Q.1- Discuss the steps of activation of vitamin D, and explain the reason for the fact that Vitamin D is toxic in excess, but excessive exposure to sunlight does not lead to vitamin D toxicity.
Answer- Vitamin D3 (cholecalciferol) can be synthesized by humans in the skin upon exposure to ultraviolet-B (UVB) radiation from sunlight, or it can be obtained from the diet. When exposure to UVB radiation is insufficient for the synthesis of adequate amounts of vitamin D3 in the skin, adequate intake of vitamin D from the diet is essential for health. Plants synthesize ergosterol, which is converted to vitamin D2 (Ergocalciferol) by ultraviolet light.
In response to ultraviolet radiation of the skin, a photochemical cleavage results in the formation of vitamin D from 7-dehydrocholesterol. Cutaneous production of vitamin D is decreased by melanin and high solar protection factor sunblocks, which effectively impair skin penetration of ultraviolet light. The increased use of sunblocks in North America and Western Europe and a reduction in the magnitude of solar exposure of the general population over the past several decades has led to an increased reliance on dietary sources of vitamin D. In the United States and Canada, these sources largely consist of fortified cereals and dairy products, in addition to fish oils and egg yolks. Vitamin D from plant sources is in the form of vitamin D2, whereas that from animal sources is vitamin D3. These two forms have equivalent biologic activity and are activated equally well by the vitamin D hydroxylases in humans.
Activation of Vitamin D
Vitamin D itself is biologically inactive, and it must be metabolized to its biologically active forms. After it is consumed in the diet or synthesized in the epidermis of skin, vitamin D enters the circulation and is transported to the liver. In the liver, vitamin D is hydroxylated to form 25-hydroxyvitamin D (calcidiol; 25-hydroxyvitamin D, the major circulating form of vitamin D. The serum 25-hydroxyvitamin D concentration a useful indicator of vitamin D nutritional status. In the kidney, the 25-hydroxyvitamin D3-1-hydroxylase enzyme catalyzes a second hydroxylation of 25-hydroxyvitamin D, resulting in the formation of 1,25-dihydroxyvitamin D (calcitriol, 1alpha,25-dihydroxyvitamin D]—the most potent form of vitamin D. Most of the physiological effects of vitamin D in the body are related to the activity of 1,25-dihydroxyvitamin D (Figure-1)
Figure-1- showing the steps of activation of vitamin D in the body.
Sunlight exposure can provide most people with their entire vitamin D requirement. Serum vitamin D concentrations following exposure to 1 minimal erythemal dose of simulated sunlight (the amount required to cause a slight pinkness of the skin) is equivalent to ingesting approximately 20,000 IU of vitamin D2
Although excess dietary vitamin D is toxic, excessive exposure to sunlight does not lead to vitamin D toxicity, because there is a limited capacity to form the precursor, 7 dehydrocholesterol, and prolonged exposure of previtamin D to sunlight leads to formation of inactive compounds. Vitamin D toxicity (hypervitaminosis D) induces abnormally high serum calcium levels (hypercalcemia), which could result in bone loss, kidney stones, and calcification of organs like the heart and kidneys if untreated over a long period of time.
Q.2- Why is vitamin D considered a hormone? Describe the mechanism of action of vitamin D.
Answer- Vitamin D is actually a hormone since -
1) Structurally it has a cyclopentano perhydrophenanthrene nucleus, like a steroid hormone.
2) Its mechanism of action resembles that of hormones (Figure-2).
Figure-2- Showing the mechanism of action of a steroid hormone. Vitamin D has a similar action. Click the link below to see the animations of mechanism of action of a steroid hormone
3) Like hormones it is required only in small amount.
4) Like hormones, the formation of the active form is subjected to feed back inhibition.
5) Like hormones, vitamin D has specific target organs like intestine, bone and kidneys.
6) Like hormones, it is produced in one organ and acts upon distant organs for its functions.
Mechanisms of Action
Most if not all actions of vitamin D are mediated through a nuclear transcription factor known as the vitamin D receptor (VDR). Upon entering the nucleus of a cell, 1,25-dihydroxyvitamin D associates with the VDR and promotes its association with the retinoic acid X receptor (RXR). In the presence of 1,25-dihydroxyvitamin D the VDR/RXR complex binds small sequences of DNA known as vitamin D response elements (VDREs) and initiates a cascade of molecular interactions that modulate the transcription of specific genes (Figure-3). More than 50 genes in tissues throughout the body are known to be regulated by 1,25-dihydroxyvitamin D.
Figure-3- showing the mechanism of action of vitamin D
Q.3- Justify the statement that “Vitamin D metabolism is both regulated by and regulates calcium homeostasis”. What are the other functions performed by vitamin D?
