Lower Levels of Vitamin D Are Associated with an Increase in Insulin Resistance in Obese Brazilian Women

Abstract

Adult women are more likely to be obese than men. Moreover, there is evidence that obesity is a risk factor for increased insulin resistance (IR) and hypovitaminosis D (VITD), conditions related to metabolic and endocrinologic disturbance. We performed a cross-sectional study with 103 women diagnosed with obesity, recruited between 2009 and 2013, in an obesity referral outpatient clinic in Bahia, Brazil. Laboratory and clinical characteristics were compared between the groups according to the degree of obesity (I, II and III), and levels of 25-hydroxyvitamin D [25(OH)D] were used to define the VITD status (insufficiency and no insufficiency). We calculated the homeostatic model assessment-IR (HOMA-IR) index to assess insulin resistance in the groups. Our analyses revealed that HOMA-IR values and VITD levels were inversely correlated. Furthermore, we observed a distinct expression profile of values of laboratory markers according to 25(OH)D levels. Negative correlations were found between HOMA-IR and body mass index (BMI) in VITD insufficient participants but not in those with the sufficiency. Furthermore, multivariate regression demonstrated independent associations between lower levels of 25(OH)D and increased values of HOMA-IR. These findings suggests that lower levels of VITD are strongly associated with the increased IR in obese women.

Keywords: HOMA-IR; insulin resistance; obesity; vitamin D; women.

Figure 1

Spearman correlation analysis of the 25-hydroxyvitamin D [25(OH)D] shows association with HOMA-IR and insulin in women with obesity. (A) Correlation plot between 25-hydroxyvitamin D and HOMA-IR. (B) Correlation plot between 25-hydroxyvitamin D and HOMA-IR by obesity class. (C) Correlation plot between 25-hydroxyvitamin D and Insulin. (D) Correlation plot between 25-hydroxyvitamin D and insulin by obesity class. Abbreviations: 25(OH)D = 25-hydroxyvitamin D; HOMA-IR = homeostatic model assessment- insulin resistance; IU = international unit; mL = microliter.

Figure 2

Biochemical profiles of obese women stratified by 25-hydroxyvitamin D status. (A) Value of each parameter was log10 transformed. Mean values for each indicated clinical group were z-score normalized, and a hierarchical cluster analysis (Ward’s method with 100xbootstrap) was performed to illustrate the overall biochemical profiles. (B) Data represent median and interquartile ranges. The Mann–Whitney U test was employed to compare the values detected between the study subgroups. Abbreviations: FPG = Fasting plasma glucose; HbA1c = Glycated hemoglobin; HDL: High-density lipoprotein; LDL = Low-density Lipoproteins; TSH = Thyroid-stimulating hormone; C-RP = C-reactive protein; 25(OH)D = 25-hydroxyvitamin D.

Figure 3

Factors associated with HOMA-IR among women with obesity. (A) Spearman rho correlation between HOMA-IR and clinical and biochemical parameters; statistically significant differences are highlighted in green (positive) and yellow (negative). (B) Spearman correlation between HOMA-IR and BMI levels in women with obesity grouped according to 25-hydroxyvitamin D status; the values of r and p shown in the figure correspond to the general population (red box), with insufficient VID D (yellow box) and without insufficient VITD (blue box). (C) A linear regression analysis was used to test independent association between clinical and biochemical parameters and HOMA-IR. Abbreviations: FPG = Fasting plasma glucose; HbA1c = Glycated hemoglobin; HDL: High-density Lipoprotein; LDL = Low-density Lipoproteins; TSH = Thyroid-stimulating hormone; CRP = C-reactive protein; 25(OH)D = 25-hydroxyvitamin D.