Background: Impaired fasting glucose (IFG) is a prediabetic condition and is a high-risk state for developing diabetes and associated complications. The aim of this study was to explore the association of vitamin D with insulin secretory function among the IFG subjects. Materials and Methods: This was a prospective cross-sectional study conducted in the Department of Biochemistry and Cell Biology, Bangladesh University of Health Sciences (BUHS), Dhaka, Bangladesh; from June 2016 to May 2017, on forty-six (46) IFG subjects. Serum blood glucose was measured by glucose-oxidase method, Fasting serum lipid profile (Total cholesterol, TG and HDL-c); liver enzyme like alanine aminotransferase (ALT) was measured by enzymatic-colorimetric method; Serum creatinine was measured by colorimetric kinetic method; Serum Insulin and vitamin D [25(OH)D] were measured by ELISA method; insulin secretory capacity (HOMA%B) and insulin sensitivity (HOMA%S) were calculated by Homeostasis Model Assessment (HOMA) using HOMA-CIGMA software. Results: Among 46 IFG subjects, 22 (47.8%) were male and rest 24 (52.2%) were female and their mean age was 40 (±8) years. Mean fasting serum glucose level of the study subjects was 6.33 (±0.23) mmol/l and mean postprandial serum glucose level was 7.23 (±0.41) mmol/l. Mean serum vitamin D level of the study subjects was 26.54 (±8.83) ng/ml. Mean HOMA%S, HOMA%B and HOMA%IR of the total study subjects were 48.34 (±16.70), 102.16 (±23.13) and 2.28 (±0.70) respectively. Insulin secretory capacity (HOMA%B) was significantly higher (p = 0.047) in the subjects having higher vitamin D [25(OH)D level ≥ 20 ng/ml] compared to the subjects having lower vitamin D [25(OH)D levels < 20 ng/ml]. Conclusion: High prevalence of hypovitaminosis D exists among IFG subjects. Vitamin D is associated with insulin secretory function in IFG subjects. Population based prospective studies using larger sample size should be done to confirm the results.
Impaired fasting glucose (IFG) is the state of an individual whose fasting blood glucose levels are consistently elevated above normal but are not high enough to be considered as having diabetes mellitus. IFG is defined as elevated fasting plasma glucose (FPG) concentration ≥ 110 mg/dl and < 125 mg/dl (6.1 - 6.9 mmol/L) [
In healthy people blood glucose is strictly regulated. Fasting glucose is maintained between 3.9 and 5.6 mmol/L and the post-meal increases rarely exceed 3 mmol/L. During the development of prediabetes, this homeostasis of fasting and post-load glucose becomes abnormal. Subjects with prediabetes manifest both core defects that are characteristic of type 2 diabetes i.e. insulin resistance and β-cell dysfunction. Study reveals that, IFG depends primarily on defective insulin secretion, and IGT on insulin resistance [
Vitamin D is a fat soluble vitamin and is obtained from diet (oily fish or fortified dairy products), dietary supplements and sun exposure. Vitamin D is a multifunctional hormone that can affect many essential biological functions, ranging from immune regulation to mineral ion metabolism. Although the major function of vitamin D is to maintain calcium and phosphate homeostasis and to promote bone mineralization, many extraskeletal roles for vitamin D have been identified [
Dietary vitamin D [25(OH)D] is absorbed in the small intestine and incorporated into chylomicrons, then travels to the liver, bound to vitamin D binding protein and in continued association with chylomicrons and lipoproteins, where it and endogenously synthesized cholecalciferol are metabolized. During exposure to sunlight, the solar ultraviolet B (UVB) photons with 290 to 315 nm wavelengths are absorbed by 7-dehydrocholesterol in the skin, which is then converted into previtamin D which is converted by a thermal reaction into vitamin D [25(OH)D]. In the kidney, the 25(OH)D can undergo two different hydroxylation reactions (catalyzed by different hydroxylases:1α-hydroxylase and 24-hydroxylase), generating the active 1,25(OH)2D (calcitriol), and 24,25(OH)2D, the inactive form.
