Vitamin D and Adiposity

Studies in adults and children have shown a link between obesity and vitamin D status. Serum 25-hydroxyvitamin D [25(OH)D], the recognized biomarker of vitamin D status, is inversely associated with clinical and laboratory measures of adiposity such as body mass index (BMI), waist circumference and percentage body fat in adults and youth.

Our research aimed to specifically examine the relationship between vitamin D status and abdominal adiposity, and lipids in children, and the impact of race (black vs. white) since body fat topography and lipid profile differ between black and white youth. Plasma 25(OH)D, adiposity [body mass index (BMI), percentage of total body fat, visceral adipose tissue (VAT), subcutaneous adipose tissue (SAT)], and fasting lipids were assessed in healthy obese and nonobese 8- to 18-yr-old black and white children.

We studied 237 children (mean ± sd age, 12.7 ± 2.2 yr; 47% black, 47% obese, and 43% male). Mean 25(OH)D concentration for the entire cohort was 19.4 ± 7.4 ng/ml. The majority of the children, 73% of blacks and 40% of whites, were vitamin D deficient [25(OH)D < 20 ng/ml]. Plasma 25(OH)D was associated inversely with BMI, BMI percentile, percentage of total body fat, VAT, and SAT and positively with HDL cholesterol in the entire cohort. VAT was higher in vitamin D-deficient whites, and SAT was higher in vitamin D-deficient blacks compared with their respective vitamin D-nondeficient counterparts.

In conclusion, vitamin D deficiency and insufficiency are highly prevalent in 8-18 year old preadolescents and adolescent children residing in the Northeast United States. Lower levels of 25(OH)D are associated with higher adiposity measures and lower HDL. Race, female gender, season, pubertal status and visceral adiposity are independent predictors of plasma 25(OH)D status.  Besides therapeutic interventions to correct the high rates of vitamin D deficiency in youth, benefits of vitamin D optimization on adiposity measures and lipid profile need to be explored.

Vitamin D and In Vivo Insulin Sensitivity and β-Cell Function 

Vitamin D is proposed to play a role in glucose homeostasis and b-cell function. In adults, low 25(OH)D concentration is found to be associated with higher risk of hyperglycemia, insulin resistance and type 2 diabetes. Data are limited in children. Therefore, we aimed to assess the relationships between plasma 25(OH)D and in vivo insulin sensitivity and β-cell function relative to insulin sensitivity, the disposition index (DI),  in youth.

Plasma 25(OH)D concentrations were analyzed in healthy youth aged 8 to 18 years who had  undergone hyperinsulinemic-euglycemic and hyperglycemic clamp evaluations to measure  insulin sensitivity and secretion. In addition body composition and body fat topography was evaluated for abdominal visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT).

A total of 183 research volunteers (mean ± SD; age, 12.6 ± 2.2 years; 98 white, 98 male, 92 obese) were studied. Analysis of HbA_1c, fasting glucose and insulin, insulin sensitivity, and b-cell function across quartiles of plasma 25(OH)D revealed no differences among whites. In blacks, the observed significance of higher insulin sensitivity and DI in the highest quartile of 25(OH)D disappeared after adjusting for any of the adiposity measures (BMI or fat mass or VAT or SAT). The difference in insulin sensitivity (9.4 ± 1.2 vs. 5.6 ± 0.5 mg/kg/min per μU/mL; P = 0.006) between 25(OH)D nondeficient (≥20 ng/mL) versus deficient (<20 ng/mL) black youth also disappeared after adjusting for adiposity differences.

We conclude that in healthy youth, plasma 25(OH)D concentrations bear no independent relationship to parameters of glucose homeostasis and in vivo insulin sensitivity and β-cell function. It remains to be determined whether in youth with dysglycemia (type 2 diabetes or pre diabetes) the relationships are different and whether vitamin D optimization enhances insulin sensitivity and β-cell function.