Vitamin D:
Structure of vitamin D:
Diagram I: Diagram showing the molecular structure of vitamin D (Norman, 2008).
Vitamin D is classified as a secosteroid as the molecular construction of vitamin D is closely related to that of steroid hormones (e.g. estradiol, cortisol, and aldosterone) in that they have the same root ring structure (Diagram I). Vitamin D2 (C28H440) differs from vitamin D3 (C27H440) in the side chain attached to the secosteroid skeleton, which contains an additional methyl group on carbon atom 24 and a double bond between carbon atoms 22 and 23 (Norman, 2008).
Types and sources of vitamin D:
The two main types of vitamin D are: vitamin D3 or cholecalciferol and vitamin D2 or Ergocalciferol. Both types are biologically inactive and must be activated in two metabolic steps before becoming physiologically active (Taylor et al., 2008) (Diagram II).
After exposure to sunlight or ultraviolet light for 15 minutes for two or three times per week, vitamin D3 is formed in the skin. The ultraviolet (UV) spectrum of sunlight (wave length 290-310 nm) invades the human skin and facilitates the transformation of 7-dehydrocholesterol present in the subcutaneous fat to pro-vitamin D, which undergoes alterations by thermal isomerization to vitamin D3. Restrictions are age, a pigmented skin, sunscreen use, and clothing (Holick, 2004).
On the other hand, vitamin D2 can be obtained by irradiation of plants or other types of food (Feldman et al., 2005).
Diagram II: Diagram showing sources and functions of vitamin D (Sung et al, 2012).
Very few types of natural food contain vitamin D. Fish (e.g., salmon, tuna, and mackerel) and fish liver oils are the greatest sources. Lesser amounts of vitamin D are found in beef liver, cheese, egg yolk, and various types of mushrooms. So vitamin D supplementation is important to maintain sufficient level (Ovesen et al., 2003). Since the amount of vitamin D gained from the diet is often low and limited, many people depend on fortified foods and dietary supplements to get their vitamin D needs during times of insufficient sunlight. Vitamin D3 supplements may be more effective than vitamin D2 supplements at increasing serum levels of 25-hydroxyvitamin D (25(OH) D). This may be because vitamin D3 has a higher affinity for vitamin D-binding protein (Armas et al., 2004).
Moreover, the use of ultraviolet lamps has been suggested. Exposure to UV-B radiation is a simple way to increase the synthesis of 25 (OH) D in the body. There is no risk of intoxication, as any excess of vitamin D3 and provitamin D3 is changed to inert metabolites. The smallest dose of UV-B radiation to the whole body supplies 20,000 IU of vitamin D in a single day (Audran and Briot, 2010).
Vitamin D Metabolism:
Vitamin D is chiefly absorbed in the small intestine. Vitamin D is a fat soluble vitamin. Once it is ingested, it is combined into the chylomicron fraction and absorbed through the lymphatic system. It is estimated that about 80% of the ingested vitamin D is absorbed through this mechanism (Holick, 1995).
Transportation of vitamin D in the blood is helped by a binding carrier protein called vitamin D binding protein (DBP). Approximately 99% of vitamin D metabolites are protein bound so there is limited access to the target cells and hence increased half-life in circulation (about 10 days to weeks). The excretion of vitamin D is primarily in the bile. Although amount of it is reabsorbed in the small intestine, this enterohepatic circulation of vitamin D is not considered to be an important mechanism for its conservation. However, since metabolism of vitamin D leads to more water-soluble compounds, a variety of vitamin D metabolites are excreted in urine by the kidney (Kochupillai, 2008).
Furthermore, as vitamin D enters the blood circulation from the skin or from the lymph via the thoracic duct, it collects in the liver within few hours. Liver converts vitamin D to 25 (OH) D, the main circulating form, so it is the usual measure of vitamin D status. This conversion to 25 (OH) D is inhibited in those with liver disease so 92% of patients with chronic liver disease have vitamin D deficiency state (Taylor et al., 2008).
