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Research Article
Open Access
Vitamin D Deficiency and Cardio-Metabolic Risk in a North Indian Community
with Highly Prevalent Type 2 Diabetes
Timothy R Braun, Latonya F Been, Piers R Blackett and Dharambir K Sanghera*
Department of Pediatrics, College of Medicine, University of Oklahoma Health Sciences Center, Oklahoma City, USA
*Corresponding author:
Dharambir K Sanghera
Associate Professor of Pediatrics
Department of Pediatrics, Section of Genetics
University of Oklahoma Health Sciences
Center
940 Stanton L. Young Blvd., Rm. D317 BMSB
Oklahoma City, OK 73104, USA Tel: 405-271-6026 Fax: 405-271-6027 E-mail: dharambir-sanghera@ouhsc.edu
Received June 22, 2012; Accepted August 25, 2012; Published August 29, 2012
Citation: Braun TR, Been LF, Blackett PR, Sanghera DK (2012) Vitamin D
Deficiency and Cardio-Metabolic Risk in a North Indian Community with Highly
Prevalent Type 2 Diabetes. J Diabetes Metab 3:213. doi:10.4172/2155-6156.1000213
Objective: The purpose of this investigation was to examine serum vitamin D status in a population of Punjabi
ancestry from Northern India with a high prevalence of type 2 diabetes (T2D) and evaluate the effects of 25(OH)D
levels on cardio-metabolic traits.
Research design and methods: We assessed cardiovascular risk factors and 25(OH)D levels in 1,765
participants (887 T2D cases, 878 normoglycemic controls).
Results: 76% of individuals were deficient (<50 nmol/L) in vitamin D. A higher percentage of T2D participants(83%)
were vitamin D deficient compared to normoglycemic controls (68%)(p<0.0001).The prevalence of vitamin D
deficiency increased progressively with body mass index (BMI) categories (p<0.0001): BMI<23 kg/m2, 65%; BMI
23-27.5 kg/m2, 75%; and BMI>27.5 kg/m2, 81%. T2D participants had significantly decreased serum 25(OH)D levels
(β=-0.41, p=2.8 × 10-20). Individuals with low serum 25(OH)D had elevated fasting glucose(β=-0.18, p=0.022), BMI
(β=-0.71, p=1.4 × 10-7) and systolic blood pressure (β=-1.68, p=0.006). A positive association of increased 25(OH)D
with HOMA-B (β=0.17, p=8.0×10-6), and C-peptide (β=0.09, 0.017) was observed. Non-medicated, normoglycemic,
non-hypertensive individuals classified as vitamin D deficient (n=289) exhibited a significant increase in fasting
glucose (p=0.02) and BMI (p<0.0001) as well as a significant decrease in C-peptide (p<0.0001) and amylin
(p<0.0001) compared to vitamin D sufficient controls (n=150).
Conclusions: Vitamin D deficiency appears to be a significant risk factor for T2D severity and associated
cardio-metabolic risk. Early intervention may be considered to improve prevention of T2D related cardiovascular
complications.
Introduction
It may not be coincidental that the prevalence of vitamin D
deficiency, estimated to affect over 1 billion people worldwide [1],
is increasing in conjunction with T2D, obesity, and cardiovascular
disease. The ubiquitous distribution of vitamin D receptors in the body,
controlled by nearly 3,000 genes [2], suggest that a deficiency could
have widespread health implications. Recent studies have examined the
physiological functions of vitamin D beyond its well established role
in musculoskeletal health [3]. In addition to findings of oncologic [4]
and immunologic [5] associations, vitamin D deficiency is associated
with cardio-metabolic risk factors including T2D [6], blood pressure
[7], and obesity [8].
Lo et al. [9] report that Asian Indians require twice as much UV-B
exposure to produce 25(OH)D levels equal to Caucasians due to
increased skin pigmentation. In addition, a cultural tendency to avoid
direct sunlight may contribute to suboptimal vitamin D status although
the climate in India is sunny throughout the year. Currently, 62.4
million people in India have T2D and 77.2 million have prediabetes
[10], representing the country with the second highest prevalence in
the world after China [11]. Asian Indians have lower body mass index
(BMI) than US whites, African Americans, and Mexican Americans
[12] but are termed “metabolically obese” due to disproportionally high
abdominal fat, an important contributing factor in T2D. Central obesity,
sedentary lifestyle, a westernized diet, and genetic predisposition are
several factors contributing to the alarming increase of T2D in India
and around the world. The high morbidity and mortality associated
with T2D presents an overwhelming health care burden necessitating
improved treatment and preventative therapy.
