A 47-year-old woman with a long history of Crohn's disease (a chronic inflammatory bowel disease) had to be fed for some months using intravenous nutrition. Initially, this treatment included intravenous fat and fat-soluble vitamins. As a result of complications in the administration of the intravenous feed, the fat component was removed and more energy supplied using a carbohydrate source. Prior to her starting intravenous feeding she had been receiving supplements of oral vitamins: these had been discontinued. Three months after the alteration of her intravenous feeding regimen she began to complain of being unable to see appropriately in dim light. Measurement of her serum vitamin A indicated a level well below the reference range. |
| Comment. Intravenous feeding solutions are highly purified and micronutrients must be added. The removal of fat from the prescription precluded the administration of fat-soluble vitamins intravenously. This was not noted at the time and the patient proceeded to develop symptoms of vitamin A deficiency, i.e. night blindness. This could have been avoided by providing a separate infusion of fat emulsion one or two days per week to act as a carrier for the fat-soluble vitamins. |
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| Figure 10.2 Structure, function and metabolism of vitamin D. Note that excess of 25(OH)D3 can mimic 1,25(OH)2D3 but at greater concentrations. PTH, parathyroid hormone. DBP, vitamin D-binding protein. See also Fig. 16.9 and Fig. 23.5. |
| Vitamin D (calciol) is really a hormone; it is only under conditions of inadequate exposure to sunlight that dietary intake is required. Vitamin D is the only vitamin that is not usually required in the diet. It is, in fact, a group of closely related sterols produced by the action of ultraviolet light (wavelength 290-310 nm) on provitamins, ergosterol in plants and 7-dehydrocholesterol in animals (Fig. 10.2). The latter is synthesized in the liver and is found in the skin. The products of the photolytic reaction are ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3), respectively. They are equipotent. Both are converted to a series of hydroxylated
derivatives, firstly at the 25-position in the liver producing 25-hydroxycholecalciferol (25(OH)D3; calcidiol) and at the 1-position in the kidney, producing the active compound 1α-,25-dihydroxycholecalciferol (1,25(OH)2D3; calcitriol)). The details of of vitamin D metabolism and action are described in Chapter 24.
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| Most of the vitamin intake is via milk and other fortified foodstuffs. Fish oils, egg yolks and liver are also rich in vitamin D. Insufficient sunlight and increased metabolism of vitamin D due to low calcium intake or absorption may lead to deficiency. Vitamin D requirements are greater in winter due to lower exposure to sunlight.
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| Deficiency of vitamin D produces rickets in children and osteomalacia in adults
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| Rickets is characterized by the soft pliable bones due to defective mineralization secondary to calcium deficiency. The characteristic bowing of the leg bones and the formation of the rickety rosary around costochondral junctions results. In the adult, demineralization of pre-existing bones takes place, increasing susceptibility to fractures. Vitamin D deficiency is also characterized by low circulating concentrations of calcium and an increased serum alkaline phosphatase activity (see Chapter 24).
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| A 42-year old devout woman of Asian origin presented to her Scottish female GP with knee and tibia pains, which had been present for 3 months. During the GP's initial screen her adjusted calcium (2.2 mmol/l) was at the low end of normal, but her alkaline phosphatase was four times the upper limit of normal, with a raised PTH of 20 pmol/l (normal range 1-6.9 pmol/l). The woman ate her normal cultural diet including curries and chapattis, and was always covered up when outside. |
| Comment. Osteomalacia is still common in certain groups in the UK. Painful tibia, pain in the groins and sore wrists are frequent symptoms. While vitamin D is produced by the action of ultraviolet light, the further away from the equator the shorter this light is available, such that in Scotland only sunlight between the end of April and early September is of the correct wavelength. With little skin exposure when out for religious reasons and chapattis, which bind vitamin D supplemented foods, vitamin D deficiency is common. Particular problems can occur during pregnancy, and both mother and baby should receive vitamin D supplementation. |
| Vitamin D is toxic in excess
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| Vitamin D excess leads to enhanced calcium absorption and bone reabsorption, leading to hypercalcemia and metastatic calcium deposition. There is also a tendency to develop kidney stones from the hypercalciuria secondary to hypercalcemia.
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