Evaluation of the relationship between microalbuminuria and 25-(OH) vitamin D levels in patients with type 2 diabetes mellitus

Abstract

Background: 25-(OH) vitamin D (VD) deficiency has been described as potential risk factor for the development of diabetes in many epidemiological studies. 25-(OH) VD deficiency and insulin resistance associated with this deficiency are common findings in patients with type 2 diabetes mellitus. The objective of this study is to evaluate the relationship between 25-(OH) VD levels and microalbuminuria.

Methods: The patients with type 2 diabetes mellitus aged between 40 and 65 years, who were admitted to the diabetes outpatient clinics of our hospital, were evaluated in two different groups. The first group consisted of 119 patients with insufficient 25-(OH) VD levels (10–30 ng/mL) and the second group consisted of 121 patients with 25-(OH) VD deficiency (≤10 ng/mL). The relationship between 25-(OH) VD levels and the level of microalbuminuria was evaluated in the two groups.

Results: The mean 25-(OH) VD level was 11.5 ng/mL and the mean HbA1c level was 9.1%. When the patient groups were evaluated according to 25-(OH) VD levels, HbA1c values were significantly higher in patients with a 25-(OH) VD level of 10 ng/mL or lower (p = .039). 25-(OH) VD levels were not significantly different between patients with different stages of renal failure (p = .119), whereas the level of microalbuminuria was significantly different (p = .030).

Conclusions: This study found that the level of microalbuminuria was significantly higher in patients with 25-(OH) vitamin D deficiency compared to patients with 25-(OH) VD insufficiency.

Keywords:

• 25-(OH) VD insufficiency
• diabetic nephropathy
• albuminuria

Introduction

25-(OH) vitamin D (VD) is a steroid hormone that plays an important role in the development of type 2 diabetes mellitus through its effects on the regulation of adipogenesis and differentiation of adipocytes, protection of pancreatic B cells, and improvement of insulin resistance in the myocytes [1]. VD deficiency is common and it is an important risk factor for impaired glucose tolerance [2,3]. The animal studies have shown a relationship between 25-(OH) VD deficiency and impaired insulin synthesis and secretion [4].

Normoalbuminuria is defined as urinary albumin excretion ≤30 mg/24 h, microalbuminuria between 31 and 299 mg/24 h, and macroalbuminuria mg 300 mg/24 h [5].

Diabetic nephropathy, one of the microvascular complications of diabetes, is the most common cause of end-stage renal disease and it is associated with increased mortality and morbidity in diabetic patients [6,7]. In previous human studies, the prevalence of albuminuria increased with decreasing levels of VD [8]. VD is known to suppress renin biosynthesis and its deficiency contributes to the development of chronic renal failure [9]. The patients with type 2 diabetes and renal failure exhibit severe 25-(OH) VD deficiency [10]. In a study of 4330 individuals with type 2 diabetes mellitus that participated in a study of 5-year follow-up, low VD status was shown to be a predictor of increase in urine albumin to creatinine ratio in the long term [11].

There are studies suggesting a relationship between VD levels and microvascular complications in diabetic patients [12,13]. However, there is not any study in the literature that evaluated the relationship between VD and microalbuminuria. The studies related to D vitamin levels and diabetic micro complications are also contradictory. The objective of this study was to evaluate the relationship between VD levels and microalbuminuria.

Materials and methods

Patient population

The present single-center study has been designed to last for one year, and an approval was issued by the Ethics Committee of Umraniye Education and Research Hospital. A total of 240 patients aged 45–60 years with the diagnosis of type 2 diabetes mellitus for at least 10 years, who were receiving therapy either with oral anti-diabetic agents and/or insulin therapy and who were admitted to the diabetes outpatient clinics at our hospital between January 1 2016 and April 1 2016, were included in the study after obtaining their written informed consent.

The patients underwent physical examination and body mass index (BMI, kg/m²) was calculated. Serum VD, fasting blood glucose, HbA1c, creatinine, total cholesterol, LDL-cholesterol, HDL-cholesterol, and triglycerides were measured after at least 8 h of fasting. Serum VD level was determined using high-performance liquid chromatography (HPLC) method. Fasting blood glucose was determined using enzymatic method and HbA1c was determined using HPLC-UV detector. Lipid levels were measured using Abbott-Aeroset auto-analyzer (Abbott Laboratories, Chicago, IL) and original measurement kits. LDL-cholesterol was calculated using the Friedewald equation.

The patients were evaluated in two groups. The first group consisted of 119 patients with insufficient VD levels (10–30 ng/mL) and the second group consisted of 121 patients with VD deficiency (≤10 ng/mL). Microalbuminuria levels were determined in the two groups.

Exclusion criteria

The patients with type 2 diabetes mellitus aged less than 40 years and above 65 years, patients who received VD supplementation, and patients with a liver disease, acute infections, hypothyroidism, hyperparathyroidism, and chronic kidney disease and those who were receiving medications that interfere with measurement of VD level (affecting renin–angiotensin-aldosterone system, glucocorticosteroids, anticonvulsants, VD, and calcium products), were excluded.