During recent years, a considerable body of evidence has emerged suggesting that vitamin D may have an impact on the development of prediabetes. Data from the third National Health and Nutrition Examination Survey (NHANES III, 1988-1994) revealed that vitamin D deficiency was associated with an increased risk of type 2 diabetes. Conversely, in the Nurses’ Health Study, Pittas AG et al., (2006) reported a 33% decreased risk of type 2 diabetes in women with high vitamin D intake compared to women with low intake [
A prospective cross-sectional study has been conducted in the Department of Biochemistry and Cell Biology, Bangladesh University of Health Sciences (BUHS), Darus Salam, Mirpur, Dhaka, Bangladesh; over a period of one year from June 2016 to May 2017. Forty-six (46) impaired fasting glucose (IFG) subjects were purposively collected. Data about socio-demographics were carefully taken; blood pressure, height, weight were appropriately measured; BMI and WHR were calculated accordingly. Serum analysis was done by appropriate procedure and methods. Serum blood glucose was measured by glucose-oxidase method; Fasting serum lipid profile (Total cholesterol, TG and HDL-c) and liver enzyme like Alanine Aminotransferase (ALT) were measured by enzymatic-colorimetric method; Serum creatinine was measured by colorimetric kinetic method; Serum Insulin and vitamin D [25(OH)D] were measured by ELISA method; insulin secretory capacity (HOMA%B) and insulin sensitivity (HOMA%S) were calculated by Homeostasis Model Assessment (HOMA) using HOMA-CIGMA software. Student’s unpaired “t” test, Pearson’s correlation coefficient and logistic regression analysis was done for statistical analysis using SPSS package.
This study was approved by the ethical review committee of Bangladesh University of Health Sciences (BUHS), Darus Salam, Mirpur, Dhaka, Bangladesh.
According to WHO criteria forty-six (46) IFG subjects were collected on Saturday, Monday, and Wednesday of the week between 8:00 am to 12:00 noon from the Out-Patient Department (OPD) of Bangladesh Institute of Health Sciences (BIHS) Hospital, Darus Salam, Mirpur, Dhaka, Bangladesh. After taking brief history, preliminary selection was done, and the purpose of the study was explained in details to each subject and their verbal consent was taken. They were advised to take unrestricted carbohydrate diet, to do normal physical activities and to avoid drugs and vitamin supplementations that significantly interfere blood glucose level (like glucocorticoids, oral contraceptives containing levonorgestrel or high-dose estrogen, phenytoin, high-dose thiazide diuretics, etc.) for 3 days. They were also advised to abandon the program if they became sick. Then they were requested to report to BUHS Biochemistry laboratory after 3 days at morning between 8.00-10.00 am following an overnight (8 - 12 hours) fasting. When the subjects reported, informed written consent was taken from all participants. A predesigned case record form was used to record relevant clinical, medical, demographic, socio-economic data (age, sex, educational status, and occupational status) from the consenting subjects.
Overnight fasting (8 - 12 hours) blood was collected between 8.00-9.00 am. Venous blood (6 ml) was obtained by venipuncture following standard procedure. Subjects were then allowed to drink glucose (75 gm in 300 ml of water). Then they were requested not to take any food & beverage and be rested for two hours. After 2 hours of glucose intake the second blood sample (3.00 ml venous blood) was taken. Fasting and postprandial blood samples were taken into plain tube (6 cc), allowed to clot for 30 minutes and serum was separated by centrifugation for 10 min at 3000 rpm and then the serum was collected at least 600 µl in each of four aliquots. Blood samples were maintained at 4˚C until separation and serum was frozen at −30˚C within an hour of sample collection. One aliquot was assayed for fasting serum glucose, serum glucose 2 hours after 75 gm oral glucose load, S. triglyceride, total cholesterol, HDL cholesterol, S. creatinine, ALT by appropriate procedure within a week of sample collection. Second aliquot was kept frozen for insulin and third aliquot for vitamin D [25(OH)D] measurement respectively. The remaining aliquot was kept frozen at −30˚C for further measurement. Serum was not allowed to be thawed until the assay was performed.
Insulin secretory function (HOMA%B) and insulin sensitivity (HOMA%S) was calculated from fasting serum glucose and insulin values by Homeostasis Model Assessment (HOMA) using HOMA-CIGMA software.