Vitamin D status is known to be poor among Asian Indians
[13]; however, limited data exists to assess the implication of vitamin
D deficiency on cardio-metabolic traits in Asian Indians. To our knowledge, this is the first large study reporting the role of 25(OH)D
deficiency in North Indians who have an increased prevalence of T2D
and cardiovascular diseases [14-16].
Materials and Methods
Participants in this diabetes-focused case-control cohort are part
of the Asian Indian Diabetic Heart Study (AIDHS) /Sikh Diabetes
Study (SDS) [17] (Table 1). The diagnosis of T2D was confirmed by
scrutinizing medical records for symptoms, use of medications, and
defining diabetes according to fasting glucose levels as defined in the
American Diabetes Association guidelines [18]. BMI was calculated
as (weight (kg)/height (meter2)). Waist and hip circumferences were
measured with a tape measure at the abdomen and at the hip. The
World Health Organizations (WHO) has recommended lower BMI
thresholds for Asians [19] therefore, obesity was defined using WHO’s
new guidelines [19]. Participants with BMI<23 kg/m2 were classified
as normal weight, BMI between 23-27.5 kg/m2 were classified as
overweight, and BMI>27.5kg/m2 were classified as obese. Individuals
with type 1 diabetes (T1D), or with rare forms of T2D such as maturityonset
diabetes of young (MODYs), or secondary diabetes (e.g., due to
hemochromatosis or pancreatitis) were excluded. Details of physical
activity, smoking, alcohol, diet, and family history are described
elsewhere [17,20]. Blood pressure was measured twice after a five
minute seated rest period with the participant’s feet flat on the floor.
Pulse pressure was calculated as: [systolic blood pressure (SBP)-diastolic
blood pressure (DBP)] and mean arterial pressure was calculated as
(2×DBP + SBP)/3. All blood samples were obtained at the baseline visit.
Among diabetics, 40% were taking oral hypoglycemic agents. Of these,
9% were treated with sulphonylurea, metformin, dianil, ayurvedic, or ‘desi’ medicine, 5% were treated with insulin, and 26% were co-treated
with insulin and oral agents. The remaining 60% were maintaining
glycemic control with diet and exercise. All participants provided
informed consent following procedures approved by Institutional
Review Boards (IRBs). All SDS protocols and consent documents were
reviewed and approved by the University of Oklahoma Health Sciences
Center (OUHSC)’s IRB as well as the Human Subject Protection (Ethics)
committees at the participating hospitals and institutes in India.
Table 1: Prevalence of vitamin D deficiency and the association of cardio-metabolic traits with 25(OH)D.
The selection of controls was based on a fasting glycemia<100.8
mg/dL or a 2 hr. glucose<141.0 mg/dL. Subjects with impaired fasting
glucose (IFG) or impaired glucose tolerance (IGT) were excluded
from the study. Insulin was measured by radio-immuno assay
(Diagnostic Products, Cypress, USA). Homeostasis model assessment
for insulin resistance (HOMA-IR) was calculated as: fasting insulin
(μIU/mL)×fasting glucose (mmol/L)/22.5. HOMA-B assessed β-cell
function using the following formula: 20×fasting insulin (μIU/mL)/
fasting glucose (mmol/L)-3.5. Serum was obtained from fasting blood
samples to quantify 25(OH)D levels in a total of 1,765 individuals
using standard monoclonal antibody based ELISA kits from ALPCO
Diagnostics (Salem, NH, USA). Samples were run in duplicate
following the manufacturer’s instructions. Vitamin D status was
classified as deficient<50 nmol/L or sufficient ≥50 nmol/L according
to recommendations of the Institute of Medicine, Food and Nutrition
Board [21] and severely deficient when <30 nmol/L. C-peptide and
amylin measures were simultaneously quantified using Millipore’s
Magnetic MILLIPLEX Human Metabolic panel (St. Charles, Missouri)
and analyzed on a Bio-plex 200 multiplex analysis system (Bio-Rad
Hercules, CA).
