Vitamin D and
Tatiana Takiishi, MSc, Conny Gysemans, PhD,
Roger Bouillon, MD, PhD*, Chantal Mathieu, MD, PhD
Diabetes mellitus is one of the most common endocrine diseases, characterized by an
increase in plasma glucose. Different forms of diabetes with very distinct pathogen-
esis exist. Over time, diabetes can lead to blindness, kidney failure, and nerve
damage. Diabetes is also an important factor in accelerating atherosclerosis, leading
to stroke, coronary heart disease, and other large blood vessel diseases, all ultimately
associated with increased mortality risks.
The most prevalent form of diabetes is type 2 diabetes (T2D), currently affecting
more than 300 million people worldwide.1T2D is characterized by the combination
of insulin resistance and failing b-cell function.2Type 1 diabetes (T1D), on the other
hand, is an autoimmune disease in which the body’s own immune system mistakenly
attacks and destroys the insulin-producing b cell in the pancreas.3T1D typically
occurs in young, lean individuals, but older patients can also be affected. This
subgroup is referred to as latent autoimmune diabetes in adults (LADA). LADA is
a slow, progressive form of T1D. Other forms of diabetes include gestational diabetes
(GD), secondary diabetes due to pancreatic diseases or surgery, and genetic forms of
Vitamin D is a secosteroid that is generated from 7-dehydrocholesterol in skin under
the influence of UV light. Therefore, by definition, vitamin D cannot be considered
a true vitamin but rather a prohormone, as the natural source of vitamin D in evolution
of vertebrates and primates is photosynthesis in the skin. Indeed, the normal human
diet is usually poor in vitamin D except for fatty fish. Regardless of the source of
vitamin D, it needs to be hydroxylated twice to become biologically active.7Vitamin
D is first hydroxylated in the liver by 25-hydroxylases (25(OH)ase), consisting of
This work was supported by a postdoctoral fellowship for C.G. and a clinical fellowship for C.M.
from the Flemish research fund ‘‘FWO Vlaanderen.’’ T.T. received an SBA scholarship from the
Katholieke Universiteit Leuven.
Laboratory for Experimental Medicine and Endocrinology (LEGENDO), Katholieke Universiteit
Leuven, UZ Gasthuisberg, O&N I Herestraat 49 - bus 902, 3000, Leuven, Belgium
* Corresponding author.
E-mail address: Roger.firstname.lastname@example.org
? Vitamin D ? Type 1 diabetes ? Type 2 diabetes ? b Cells
?Deficiency ?Immune system
Endocrinol Metab Clin N Am 39 (2010) 419–446
0889-8529/10/$ – see front matter ª 2010 Elsevier Inc. All rights reserved.
cytochrome P450 (CYP) isoforms (the mitochondrial CYP27A1 and the microsomal
CYP2R1 [the most critical enzyme], CYP3A4, and CYPJ3) into 25-hydroxyvitamin D
(25(OH)D). The second hydroxylation occurs in the kidney, by 1a-hydroxylase
(1a(OH)ase, CYP27B1), as this tissue is normally the only one capable of the secretion
of its end product, 1,25(OH)2D, into the blood circulation. Vitamin D and its metabolites
are bound to a carrier protein (vitamin D binding protein; DBP) when transported
(24(OH)ase, CYP24A1), catabolizes vitamin D metabolites. Vitamin D exerts its action
via a nuclear receptor (vitamin D receptor; VDR), present in nearly all nucleated cells,
but with the highest concentration in the epithelial cells of the gut. However, most of
these enzymes and proteins essential for the action of vitamin D are also present in
many tissues not related to bone and calcium metabolism, such as the immune
Over many years, links between vitamin D status and diabetes mellitus have been
identified. As early as the 1980s, it was shown that vitamin D deficiency in rodents
and rabbits inhibits pancreatic insulin secretion, indicating that vitamin D is essential
for the function of the endocrine pancreas.9Later, the connection between vitamin
D and diabetes was reinforced by the discovery of the VDR and DBP in pancreatic
tissue (more specifically in the insulin-producing b cells) and also in various cell types
of the immune system. Thus, vitamin D has been proposed as a possible therapeutic
agent in the prevention and treatment of T1D and T2D.8
TYPE 1 DIABETES
T1D is a chronic autoimmune disease that results from the immune-mediated
destruction of pancreatic b cells, thus resulting in insulin deficiency. The autoimmune
process most commonly initiates in childhood and progresses for a variable period of
months and even years before it leads to hyperglycemia and, thus, diagnosis. By the
time of diagnosis only 10% to 30% of functional b-cell mass remains.10,11T1D is
the second most common chronic disease in children, second only to asthma, and
is considered as a complex genetic trait; not only do numerous genetic loci
contribute to susceptibility, but environmental factors also play an important role in
Vitamin D and Genetic Predisposition to T1D
In T1D the major genetic determinant is located in the major histocompatibility
complex (MHC) region on chromosome 6p21,12although multiple non-MHC genes
also contribute to T1D disease susceptibility.13Refining genetic mapping particularly
of the non-MHC loci may improve the ability to predict the risk of T1D and facilitate the
testing of more aggressive preventive therapies.14In this context, associations of T1D
with polymorphisms in the CYP27B1 gene on chromosome 12q13.1-q13.315–17may
be useful. It is hypothesized that presence of polymorphisms in the CYP27B1 gene
may reduce the (local) expression of 1a(OH)ase and consequently the conversion of
25(OH)D to 1,25(OH)2D, leading to increased predisposition to T1D.
In the past, it has been shown that allelic variations in the VDR gene are an important
determinant for the amount of VDR expressed,18and which in turn may influence the
immune-modulatory function of the VDR. Several authors have demonstrated that
VDR polymorphisms are able to influence the immune response in either healthy indi-
viduals or T1D patients.19,20In epidemiologic studies, clear associations between VDR
polymorphisms and T1D have been reported in South Indian, German, and Taiwanese
populations,21–23but were not found in a large combined population sample of British,
Takiishi et al
Portuguese, and Finnish origin.24–26In a recent study it was shown that specific VDR
polymorphisms interact with the predisposing HLA DRB1 allele through vitamin D
response element present in the promoter region of the DRB1*0301 allele,27which
may be detrimental for the manifestation of T1D, particularly in the case of vitamin
D deficiency in early childhood due to poor expression of DRB1 0301 in the thymus.
