The relationship between vitamin D deficiency and metabolic syndrome in obese individuals

Implication for health policy/practice/research/medical education: The results of this study suggest that examining the serum levels of vitamin D allows screening people at risk for metabolic syndrome development. Please cite this paper as: Mirhoseini M, Daemi H, Masoom Babaiee M, Asadi-Samani M, Mirhoseini L, Sedehi M. The relationship between vitamin D deficiency and metabolic syndrome in obese individuals. J Renal Inj Prev. 2018;7(4):275-279. DOI: 10.15171/jrip.2018.6


Introduction
Metabolic syndrome (insulin resistance syndrome) includes a series of metabolic disorders that increase the risk of cardiovascular diseases and type II diabetes (1)(2)(3).Following increased urbanization, inactivity, stress, and consumption of fatty, high-carbohydrate and high-calorie foods, the incidence of metabolic syndrome has been growing steadily throughout the world over the past 20 years, in which up to 25% of the population has been involved according to the latest research (4).In fact, changes in the lifestyle of people over the past decades have contributed to the prevalence of obesity DOI: 10.15171/jrip.2018.61 and metabolic syndrome in societies.Along with disease complications, special attention to studies on obesity, its complications, and related factors has attracted in recent years.Although the cause of metabolic syndrome has not been fully understood, insulin resistance and visceral fat accumulation have been suggested as the underlying causes (5).It seems that a combination of genetic, metabolic, and environmental factors such as diet have an important role in the development of metabolic syndrome (6).Some studies have shown that decreased levels of vitamin D may be involved in the development of metabolic syndrome owing to its effect on each component of the metabolic syndrome, or because of insulin resistance (7)(8)(9)(10).However, such a role has not been confirmed in some other studies (11,12).

Objectives
The present study, therefore, aimed to study more precisely a possible cause of metabolic syndrome by examining the serum levels of vitamin D and its relationship with metabolic parameters.

Study population
In this descriptive-analytic study, all obese subjects with body mass index (BMI) values of ≥25% were randomly selected through convenient sampling at Hajar and Kashani hospitals in Shahrekord.The inclusion criteria were BMI values of ≥ 25 kg/m 2 , ages of >18 years, and presence of metabolic syndrome.The exclusion criteria included kidney disease, liver disease, and use of weight loss drugs or weight loss efforts.Additionally, vitamin D intake, pregnancy, breast feeding, withdrawal from the continued cooperation or non-participation in tests and examinations, and intake of estrogen and progesterone pills in women were the exclusion criteria.Accordingly, a total of 192 individuals were selected as the sample size.Lipid profile and fasting blood sugar (FBS) of all subjects were determined by blood sample tests.Blood glucose was measured on the sampling day by enzymatic colorimetry using glucose oxidase.Triglyceride levels were measured by BIONIK commercial kits with a BT-3000 device by cholesterol esterase colorimetric tests, cholesterol oxidase, and glycerol phosphate oxidase.The laboratory reported a normal cholesterol range of 130-200 mg/dL.Serum HDL cholesterol was measured after the precipitation of APoB protein lipids using phosphotungstic acid solution.Serum LDL cholesterol was assessed after determining total cholesterol concentration using the Friedewald formula.Serum LDL-C was evaluated by an enzyme kit when serum triglyceride levels exceeded 400 mg/dL.All samples were evaluated with an acceptable standard after an internal quality control.The internal and external changes were 1.6 and 0.6% for triglyceride (TG), and 2% and 0.5% for total cholesterol, respectively.Serum triglyceride levels equal to or greater than 150 mg/dL, HDL-C concentrations less than 40 mg/dL in men and less than 50 mg/dL in women, and fasting glucose values of 100 mg/dL or higher were defined according to National Cholesterol Education Program (NCEP), Panel III NCEP-ATP III (13).Metabolic syndrome was defined by the NCEP-ATP III criteria.After identification of subjects with metabolic syndrome, they were divided into two groups of 96 patients (those with metabolic syndrome during obesity) and 96 healthy subjects (those without metabolic syndrome).The two groups were selected based on gender, age, occupation, physical activity, smoking, type of urban and rural populations, and similar education levels.Blood samples were collected from all participants in the study after 12 hours of fasting to measure serum vitamin D 25(OH) levels at 8 am.The following vitamin D levels were defined based on the laboratory report; vitamin D 25 (OH) deficiency when it is less than 20 ng/mL.Insufficient vitamin D 25(OH) level; 20-30 ng/mL, and toxic doses of vitamin D 25(OH) when it is over 100 ng/ mL.After measuring levels of vitamin D in both groups, direct correlations of the vitamin values were determined with metabolic syndrome and its indices in obesity.

Ethical issues
1) The research followed the tenets of the Declaration of Helsinki; 2) informed consent was obtained; and 3) This study was approved by the Ethics Committee of Shahrekord University of Medical Sciences.This study is the result of a dissertation of Hamid Daemi (MD dissertation).

Data analysis
Data were entered into an Excel database and then analyzed using the SPSS software version 22. Descriptive and inferential statistics including mean, standard deviation, t test, chi-square, and Pearson's correlation coefficient was conducted by SPSS software.The threshold of P < 0.05 was considered significant.