Statistical Analysis
Statistics were performed using SPSS for windows statistical
package (SPSS INC., Chicago, USA). Serum 25(OH)D levels were
analyzed as a categorical variable deficient [<50 nmol/L] and sufficient
[≥50 nmol/L] or a continuous variable after log transformation.
As a categorical variable, difference in frequency distribution
between vitamin D deficient participants and vitamin D sufficient
participants were analyzed using χ2-test. As a continuous variable,
multivariate logistic regressions tested association of log transformed
serum 25(OH)D levels with T2D adjusting for age, BMI and gender.
Multivariate linear regressions were performed to test association
between log transformed serum 25(OH)D as the independent variable
with quantitative traits and biomarkers related to T2D (adjusted for
T2D, age, BMI and gender), obesity (adjusted for T2D, age, gender)
and cardiovascular risk (adjusted for T2D, age, BMI, gender) as the
dependent variable. Variables with skewed distributions were log
transformed (fasting glucose, fasting insulin, C-peptide, and amylin).
Untransformed measures were reported as arithmetic means while
arithmetic means of the transformed variables were retransformed
into the original measurement scale and reported as geometric means.
Independent t-tests were used to analyze the association between
serum 25(OH)D (continuous variable) with BMI tertiles in participants
with and without T2D and to evaluate the effect of vitamin D deficiency
(categorical variable) on mean levels of quantitative traits (continuous
variable).
Results
Prevalence of vitamin D deficiency
25(OH)D was quantified in a total of 1,765 participants with a mean age of 52.7 ± 12.4 years. Among the participants, 52.6% were male,
50.2% had T2D, 21.6% were normal weight (BMI<23 kg/m2), 36.4%
overweight (BMI 23-27.5 kg/m2) and 42.0% were obese (BMI>27.5
kg/m2). Table 1 summarizes the vitamin D status of our population. A
total of 75.8% of participants were classified as vitamin D deficient (<50
nmol/L) and only 24.2% of individuals had sufficient (≥50 nmol/L)
25(OH)D levels (p<0.0001). Of particular note was the increased
prevalence of vitamin D deficiency in T2D, obese, and hypertensive
participants. The T2D cases had a significantly higher prevalence of
vitamin D deficiency (83.5%) when compared to normoglycemic
controls (68%) (p<0.0001). Similarly, a significant linear increase
(p<0.0001) in the frequency of vitamin D deficient individuals was
observed (65.2%, 75.5%, 81.4%) as the BMI increased (<23 kg/m2,
23-27.5 kg/m2, >27.5 kg/m2), respectively. Figure 1 highlights the
significant decrease in serum 25(OH)D levels as BMI increases in the
entire AIDHS/SDS cohort stratified by T2D. Serum 25(OH)D levels
were markedly reduced in over-weight and obese individuals even
without T2D.
Figure 1: Association of BMI Tertiles with 25(OH)D in participants with and
without T2D.
Association of vitamin D deficiency with cardio-metabolic
traits
Multivariate regression analyses were performed to assess the
impact of vitamin D status on T2D, obesity and related traits and
biomarkers (Table 1). We report a highly significant, inverse association
of serum 25(OH)D with T2D (β=-0.41, p=2.8×10-20) and BMI (β=-
0.71, p=1.4×10-7) in males and females combined. Other traits related
to T2D and β-cell function showing gender-specific associations with
25(OH)D levels in female participants include fasting glucose (β=-
0.03, p=0.03), fasting insulin (β=0.08, p=0.044),C-peptide (β=0.21,
p=4.9×10-4), and amylin (β=0.04, p=0.009). HOMA-B was significantly elevated with increased vitamin D levels in female participants (β=0.2,
p=1.4 × 105).The association of serum 25(OH)D with quantitative traits
related to cardiovascular function were restricted to male participants.