DBP linked vit. D is actively reabsorbed by receptor-mediated uptake in renal proximal convoluted tubules. DBP is degraded and 25 (OH) D is released for metabolism by either 1α or 24 hydroxylases to form the physiologically active form of the hormone 1, 25-dihydroxyvitamin D [1, 25 (OH)2 D] or undergo hydroxylation on the 24-carbon to form their 24-hydroxy counterparts, 24, 25-dihydroxyvitamin D (24, 25 (OH)2 D) is thought to be the first step in the metabolic degradation of 25 (OH) D and 1, 25- (OH)2 D. This second activation metabolic step occurs also in many other tissues (i.e., heart, brain, skin, reproductive tissue, skeletal muscle, spinal cord, and the placenta) (Holick, 1995).
The half-life of each of 25 (OH) D2 and 25 (OH) D3 in the human circulation is moderately long, approximately 2 to 3 weeks. The rates of hepatic transformation of circulating unhydroxylated vitamin D2 to 25 (OH) D2 or of circulating unhydroxylated vitamin D3 to 25-(OH) D3 are dependent only on the supply of unhydroxylated vitamin D2 or vitamin D3 to the liver, making the blood concentration of 25 (OH) D is an excellent reflection of vitamin D status (Glade, 2012).
Role of vitamin D:
It is widely recognized that vitamin D expresses many actions. Beside to its classic role in mineral absorption and skeletal remodeling, the higher steady-state serum 25 (OH) D concentrations are associated with improved health (Autier and Gandini, 2007).
Basically, the main biological function of vitamin D in humans is to maintain serum calcium and phosphorus concentrations within normal range by improving the efficiency of the small intestine to absorb these minerals from the diet. 1, 25 (OH)2 D increases the efficiency of calcium absorption along the whole small intestine, but mainly in the duodenum and jejunum. Also, it enhances dietary phosphorus absorption along the entire small intestine, but mainly in the jejunum and ileum (Feldman et al., 2005).
Active vitamin D function is mediated via vitamin D receptors (VDR), leading to gene expression, either upregulation or downregulation of gene products for example calcium binding protein or osteocalcin (Lips, 2006).
Moreover, vitamin D receptor (VDR) is the ligands-inducible nuclear receptor for vitamin D, and is involved in many physiological processes, including bone metabolism, immune regulation, cell proliferation and differentiation. VDR and its ligands have important roles not only in the pathogenesis but also the treatment of many diseases such as osteoporosis, arthritis, psoriasis and cancers (Wongmayura et al., 2012).
Furthermore, the active metabolite 1, 25 (OH)2 D decreases parathyroid hormone (PTH) secretion, enhances osteoclastic bone resorption, stimulates the osteoblasts, reduces the production of collagen type 1, affects muscular function, stimulates cell differentiation and the immune system and influences insulin secretion. Locally synthesized 1, 25 (OH)2 D stimulates cell differentiation in a paracrine way and may play an important role in cancer avoidance (Lips, 2006).
Vitamin D deficiency:
Vitamin D deficiency was formerly defined by clinical signs and symptoms such as bone pain and proximal muscle weakness and by measuring serum calcium, phosphate and alkaline phosphate levels (Holick et al., 2011).
Using serum 25 (OH) D concentrations as a biomarker for vitamin D intake, severe classic vitamin D deficiency is associated with serum 25 (OH) D concentrations lower than 20 ng/mL (Harinarayan, 2014).
Disorders associated with hypovitaminosis D include increased risks of sepsis [bacterial, mycobacterial and viral], cardiovascular and metabolic disorders [e.g. hyperlipidemia, type 2 diabetes mellitus, acute vascular events, dementia, stroke and heart failure]. Several risks of cancers are associated with vitamin D inadequacy, though causality is accepted only for colo-rectal cancer (Boucher, 2012).
Factors contributing to vitamin D deficiency:
When skin is exposed to ultraviolet (UV) sunlight, it synthesizes vitamin D3, the most readily available form. However, the ability of skin to synthesize vitamin D3 from sunlight exposure is adversely affected by factors that decrease the intensity of the exposure, such as poor air quality, extreme latitudes, and the winter season. In addition, factors that limit the skin’s absorption of sunlight, such as increased pigmentation, the use of sunscreen, and advanced age, can also reduce vitamin D3 synthesis (Holick, 2007).