1,25(OH)2D is biologically inactivated through a series of events starting with
24-hydroxylation. The 24(OH)ase enzyme is encoded by the CYP24A1 gene located
on chromosome 20q13.2-q13.3. At present, no associations between CYP24A1
gene polymorphisms and T1D have been found.17
Vitamin D as an Environmental Risk Factor
T1D has been linked to a clear north-south gradient aswell as to a deficiency in vitamin
D concentrations. Indeed, the incidence of T1D is higher in countries of northern lati-
tude,28–30this trend being reversed in the southern hemisphere.31Latitude itself is
unlikely to be an independent risk factor for T1D onset. On the other hand, UV-B irra-
diation, which follows a north-south gradient, is known to convert 7-dehydrocholes-
terol to vitamin D in the skin, and has protective properties against autoimmunity.32
A more significant correlation of T1D has been observed with erythemal UV-B irradi-
ation than with latitude. Recently, Mohr and colleagues33assessed the T1D incidence
data for children younger than 14 years during 1990 to 1994 in 51 regions worldwide.
This investigation found that incidence rates of T1D approached zero in regions world-
wide with high UV-B irradiance. Furthermore, seasonality of T1D onset is well
known.34,35Kahn and colleagues36reported that spring births were associated with
increased likelihood of T1D, which might reflect insufficient maternal/neonatal vitamin
D levels during a critical fetal/neonatal programming period. Indeed, vitamin D has
been shown to have a role in development and function of the immune system.37In
fact, inadequate vitamin D and other nutrients during immune system development
(from gestation up to the second year of life) may play a critical role in the development
of autoimmune diseases. Therefore, it is thought that restoration of vitamin D levels
(either by supplementation with vitamin D or by administration of [less hypercalcemic
analogues of] the active hormone 1,25(OH)2D) may reduce the risk of T1D. How
vitamin D interferes with the pathogenesis of T1D is still not fully elucidated, though
some possible mechanisms have been suggested (Fig. 1).
Vitamin D as an Immune System Modulator
There is increasing evidence that active vitamin D (1,25(OH)2D) acts as a modulator of
the immune system. One of the first indications for this role was the finding of VDR
expression in a wide range of immune cells (eg, monocytes, activated lympho-
cytes).38,39Also, activation of the nuclear VDR is known to modify transcription via
several intracellular pathways and influence proliferation and differentiation of immune
Antigen-presenting cells: dendritic cells
Dendritic cells (DCs), which are highly specialized antigen-presenting cells, are known
to be important for the priming of CD41T cells. DCs act as sentinels in lymphoid and
nonlymphoid organs, capturing and processing antigens; once the antigen is captured
the DCs will mature, increasing the expression of costimulatory molecules.42For acti-
vation of T cells, appropriate interaction between the T-cell receptor and antigen/MHC
complex as well as costimulatory signals are necessary. However, when there is
a disruption in these interactions, T cells become anergic.43There is increasing
evidence that by hampering the costimulatory capacity of DCs, a shift from
Vitamin D and Diabetes
immunogenicity to tolerance can be achieved. 1,25(OH)2D has dramatic effects on
antigen presentation, whereby it reduces antigen presentation by suppressing the
decrease their production of interleukin (IL)-12, and increase the production of IL-10,
thus leading to an immune-modulatory DC. 1,25(OH)2D3not only inhibits maturation
of DCs but also increases the apoptosis of mature DCs.47Furthermore, coculture of
1,25(OH)2D3-treated DCs with autoreactive T-cell clones isolated from T1D patients
sive proteomic analysis of DCs treated with TX527, a 14-epivitamin D3analogue,
showed that DCs are not merely locked in an immature state but adopt a tolerogenic
phenotype with special migratory and endocytic properties compared with mature or
immature DCs.49Furthermore, these 1,25(OH)2D3-treated DCs may induce Treg cells
and inhibit autoimmune diseases such as T1D.50,51However, most of these studies
VDR expression was first described in activated T cells, and early work suggested that
vitamin D exerted its immune-modulatory effects on these cells. In the 1980s, several
Fig. 1. Mechanisms of action of (active) vitamin D in the protection against diabetes.
1,25(OH)2D3 plays an important role in glucose homeostasis via different mechanisms.
1,25(OH)2D3not only enhances and improves the b-cell function but also improves insulin
sensitivity of the target cells (liver, skeletal muscle, and adipose tissue). In addition,
1,25(OH)2D3protects the b-cell from detrimental immune attacks, directly by its action on
the b-cell but also indirectly by acting on different immune cells, including inflammatory
macrophages, dendritic cells, and a variety of T cells. In addition, macrophages, as well as
dendritic cells, T lymphocytes, and B lymphocytes can synthesize 1,25(OH)2D3, all contrib-
uting to the regulation of local immune responses.