No association was observed between 25(OH)D levels and diastolic
blood pressure. We also tested the association of serum 25(OH)D with
high density lipoproteins (HDL), low density lipoproteins (LDL), total
cholesterol (TC), and triglycerides (TG). No significant associations
were found (Supplemental Table 1). Figure 2 illustrates these highly
significant gender-specific associations of C-peptide, amylin, HOMA-B
and systolic blood pressure by comparing mean levels of these
quantitative traits in vitamin D deficient and vitamin D sufficient
participants.
Figure 2: Vitamin D deficient (<50 nmol/L) gender specific associations of C-peptide, amylin, HOMA-B and systolic blood pressure.
Impact of medication on vitamin D deficiency
To assess the impact of medication on vitamin D status among diabetic patients, we stratified the data based on medication use
(Supplemental Table 2). Of the 887 diabetic cases, 359 (40%) were taking
medication to control glucose levels at the time of inclusion into this
study. We observe significantly higher 25(OH)D levels with improved
metabolic and cardiovascular traits among non-medicated, diabetic
patients. While this may seem unexpected, significantly earlier ageof-
onset of T2D (p<0.0001) and longer duration of T2D (p<0.0001) in
medicated individuals was associated with lower 25(OH)D levels when
compared to the non-medicated group. Furthermore, significantly
higher fasting glucose in treated participants suggests uncontrolled
T2D. To further assess the impact of medication and disease status
on 25(OH)D levels, we stratified the data to compare non-medicated
healthy participants (normoglycemic and non-hypertensive, n=434)
with diseased participants (n=1331). Mean levels of fasting glucose and BMI were significantly lower whereas C-peptide and amylin were
significantly higher in individuals classified as having sufficient serum
25(OH)D levels regardless of disease status (Figure 3).
Figure 3:Impact of vitamin D deficiency (<50nmol/L) on non-medicated healthy controls and diabetic patients.
Conclusions
In this investigation, we report a very high prevalence of vitamin
D deficiency in this Punjabi diabetic cohort from Northern India. The
inverse association of serum 25(OH)D with T2D and related metabolic
traits complements previous reports [6] and the proposed actions of
vitamin D deficiency-mediated decrease in insulin secretion and
sensitivity [22]. Consistent with reports from other epidemiological
studies, we also observed a significant association of low serum 25(OH)
D with obesity [23,24] and increased systolic blood pressure [25].
Due to our case-control study design, we cannot determine evidence
for a causal relationship between vitamin D deficiency and declining
cardio-metabolic traits related to T2D; however, low serum 25(OH)D
in healthy, non-medicated controls was significantly associated with
increased fasting glucose and BMI as well as decreased C-peptide
and amylin (Figure 3). These results suggest vitamin D deficiency
may increase susceptibility to T2D and therefore provide rationale for
investigating preventive measures, such as improving vitamin D intake,
as part of an overall strategy to improve glycemic outcomes in the
Punjabi community.
Our findings of a significant negative association of fasting glucose
as well as a significant positive association of HOMA-B with serum
25(OH)D add to the mounting evidence that proposes vitamin D plays
a critical role in insulin secretion [26,27]. The biologically active form
of vitamin D [1,25-(OH)2D] may directly stimulate insulin release by
binding to the vitamin D receptors of β-cells, thus influencing pathways
such as calcium-mediated insulin secretion or by affecting other genes
associated with β-cell metabolism and development. While we did
not observe a significant association of insulin resistance, assessed by
HOMA-IR, with serum 25(OH)D, studies suggest that vitamin D may
directly enhance insulin response of peripheral tissues by stimulating
expression of insulin receptors. In addition to these direct actions,
vitamin D may indirectly regulate calcium homeostasis in β-cells and
insulin-responsive tissues [22]. Since calcium is critical for skeletal
muscle function [28], increased vitamin D status may improve muscle
function [29] thus increasing insulin sensitivity.