On the other hand, the amount of vitamin D obtained through diet is often low and is more limited by malabsorption syndromes. Therefore, many people depend on fortified foods and dietary supplements to obtain their vitamin D needs during times of insufficient sunlight (Armas et al., 2004).
Moreover, special populations with increased demands of vitamin D may also suffer a deficiency. The National Health and Nutrition Examination Survey found that 4% of white and 42% of black women of childbearing age living in the United States have serum 25 (OH) D levels less than 10 ng/mL, consistent with a diagnosis of severe vitamin D deficiency. More than 1000 IU per day (25 mcg/day) may be needed during pregnancy and lactation to reach adequate levels of serum 25 (OH) D (Kaludjerovic and Vieth, 2010).
The elderly people are particularly at risk of developing clinical complications related to decreased vitamin D level. Their ability to generate the precursor of vitamin D in the skin is reduced with the advance of age, which is further worse by the change in lifestyle that reduces outdoor physical activities and by immobility. Moreover, the messages and widely spread information of adverse effect of chronic unprotected sunlight exposure, at times, lead to overprotection, either by avoiding sunlight or by using sun protective agents to reduce exposure. Vitamin D supplementation, therefore, is one of the most commonly prescribed medications for the elderly (Lanske and Razzaque, 2007).
Structure of Pancreas:
• Macroscopic pancreatic structure :
Admas and Harrison (1953) described the pancreas in the rat as of a diffuse dendritic type, that extends from the duodenal loop into the gastro-splenic omentum. Also, Kara M. (2005) stated that the pancreas in rats is a large diffuse organ and is divided into three parts; i.e. the biliary, duodenal and gastrosplenic portions. The biliary portion is situated around the biliopancreatic duct mostly between this duct and descending duodenum. The duodenal portion is located between the beginning of the mesojejenum and biliopancreatic duct. These two portions are present in the mesoduodenum. The gastrosplenic portion, the greatest portion, continued along the dorsal aspect of the stomach towards the visceral surface of the spleen and transverse colon.
Treuting and Dintzis (2012) described the difference between the pancreas in mice and in human. They said that the gross anatomical pattern of the pancreas presents as either a “mesenteric†or a “compact†type. The mesenteric pattern describes the pancreas’ relatively diffuse dispersion within the duodenal mesentery, in this pattern, lobules and segments are separated, sometimes widely, by mesenteric fat and stromal elements including lymphoid aggregates in mice. This pattern is present in the rabbit and in the mouse. In the typical compact presentation which occurs in human, monkey, dog and hamster-the organ is denser and more closely confined within the curve of the duodenum. Unlike the mesenteric type, this compact presentation permits distinct regions of the pancreas to be recognized more readily by the gross anatomic terms: head, neck, body, and tail.
• Microscopic pancreatic structure :
William et al. (1985) described the pancreas as an organ containing two different populations of cells, the exocrine cells that secrete enzymes into the digestive tract, and the endocrine cells that secrete hormones into the blood. Dolensek et al. (2015) stated that the exocrine pancreas amounts to 96–99% of TPV (total pancreatic volume). Each lobe of pancreas consists of several smaller lobules. Each pancreatic lobule is composed of structures named acini.
Craigmyle (1986) found that each acinus consisted of a single layer of pyramidal cells having their broad base lying on a thin basement membrane. Villa et al. (1991) revealed that the endoplasmic reticulum of the acinar cells appeared, as a continuous tubular network structure. Electron microscopic examination of human acinar cells by (Kern, 1993) showed that an extensive rough endoplasmic reticulum occupied the basolateral parts and surrounded the nucleus of the acinar cells, whereas, the apical region was closely packed with secretory granules.
