Introduction
Vitamin 25(OH) D deficiency is a clinical condition when the level is lower than 50 nmol/L (20 ng/mL).1, 2 Globally, vitamin D deficiency is a common condition and impose major public health problem.3 Mounting evidences show that apart from vitamin D functions on bone health, it alters immune function, renin–angiotensin system and decreases insulin resistance in pregnant women.4 The vitamin D receptor is localized in wide range of cells cardiomyocytes, endothelial and immune cells.5 Mounting studies shows that vitamin D insufficiency may impose risk of hypertension, left ventricular hypertrophy, cardiac failure, diabetes and metabolic syndrome.6, 7 Raised levels of lipid profiles such as cholesterol, triglyceride, and low density lipoprotein (LDL), as reduced level of high density lipoprotein (HDL) contribute to the risk of developing atherosclerosis and CVD.8 Lowering serum cholesterol has been shown to reduce cardiovascular morbidity and mortality. Vitamin D ability to affect CVD via changing lipid profiles has yet to be extensively studied. According to prior studies, there appears to be a connection between 25 (OH) D levels and serum lipids. The results, on the other hand, are mixed. Mounting studies displayed the significant relationship between serum vitamin D levels and lipid profiles.9, 10 In addition, compared to LDL-C,11 small and dense LDL (sdLDL) is observed to lodge more easily on the artery wall. Despite the significant high level of vitamin D insufficiency for India's healthy population, no research has looked at its relationship with lipid profile. By studying the association between serum levels of 25(OH) D and lipid profile among the Indian subjects, this study will address a knowledge vacuum. The goal of this research was to see if there was a link between 25(OH) vitamin D levels and lipid profiles in individuals with vitamin D insufficiency and those with normal lipid profiles.
Materials and Methods
Healthy adults, obese (n=63) and non-obese (n=20), aged more than 18 years, were recruited from the outpatient department of biochemistry government medical college and hospital Aurangabad between July 2016 to July 2018.
Inclusion criteria
Obese subjects had a BMI ≥95th percentile for their age and gender, while non-obese subjects had a BMI in the range of 5th to 85th percentile for their age and gender.
Exclusion criteria
Patients having previous history of cancer, heart disease, diabetes, renal or hepatic disorders were excluded from the study.
Patients suffering from the disease which affects the serum vitamin D levels such as thyroid disorders were excluded from the study.
Blood pressure, height, and weight were all measured using standard methods. Fasting insulin, lipid panel (total cholesterol, low density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, very low density lipoprotein (VLDL) cholesterol, and triglycerides), Homeostatic Model Assessment HOMA, bone density (T and Z scores), levels were all measured in serum . The enzyme-linked fluorescence assay (ELFA) technique was used to detect serum 25(OH)D using the auto analyser VIDAS (Marcy l'Étoile, France). The vitamin D levels were classified as normal (≥30 ng/mL), inadequate (>20 to 29.9 ng/mL), and deficient (≤20 ng/mL) as per Endocrine Society's Clinical Practice Guidelines, 2011. 12
Statistical analysis
The statistical analysis was performed using SPSS v 18 (IBM Corp., Armonk, NY, USA). The comparison of biochemical parameters between obese and non-obese group was performed using Student’s t-test. The relationship between serum vitamin D levels, lipid profiles, and other clinical factors was estimated using simple linear regression analysis A p value <0.05 was considered as statistically significant.
Results
The overall demographics and biochemical profile of study participants (n=83) encompassing obese and non-obese individuals were shown in Table 1.
Table 1
Demographics and clinical characteristic of the study population
In this study the age, systolic and diastolic blood pressure among the obese and non-obese individuals was found to be non-significant. There was significant (p<0.0001) increase in height among the non-obese subjects as that of the obese subjects (157.75 ± 5.5 vs 151.44 ± 7.12 cms). Further, weight, BMI, vascular stromal fraction (VSF) and total body Fat% (TBF) was shown to be substantially elevated in obese individuals than in non-obese ones (Table 2).
Table 2
Comparison of Demographics among the obese and non-obese subjects
T-scores and Z-scores were used to report DEXA results. Obese participants had significantly higher T-scores and Z-scores than non-obese ones in this research. Furthermore, obese patients displayed significantly higher insulin levels, blood sugar levels, and Homeostatic Model Assessment-Insulin Resistance HOMA-IR than non-obese ones. In the meanwhile, vitamin D levels between obese and non-obese patients was found to be non-significant (p=0.14) (20.65± 4.53 vs 22.65± 6.62nmol/L). The results were shown in (Table 3).
Table 3
Comparison of Biochemical variables among the obese and non-obese subjects
Values are represented as mean ± SD. P value <0.05 was found to be significant. The association between vitamin D levels and lipid profiles was evaluated using regression coefficient analysis. The correlation was done based on the various grade of vitamin D levels such as Deficiency <20nmol/L, Insufficiency 20-30nmol/L and Sufficiency >30nmol/L. There was a strong negative connection between triglycerides and vitamin D levels in vitamin D insufficiency.(R=-0.642; p=0.01) and VLDL and vitamin D (R= -0.504; p=0.01). Meanwhile, there was no significant correlation between total cholesterol, HDL, LDL and Non-LDL and vitamin D level. In Vitamin D insufficiency, there was positive correlation between total cholesterol and found to be significant (R=0.566; p=0.001), Non-LDL (R= 0.729; p=0.000) and vitamin D levels.
In Vitamin D sufficiency, there was a substantial negative association between total cholesterol, positive correlation between triglycerides, HDL and VLDL and vitamin D level respectively (Table 4).