Takiishi et al
studies reported that 1,25(OH)2D3could inhibit proliferation of mitogen-stimulated
T-cell cultures.52–55In 1987, Rigby and colleagues56described that 1,25(OH)2D3treat-
ment inhibited IL-2 and interferon (IFN)-g production by human T cells. Others demon-
strated inhibition of the aforementioned cytokines as well as IL-12, a known T-cell
stimulating factor that is involved in the differentiation of naı ¨ve T cells into Th0 cells,
which further develop into either Th1 cells or Th2 cells. In addition, an enhancement
of Th2-related cytokines (IL-4, IL-5, and IL-10) was observed.57,58As already
mentioned, 1,25(OH)2D3can also induce Treg cells in vitro and in vivo.51,59,60
Until recently, most of these studies were performed on peripheral blood mononu-
clear cells (PBMC) consisting of lymphocytes and monocytes. As such, direct effects
of 1,25(OH)2D3on T cells could not be proven.61,62Work by Jeffery and colleagues63
on isolated CD41cells demonstrated that 1,25(OH)2D3can directly modulate T-cell
responses. 1,25(OH)2D3inhibited the production of IFN-g, IL-17, and IL-21 inflamma-
tory cytokines by CD41T cells, and induced development of Treg cells expressing
CTLA-4 and FoxP3. T cells cultured in the presence of both 1,25(OH)2D3and IL-2
expressed the highest levels of CTLA-4 and FoxP3, and possessed the ability to
suppress proliferation of resting CD41T cells. Of interest, a different study showed
that exposure of the skin to the topical vitamin D analogue calcipotriol before immu-
nization with ovalbumin (OVA) and CpG DNA as an immune-stimulatory adjuvant
induces Treg cells that prevent consequent antigen-specific CD81T-cell proliferation
and IFN-g production.60
Resting B lymphocytes normally do not express VDR.39On activation, however, VDR
expression has been reported.64Administration of 1,25(OH)2D3decreases prolifera-
tion and immunoglobulin (Ig) production, and induces apoptosis.64Although
1,25(OH)2D3has potent direct effects on B lymphocytes in vitro, indirect mediation
by T cells and monocytes or macrophages has been suggested as its most important
mechanism of action.64,65
Active vitamin D has also been reported to down-regulate the production of several
cytokines, in particular, inflammatory cytokines such as IL-2, IL-6, IL-12, IFN-g, tumor
necrosis factor (TNF)-a, and TNF-b, while enhancing anti-inflammatory cytokines such
as IL-4, IL-10, and TGF-b.66,67This finding is of great relevance for the pathogenesis of
T1D, as especially IFN-g and IL-12, which are markers of Th1 immune responses, are
known to enhance inflammatory processes and participate in immune-mediated
destruction of insulin-producing b cells.68Recently, low-intensity chronic inflammation
related to obesity has been linked to insulin resistance, the major cause of T2D. It is
suggested that the relationship between vitamin D and low-intensity chronic inflam-
mation and insulin resistance in T2D can be mediated in part by the immune-modu-
lating properties of 1,25(OH)2D3.69
Vitamin D and the b cell
Exposure of pancreatic b cells to proinflammatory cytokines induces endoplasmic
reticulum stress, leading to death by apoptosis.70Treatment of b cells with
1,25(OH)2D has been reported to directly protect against b-cell death by reducing
expression of MHC class I molecules,71inducing expression of antiapoptotic A20
protein and decreasing expression of Fas.72,73The latter is a transmembrane cell
surface receptor, transducing an apoptotic death signal and contributing to the path-
ogenesis of several autoimmune diseases including T1D. Of note, in vitro treatment of
Vitamin D and Diabetes
pancreatic islets with 1,25(OH)2D3was also shown to decrease IL-1b and IL-15 cyto-
kine as well as IP-10 chemokine (IFN-g inducible protein 10) expression in pancreatic
b cells, indicating that 1,25(OH)2D3treatment could reduce the migration and recruit-
ment of effector T cells and macrophages to the islets.74
The immune-modulatory properties of active vitamin D suggest that vitamin D (metab-
olites or analogues) could be potential therapeutic agents for the prevention or cure of
T1D. Treatment of NOD mice, an animal model for T1D, with high doses of
1,25(OH)2D3(5 mg/kg/2 d) showed a decrease in insulitis and diabetes develop-
ment.75,76Decreased numbers of effector T cells, as well as induction of Treg cells,
was shown to be the basis for this protection.59Later, the authors demonstrated
that the arrest of insulitis and block of T-cell infiltration into the pancreas by treatment
of prediabetic NOD mice with 1,25(OH)2D3was associated with reduced chemokine
production by islet cells.74Treatment of prediabetic NOD mice with 1,25(OH)2D3
increased deletion of T lymphocytes in the thymus, allowing activation-induced cell
death (AICD)-sensitive T lymphocytes to reach the periphery.77Culture of DCs and
thymic T lymphocytes from 1,25(OH)2D3-treated animals separately, however,
demonstrated that both cell types needed to be exposed to 1,25(OH)2D3to obtain
the apoptosis-restorative effect. Moreover, transfer experiments demonstrated that
T lymphocytes from 1,25(OH)2D3-treated NOD mice were not able to transfer diabetes
into young irradiated NOD mice, in contrast to age-matched untreated mice.78The
latter indicates that 1,25(OH)2D3is able to directly modulate immune cell responses.
The authors also found that 1,25(OH)2D3is a potent inducer of thymic DC differentia-
tion in NOD mice, consisting in modulation toward a more pronounced lymphoid
phenotype and up-regulation of CD86.77Moreover, NOD DCs generated from bone
marrow in the presence of in vitro 1,25(OH)2D3exhibit dedifferentiation features of tol-
Unfortunately, the doses needed for disease prevention in NOD mice lead to hyper-
calcemia and bone decalcification.37This issue can be (partially) solved by using
structural analogues of 1,25(OH)2D3.37In fact, administration of vitamin D3analogues,
specifically selected for their enhanced immune effects and decreased calcemic
effects, was shown to delay or even inhibit the development of insulitis and the onset
of T1D in NOD mice.79The proposed mechanism of action was a restoration of
In the streptozotocin-induced diabetes model, which is an inflammation-driven
model of diabetes, 1,25(OH)2D3also induced a reduction in the incidence of dia-
betes.80In the case of overt diabetes, 1,25(OH)2D3has little effect in reverting the
disease. It is possible that at this point the number of remaining b-cell mass is simply
not sufficient to restore insulin needs.81Therefore, an alternative option to insulin
therapy is islet or b-cell transplantation. In that regard, it has been shown that overtly
diabetic NOD mice that received syngeneic islets and were treated with KH1060 (a 20-
epivitamin D3analogue) together with cyclosporine displayed a significant prolonga-
tion of islet graft survival compared with untreated controls.82The authors also
demonstrated that this analogue in combination with cyclosporine was able to prevent
early graft failure and delay graft rejection of xenogeneic islets transplanted in spon-
taneously diabetic NOD mice.83More recently, a combination therapy using TX527,
a 14-epivitamin D3analogue, with cyclosporine or IFN-b also induced a significant
delay in diabetes recurrence after syngeneic islet transplantation, with an increase
of IL-10 expression in islet grafts.84Work by Adorini50in an allogeneic islet transplan-
tation model also confirmed that 1,25(OH)2D3 treatment in combination with
Takiishi et al
mycophenolate mofetil was able to reduce graft rejection, possibly by inducing tolero-
genic DCs and/or Treg cells. Taken together, 1,25(OH)2D3and its structural analogues
display attractive anti-inflammatory properties that open new avenues for the primary,
secondary, or tertiary prevention of T1D and other autoimmune diseases.
Nevertheless, despite all the work showing the beneficial effects of vitamin D in T1D
prevention, data from VDR knockout mice show conflicting results. Zeitz and
colleagues85showed in their model that mice presented higher concentrations of
blood glucose and lower levels of circulating insulin, whereas Mathieu and
colleagues83and Gysemans and colleagues86did not observe major alterations in
glucose tolerance or diabetes incidence in their VDR knockout mouse models.