Slack (1995) added that the exocrine secretory cells contain several secretory (zymogen) granules, containing the digestive enzymes. They are secreted in inert form and become activated after they enter the duodenum. At the junction between the acini and ducts, low cuboidal cells are located and they are rich in mitochondria, they are thought to secrete non-enzymatic constituents of the pancreatic juice, including bicarbonate.
Furthermore, Bannykh et al. (1998) described the presence of Golgi complex in the acinar cells of rat, it was located mainly in the supra nuclear region and consisted of large flat sacs.
Regarding the zymogen granules, Wheater et al. (1993), described that the newly formed zymogen granules in human acini were large and much less electron dense than the mature granules which were smaller and usually aggregated in the apical cytoplasm. Gaisano et al. (2001) added that the zymogen granules occupied 30 % of acinar cell volume while exocytosis happened only through the apical membrane surface that was limited to less than 10 % of the total membrane area.
Dolensek et al, (2015) stated that the remaining 1–4% of TPV (total pancreatic volume) comprises endocrine micro-organs called islets of Langerhans. They are compact spheroidal clusters surrounded by the exocrine tissue. There are four main types of endocrine cell. The β cells that secrete insulin, and also an insulin antagonist called amylin, and form the majority of cells in the islets, the α cells that secrete glucagon, the δ cells that secrete somatostatin (SS), and the PP cells that secrete pancreatic polypeptide. PP-rich islets are found mainly in the posterior part of the head of the pancreas (Rachedi et al., 1984).
The rat pancreatic tissue contains around 5000 islets scattered throughout the exocrine pancreas (William and Goldfine, 1993). While about one million islets exist in a healthy adult human pancreas, which is distributed in the whole organ; their mass is equal to 1 – 1.5 gram, each of which measures about 0.2 mm in diameter. Each one is separated from the surrounding pancreatic tissue by a thin fibrous connective tissue capsule (Sleisenger et al., 2009).
Most mammalian islets had the same cell organization of beta cell core (center) and non-beta cell mantle (periphery) (Elayat et al., 1995).
They are
• Alpha cells secreting glucagon (15-20% of total Islet cells)
• Beta cells secreting insulin and amylin (65-80%)
• Delta cells secreting somatostatin (3-10%)
• PP cells secreting pancreatic polypeptide (3-5%)
• Epsilon cells secreting ghrelin (≤ 1%)
N.B: Ghrelin is a 28 amino acid hunger-stimulating peptide and a hormone that is produced primarily by P/D1 cells lining the fundus of the human stomach and epsilon cells of the pancreas (Inui et al., 2004).
Regarding the arrangement of the cells in each islet, Githens (1988) found that in rodents there is quite a sharp arrangement within the islets such that the β cells lie in the center and the other types at the periphery, while in humans this arrangement, although present, is less clear cut. The percentage of endocrine cells is a small fraction of the total, about 4% of total cells in the adult rat. Furthermore, Cabrera et al. (2006) demonstrated that the rodent islets are characterized by a major proportion of Insulin-producing beta cells in the core of the cluster and by limited alpha, delta and PP cells in the periphery. Human islets show alpha and beta cells in close relationship with each other throughout the cluster.
β-cells are simply identified by their immunohistochemical reactivity for insulin, proinsulin, C-peptide, and amylin. β-cells contain secretory granules either with a typical crystalloid core or a non-crystalline, finely granular compact core. Crystalline granules contain chiefly insulin while compact granules are considered immature and contain proinsulin (Lloyd, 2010).
Siddle (1992) found that insulin is synthesized as a single chain precursor which first loses its signal peptide, then loses a segment, known as the C-peptide, before becoming the mature hormone molecule. The mature insulin is stored in secretory granules and the level of glucose in the perfusing blood control its release.
Slack (1995) found that in addition to the glandular constituents, the pancreas has a rich blood supply, the arterial blood passing in the lobule first to the islets and then to the nearby acini. There is also extensive lymphatic drainage, and a rich sympathetic and parasympathetic nerve supply. Smooth muscles surround the larger ducts and are present in the sphincter muscles of the two ampullae.
