Vitamin D deficiency and insufficiency are epidemic but commonly undiagnosed among obese children. Obese children in turn have higher risk of hypovitaminosis D. Turkey's vitamin D prophylaxis augmentation program (started in 2005) has resulted in a marked decrease in vitamin D deficiency in healthy children under 1 year of age [26
]; yet despite these improvements, most Turkish adolescents are still vitamin D deficient. In our study, 25-OHD levels were significantly lower in the obese group compared with healthy subjects. This finding revealed that obesity could be a risk factor of hypovitaminosis D in Turkish children and adolescents. Recent studies from different countries have also demonstrated that vitamin D deficiency is common in obese children [27
], possibly due to the low quality of diet [28
]. Here in Turkey, rates of hypovitaminosis D in healthy adolescents were 59.4% and 65% in two different studies [26
Vitamin D plays an important role in glucose homeostasis in the mechanism of insulin release. Most of the studies suggested vitamin D deficiency as a risk factor of disturbed glucose homeostasis in adults [30
], but this hypothesis is still controversial in relation to children. It remains unclear if vitamin D deficiency and insufficiency are associated with insulin resistance in obese children and adolescents.
Insulin resistance was estimated from fasting plasma measurements using HOMA-IR. In our study, insulin resistance values were higher in obese subjects compared with the healthy, vitamin-D-sufficient, age- and gender-matched children and adolescents. In order to determine the role of 25-hydroxyvitamin D levels in glucose intolerance and insulin resistance in obese children, we compare the HOMA-IR levels of vitamin-D-sufficient obese children and adolescents with the vitamin-D-deficient and -insufficient obese ones. There were no significant differences in HOMA-IR levels in obese children and adolescents due to 25-hydroxyvitamin D levels. In correlation analyses, we found that HOMA-IR depended on the degree of obesity and correlated with serum lipid profile. There was a significant trend towards higher insulin concentrations and insulin resistance in subjects with higher body mass index independent from 25-hydroxyvitamin D levels.
Recent studies researching the relationship between vitamin D deficiency and insulin resistance in obese children revealed controversial results. In two studies, no relationship was found between low vitamin D status and insulin resistance [34
]. On the other hand, the literature mostly supported the hypothesis that low vitamin D status is associated with worse glucose tolerance. Kelly et al. [36
] showed that vitamin D deficiency is associated with increased insulin resistance in children. Alemzadeh et al. [37
] observed a positive relationship between vitamin D status and insulin sensitivity in children. The study by Reis et al. [16
] showed that low serum vitamin D in adolescents was strongly associated with increased risk for fasting hyperglycemia, hypertension, and metobolic syndrome. In the study conducted by Garanty-Bogacka et al., fasting insulin levels and HOMA-IR were found correlated with low vitamin D levels [38
A limitation of this study is that vitamin D deficiency and insufficiency was defined only by 25-OHD levels. It is 1,25-dihydroxyvitamin D which is active on the vitamin D receptor in the insulin producing cells. It was also, not possible to evaluate other parameters modifying bone health such as subjects' dietary calcium intake or the responsiveness of the vitamin D receptor. In addition, with the lack of consensus regarding the definition of optimal vitamin D status in children, it is not possible to determine the level of vitamin D which disturbs glucose homeostasis and causes metabolic effects.
In conclusion, in our study, insulin resistance correlated mostly with BMI but not with 25-hydroxyvitamin D levels in the obese children and adolescents. We found significant association between the degree of obesity and some biochemical parameters with insulin resistance, but different levels of 25 hydroxyvitamin D status among obese children were not an independent predictor of insulin resistance.