However, these contradictory data could result from the different genetic background
of the mice or the control of serum calcium homeostasis, but could also point toward
redundancy of vitamin D signaling pathways, and may suggest that compensatory
mechanisms are taking place when VDR is completely abrogated from early life
Large-scale trials evaluating the efficacy of vitamin D in the prevention of T1D are still
lacking, but interesting data can be obtained from epidemiologic observations and
Several retrospective studies found beneficial effects of supplementation with
regular vitamin D in early life on the later lifetime risk of T1D. Hypponen and
colleagues87found a significantly reduced risk of T1D development in a birth-cohort
study when high doses of vitamin D supplementation (up to 2000 IU/d) were given
during infancy. In addition, 2 studies by Stene and colleagues88,89showed that use
of cod liver oil, which is rich in vitamin D, during the first year of life was associated
with a lower risk of developing T1D later in life. Finally, the European Community–
sponsored Concerted Action on the Epidemiology and Prevention of Diabetes showed
a 33% reduction of T1D in children who received vitamin D supplementation early in
life.90A meta-analysis of data from 4 case-control studies and one cohort study
revealed lately that the risk of T1D was significantly reduced (29% reduction) in infants
who were supplemented with vitamin D as compared with those who were not supple-
mented (pooled odds ratio 0.71, 95% confidence interval [CI] 0.60–0.84).91There was
also some evidence of a dose-response effect, with those using higher amounts of
vitamin D being at lower risk of developing T1D. Some studies were not able to
show an association with a reduced T1D risk, but none of them were associated
with an increased risk.
Vitamin D supplementation during pregnancy has also yielded contradictory
results. In 2000, Stene and colleagues88documented in a pilot case-control study
that cod liver oil, taken by the mother during pregnancy, was associated with a lower
risk of T1D in their offspring. In a larger case-control study, Stene and colleagues89
could not confirm their initial results and were unable to find an association between
the use of cod liver oil or other vitamin D supplements during pregnancy and T1D risk.
Nevertheless, Fronczak and colleagues92reported lower levels of anti-islet cell auto-
antibodies in almost two-thirds of children whose mothers had higher vitamin D intake
during the third trimester. Taking into account that T1D susceptibility has been linked
to certain HLA genotypes, Wicklow and Taback intend to pursue a trial using 2000 IU
of regular vitamin D per day in newborn babies with increased HLA-associated risk.
So far they have shown in a few babies that this dose of supplementation seems
safe, and did not cause alterations in serum and urine calcium measurements93
Vitamin D and Diabetes
Data on intervention with active vitamin D (1,25(OH)2D) starting when b-cell damage
is already present are disappointing. A small intervention trial in which newly-onset
diabetic children were given a small dose (0.25 mg) of 1,25(OH)2D or nicotinamide
showed that although insulin requirements decreased in the group treated with
1,25(OH)2D, they had no improvement in C-peptide levels.94LADA is considered to
be a subtype of T1D, in which the clinical manifestation begins and progresses slowly
in adulthood. As in T1D, patients with LADA exhibit the presence of autoantibodies to
the islets, especially those against glutamic acid decarboxylase.95Li and colleagues96
described results of a pilot study in which LADA patients were given a synthetic
analogue of vitamin D3, 1a(OH)D3, in addition to insulin treatment. The patients who
received the analogue exhibited a better ability to preserve b-cell function in compar-
ison with patients treated with insulin alone (see Table 1).
TYPE 2 DIABETES
T2D is a disorder that results from defects in both insulin secretion and insulin sensi-
tivity, and accounts for 90% of all diabetes cases. The growing rate of T2D is
Reported effects of vitamin D supplementation in humans and etiology of T1D
Intervention Subjects’ CharacteristicsStudy Results Ref.
Vitamin D supplements
(first year of life)
1429 cases; 5026 controls
Age <14 y
Risk T1D Y (OR 5 0.83)
age <5 y
Risk T1D Y (OR 5 0.81)
age 5–9 y
Risk T1D Y (OR 5 0.47)
age 10–14 y
Cod liver oil
(first year of life)
78 cases; 980 controls
Offspring <15 y
Risk of T1D Y offspring
(OR 5 0.63)
Risk of T1D Y cod-liver oil
fed-infants (OR 5 0.82)
(first year of life)
81 cases; 10,285 controls
Age 1–31 y
Insulin and C-peptide [
Risk of T1D Y (RR 5 0.22)
Cod liver oil
(first year of life)
95 cases; 346 controls
Age <15 y
Risk of T1D Y
7–12 mo (OR 5 0.55)
0–6 mo (OR 5 0.80)
Vitamin D (food)
Age <5 y
Insulin autoantibodies Y
(OR 5 0.49)
0.25 mg 1,25(OH)2D3every
2 d or 25 mg
70 Recently diagnosed T1D
Age >5 y
Insulin needs Y 1,25(OH)2D3-
treated group at 3–6 mo
Vitamin D supplements
159 cases; 318 controls
Age 0–29 y
Risk of T1D Y (OR 5 0.33)
0.5 mg 1a(OH)D3/d
b-Cell function [
2000 IU vitamin D
first year of life
HLA-associated T1D risk
Age >1 y
Serum and urine calcium 5
In column 1, intervention period is given in parentheses.
Abbreviations: OR, odds ratio; RR, relative risk.
Takiishi et al
worrisome, with in the United States alone an estimated 1 million new cases every
year.97Initially, patients counteract their increased insulin resistance and stabilize
circulating glucose levels through increased insulin production by pancreatic b cells.