Results
The selected participants (n = 192) in this study had an average age of 51.37 ± 19.22 years and were assigned to two groups after distribution normality of data was confirmed.The first group comprised 96 obese people with metabolic syndrome and the second group consisted of 96 non-obese subjects with metabolic syndrome.There were 40 male smokers, 20 of which were in the metabolic syndrome group and 20 individuals in the healthy group (Table 1).The metabolic syndrome parameters are presented in Table 2.The prevalence of vitamin D deficiency was 58.85%.A mean vitamin D levels of 18.88 ± 1.24 ng/mL was found in subjects with metabolic syndrome.The vitamin level in the obese group without metabolic syndrome was 40.38 ± 2.15 ng/mL.The decrease in the mean level of vitamin D in our patients was significant when compared to healthy participants (P < 0.001).Distribution of vitamin D status Vitamin D and metabolic syndrome in the studied groups is presented in Table 3. Table 4 shows the comparison of metabolic syndrome indices at different levels of vitamin D. The mean serum HDL-C level increased with increasing vitamin D levels but the mean serum levels of TG, and FBS and systolic blood pressure decreased significantly (P < 0.05).However, BMI remained almost constant with increasing serum vitamin D levels with no significant relationships (P = 0.999).Additionally, the waist size was not significantly correlated with increased serum vitamin D levels (P = 0.825).

Discussion
The present study aimed to determine the prevalence of vitamin D deficiency in obese individuals and the relationship between vitamin D deficiency and metabolic syndrome with some indices in obese patients with metabolic syndrome and healthy subjects.Vitamin D was found to be rather highly deficient in the subjects.Serum vitamin D levels were significantly lower in the patients than in control group.The relationship between each of the metabolic syndrome indices at different levels of vitamin D showed that levels of TG and FBS decreased with increasing serum vitamin D, and that HDL-C levels were consistent with vitamin D levels showing significant changes.However, the level of vitamin D was not significantly correlated with waist and BMI.The prevalence and incidence of metabolic syndrome is increasing worldwide.One of the latest studies in Iran reported a metabolic syndrome prevalence of over 30%, which was significantly higher in women than in men (42% versus 24%) (14).Our results revealed a prevalence of 85.58% for vitamin D deficiency.A study by Soltani et al reported a prevalence of 62% and 21.2% for vitamin D deficiency and metabolic syndrome, respectively (15).In addition, Ford et al evaluated a vitamin D deficit prevalence of 72% for serum vitamin D and metabolic syndrome in a total of 8421 adult Americans (16).In another study on 646 Iranian adults, the frequencies of metabolic syndrome and vitamin D deficiency were detected to be 18.3% and 73.3%, respectively (17).A study in South Korea by Park et al revealed a relationship between serum vitamin D levels and metabolic syndrome, with a 76.6% increased incidence of vitamin D deficiency in patients with metabolic syndrome (18).The frequency of vitamin D deficiency in our study is lower than those found in other studies, which can be due to differences in the method of measuring serum vitamin D levels, culture, coverage, solar radiation levels, exposure to sunlight, sample collection season, and geographical coordinates.Our findings also indicate significantly lower serum vitamin D levels in the patients than in the normal group.Such a significant relationship could be related to the role of vitamin D in blood pressure regulation and diabetic insulin resistance.This relationship can also depend on the degree of mobility and exposure to sunlight.In fact, vitamin D is one of the essential fat-soluble vitamins in the body that helps bone growth and strength through the control of calcium and phosphorus balance.The vitamin D increases the absorption of phosphorus and calcium from the intestine, thereby, regulate bone metabolism.Other studies have also demonstrated that decreased vitamin D levels can contribute to the development of metabolic syndrome by affecting either metabolic syndrome components or through insulin resistance (7).A suggested role for vitamin D is participation of insulin synthesis and secretion thus reduced vitamin D levels is a possible risk factor for diabetes (19).
In the present study, levels of blood pressure, vitamin D, TG, and FBS were significantly higher in the patients than in the normal group.The study by Borzouei et al on 186 adults, revealed that people with vitamin D deficiency had a higher chance of metabolic syndrome development, with significantly higher waist size, systolic blood pressure, and FBS in the patients than in the healthy group (20).Roshanzamir et al found that HDL-C had a significant, direct correlation with vitamin D level (21).Ford et al evaluated serum levels of vitamin D and metabolic syndrome in 8421 adults and found a significant, inverse correlation between serum vitamin D and TG and FBS levels and waist size.An inverse relationship was also shown between serum vitamin D levels and metabolic syndrome (16).Besides, Moy and Bulgiba investigated the relation of vitamin D with the prevalence of metabolic syndrome.They showed that people with lower levels of vitamin D had higher incidences of metabolic syndrome (22).However, in some studies, no significant correlations were found between vitamin D deficiency and metabolic syndrome components (15,23,24).The disparate results can be attributed to the role of vitamin D in 3% expression of the human genome, which may help explain the increased prevalence of metabolic syndrome in patients with vitamin D deficiency (25).

Conclusion
The results of this study suggest that examining the serum levels of vitamin D allows screening people at risk for metabolic syndrome development, especially in high-risk individuals.

Limitations of our study
A limitation of this study was relatively small size studied population.Further investigation in this regard suggests.This study supported and funded by the Research Deputy

Table 1 .
Demographic data of individuals in the two groups studied

Table 2 .
Metabolic syndrome indicators in the studied subjects

Table 3 .
Frequency distribution of vitamin D status in the studied groups

Table 4 .
Relationships between metabolic syndrome indicators and different levels of vitamin D in the subjects