The patients with serum creatinine level above 2.0 mg/dL were also excluded.

Statistical analysis

Descriptive statistics were used to describe continuous variables (mean, standard deviation, minimum, median, and maximum). The Student’s t-test was used to compare two independent continuous variables that showed normal distribution and the Mann–Whitney U test was used to compare two independent variables without normal distribution. The categorical variables were analyzed using chi-square test or Fisher’s exact test, where appropriate. The Spearman and Pearson correlation coefficients were used to evaluate correlations. Multivariate linear regression analysis was performed to determine the presence of microalbuminuria in relation to VD levels, HbA1c, and blood pressure measurements. The level of statistical significance was set at p < .05. The statistical analysis was performed using MedCalc Statistical Software version 12.7.7 (MedCalc Software BVBA, Ostend, Belgium).

Results

The study included 240 patients. Of these patients, 102 (42.5%) were males and 138 (57.9%) were females. The mean age was 55.1 ± 6.3 years. The mean height was 163.9 ± 9.3 cm, the mean weight was 84.9 ± 14.8 kg, the mean waist circumference was 101.8 ± 13.07 cm, and the mean BMI was 32 ± 6.9 kg/m². The mean duration of diabetes was 7.6 ± 6.7 years. In the study group, the mean VD level was 11.5 ± 5.3 ng/mL and the mean HbA1c level was 9.1 ± 2.5%. Of these patients, 50.4% (n = 121) had VD levels less than 10 ng/mL and 49.6% (n = 119) had levels between 10 and 30 ng/mL (Table 1).

Table 1. Degree of renal failure according to vitamin D levels.

	n	%
Vitamin D Level
≤10 ng/mL	121	50.4
10–30 ng/mL	119	49.6
Renal failure
None	4	1.7
Stage 1	138	57.3
Stage 2	87	36.1
Stage 3	12	5
Total	240	100

Of the study patients, 22% were smokers and 18.2% were ex-smokers. The family history was remarkable for diabetes mellitus in 53.8% of the patients and family history was remarkable for coronary artery disease in 12% of the patients. There was no difference between the two groups in terms of anti-diabetic treatments. The rates of insulin therapy and oral anti-diabetics were similar in both groups. Of the study patients, 27.4% were using anti-hypertensive medications, and patients using anti-hypertensive agents acting on renin–angiotensin-aldosterone system were excluded from the study. The stage of renal disease was determined by the calculation of glomerular filtration rate (GFR) and 57.3% of the patients had stage 1 renal disease (Table 1). There was no significant difference between the two groups in terms of the stage of renal impairment.

Microalbuminuria and creatinine values and microalbuminuria/creatinine ratio in a spot urine sample were calculated and the values are presented in Table 2.

Table 2. Microalbumin, creatinine, and microalbumin/creatinine ratio in the patients.

	Microalbumin	Creatinine	Mic/Cre
n	203	174	174
Mean	740.821	360.751	12.4090
Median	27.000	105.500	0.2362
Standard deviation	5136.2774	3207.4078	103.63767
Minimum	0.0	12.5	0.00
Maximum	42,553.0	42,416.0	1062.60

VD levels were more likely to be less than 10 ng/mL in females than in males (p = .028). The HbA1c and microalbuminuria levels were significantly higher in patients with VD deficiency (p = .039 and p = .030, respectively). Systolic blood pressure (SBP) and diastolic blood pressure (DBP) were significantly higher in patients with VD deficiency (p = .010 and p = .013, respectively). Patient characteristics according to VD levels are summarized in Table 3.

Table 3. General characteristics of the two groups.

	≤10 ng/mL (n = 121)	10–30 ng/mL (n = 119)
HbA1c
Mean ± SD	9.4 ± 2.5	8.8 ± 2.5
Median (min–max)	9.1 (5.5–15.1)	8.1 (5.5–17.4)
p Value	.039
Stage of renal disease, n (%)
0	2 (1.7)	2 (1.7)
1	66 (54.5)	71 (59.7)
2	43 (35.5)	44 (37)
3	10 (8.3)	2 (1.7)
p value	.119
Systolic blood pressure
Mean ± SD	136.5 ± 17.1	131.4 ± 16.1
Median (min–max)	140 (80–180)	130 (100–180)
p value	.010
Diastolic blood pressure
Mean ± SD	82.0 ± 12.7	78.7 ± 10.5
Median (min–max)	80 (40–100)	80 (60–100)
p value	.013
Microalbuminuria
Mean ± SD	514.8 ± 4209.4	969.1 ± 5940.5
Median (min–max)	36 (2–42,553)	25 (0–42,528)
p value	.568
Level of microalbuminuria
Normoalbuminuria	48 (47.1)	56 (55.4)
Microalbuminuria	45 (44.1)	28 (27.7)
Macroalbuminuria	9 (7.4)	17 (16.8)
p value	.030

Bold values represent the statistically significant p < .05.

The relationship of VD levels with the level of microalbuminuria and microalbuminuria/creatinine ratio was not statistically significant. There was no statistically significant relationship between VD levels and microalbuminuria level in patients with VD insufficiency.