Biochemical measurements were carried out at the Biochemistry Lab, Dept of Biochemistry & Cell Biology, Bangladesh University of Health sciences (BUHS), Darus Salam, Mirpur, Dhaka, Bangladesh.
The following laboratory investigations were done for each of the study subject-
・ Serum blood glucose was measured by glucose-oxidase method in the automatic analyzer (Siemens Dimension-RXL, max, Siemens, USA).
・ Fasting serum lipid profile (Total cholesterol, TG and HDL-c) and liver enzyme like Serum Alanine Aminotransferase (ALT) were measured by enzymatic-colorimetric method in the automatic analyzer (Siemens Dimension-RXL, max, Siemens, USA).
・ Serum creatinine was measured by colorimetric kinetic method in the automatic analyzer (Siemens Dimension-RXL, max, Siemens, USA).
・ Serum Insulin and vitamin D [25(OH)D] were measured by ELISA method (DRG-International, Germany).
・ Insulin secretory function (HOMA%B) and insulin sensitivity (HOMA%S) were calculated by Homeostasis Model Assessment (HOMA) using HOMA-CIGMA software.
All the collected data were rechecked, coded and entered into a database using Statistical Package for Social Science (SPSS) for Windows version 19 software (SPSS Inc., Chicago, ILL). Analysis was done targeting the study objectives by considering the indicators. Simple descriptive method was used for representing variables that is, frequencies along with percentages or mean with standard deviation as necessary. Comparison between two groups was done using Student’s unpaired “t” test for normally distributed continuous variables. Correlation analysis between variables was examined by the Pearson’s correlation test. Multiple linear regression analysis was done where appropriate. p value < 0.05 was considered as significant.
Among forty-six (46) impaired fasting glucose (IFG) subjects, 22(47.8%) were male and rest 24 (52.2%) were female and their mean (±SD) age was 40 (±8) years. Mean (±SD) BMI (kg/m2) and WHR of the study population was 26.68 (±4.99) kg/m2 and 0.95 (±0.07). Mean (±SD) systolic blood pressure (SBP) and diastolic blood pressure DBP) of the study population was 113.26 (±9.67) mm of Hg and 75.43 (±7.52) mm of Hg respectively.
The anthropometric and clinical characteristics of the study subjects shown in following table (
Mean (±SD) fasting serum glucose level of the study subjects was 6.33 (±0.23) mmol/l and mean (±SD) postprandial (2 hour after glucose-AG) serum glucose level was 7.23 (±0.41) mmol/l. There was no significant abnormality in serum lipid profile of the study subjects. Their renal function [mean (±SD) S. creatinine 1.04 (±0.15) mg/dl] and liver function [mean (±SD) Serum Alanine Aminotransferase (ALT) 29.13 (±8.94) IU/L] were within normal limits. Mean (±SD) vitamin D level of the study subjects was 26.54 (±8.83) ng/ml.
Mean (±SD) fasting serum insulin of the total IFG subjects was 16.93 (±5.37) µIU/ml. Mean (±SD) HOMA%S, HOMA%B and HOMA%IR of the total study subjects were 48.34 (±16.70), 102.16 (±23.13) and 2.28 (±0.70) respectively (
Variables | Mean ± SD |
---|---|
Age (Years) BMI (kg/m2) | 40 ± 8 26.68 ± 4.99 |
WHR | 0.95 ± 0.07 |
SBP (mm of Hg) | 113.26 ± 9.67 |
DBP (mm of Hg) | 75.43 ± 7.52 |
BMI = Body Mass Index; WHR = Waist Hip Ratio; SBP = Systolic Blood Pressure; DBP = Diastolic Blood Pressure.
Variables | Mean ± SD |
---|---|
FBG (mmol/l) | 6.33 ± 0.23 |
AG (mmol/l) | 7.23 ± 0.41 |
TG (mg/dl) | 165.48 ± 44.39 |
Chol (mg/dl) | 197.04 ± 40.92 |
HDL-c (mg/dl) | 44.07 ± 5.02 |
LDL-c (mg/dl) | 119.87 ± 38.17 |
ALT (IU/L) | 29.13 ± 8.94 |
S. Creatinine (mg/dl) | 1.04 ± 0.15 |
S. Vitamin D (ng/ml) | 26.54 ± 8.83 |
FBG = Fasting Blood Glucose; AG = After Glucose (2 hours after glucose)/PPBS (Post-Prandial Blood Sugar; TG = Triglyceride; Chol = Cholesterol; HDL-c = High Density Lipoprotein Cholesterol; LDL-c = Low Density Lipoprotein Cholesterol; ALT = Alanine Aminotransferase.