Diabetes Mellitus:
Diabetes mellitus is one of the most common endocrine diseases, characterized by an elevation in plasma glucose. Different types of diabetes with very distinct pathogenesis exist. Over time, diabetes can cause blindness, kidney failure, and nerve damage. Diabetes is also a significant factor in accelerating atherosclerosis, leading to stroke, coronary heart disease, and other large blood vessel diseases (Takiishi et al., 2010).
Type I diabetes is a multifactorial disease resulting from a complex interaction between host genetics, the immune system, and the environment, that ends in the destruction of insulin-producing beta cells. The incidence of type 1 diabetes is increasing at a worrying rate, especially in children under the age of 5.Genetic predisposition, although clearly important, can not explain this rise and so it has been
proposed that changes in the ‘environment’ and/or changes in ‘how we respond to our environment’ must contribute to this increasing incidence (Richardson et al., 2014).
Type 2 diabetes mellitus is a chronic and progressive disease, the underlying pathology of which includes abnormal insulin secretion caused by impaired β-cell function as well as insulin resistance in target tissues. The β-cell function is generally reduced by more than 50% by the time an individual is diagnosed with type 2 diabetes. To maintain adequate long-term glycemic control, preventive intervention against
the progress of pancreatic β-cell dysfunction is most crucial (Hamamoto et al.,2013)
Vitamin D and Diabetes Mellitus:
In vivo studies examining the influence of early onset, long-term vitamin D deficiency in a mouse model support the existence of an inverse relationship between vitamin D levels and the incidence of T1DM (type 1 diabetes mellitus) (Diagram III). 1, 25(OH) 2D3 or non-hypercalcemic analogues were able to inhibit the development of insulitis and prevent the onset of diabetes (Wolden-Kirk et al., 2012).
Country Mean 25 (OH) D levels (nmol/L)
Australia 78.7
Egypt 46.75
Florida 53
Qatar 39.8
Sweden 82.5
Switzerland 45.7
USA (North Eastern) 67
Diagram III: Diagram showing mean 25 (OH) D level in T1D in different countries. (Chakhtoura and Azar, 2013)
Besides, variations in UV exposure and seasonal timing may correlate with the incidence and prevalence of impaired glucose tolerance and T2DM (type 2 diabetes mellitus), suggesting a role for vitamin D. A correlation between vitamin D deficiency and decreased β -cell function, impaired glucose tolerance, T2DM, or mortality from T2DM has been observed across age and ethnic groups. Vitamin D deficiency has been linked to impaired glucose clearance and insulin secretion in rats and rabbits, with improvement after vitamin D repletion independently of dietary intake and calcium homeostasis. A few intervention studies in rodent experimental models of T2DM support the hypothesis that vitamin D treatment could contribute to improved glucose homeostasis and T2DM (Wolden-Kirk et al., 2012).
Furthermore, supplementation with vitamin D was associated with improved pancreatic β-cell function in adults at high risk of T2DM, and there was a trend toward decreasing the rise in glycated hemoglobin (Hb A1c) that occurs over time (Mitri et al, 2011).
1, 25-dihydroxyvitamin D plays an important role in glucose homeostasis by different mechanisms. It not only improves insulin sensitivity of the target cells (liver, skeletal muscle, and adipose tissue) but also enhances and improves β-cell function. In addition, 1, 25-dihydroxyvitamin D protects β-cells from detrimental immune attacks, directly by its action on β-cells, and indirectly by acting on different immune cells, including inflammatory macrophages, dendritic cells, and a variety of T cells. Macrophages, dendritic cells, T- lymphocytes, and B-lymphocytes can synthesize 1, 25 dihydroxyvitamin D, all contributing to the regulation of local immune responses (Sung et al., 2012).
Additionally, there is an evidence that vitamin D may stimulate pancreatic insulin secretion directly. Vitamin D exerts its effects through nuclear vitamin D receptors (VDR). The stimulatory effects of vitamin D on insulin secretion may become only established when calcium levels are adequate. Insulin secretion is a calcium-dependent process, and therefore alterations in calcium flux can have adverse effects on β-cell secretory function (Sung et al, 2012).