As the disease progresses and functional alterations are accentuated, patients
show decreased insulin secretion, and eventually they can also present loss of b-
cell mass.4,98The exact mechanisms involving T2D development are still unknown,
but lifestyle (eg, obesity, sedentary lifestyle, and unhealthy eating habits) and genetic
components (eg, PPARg and CAPN10 genes, and a whole set of gene polymorphisms
each with small contributing effects) seem to be involved. This disease is most prev-
alent in obese, sedentary individuals with a concomitant elevation in free fatty acids
and proinflammatory cytokines, and relatives of T2D patients also have an increased
probability of developing this disease.99
Vitamin D and Lifestyle in T2D
The number one risk factor for T2D is obesity. However, weight loss is difficult to
achieve and maintain in a long term. Identification of easily modifiable risk factors
is, therefore, urgently needed for primary prevention of T2D. It is interesting that
obesity is often related to hypovitaminosis D.4,100Indeed, the absolute fat mass has
an inverse relation with the serum 25(OH)D concentration and correlates positively
with the serum parathyroid hormone (PTH) level. This relationship may be caused
by the great capacity of adipose tissue of storing vitamin D, thus making it biologically
unavailable. An increased PTH level and a decreased amount of serum 25(OH)D3as
well as 1,25(OH)2D3can increase intracellular calcium in adipocytes, which then stim-
ulates the lipogenesis and predisposes to further weight gain. Therefore, it is presently
unclear whether (mild) vitamin D deficiency is contributing to or is the consequence of
Vitamin D deficiency has been associated with higher risks for metabolic syndrome
and T2D.101–103Population studies suggest that vitamin D (and calcium) may play
a significant role in promoting b-cell function and insulin sensitivity, important issues
in the pathogenesis of T2D. The National Health and Nutrition Examination Survey
(NHANES), a large cross-sectional study, showed an inverse correlation between
serum 25(OH)D and incidence of T2D and insulin resistance.102,104In a prospective
examination of the Medical Research Council Ely Study 1990 to 2000, an inverse rela-
tionship between 25(OH)D and glycemic status was found.105Supporting these data,
a positive correlation between plasma 25(OH)D and insulin sensitivity in healthy
subjects subjected to glucose tolerance tests was also reported.103On the other
hand, prolonged treatment of osteomalacia with vitamin D can increase insulin secre-
tion and improve glucose tolerance.106,107
Vitamin D deficiency in obese patients has been linked to secondary hyperparathy-
roidism, which can contribute to T2D development, as elevated levels of PTH have
been associated to glucose intolerance and cardiovascular complications.4Current
data suggest that T2D patients with vitamin D insufficiency have increased C-reactive
protein, fibrinogen, and hemoglobin A1c compared with healthy controls,108indicating
that inflammation provoked by immune cells (eg, macrophages) are implicated in
insulin resistance and T2D. The authors reported that administration of vitamin D
ameliorates markers of systemic inflammation, which are typically found in T2D
patients, thereby possibly improving b-cell survival.109
tive correlation with T2D,110for which a possible explanation has recently been
proposed.111Oh and colleagues112observed that up-regulation of caveolin-1 (highly
likely to be involved in nongenomic vitamin D signaling113) significantly improved
Vitamin D and Diabetes
insulin sensitivity and improved glucose uptake in the skeletal muscle. Therefore, it
would be interesting to investigate whether vitamin D supplementation in the nonob-
ese T2D mouse model would improve insulin sensitivity and whether this would be
correlated to caveolin-1 expression.
Vitamin D and Genetic Predisposition to T2D
T2D is a polygenic disorder, but monogenic disorders closely related to T2D also exist.
Monogenic forms of T2D are rare and include subtypes of maturity onset diabetes
(MODY). The majority of proteins that are linked to MODY are transcription factors
(such as HNF-4a, HNF-1a, IPF-1, HNF-1b, and NEUROD1).114On the other hand,
causative genes in the more common polygenic forms of T2D are harder to be iden-
tified. However, there are several indications that polymorphisms of the DBP and
VDR are related to impaired glucose tolerance and obesity. Even though these corre-
lations vary according to age, lifestyle, and ethnicity of the subjects, there seems to be
a fair amount of evidence to support this theory.
The DBP protein (also known as group-specific component protein, or GC) located
on chromosome 4q12 is a highly polymorphic serum protein, mainly produced in the
liver, with 3 common alleles (Gc1F, Gc1S, and Gc2) and more than 120 rare vari-
ants.115Few studies have examined DBP polymorphisms and the risk of vitamin D–
related diseases. In this regard, the DPB protein has been linked to abnormalities in
glucose metabolism and obesity-related traits in different populations. Hirai and
colleagues116evaluated the variations of the DBP gene (Gc1F, Gc1S, and Gc2) in
Japanese individuals with normal glucose tolerance. These investigators demon-
strated that people with Gc1S/Gc2 and Gc1S/Gc1S had significantly higher fasting
plasma concentrations than those with Gc1F/Gc1F. The same group also reported
that Japanese T2D patients had higher frequencies of the Gc1S/Gc2 genotype and
lower frequencies of the Gc1F allele in comparison with control subjects.117Other
studies in Caucasian patients of American or European origin could not confirm the
relation between genetic variants of the DBP gene and the susceptibility to
T2D.118,119It has been suggested that DBP polymorphisms can perhaps influence
bioactive 25(OH)D levels through changes in the ratio of free/bound hormones, by
a differential affinity, or through effects on concentrations of the DBP/25(OH)D
complex that can be internalized by receptor-mediated endocytosis and activate
the VDR pathway.120
The VDR gene is located on chromosome 12q13.11 and consists of 11 exons. Most
VDR polymorphisms are located at the 30untranslated region of the VDR gene, such as
the BsmI, ApaI, and TaqI restriction fragment length polymorphisms.121Several
observational studies have reported associations between VDR polymorphisms and
T2D, fasting glucose, glucose intolerance, insulin sensitivity, insulin secretion, and cal-
citriol levels.4,122,123In the Rancho Bernardo study, polymorphism in ApaI, BsmI, and
TaqI in older Caucasian men was verified. The investigators observed that the
frequency of aa genotype of ApaI polymorphism was marginally higher in T2D
patients. Also, fasting plasma glucose and prevalence of glucose intolerance were
significantly higher in nondiabetic persons with aa genotype compared with those
with AA genotype. Moreover, the bb genotype of BsmI polymorphism was associated
with insulin resistance.124Ortlepp and colleagues125investigated the association of
fasting glucose, low physical activity, and BsmI VDR polymorphism. In this study,
males with low physical activity and gene carriers with the genotype BB had signifi-
cantly higher levels of fasting glucose than gene carriers with the genotype Bb or
bb. Of note, this effect was not seen in individuals with high physical activity. A recent
study found that the BsmI polymorphism seems to influence body mass index (BMI;
Takiishi et al
weight in kilograms divided by height in meters squared), whereas the FokI seems to
affect insulin sensitivity and serum high-density lipoprotein cholesterol (cHDL) level. It
was found that BB carriers tend to have higher BMI and waist circumference
compared with the bb genotypes. Similarly, FF and Ff carriers had higher fasting
insulin levels than the ff carriers, and lower cHDL levels in comparison with ff geno-
types.126Ye and colleagues127also describe a correlation between BMI/obesity
and VDR polymorphism. This study found that T2D patients who were diagnosed at
45 years or younger with T-allele of TaqI and the b-allele of BsmI had higher BMI.