Variables | Mean ± SD |
---|---|
S Insulin (µIU/ml) | 16.93 ± 5.37 |
HOMA%S | 48.34 ± 16.70 |
HOMA%B | 102.16 ± 23.13 |
HOMA% IR | 2.28 ± 0.70 |
The Homeostasis Model Assessment (HOMA) estimates steady state beta cell function (%B) and insulin sensitivity (%S), as percentages of a normal reference population. HOMA IR (insulin resistance) which is the reciprocal of %S (100/%S).
Among the total study population (IFG subjects, n = 46), 13 subjects (28.26%) were serum vitamin D deficient [serum 25(OH)D < 20 ng/ml], 17 subjects (36.95%) had insufficient serum vitamin D [serum 25(OH)D = 20.1 ~ 30 ng/ml] level and rest 16 subjects (34.78%) were sufficient serum vitamin D [serum 25(OH)D > 30.1 ng/ml] level (
Subjects were sub-grouped according to their vitamin D [Serum 25(OH)D concentration] level (cut-off point 20 ng/ml). No significant variation was found in anthropometric & clinical characteristics according to the cut-off point [< 20 ng/ml of 25(OH)D levels and ≥ 20 ng/ml of 25(OH)D levels] among the IFG subjects (
There was no significant difference in biochemical characteristics according to cut-off point [< 20 ng/ml of 25(OH)D levels and ≥20 ng/ml of 25(OH)D levels] among the total IFG subjects (
Insulin secretory capacity (HOMA%B) was significantly higher (p = 0.047) in the subjects having higher vitamin D [25(OH)D level ≥ 20 ng/ml (n = 33)] compared to the subjects having lower vitamin D [25(OH)D levels < 20 ng/ml (n = 13)].
S. vitamin D level (ng/ml) | Number of subjects | Percent (%) |
---|---|---|
< 20 | 13 | 28.26 |
20 - 30 | 17 | 36.95 |
>30 | 16 | 34.78 |
Total | 46 | 100.00 |
Variables | Serum 25(OH)D concentration | t/p value | |
---|---|---|---|
<20 ng/ml (n = 13) | ≥20 ng/ml (n = 33) | ||
BMI (kg/m2) | 26 ± 3 | 26 ± 6 | −0.305/0.762 |
WHR | 0.92 ± 0.05 | 0.95 ± 0.07 | −1.364/0.179 |
SBP (mm of Hg) | 112 ± 11 | 114 ± 9 | −0.754/0.455 |
DBP (mm of Hg) | 74 ± 8 | 76 ± 7 | −0.898/0.374 |
The level of significance at p < 0.05.
Variables | Serum 25 (OH) D concentration | t/p value | |
---|---|---|---|
<20 ng/ml (n = 13) | ≥20 ng/ml (n = 33) | ||
FBG (mmol/l) | 6.43 ± 0.31 | 6.29 ± 0.18 | 1.541/0.062 |
AG (mmol/l) | 6.96 ± 0.43 | 7.34 ± 0.35 | −2.873/0.003 |
TG (mg/dl) | 150.77 ± 44.0 | 171.27 ± 43.85 | −1.427/0.161 |
Chol (mg/dl) | 197.62 ± 35.23 | 196.82 ± 43.46 | 0.059/0.953 |
HDL-c (mg/dl) | 45.54 ± 4.14 | 43.48 ± 5.27 | 1.257/0.215 |
LDL-c (mg/dl) | 121.89 ± 31.22 | 119.08 ± 41.0 | 0.223/0.825 |
ALT (U/L) | 24.85 ± 5.63 | 30.82 ± 9.49 | −2.629/0.040 |
S. Creatinine (mg/dl) | 0.92 ± 0.16 | 1.08 ± 0.11 | −3.254/0.1 |
S. Vitamin D (ng/ml) | 24.85 ± 5.63 | 30.82 ± 9.49 | −10.963/0.1 |
The level of significance at p < 0.05.
