Metabolism of vitamin D, Calcium and Phosphorus and their disorders in goats
Authors: Sakhniuk V. , Hotsuliak M. 
Published: February 2024

The review article presents materials on the metabolism of the mostcommon vitamins of group D ergocalciferol (D2), cholecalciferol (D3), aswell as essential macronutrients Ca and Phosphorus and their disorders ingoats. Since the primary forms of vitamin D (D2 and D3) are biologicallyinactive and must undergo several stages of hydroxylation to be activated,the biological role and importance for the body of active metabolites ofvitamin D3 25OH D3 (synthesised mainly in the liver under the influenceof hepatic cytochromes P450) and 1, 25(OH)2 D3 and 24,25(OH)2 D3 (theirsynthesis occurs via 1α-hydroxylase in the mitochondria of proximal cellsof the convoluted tubules of the kidneys).It is believed that the liver, while playing an important role in themetabolism of vitamin D and its metabolites and producing 25OH D3, isalso the only organ that synthesises DBP, which transports 25OH D3 totissues and maintains its concentration in the circulatory system.Vitamin D becomes biologically active only after the second stageof hydroxylation is completed. Renal 1α-hydroxylase (CYP27B1),regulated by parathyroid hormone (PTH), plays an important role inthe transformation of the extracellular substrate 25OH D3 to 1,25(OH)2D3, which exerts its effect on target cells and tissues by binding to thenuclear vitamin D receptor. Alternatively, 1,25(OH)2 D3 can bind to theplasma membrane VDR and induce non-genomic actions, in particular,stimulation of intestinal calcium transport.Vitamin D is a steroid substance that is essential for all vertebratesto maintain calcium and phosphorus metabolism within optimal limits, ahealthy skeleton, muscle contraction, modulation of cell growth and neuromuscular function. Calciferol also regulates the immune system, inhibits the development of pathological cells, angiogenesis and inflammatoryreactions. The active form of vitamin D, 1,25(OH)2 D3, stimulates intestinal absorption and renal Ca reabsorption and maintains its minimumphysiological level in the blood.Vitamin D deficiency in goats leads to a decrease in productivity, causesa decrease in intestinal and renal calcium reabsorption, which leads to theincrease in parathyroid hormone levels. This process leads to activation ofosteocytes and, as a result, accelerates bone demineralisation, causing thedevelopment of many diseases in adults, including nutritional and fibrousosteodystrophy, secondary osteodystrophy, endocrine dysfunction), as wellas rickets in young animals. The development of non-skeletal pathologies,in particular, inflammatory, neoplastic and autoimmune diseases, is also associated with cholecalciferol deficiency in the body. In addition, disordersof D-vitamin and calcium-phosphorus metabolism in goats cause the development of postpartum hypocalcaemia and postpartum hypophosphatemia.In the animal body, calcium and phosphorus homeostasis is maintained by a coordinated interaction of absorption and reabsorptionthrough the gastrointestinal tract and kidneys, as well as by storage and mobilization from bone tissue and is regulated mainly by biologically active cholecalciferol metabolites - 25OH D3, 1,25(OH)2 D3, as wellas parathyroid hormone (PTH; synthesised by the pineal glands) andcalcitonin (CT; produced by sparafollicular (light) thyroid C cells) andfibroblast growth factor-23 (FGF23).In contrast to monogastric animals, small ruminants do not modulaterenal calcium excretion in response to calcium limitation in the diet. Themobilization of Ca and P from the skeleton is stimulated by PTH throughosteoclast activation mediated by receptor activator of nuclear factor-κB(RANK). Vitamin D maintains Ca (by stimulating CaZB) and P homeostasis (the direct rapid action of 1,25(OH)2 D3 has been proven to have a directeffect on the absorption of these vital elements in the intestine, reabsorptionof these cations in the renal tubules and their mobilization from bone tissue).Phosphorus is a component of adenosine triphosphate (ATP) and nucleotides. Macroergic phosphate compounds, among which the main oneis adenosine triphosphate acid, provide both the accumulation of energyreserves and its consumption (ATP, ADP, creatine phosphate), affectingprotein, lipid, carbohydrate, mineral, and energy metabolism. An interaction between vitamin D and fibroblast growth factor 23 (FGF23), a bonehormone that causes the development of phosphaturia and reduces thesynthesis of 1,25(OH)2 D3, has been identified.Despite the multidirectionality of etiological factors, common to allforms of osteodystrophy is a disruption of the processes of bone formation and renewal, which is manifested by increased mobilization of calcium, phosphorus and other elements from bone tissue, so the pathologyis accompanied by osteomalacia, osteoporosis and osteofibrosis, and aviolation of the mechanism of maintaining their homeostasis.The main factors of osteodystrophy in animals are feeding disordersand physical inactivity, and the leading links in its pathogenesis are theimbalance between bone formation and resorption.The main methods for diagnosing disorders of D-vitamin and calcium-phosphorus metabolism in goats are clinical, physical, biochemical,enzyme-linked immunosorbent assays and pathological and morphological studies. Biochemical analysis in goat serum determines the content oftotal calcium, inorganic phosphorus, activity of alkaline phosphatase andits isozymes, and immunoassay the concentration of 25OH D3, 1,25(OH)2D3, calcitonin and parathyroid hormone.
Keywords: goats, vitamin D, metabolites, metabolism, calcium,phosphorus, liver, kidneys.