More recently, Dilmec and colleagues128could not find a correlation between VDR
polymorphisms (ie, ApaI and TaqI) and T2D risk in a study of 241 individuals (72
patients with T2DM and 169 healthy individuals). Regarding the matter of ethnicity,
studies are inconclusive, as associations between VDR polymorphisms and the risk
of T2D in different ethnic populations have produced variable results.129,130
The pathophysiological mechanisms of these associations remain unexplained, but
there seems to be a relation between the VDR genotype and certain traits of suscep-
tibility to T2D. For instance, VDR polymorphisms are linked to obesity, and vitamin D
itself has been reported to participate in adipocyte differentiation and metabolism.
Moreover, polymorphisms of VDR might play a role in the pathogenesis of T2D by
influencing the secretory capacity of b cells.131The VDR genotype was associated
with altered fasting glucose, confirming the importance of vitamin D in the modulation
of insulin secretion (see next section).
Vitamin D and the b cell
There is strong evidence that vitamin D is important for glucose homeostasis and that
this could be mediated by its direct action on b-cell function. Several studies in
animals and humans indicate a positive correlation between vitamin D deficiency
and glucose intolerance as well as impaired insulin secretion.132–135Further, this defi-
ciency seems to have a specific effect on insulin and not on other islet hormones such
as glucagon.136For instance, experiments on glucose- and sulfonylurea-stimulated
islets obtained from rats kept on a vitamin D–deficient diet showed impaired insulin
secretion and glucose tolerance. These defects were partially corrected by vitamin
D replenishment.132,137,138However, whether these defects are directly caused by
lack of vitamin D or indirectly by hypocalcemia is not clear.
More convincing data on the beneficial effects of vitamin D on insulin secretion were
obtained in experiments demonstrating that synthesis and release of insulin by islets
isolated from normal animals could be enhanced by glucose challenge in the presence
of high doses of 1,25(OH)2D3.139,140Stimulation of islets by 1,25(OH)2D3was shown to
significantly increase the levels of cytosolic Ca21, indicating that this could be a mech-
anism by which 1,25(OH)2D3is able to stimulate insulin secretion.141–143Ca21is
known to be important for the exocytosis of insulin from the b cell and for b-cell glycol-
ysis, which participates in translating circulating glucose levels.144,145Moreover,
vitamin D could regulate insulin secretion and synthesis by facilitating the conversion
of proinsulin to insulin, which is known to be dependent on the cleavage by b-cell
calcium-dependent endopeptidases.146,147It is also possible that high intracellular
Ca21improves the binding of calmodulin to the insulin receptor substrate-1, thereby
interfering with insulin-stimulated tyrosine phosphorylation and phosphoinositide-3
kinase activation.148,149In this regard, PTH has been found to be inversely associated
with insulin sensitivity.150Another possible mechanism that has been suggested is
that vitamin D could directly modulate b-cell growth and differentiation.151,152
The effects of 1,25(OH)2D3 and its analogues have been examined regarding
binding to nuclear VDR and membrane VDR, through which they induce genomic
Vitamin D and Diabetes
and nongenomic responses, respectively. Among these studies, Sergeev and Rho-
ten153have reported that the administration of 1,25(OH)2D3evoked oscillations of
intracellular Ca21in a pancreatic b-cell line within a few minutes. Later, Kajikawa
and colleagues154demonstrated that the 6-s-cis analogue, 1,25(OH)2lumisterol3,
has a rapid insulinotropic effect, through nongenomic signal transduction via putative
membrane VDR, which would be dependent on the augmentation of Ca21influx
through voltage-dependent Ca21channels on the plasma membrane, being also
linked to metabolic signals derived from glucose in pancreatic b cells.
As T2D has been recently associated with systemic inflammation, which is linked
primarily to insulin resistance, vitamin D may also improve insulin sensitivity and b-
cell function by directly modulating the generation and effects of inflammatory cyto-
kines, as discussed previously.155
Experimental studies in animal models also suggest a role for vitamin D in the patho-
genesis of T2D. Animal studies reveal that vitamin D deficiency is associated with
impaired insulin sensitivity, while insulin secretion increases through vitamin D
Chang-Quan and colleagues156reported that T2D was associated with an abnormal
vitamin D metabolism that was characterized by deficiency in 1,25(OH)2D and was
related to renal injury. In the ob/ob mouse model, treatment with 1a(OH)D improved
hyperglycemia, hyperinsulinemia, and fat tissue responsiveness to hormones.157
Anderson and Rowling158demonstrated in Zucker Diabetic Fatty rats that vitamin D
status was compromised due to poor vitamin D reabsorption in the kidney.
Considering that obesity seems to be an important risk factor to T2D, it was inves-
tigated whether vitamin D3had a beneficial effect on blood glucose in obese SHR and
Wistar rats. Although vitamin D3supplementation in SHR rats did not alter the blood
glucose levels in all rats, 40% of those rats had a reduction in glucose by 60%. In Wis-
tar rats, a significant reduction in glucose levels in all animals supplemented with
vitamin D3was found.159In addition, feeding of cod liver oil to streptozotocin-induced
diabetic rats partially improved their blood glucose levels as well as their cardiovas-
cular and metabolic abnormalities.160In another study, vitamin D3supplementation
of spontaneously hypertensive rats normalized the membrane potential and contrac-
tility of aorta.161
In view of the cellular, preclinical data and observational studies in man, it seems
reasonable to consider that vitamin D status influences the incidence of T2D and
that vitamin D supplementation could prevent or ameliorate the disease, at least in
cases of (mild) vitamin D deficiency. Despite this, trials in patients have yielded con-
flicting results (Table 2).
Vitamin D3supplementation of vitamin D–deficient T2D patients tended to reduce
insulin requirements and lower serum triglycerides.177Boucher and colleagues167
showed transient improvement of insulin secretion and C-peptide levels in at-risk
patients treated with intramuscular vitamin D. In support to these findings, a small
study in which a group of T2D women received 1332 IU of vitamin D3daily for 1 month
showed an improvement on first-phase insulin secretion and a trend toward
decreased insulin resistance.172Moreover, the Nurses’ Health Study, which included
1,580,957 women over a period of more than 20 years with no history of diabetes,
cardiovascular disease, or cancer at baseline, showed that a combined daily intake
of greater than 1200 mg calcium and greater than 800 IU vitamin D was associated
Takiishi et al
Reported effects of vitamin D supplementation in humans and etiology of T2D
InterventionSubjects’ CharacteristicsStudy Results Refs.