There was no significant difference in Insulinemic status according to cut-off point [<20 ng/ml of 25(OH)D levels and ≥20 ng/ml of 25(OH)D levels] among the total IFG subjects (
Pearson’s correlation analysis were performed to cut-off point < 20 ng/ml for serum vitamin D [25(OH)D] with other anthropometric & biochemical variables among the IFG subjects (n = 13). Where <20 ng/ml of 25(OH)D levels showed no significant association between vitamin D [25(OH)D] and other variables among these IFG subjects (
Variables | Serum 25(OH)D concentration | t/p value | |
---|---|---|---|
<20 ng/ml (n = 13) | ≥20 ng/ml (n = 33) | ||
S Insulin (µIU/ml) | 16.93 ± 5.37 | 17.60 ± 5.57 | −1.353/0.183 |
HOMA%S | 48.34 ± 16.70 | 52.38 ± 16.01 | 1.029/0.309 |
HOMA%B | 102.16 ± 23.13 | 106.05 ± 23.83 | −2.081/0.047 |
HOMA IR | 2.28 ± 0.70 | 2.37 ± 0.72 | −1.301/0.200 |
The level of significance at p < 0.05.
Variables | r value | p value |
---|---|---|
Age (yrs) | 0.149 | 0.627 |
BMI (kg/m2) | −0.111 | 0.717 |
FBG (mmol/l) | 0.188 | 0.539 |
AG (mmol/l) | 0.409 | 0.165 |
TG (mg/dl) | 0.198 | 0.516 |
ALT (IU/L) | 0.222 | 0.467 |
S. Insulin (µIU/ml) | 0.311 | 0.301 |
HOMA%S | −0.163 | 0.595 |
HOMA%B | 0.223 | 0.464 |
HOMA IR | 0.310 | 0.303 |
The level of significance at p < 0.05; r = correlation coefficient.
Pearson’s correlation analysis were performed to cut-off point ≥ 20 ng/ml for serum vitamin D [25(OH)D] with other anthropometric & biochemical variables among the IFG subjects (n = 33). Where ≥20 ng/ml of 25(OH)D levels showed that there is no significant association between vitamin D [25(OH)D] and other variables among these IFG subjects (
Multiple linear regression analysis was done by considering HOMA%B as dependent variable and BMI, TG and S. vitamin D as independent variables. Where BMI had a positive association (p = 0.009) with HOMA%B but TG and S. vitamin D showed no significant association with HOMA%B after adjusting the confounding variables (
Multiple linear regression analysis was done by considering HOMA%S as dependent variable and BMI, TG and S. vitamin D as independent variables. Where BMI, TG and S. vitamin D showed no significant association with HOMA%S after adjusting the confounding variables (
Impaired fasting glucose (IFG) is a prediabetic condition and is a transitory state
Variables | r value | p value |
---|---|---|
Age (yrs) | 0.287 | 0.106 |
BMI (kg/m2) | 0.034 | 0.851 |
FBG (mmol/l) | −0.016 | 0.930 |
AG (mmol/l) | 0.344 | 0.050 |
TG (mg/dl) | 0.054 | 0.766 |
ALT (IU/L) | 0.182 | 0.310 |
S Insulin (µIU/ml) | −0.075 | 0.679 |
HOMA%S | 0.055 | 0.762 |
HOMA%B | −0.068 | 0.707 |
HOMA IR | −0.076 | 0.674 |
The level of significance at p < 0.05; r = correlation coefficient.
Variable | β value | p value |
---|---|---|
BMI | 0.385 | 0.009 |
TG | 0.66 | 0.651 |
Vitamin D | 0.168 | 0.248 |
β for standardized regression coefficient, Dependent Variable: HOMA% B, p value < 0.05 was considered statistically significant.