Table 4
Correlation between Vitamin D level and lipid profiles
Further, we have observed non- significant correlation between vitamin D level and Z scores and T-scores. However, there was significant negative correlation between Z score and Vitamin D insufficiency (R=-0.536; p=0.002) (Table 5).
Table 5
Correlation between T and Z Scores and Vitamin D level
In this study, there was no significant correlation between Insulin, HOMA-IR and blood glucose level and Obese and Non-Obese subjects”. The results were shown in Table 6.
Table 6
Correlation between Insulin, HOMA-IR and blood glucose level and Obese and Non-Obese subjects
|
BMI Category |
Parameters |
Regression Coefficient (R) |
P-Value |
|
Obese |
Insulin |
0.188 |
0.495 |
|
|
HOMA-IR |
0.196 |
0.526 |
|
|
BSL. |
-0.159 |
0.351 |
|
|
|
|
|
|
Non-Obese |
Insulin |
-11.64 |
0.228 |
|
|
HOMA-IR |
14.26 |
0.229 |
|
|
BSL. |
-4.58 |
0.246 |
The correlation regression analysis reveals that there was negative association between vitamin D levels and Total abdominal adipose tissue (TAAT) in obese subjects (Table 7).
Discussion
Vitamin D is a multipurpose nutrient. Vitamin D deficiency has been linked to a variety of health problems, including CVD. 13 Dyslipidaemia, which is defined as an increase in LDL, a rise in TG, and a reduction in HDL-C, has long been recognised as a CVD risk factor. 14 Vitamin D may have an effect on cardiovascular health via affecting blood lipids. In the present study, the obese patients displayed significant increase in BMI, VSF and TBF% in comparison to the non-obese individuals. Related to our report in a Nigerian study, the BMI is considerably elevated in obese subjects as compared to the control patients. 15
Obesity and being overweight might strengthen bones, perhaps lowering the incidence of osteoporotic fractures by increasing bone mineral density (BMD), a well-known indication of osteoporosis. In our study, the T and Z scores are greater in obese as that of the non-obese subjects (p<0.05) and in both the cases the T and Z scores were in the normal range. Similar to the present study, Salamat et al.16 reported normal BMD values in subjects with BMI <25 kg/m2 and BMI >30 kg/m2, however, the BMD values were lower in subjects with BMI<25 as that of the subjects with BMI >30 kg/m2. The vitamin D level in obese patients is decreased as compared to non-obese patients, but found to be non-significant (20.65± 4.53 vs 22.65± 6.62nmol/L; p=0.14). Previous reports shows in obese subjects the serum vitamin D levels is 20% lower as compared to the normal individuals. 17 Low level of vitamin D is mainly due to the adverse effect of obesity rather its etiology. Previous genetic study shows that high BMI and genes that provokes obesity reduces serum vitamin D level. However, there is a contrasting report that the decreased vitamin D levels and the genes which predisposes low vitamin D level has only minimum effect on obesity.18 The other mechanism of decreased vitamin D level in obese subjects might be due to changes in protein binding or increased metabolic clearance.17
Further, insulin level and HOMA-IR displayed significant elevation in obese as compared to non-obese subjects. Similar to our report, in a study done by ter Horst et al.19 the plasma insulin level and HOMA-IR are significantly elevated in obese subjects as compared to non-obese subjects.
In this study based on the vitamin D requirement the patients are classified as Deficiency <20nmol/L, Insufficiency 20-30nmol/L and Sufficiency >30nmol/L. There is a substantial negative association between triglycerides and vitamin D insufficiency in individuals (R= - 0.642; p=0.01) and VLDL (R=-0.504; p=0.01). Our results are in corroboration with Chaudhuri et al. 20 study where there is a significant (p<0.001) elevation of triglycerides and VLDL in vitamin D deficiency subjects. Vitamin D status was shown to be substantially linked with the development of metabolic syndrome in another study done in Lebanon [OR:2.496; 95% CI (1.31-4.73); p=0.005)21 Insulin resistance plays a role in the link between vitamin D and triglycerides. Vitamin D deficiency accelerates the evolution of insulin resistance, which raises VLDL cholesterol and triglyceride levels.17
The present study also reported, significant positive correlation in vitamin D insufficiency and cholesterol level (R=0.566; p=0.001). Similarly, Bashir et al. 22 reported that vitamin D insufficiency displays a significant effect on total cholesterol level and it elevates upto two-fold. Low vitamin D levels was associated with a strong negative correlation with total cholesterol and a positive correlation with HDL.Studies show that sufficient vitamin D levels reduce the intestinal fatty acid absorption by increasing the intestinal calcium absorption. 23 By lowering hepatic triglyceride production and release, the increased calcium level lowers blood triglycerides. A study done by Karhapaa et al. 24 displayed significant association of elevated HDL cholesterol with sufficient vitamin D level. Further, vitamin D levels are also linked to lower total cholesterol and triglycerides, according to the researchers.
In our study, only vitamin D insufficiency subjects displayed significant negative correlation with Z score (R= -0.536; p=0.002). However, in contrast Kirba et al. 25 showed no correlation between the serum vitamin D levels and BMD.
In addition, we have observed no significant correlation between Insulin, HOMA and BSL in obese and non-obese patients. However in contrast, in De Luis et al. 26 study obese persons had greater HOMA-IR levels than people with normal BMI, and there was a strong positive connection between HOMA-IR and BMI.
Conclusion
Thus the present study shows that the Vitamin D levels are markedly decreased in obese patients as compared to non-obese individual but it was found to be non-significant. Further, vitamin D deficiency displayed significant association with triglycerides and VLDL level. In addition, vitamin D insufficiency is significantly associated with total cholesterol and HDL levels. Vitamin D level has no significant effects on BMD based on Z and T scores.