Answer- The main function of vitamin D is to maintain calcium homeostasis, and in turn, vitamin D metabolism is regulated by factors that respond to plasma concentrations of calcium and phosphate.
Maintenance of serum calcium levels within a narrow range is vital for normal functioning of the nervous system, as well as for bone growth and maintenance of bone density.
Flow chart- showing role of vitamin D in the absorption of calcium from gut.
The parathyroid glands sense the serum calcium level, and secrete parathyroid hormone (PTH) if it becomes too low, for example, when dietary calcium intake is inadequate. PTH stimulates the activity of the 1-hydroxylase enzyme in the kidney, resulting in increased production of calcitriol, the biologically active form of vitamin D3. Increased calcitriol production restores normal serum calcium levels in three different ways:
1) by activating the vitamin D-dependent transport system in the small intestine, increasing the absorption of dietary calcium(Flow chart),
2) by increasing the mobilization of calcium from bone into the circulation; and
3) by increasing the reabsorption of calcium by the kidneys (Figure-4).
PTH is also required to increase calcium mobilization from bone and calcium reabsorption by the kidneys.
When there is adequate calcium concentration, Calcitriol acts to reduce its own synthesis by inducing the 24-hydroxylase and repressing the 1-hydroxylase in the kidney.
Other functions of vitamin D
In addition, calcitriol is involved in insulin secretion, synthesis and secretion of parathyroid and thyroid hormones, inhibition of production of interleukin by activated T-lymphocytes and of immunoglobulin by activated B-lymphocytes, differentiation of monocyte precursor cells, and modulation of cell proliferation. In most of these actions, it acts like a steroid hormone, binding to nuclear receptors and enhancing gene expression, although it also has rapid effects on calcium transporters in the intestinal mucosa.
Figure-4- showing the role of vitamin D in maintaining calcium homeostasis.
Q.4 - Discuss the causes, clinical manifestations, laboratory diagnosis and treatment of vitamin D deficiency in children.
Rickets is a disease of growing bone that is unique to children and adolescents. It is caused by a failure of osteoid to calcify in a growing person. Failure of osteoid to calcify in adults is called osteomalacia. Vitamin D deficiency rickets occurs when the metabolites of vitamin D are deficient. Less commonly, a dietary deficiency of calcium or phosphorus may also produce rickets.
In the vitamin D deficiency state, hypocalcemia develops, that stimulates excess parathyroid hormone, which acts to stimulate renal phosphorus loss, further reducing deposition of calcium in the bone. Excess parathyroid hormone also produces changes in the bone similar to those occurring in hyperparathyroidism. Early in the course of rickets, the calcium concentration in the serum decreases. After the parathyroid response, the calcium concentration usually returns to the reference range, though phosphorus levels remain low. Alkaline phosphatase, which is produced by overactive osteoblast cells, leaks to the extracellular fluids so that its concentration rises to anywhere from moderate elevation to very high levels.
Vitamin D deficiency rickets does not occur in formula-fed infants because formula and milk sold in the United States contains 400 IU of vitamin D per liter. Except in pediatric patients with chronic malabsorption syndromes or end-stage renal disease, nearly all cases of rickets occur in breastfed infants who have dark skin and receive no vitamin D supplementation.
Causes of vitamin D deficiency
The clinical syndrome of vitamin D deficiency can be a result of-
a) deficient production of vitamin D in the skin,
b) lack of dietary intake,
c) accelerated losses of vitamin D,
d) impaired vitamin D activation or
e) resistance to the biologic effects of 1,25(OH)2D.
The elderly people are particularly at risk for vitamin D deficiency, since both the efficiency of vitamin D synthesis in the skin and the absorption of vitamin D from the intestine decline with age.
Similarly, intestinal malabsorption of dietary fats leads to vitamin D deficiency. This is further exacerbated in the presence of terminal ileal disease, which results in impaired enterohepatic circulation of vitamin D metabolites. In addition to intestinal diseases, accelerated inactivation of vitamin D metabolites can be seen with drugs that induce hepatic cytochrome P450 mixed function oxidases, such as barbiturates, phenytoin, and rifampin.
Impaired 25-hydroxylation, associated with severe liver disease or isoniazid, is an infrequent cause of vitamin D deficiency. Impaired 1-hydroxylation is prevalent in the population with profound renal dysfunction and a decrease in functional renal mass. Thus, therapeutic interventions should be considered in patients whose creatinine clearance is <0.5 mL/s (30 mL/min).
