OH)D level and anthropometric measures. This finding may be, at least in part, because of considerably high prevalence of hypovitaminosis D in the study population.

AUTHOR'S CONTRIBUTION

MJ contributed in the study design, drafting, and revising the manuscript. MQ contributed in the study design, statistical analysis, drafting, and revising the manuscript. MM contributed in the study design, drafting, and revising the manuscript. MEM contributed in the study design and conduct, drafting, and revising the manuscript. MK contributed in the study design, drafting, and revising the manuscript GA contributed in the study design and conduct, drafting, and revising the manuscript. RK was the study PI, contributed in the study design, drafting, and revising the manuscript. All authors read and approved the final version of the manuscript.