Variable | β value | p value |
---|---|---|
BMI | −0.277 | 0.070 |
TG | −0.166 | 0.285 |
Vit-D | −0.063 | 0.677 |
β for standardized regression coefficient, Dependent Variable: HOMA% S, p value < 0.05 was considered statistically significant.
between normoglycemia and diabetes. Therefore IFG carries a variable risk of progression to diabetes. The progression from IFG to diabetes is a time dependent phenomenon and biochemical mechanism of the pathophysiological progression is still unclear. Since insulin resistance and reduced insulin secretion is a risk factor for prediabetes, understanding the role of various nutritional and other modifiable risk factors that may contribute to the pathogenesis of IFG is important.
Vitamin D is a multifunctional hormone that can affect many essential biological functions, ranging from immune regulation to mineral ion metabolism. Although the major function of vitamin D is to maintain calcium and phosphate homeostasis and to promote bone mineralization, many extra-skeletal roles for vitamin D have been identified [
During recent years, a novel association between vitamin D [25(OH)D] deficiency and prediabetes has been proposed. There are evidences to suggest that altered vitamin D and calcium homeostasis may also play a role in the development of type 2 Diabetes Mellitus (DM) [
It is known that β-cell dysfunction is an important phenomenon of IFG subjects. There are several lines of evidence supporting a role for vitamin D on pancreatic β-cell function [
A group of middle aged (mean age 40 years) IFG subjects were included in this study to investigate the association of vitamin D [25(OH)D] with insulin secretion and insulin sensitivity. Among the total study population 13 subjects (28.3%) were serum vitamin D deficient, 17 subjects (37%) had insufficient serum vitamin D level and rest 16 subjects (34.8%) were sufficient serum vitamin D level. So majority (65.21%) of impaired fasting glucose (IFG) subjects have low serum vitamin D [25(OH)D] level. The Mean (±SD) vitamin D [25(OH)D] level of our study subjects was 26.54 ± 8.83 ng/ml, which reflects the usual vitamin D status of our healthy population. This finding was consistent with study done on Asian population and they demonstrated that in South Asia, 80% of the apparently healthy population is deficient in vitamin D (<20 ng/ml) and up to 40% of the population is severely deficient (<9 ng/ml) [
Many studies have revealed an association of low vitamin D [25(OH)D] with obesity. A recent study reported that body mass index (BMI) is inversely related to circulating levels of vitamin D [25(OH)D] [
Vitamin D deficiency is thought to influence the pathogenesis of prediabetes by affecting either insulin sensitivity and β-cell dysfunction or both. In present study HOMA%B of subjects having vitamin D [25(OH)D] level ≥ 20 ng/ml (n = 33) had significant positive (p = 0.047) association with serum vitamin D [25(OH)D] in comparison to subjects (n = 13) with vitamin D [25(OH)D] levels < 20 ng/ml. It has been demonstrated that the subjects who have higher vitamin D level had better β (beta)-cell function compared to the subjects having lower vitamin D level. So vitamin D is associated with insulin secretory function in IFG subjects. This finding was consistent to previous reports, as showed that higher baseline vitamin D [25(OH)D] independently predicted better β-cell function and serum vitamin D [25(OH)D] is associated with insulin sensitivity and β-cell function [
Majority of impaired fasting glucose (IFG) subjects have insufficient vitamin D. No significant correlation exists between BMI, FBS, PPBS & fasting insulinemic status with vitamin D [25(OH)D] levels. IFG subjects having higher vitamin D level have higher insulin secretory capacity. Vitamin D is associated with insulin secretory function in IFG subjects.
This was a cross sectional study with small sample size, thus our results cannot establish cause & effective relationship between vitamin D with insulin secretory function in impaired fasting glucose (IFG) subjects.
Population based prospective studies using larger sample size should be done to confirm the results and to establish the association between vitamin D with insulin secretory function in impaired fasting glucose (IFG) subjects.
The authors declare no conflicts of interest regarding the publication of this paper.
Shahidur Rahman, A.K.M., Sohel, Md.A., Bhuiyan, F.R., Ashrafee, F. Al S., Hossain, Md.K., Islam, S.F., Ali, M.I. and Zinnat, R. (2019) Serum Vitamin D Associated with Insulin Secretory Function in Impaired Fasting Glucose Subjects. Journal of Biosciences and Medicines, 7, 83-98. https://doi.org/10.4236/jbm.2019.71008