2000 IU vitamin D3/d (2 y),
n 5 25
0.25 mg 1a(OH)D3/d (2 y),
n 5 23
>0.25 mg 1,25(OH)2D3/d
(1 y), n 5 40
n 5 238
Age 45–54 y
Fasting glucose levels 5
2 mg 1a(OH)D3/d
7 cases; 7 controls
T2D Japanese patients
Age w54 y
Insulin secretion [
Free fatty acid Y
2000 IU vitamin D3/d
4 cases; 10 controls
Vitamin D deficient subjects
Age w32 y
Insulin secretion [
0.75 mg 1a(OH)D3/d
65 Caucasian vitamin D
sufficient men with
Age 61–65 y
Glucose levels 5
Insulin secretion 5
Body weight Y
Urinary calcium [
2000 IU vitamin D/d
1 Vitamin D deficient
Age w65 y
Glucose tolerance [
b-Cell function [
Single IV injection of
100,000 IU vitamin D3
22 Vitamin D deficient
Age w45 y
Insulin and C-peptide [
500 mg Ca21and/or 0.5 mg
1,25(OH)2D3/d (21 d)
Oral 1 mg 1,25(OH)2D3/d
17 Uremic men and women
Age w50 y
First-phase insulin secretion
and insulin sensitivity [
20 T2D men and
Age w60 y
Insulin secretion and
Urinary calcium [ in
patients with short
duration of diabetes
1.5 mg 1,25(OH)2D3/d
18 Healthy young men
Age w26 y
PTH concentration Y
Urinary calcium [
Single IM injection
300,000 IU vitamin D2
1332 IU vitamin D3/d
3 T2D men and women Insulin resistance [
10 T2D women
Age w54 y
First-phase insulin secretion
Vitamin D and/or calcium
4843 cases; 1,576,114
Age w46 y
23% Y risk of T2D when
vitamin D consumption/d
is R800 IU compared
with <200 IU/d
500 mg calcium and 700 IU
vitamin D3/d (3 y)
314 Nondiabetic Caucasians
Age w71 y
Rise glycemia and insulin
resistance Y in patients
with impaired fasting
1000 mg Calcium and
400 IU vitamin D3/d (6 y)
2291 cases; 31,660 controls
Age 50–79 y
Insulin or glucose levels 5
Diabetes incidence 5
3 doses of 120,000 IU
1000 healthy, centrally
Age R35 y
Insulin secretion 5
Insulin sensitivity [
In column 1, intervention period is given in parentheses.
Vitamin D and Diabetes
with a 33% lower risk of T2D compared with an intake of less than 600 mg and 400 IU
calcium and vitamin D.173In 2008, 2 nested case-control studies, collected by the
Finnish Mobile Clinic from 1973 to 1980, were pooled for analysis. These results sup-
ported the hypothesis that high vitamin D status provides protection against T2D.178
Recently, a New Zealand study found that South Asian women with insulin resistance
improved markedly after taking vitamin D supplements.179Optimal vitamin D concen-
trations for reducing insulin resistance were shown to be 80 to 119 nmol/L, providing
further evidence for an increase in the recommended adequate levels.
On the other hand, some studies show no effect of vitamin D supplementation and
improvement of T2D. For instance, the Women’s Health Initiative, in which low-dose
calcium and 400 IU/d of vitamin D supplementation were given, did not show protec-
tion against diabetes.175It was recently reported that daily oral administration of 800
IU (20 mg) vitamin D3alone or in association with 1000 mg calcium to older people also
failed to prevent T2D.180In one study, Asian T2D patients with vitamin D deficiency
even had a worsening of their condition through increased insulin resistance and dete-
rioration of glycemic control.171Another point to note is that, in general, no benefits in
glucose tolerance have been seen with vitamin D supplementation in patients who are
not vitamin D deficient.108Of importance is that some of these studies reported eleva-
tion in calcium urinary excretion in vitamin D–supplemented individuals.165,169,170The
contradictory results of vitamin D supplementation in T2D suggest that dose and
method of supplementation, as well as the genetic background and baseline vitamin
D status of individuals, appear to be important for the efficacy of vitamin D supplemen-
tation against development of T2D.
GD is defined by b-cell dysfunction and insulin resistance during pregnancy. Women
who have had GD have a 20% to 50% chance of developing T2D within 5 to 10 years.
Several studies demonstrated that pregnant women are more susceptible to hypo-
vitaminosis D181,182and can suffer from insulin resistance.4,183Studies on the role of
vitamin D and the regulation of glucose homeostasis in pregnancy are scarce, and
data are not always consistent. Nevertheless, Zhang and colleagues184reported
that each 5 ng/mL decrease in 25(OH)D levels relates to a 1.29-fold increase in GD
risk. In addition, vitamin D depletion during pregnancy, aside from the classically
known consequences such as decreased bone density and development of rickets
in offspring, has also been associated with nonclassic consequences such as reduced
fetal growth, disturbed brain development, and induction of T1D development.185
A study by Rudnicki and Molsted-Pederson,186in which they injected pregnant GD-
diagnosed women intravenously with 1,25(OH)2D, showed that these women had
a transient decrease in fasting glucose levels but surprisingly also a decrease in insulin
levels. These apparent contradictory results suggest that vitamin D could directly
increase cellular glucose absorption by increasing insulin sensitivity.
VITAMIN D AND DIABETES COMPLICATIONS
Over time, hyperglycemia can have a damaging effect on the kidneys. Zhang and
colleagues187reported that the prodrug vitamin D analogue, doxercalciferol
(1a(OH)D2), may protect kidneys in mice with diabetic nephropathy. This result
suggests that vitamin D might be useful and preventative for the kidneys. The
NHANES survey found that 25(OH)D levels were significantly lower in persons with
severely decreased glomerular filtration rate when compared with healthy individuals.
Takiishi et al
In addition, persons with higher levels of 25(OH)D had decreased glucose homeo-
stasis model assessment of insulin resistance (HOMA-IR), but 25(OH)D levels did
not correlate with b-cell function (also estimated by HOMA).188In a cross-sectional
analysis of the 2001 to 2006 NHANES study, diabetic patients with nephropathy
had a high prevalence of vitamin D deficiency and insufficiency.189This finding may
be worrisome, as recent work by Wolf and colleagues190suggested that vitamin D
deficiency in hemodialysis patients was associated with increased mortality risks.
Of note, an independent association between vitamin D deficiency and insufficiency
with the presence of diabetic nephropathy was seen.189Given these findings, the
improvement of the vitamin D status or pharmacologic intervention with vitamin D
analogues for the prevention or treatment of renal failure needs further study.
