Vitamin D Deficiency in Obese Children and Its Relationship to Glucose Homeostasis

Abstract

Obesity in adults and children is associated with high rates of vitamin D deficiency. The relationship of mechanisms of glucose homeostasis to this association is poorly understood. This cross-sectional study was designed to compare the prevalence of vitamin D deficiency in obese and nonoverweight children in North Texas, and also to examine the association between dietary habits and serum 25-hydroxyvitamin D (25(OH)D) levels and correlations between 25(OH)D levels and markers of glucose homeostasis and blood pressure in obese children. The clinical parameters examined included serum 25(OH)D, dietary information, homeostasis model assessment of insulin resistance, 2-hour glucose from oral glucose tolerance test, hemoglobin A1c, fasting glucose and insulin, and systolic and diastolic blood pressures. Participants were 411 obese subjects (aged 6–16 years) examined at an obesity referral clinic. Levels of 25(OH)D also were measured in 87 matched control nonoverweight subjects (aged 6–16 years). Obese subjects with a 25(OH)D level below 50 nmol/L were considered to be vitamin D deficient, and those with a level below 75 nmol/L were classified as vitamin D insufficient. More than 90% of obese subjects were vitamin D insufficient (25(OH)D level <75 nmol/L) and 50% were vitamin D deficient (25(OH)D level <50 nmol/L). Significantly fewer control subjects were vitamin D insufficient or deficient (both P < 0.01 compared with obese subjects). A negative association was found between 25(OH)D levels and the dietary measures of soda intake, juice intake, and skipping breakfast (each comparison P < 0.001). After adjustment for body mass index and age, 25(OH)D levels were negatively correlated with homeostasis model assessment of insulin resistance (r = −0.19; P = 0.001) and 2-hour glucose (r = −0.12; P = 0.04. There was no correlation between 25(OH)D and hemoglobin A1c, systolic blood pressure Z score, or diastolic blood pressure Z score. These findings show that vitamin D deficiency is significantly more prevalent in a population of obese children than in nonoverweight children. The inverse association between 25(OH)D levels with both insulin resistance and 2-hour glucose levels suggest that low 25(OH)D levels may play a role in the pathophysiology of impaired glucose tolerance or type 2 diabetes in obese children.