One of the strengths of our study is the assessment of serum
biomarkers related to β-cell function, such as C-peptide and amylin
with 25(OH)D. C-peptide, secreted in equimolar concentration to
insulin, is widely considered a better marker of residual β-cell function
than insulin due to its longer half-life [30]. In addition to C-peptide,
amylin, a glucoregulatory hormone, is co-secreted with insulin in
response to food intake to complement insulin-dependent maintenance
of postprandial glucose homeostasis [31]. Interestingly, the negative
association of insulin, C-peptide, and amylin with 25(OH)D was only
seen in females. The higher prevalence of females with T2D in South
Asian populations, in part accounted for by previous pregnancies with
gestational diabetes [32], may influence the observed association in
females but not in males. Another plausible explanation for the genderspecific
associations found in this population is the restricted use of
alcohol. Due to cultural and religious reasons, a vast majority of Punjabi
females do not consume alcohol. Alcohol has been shown to increase
systolic blood pressure [33,34] which could account for increased
systolic blood pressure and pulse pressure observed in males. However,
adjusting for the confounding effect of alcohol with systolic blood pressure did not diminish the significant association observed with
25(OH)D in males, indicating that alcohol could be a compounding
factor in addition to other factors.
Our study supports the decreased bioavailability of serum vitamin D
observed in obese individuals from previous studies [23,24]. Wortsman
et al. [24] found obesity did not affect the skin’s ability to produce vitamin
D, but may have altered the release of vitamin D into the circulation
from adipose stores due to increased subcutaneous fat. As a fat soluble
molecule, decreased 25(OH)D levels among obese individuals may
be due to enhanced uptake in adipose tissue and metabolic clearance
[23]. We did not measure parathyroid hormone, a limitation in our
study since hyperparathyroidism, secondary to hypovitaminosis D is
augmented by obesity [35] and could be responsible for gender-specific
associations. Additionally, vitamin D deficiency is proposed to increase
cardiovascular disease through secondary increase in parathyroid
hormone which activates the renin-angiotensin system and stimulates
systemic and vascular inflammation [36]. Further limitations of our
study include the case-control study design and lack of nutritional
status of participants as vitamin D levels are influenced by dietary
factors. We also did not adjust for seasonal changes which may or
may not influence this population since India is below 35°N latitude
making it possible to synthesize vitamin D in the skin throughout the
year [37]. However, strength of our study includes a relatively large and
well characterized cohort from a relatively homogenous population
available with demographic, clinical, and metabolic traits.
Cardiovascular disease remains the leading cause of death among
individuals with T2D. A growing body of evidence is emerging that
may implicate vitamin D deficiency as an important risk factor for
hypertension. Our study reports a significant increase in systolic blood
pressure and pulse pressure in vitamin D deficient males. One proposed
mechanism has linked vitamin D deficiency to cardiovascular disease
in T2D patients. Oh et al. [38] found that the active form of vitamin
D inhibits foam cell formation and suppresses macrophage cholesterol
uptake in patients with T2D. In addition, a direct protective effect of
vitamin D on the endothelium is possible since vitamin D deficiency
has been associated with impaired endothelial function [39].
Based on the data, it is reasonable to conclude that vitamin D
deficiency may increase development of risk factors that could lead to
T2D and cardiovascular disease. Furthermore, overwhelming evidence
exists to suggest decreased 25(OH)D levels seem to increase the severity
of T2D. This could be the reason that individuals with low vitamin D in
our cohort showed poor response to diabetic treatment (Supplemental
Table 2). Since the Punjabi are known to be susceptible to the metabolic
syndrome, T2D, and early cardiovascular disease, vitamin D deficiency
may put them at additional risk, making a strong case for early
intervention. Well-designed observational studies are needed to assess
the use of vitamin D supplementation as a preventative agent.
Acknowledgements
This work was partly supported by NIH grants-R01DK082766 funded by
the National Institute of Diabetes and Digestive and Kidney Diseases and NOTHG-
11-009 funded by National Human Genome Research Institute (NHGRI), USA.
We thank the participants and research staff, and the Phen X Rising Consortium
(NHGRI) who made the study possible.
Author Contribution
Conceived and designed the experiments: DKS. Performed the experiments:
TRB, LFB. Analyzed the data: TRB, LFB. Contributed reagents/materials/analysis
tools: DKS. Wrote the paper: TRB, PRB, DKS. Guarantors: TRB and DKS.
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