Vision Loss and Blindness
Although not a sudden process, subjects with diabetes face a very real threat of vision
loss, including blindness (diabetic retinopathy). Diabetes also increases the risk of
developing cataracts (clouding of the eyes lenses) and glaucoma (damage to the optic
nerves). In T2D patients, severity of retinopathy was inversely correlated with serum
1,25(OH)2D3levels.191Age-related macular degeneration (AMD) occurs when the
macula, the area at the back of the retina that produces the sharpest vision, deterio-
rates over time. AMD is the most common cause of blindness among individuals older
than 50 years. Levels of serum vitamin D were inversely associated with early AMD.192
These data suggest that vitamin D supplementation might have a beneficial effect on
Hypertension, Heart Attack, and Stroke
ofrecent datasuggests acentralroleofthe vitaminDendocrine systemonbloodpres-
sure regulation and cardiovascular health. For this important topic, recent reviews dis-
addition, mild vitamin D deficiency was associated with higher blood pressure in
Caucasians, Hispanics, and African Americans.196In recent years, Pilz and
colleagues197have demonstrated a clear association between low levels of 25(OH)D
as well as of 1,25(OH)2D with prevalent myocardial dysfunction, deaths due to heart
failure, and sudden cardiac death. In the Multi-Ethnic Study of Atherosclerosis, low
25(OH)D levels were linked to increased risk for developing incident coronary artery
calcification.198Also, direct effects of vitamin D on the cardiovascular system may be
cells,200and endothelial cells express the VDR and vitamin D affects inflammation as
well as cellular proliferation and differentiation, vitamin D may lower the risk of devel-
oping cardiovascular disease. A recent meta-analysis of 18 independent randomized
controlled trials for vitamin D, including 57,311 participants, described that intake of
regular vitamin D supplements (from 300 IU to 2000 IU) was associated with reduced
mortality risk (relative risk 0.93; 95% CI, 0.87–0.99).201Interventional trials are war-
ranted to elucidate whether vitamin D replenishment is useful for prevention or treat-
ment of cardiovascular diseases and other health outcomes.
Nerve Damage and Dementia
Neuropathy is a common complication in diabetic patients, with a hallmark of sensory
neuropathy being the loss of sensation in feet, a risk factor for limb amputation.
Vitamin D and Diabetes
Recently, diabetic neuropathy has been linked with low levels of 25(OH)D.202In this
study, a total of (only) 51 patients with T2D (all vitamin D insufficient) with typical neuro-
pathic pain were included and given vitamin D3treatment (mean dose, approximately
2000 IU). Serum concentrations of 25(OH)D increased from 18 to 30 ng/mL, and the
intervention was associated with significant pain reduction. Whether vitamin D can
be useful as therapeutic application for neuropathic pain needs to be elucidated in
adequately powered prospective clinical studies.
Diabetes also increases the risk of Alzheimer disease and vascular dementia.203
There is ample biologic evidence to support a role for vitamin D in neuroprotection
and reducing inflammation, and moreover to put forward a role for vitamin D in brain
development and function.30Whether vitamin D can reduce the risk of diseases linked
to dementia, such as vascular and metabolic diseases like diabetes, needs further
Large cross-sectional studies have indicated that patients with T2D have significantly
increased risk of bone fractures, predominantly hip fractures.204This group of patients
frequently displayed loss of vision caused by diabetic eye disease, peripheral neurop-
athy, arterial hypertension, orthostatic hypotonia, and ischemic disease of the brain,
heart, and lower extremities—conditions that predispose to falls. Lately, frequently
used drugs in T2D (thiazolidinediones) have been implicated in an increase in bone
fractures. Implication of the vitamin D system in this issue is unlikely, but data are
scarce. The ADOPT (A Diabetes Outcome Progression Trial) group recently reported
slightly reduced vitamin D levels in rosiglitazone-treated patients compared with met-
formin-treated patients.205Moreover, as vitamin D exerts a direct action on skeletal
muscle function,206it was suggested that T2D patients might benefit from eliminating
unfavorable diet and environmental factors, such as low physical action and low
vitamin D intake. Several meta-analyses of randomized controlled trials showed that
vitamin D supplementation (>400 IU/d) reduces the risk of nonvertebral fractures by
20% and hip fractures by 18%.207,208These studies also pointed out that vitamin D
deficiency is common in patients with hip fractures, and truly contributes to the risk
There is no doubt that vitamin D deficiency is the cause of several metabolic bone
diseases, but vitamin D status is also linked to many major human diseases including
immune disorders. Mounting data strengthen the link between vitamin D and diabetes,
in particular T1D and T2D. Despite some inconsistencies between studies that asso-
ciate serum 25(OH)D levels with the risk of developing T1D or T2D, there seems to be
an overall trend for an inverse correlation between levels of 25(OH)D and both disor-
ders. There is also compelling evidence that 1,25(OH)2D regulates b-cell function by
different mechanisms, such as influencing insulin secretion by regulating intracellular
levels of Ca21, increasing b-cell resistance to apoptosis, and perhaps also increasing
The capacity of vitamin D, more specifically 1,25(OH)2D, to modulate immune
responses is of particular interest for both the therapy and prevention of diabetes.
In the case of T1D, vitamin D supplementation in prediabetic individuals could help
prevent or reduce the initiation of autoimmune processes possibly by regulating
thymic selection of the T-cell repertoire, decreasing the numbers of autoreactive T
cells, and inducing Treg cells. Although immune modulation is generally discussed
Takiishi et al
for the treatment of T1D, it is also relevant for T2D. Indeed, recent studies have shown
that T2D patients have increased systemic inflammation and that this state can induce
b-cell dysfunction and death.
Supplementation trials with regular vitamin D for the protection against the develop-
ment of T1D and T2D have generated some contradictory data, but many weaknesses
can be identified in these trials as most were underpowered or open-labeled.
However, the overwhelming strength of preclinical data and of the observational
studies make vitamin D or its analogues strong candidates for the prevention or treat-
ment of diabetes or its complications. However, proof of causality needs well-
designed clinical trials and if positive, adequate dosing, regimen, and compound
studies are needed to define the contribution of vitamin D status and therapy in the
global diabetes problem. There are many confounding factors that need to be taken
into consideration when translating successful vitamin D therapies in animal models
into humans, for example, gender, age, lifestyle, and genetic background. To come
to solid conclusions on the potential of vitamin D or its analogues in the prevention
of or therapy for all forms of diabetes, it is clear that large prospective trials with care-
fully selected populations and end points will be needed, but should also receive high
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