Mutations in the renal -1-hydroxylase are the basis for the genetic disorder,pseudo-vitamin D–deficiency rickets. This autosomal recessive disorder presents with the syndrome of vitamin D deficiency in the first year of life. Patients present with growth retardation, rickets, and hypocalcemic seizures. Serum 1,25(OH)2D levels are low, despite normal 25(OH)D levels and elevated PTH levels. Treatment with vitamin D metabolites that do not require 1-hydroxylation results in disease remission, although lifelong therapy is required.
A second autosomal recessive disorder, hereditary vitamin D–resistant rickets, a consequence of vitamin D receptor mutations, is a greater therapeutic challenge. These patients present in a similar fashion during the first year of life, but alopecia often accompanies the disorder, demonstrating a functional role of the VDR ( Vitamin D receptor) in postnatal hair regeneration. Serum levels of 1,25(OH)2D are dramatically elevated in these individuals, both because of increased production due to stimulation of 1-hydroxylase activity as a consequence of secondary hyperparathyroidism and because of impaired inactivation, since induction of the 24-hydroxylase by 1,25(OH)2D requires an intact VDR. Since the receptor mutation results in hormone resistance, daily calcium and phosphorus infusions may be required to bypass the defect in intestinal mineral ion absorption.
Occasionally, deficiency severe enough to cause maternal Osteomalacia results in rickets with metaphyseal lesions in neonates.
Summary of Causes of Impaired Vitamin D action
Figure-5 -showing bending of bones( bow legs).
In the chest, knobby deformities results in the rachitic rosary along the costochondral junctions (Figure-6)
Figure-6- showing rachitic rosary along the costochondral junctions.
The weakened ribs pulled by muscles also produce flaring over the diaphragm, which is known as Harrison groove.
Figure-7-showing Harrison groove and pot belly
The sternum may be pulled into a pigeon-breast deformity (Figure-7).
Figure-8- showing chest deformity in Rickets
In more severe instances in children older than 2 years, vertebral softening leads to Kyphoscoliosis (Figure-8)
The ends of the long bones demonstrate that same knobby thickening.
At the ankle, palpation of the tibial malleolus gives the impression of a double epiphysis (Marfan sign).
Because the softened long bones may bend, they may fracture one side of the cortex (ie, greenstick fracture).
Figure-10-Showing the clinical findings in Rickets
Regardless of the cause, the clinical manifestations of vitamin D deficiency are largely a consequence of impaired intestinal calcium absorption. Mild to moderate vitamin D deficiency is asymptomatic, whereas long-standing vitamin D deficiency results in hypocalcemia accompanied by secondary hyperparathyroidism, impaired mineralization of the skeleton (osteopenia on x-ray or decreased bone mineral density), and proximal myopathy. In the absence of an intercurrent illness, the hypocalcemia associated with long-standing vitamin D deficiency rarely presents with acute symptoms of hypocalcemia, such as numbness, tingling, or seizures. However, the concurrent development of hypomagnesemia, which impairs parathyroid function, or the administration of potent bisphosphonates, which impair bone resorption, can lead to acute symptomatic hypocalcemia in vitamin D–deficient individuals.
Bone changes, seen on x‑rays, precede clinical signs. In rickets, changes are most evident at the lower ends of the radius and ulna. The diaphyseal ends lose their sharp, clear outline; they are cup-shaped and show a spotty or fringy rarefaction. Later, because the ends of the radius and ulna have become noncalcified and radiolucent, the distance between them and the metacarpal bones appears increased. The bone matrix elsewhere also becomes more radiolucent. Characteristic deformities result from the bones bending at the cartilage-shaft junction because the shaft is weak. As healing begins, a thin white line of calcification appears at the epiphysis, becoming denser and thicker as calcification proceeds. Later, the bone matrix becomes calcified and opacified at the subperiosteal level.
Ca deficiency (which is common) and P deficiency should be corrected. As long as Ca and P intake is adequate, adults with Osteomalacia and children with uncomplicated rickets can be cured by giving vitamin D 40 μg (1600 IU) po once/day. Serum 25(OH)D and 1,25(OH)2D begin to increase within 1 or 2 days. Serum Ca and phosphate increase and serum alkaline phosphatase decreases within about 10 days. During the 3rd wk, enough Ca and P are deposited in bones to be visible on x‑rays. After about 1 mo, the dose can usually be reduced gradually to the usual maintenance level of 10 to 15 μg (400 to 600 IU) once/day. If tetany is present, vitamin D should be supplemented with IV Ca salts for up to 1 wk.
Vitamin D Toxicity Usually, vitamin D toxicity results from taking excessive amounts. Marked hypercalcemia commonly causes symptoms. Diagnosis is typically based on elevated blood levels of 25(OH)D. Treatment consists of stopping vitamin D, restricting dietary Ca, restoring intravascular volume deficits, and, if toxicity is severe, giving corticosteroids or for children with increased skin pigmentation.
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