Serum 25-hydroxyvitamin D and insulin resistance in people at high risk of cardiovascular disease: a euglycaemic hyperinsulinaemic clamp study

Serum 25-hydroxyvitamin D is associated with insulin resistance independently of obesity in children ages 5-17

AIM

To determine the association of vitamin D with insulin resistance and obesity in children.

METHODS

A total of 92 obese and 58 non-obese children aged 5-17 years were evaluated. Data were collected related to anthropometric (weight, height), and biochemical parameters (fasting plasma glucose, serum insulin, serum 25-hydroxyvitamin D, lipid profile, vitamin B12, parathormone) and physical examination (blood pressure, acanthosis nigricans, stria, lipomastia). Insulin resistance (IR) was calculated using the homeostasis model assessment (HOMA). HOMA-IR = fasting insulin level (μU/ml) × fasting glucose (mg/dL)/405. A HOMA-IR value >2.5 was defined as insulin resistance.

RESULTS

According to the US Endocrine Society classification, vitamin D deficiency (0-20 ng/ml) was determined at significantly higher rates in the obese group than in the control group (p < 0.001). The rate of subjects with a vitamin D level of 20-30 ng/ml was significantly lower in the obese group than in the control group (p < 0.001) Within the obese group a statistically significant difference was determined between the insulin resistant and non-insulin resistant groups in respect of serum 25-hydroxyvitamin D levels (p = 0.001) and vitamin B12 levels (p = 0.001). A significant negative correlation was determined between serum 25-hydroxyvitamin D and HOMA-IR (r=-0.256, p = 0.016) and insulin (r = -0.258, p = 0.015). The systolic blood pressure (p = 0.001) and diastolic blood pressure (p = 0.003) values were significantly different in the control and obese groups. A statistically significant difference was determined between the control and obese groups in terms of the levels of insulin, HOMA-IR, HbA1c, cortisol, LDL, total cholesterol, HDL, triglyceride, hemoglobin, MCV, MPV, and calcium.

CONCLUSION

The prevalence of vitamin D deficiency was higher in obese children compared to normal-weight and overweight children. Serum 25(OH)D levels showed a negative correlation with insulin and HOMA-IR. Serum 25(OH)D is associated with insulin resistance independently of obesity.

Insulin-sensitizing effects of vitamin D repletion mediated by adipocyte vitamin D receptor: Studies in humans and mice

Objective

Adipose tissue inflammation and fibrosis appear to contribute to insulin resistance in obesity. Vitamin D receptor (Vdr) genes are expressed by adipocytes, macrophages, and fibroblasts, all of which could potentially play a role in adipose tissue inflammation and fibrosis. As vitamin D has been shown to have direct anti-inflammatory effects on adipocytes, we determined whether specific vitamin D receptor-mediated effects on adipocytes could impact adipose tissue inflammation and fibrosis and ultimately insulin resistance.

Methods

We examined the effects of repleting vitamin D in 25(OH)D-deficient, insulin resistant, overweight-to-obese human subjects (n = 19). A comprehensive assessment of whole-body insulin action was undertaken with stepped euglycemic (∼90 mg/dL) hyperinsulinemic clamp studies both before and after the administration of vitamin D or placebo. Adipose tissue fibrosis and inflammation were quantified by real-time rt-PCR and immunofluorescence in subcutaneous abdominal adipose tissue.

To determine whether vitamin D's effects are mediated through adipocytes, we conducted hyperinsulinemic clamp studies (4 mU/kg/min) and adipose tissue analysis using an adipocyte-specific vitamin D receptor knockout (VDR-KO) mouse model (adiponectin-Cre + VDR+/fl) following high-fat diet feeding for 12 weeks.

Results

25(OH)D repletion was associated with reductions in adipose tissue expression of pro-inflammatory and pro-fibrotic genes, decreased collagen immunofluorescence, and improved hepatic insulin sensitivity in humans. Worsening trends after six months on placebo suggest progressive metabolic effects of 25(OH)D deficiency. Ad-VDR-KO mice mirrored the vitamin D-deficient humans, displaying increased adipose tissue fibrosis and inflammation and hepatic insulin resistance.

Conclusions

These complementary human and rodent studies support a beneficial role of vitamin D repletion for improving hepatic insulin resistance and reducing adipose tissue inflammation and fibrosis in targeted individuals, likely via direct effects on adipocytes. These studies have far-reaching implications for understanding the role of adipocytes in mediating adipose tissue inflammation and fibrosis and ultimately impacting insulin sensitivity.

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Vitamin D repletion improved hepatic insulin sensitivity in obese insulin-resistant and vitamin D deficient human.

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Correcting vitamin D deficiency concomitantly reduced adipose tissue expression of pro-inflammatory and pro-fibrotic genes.

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Worsening trends in these metabolic parameters were observed following 6 months of uncorrected vitamin D deficiency.

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Adipocyte-specific depletion of VDR in mice induced adipose tissue inflammation and fibrosis and hepatic insulin resistance.

Maternal vitamin D and markers of glycaemia during pregnancy in the Belfast centre of the Hyperglycaemia and Adverse Pregnancy Outcome study

AIMS

To measure total 25-hydroxyvitamin D levels in women in mid-pregnancy who participated in the Belfast centre of the Hyperglycaemia and Adverse Pregnancy Outcome (HAPO) observational study, and to investigate the associations between levels of 25-hydroxyvitamin D and markers of gestational diabetes mellitus and lipid biomarkers.

METHODS

A total of 1585 pregnant women had serum samples available for measurement. Participants were recruited from the Royal Jubilee Maternity Hospital, Belfast, Northern Ireland, at 24-32 weeks' gestation, as part of the HAPO study. 25-hydroxyvitamin D concentrations were measured using liquid chromatography tandem mass spectrometry. Glucose, C-peptide and lipid levels were previously analysed in a central laboratory. Statistical analysis was performed.

RESULTS

The median (interquartile range) 25-hydroxyvitamin D concentration during pregnancy was 38.6 (24.1-60.7) nmol/l, with 65.8% of women being vitamin D-deficient (≤50 nmol/l). In regression analysis, the association between maternal 25-hydroxyvitamin D and fasting plasma glucose levels approached significance [regression coefficient -0.017 (95% CI -0.034 to 0.001); P=0.06], and a significant positive association was observed between maternal 25-hydroxyvitamin D and β-cell function [1.013 (95% CI 1.001 to 1.024); P=0.031]. Maternal 25-hydroxyvitamin D level was positively associated with HDL [0.047 (95% CI 0.021 to 0.073) P≤ 0.001] and total cholesterol [0.085 (95% CI 0.002 to 0.167); P=0.044] in regression analysis.

CONCLUSIONS

These results indicate a high prevalence of vitamin D deficiency during pregnancy, which requires identification and treatment; however, only weak associations were observed between 25-hydroxyvitamin D level and markers of glucose and insulin metabolism. This would suggest that these are of doubtful clinical significance.