There was a statistically significant relationship between VD levels and the presence/absence of microalbuminuria using chi-square test (p = .015) (Table 4). Logistic regression analysis showed that microalbuminuria level was 2.1-fold higher in patients with a VD level of 10 ng/mL or lower.

Table 4. Presence of microalbuminuria according to vitamin D levels.

	Microalbuminuria absent	Microalbuminuria present	Total
VD ≤10 ng/mL	57 (55.9)	45 (44.1)	102 (100)
VD >10 ng/mL	73 (72.3)	28 (27.7)	101 (100)
Total	130 (64)	73 (36)	203 (100)

Chi-square, p values = .015.

Discussion

This study showed that microalbuminuria level was 2.1-fold higher in diabetic patients with a VD level of 10 ng/mL or lower, and there was a statistically significant relationship between VD deficiency and the presence/absence of microalbuminuria.

As it is well known, there is an increased risk of developing diabetes and occurrence of metabolic problems related to diabetes in patients with VD deficiency [2]. VD deficiency is more common in patients with type 2 diabetes mellitus compared to normal population [10]. In a meta-analysis by Pittas et al., VD deficiency was found to be more common in diabetic patient population and VD replacement therapy could prevent the development of diabetic complications [2].

Various studies have been conducted to evaluate the effects of VD levels on the development of diabetic nephropathy occurring as one of the microvascular complications of diabetes. In National Health and Nutrition Examination Survey study [14] that evaluated insulin resistance, renal functions, and VD levels in 14,679 patients, VD deficiency was found to be associated with increased risk of microvascular and macrovascular complications in patients with type 1 and type 2 diabetes mellitus. In their study, VD levels were found to be lower in diabetic patients with nephropathy compared to those without nephropathy. Several studies have shown that VD levels are associated with diabetic microvascular and macrovascular complications [15–17]. A 50 nmol/L decrease in VD levels was found to be associated with 23% increase in macrovascular complications (p = .007), while decreased VD levels were associated with increased rate of macrovascular and microvascular complications in patients with type 2 diabetes mellitus [12]. However, there is no study that directly established the relationship between microalbuminuria and VD levels. This study concluded that microalbuminuria levels were significantly higher in patients with VD deficiency compared to patients with VD insufficiency.

The prevalence of VD deficiency increases in the presence of diabetic renal disease [18]. Low VD levels are also associated with diabetic nephropathy [19]. A relationship has been demonstrated between low VD levels and increased mortality in patients with type 1 and type 2 diabetes mellitus, who were followed due to mild-to-moderate impairment in renal function [1,20].

When we examine the effects of VD on the development of diabetic nephropathy, preclinical studies have suggested renoprotective effects of VD [21]. In a Chinese study, VD replacement therapy was shown to alleviate microalbuminuria and ameliorate diabetic nephropathy [13]. Renin angiotensin aldosterone system (RAAS) is known to play a major role in diabetic nephropathy and VD suppresses renin release. In these patients, VD deficiency increases urinary albumin levels. The studies have also demonstrated favorable effects of VD replacement on other risk factors such as hypertension and hyperlipidemia that play a role in the development of nephropathy in diabetic patients [8]. This study did not find any difference in lipid profiles of patients with severe VD versus patients with VD insufficiency.

In animal models, VD deficiency was shown to play a role in the development of type 2 diabetes mellitus through its effects on insulin synthesis and secretion [4]. In this study, HbA1c levels were significantly higher in patients with a VD level of 10 ng/mL or lower, and VD deficiency was associated with poorly controlled diabetes.

When blood pressure measurements are evaluated, systolic and diastolic blood pressures (DBPs) were found to be significantly higher in patients with VD deficiency. There is a strong relationship between VD levels and hypertension. Controlled studies and meta-analyses have shown protective effects of high calcium intake both in gestational and essential hypertension, while increased risk of hypertension is associated with low VD levels [22]. In a 4-year prospective study by Forman et al. [23]. VD levels less than 15 ng/mL were associated with a 3.18-fold increase in relative risk for the development of hypertension when compared to patients with a VD level > 30 ng/mL.

This study showed that development of microalbuminuria was 2.1-fold higher in diabetic patients with VD deficiency compared to patients with VD insufficiency. VD deficiency is more commonly encountered in diabetic patients, and VD levels must be taken into consideration both in terms of their contribution to the regulation of blood glucose and its effects on the complications; therefore, patients must be evaluated for replacement therapy.

In this study, microalbuminuria was more common in patients with VD deficiency. Glycemic control and blood pressure control were poorer in this group of patients. It may be suggested that impaired glycemic control and poor regulation of blood pressure might have further contributed to the development of microalbuminuria in patients who are already devoid of renoprotective effects of VD.

Demonstration of the role of VD deficiency in early stages of renal damage in the course of diabetic nephropathy highlights the importance of VD levels in preventing microvascular complications.

This study had a few limitations that should be considered. The study design was cross-sectional, it was a single-center study, and it did not obtain direct evidence of casual relationship. Further studies would be beneficial to confirm these findings.

Disclosure statement

No potential conflict of interest was reported by the authors.