Topic: Vitamins and minerals.
Question: Briefly, write about the role played by vitamin D in the absorption of calcium. Also mention the effect of vitamin D on born renal tubules.
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Vitamin D plays a pivotal role in calcium homeostasis and bone health. Its major actions include:
Intestinal calcium absorption: Vitamin D stimulates active transcellular transport of calcium across enterocytes into the circulation. It upregulates epithelial calcium channels and binding proteins (e.g. calbindin 9K) to facilitate passage of calcium ions into blood. Inadequate vitamin D results in reduced fractional calcium absorption (typically 10-15% vs. 30-40% with sufficiency) and increased fecal calcium loss.
Bone mineralization: Vitamin D, in conjunction with parathyroid hormone (PTH), helps maintain serum ionized calcium in a physiologic range (1.13-1.32 mmol/L) through a balance of bone formation and resorption. When calcium levels fall, vitamin D promotes bone resorption to increase serum concentrations, while also stimulating osteoblasts to lay down new bone matrix when levels rise. This is essential for bone density preservation and disorders like rickets or osteomalacia are associated with deficiency.
Renal calcium reabsorption: In the kidneys, vitamin D upregulates epithelial calcium channels (e.g. TRPV5) and binding proteins in the thick ascending limb of Henle’s loop (TAL) and distal convoluted tubule (DCT) to enhance active transcellular reabsorption of filtered calcium from the urinary space back into circulation. Approximately 98% of filtered calcium is reabsorbed, preventing hypercalciuria and excessive urinary loss. Deficiency impairs reabsorption.
Other effects: Vitamin D may also enhance intestinal and renal reabsorption of inorganic phosphate, protect podocytes from damage, and suppress PTH release from chief cells in parathyroid glands when serum ionized calcium levels are normal or elevated. It has possible roles in muscle function, immunity and neuronal health which require further elucidation.
The effects of vitamin D on the kidneys include:
– Stimulating the active reabsorption of calcium from the filtrate in renal tubules. About 98% of filtered calcium is reabsorbed, mostly in the proximal tubule and thick ascending limb of Henle’s loop. Vitamin D deficiency impairs calcium reabsorption, causing excess urinary calcium loss.
– Promoting reabsorption of inorganic phosphate. Phosphate moves passively with calcium, so vitamin D is also important for phosphate homeostasis.
– May enhance potassium and magnesium reabsorption. While less well-established, vitamin D may play some role in maintaining balance of these other electrolytes.
– Reducing parathyroid hormone release. By facilitating intestinal calcium absorption and bone/renal calcium mobilization, vitamin D helps normalize serum calcium levels which inhibits PTH secretion from the parathyroid glands.
– Protecting podocytes in glomeruli. Vitamin D deficiency may accelerate age-related podocyte damage, contributing to glomerulosclerosis and reduced glomerular filtration rate later in life. Supplementation helps maintain podocyte health and renal function.
The endocrine functions of vitamin D are primarily mediated through binding of its active form, calcitriol, to the nuclear vitamin D receptor (VDR) which regulates the transcription of target genes involved in calcium transport and homeostasis. Adequate levels of vitamin D are essential for health, growth and development across the lifespan. Both deficiency as well as excess can have detrimental effects on skeletal integrity, nephrolithiasis risk, and potentially other long term outcomes.
Maintaining appropriate vitamin D status through diet, supplementation and monitoring is imperative for optimal bone and renal health.In summary, vitamin D enhances intestinal calcium absorption and bone calcium resorption while promoting renal calcium reabsorption to maintain serum calcium levels in a tight physiologic range. Its actions in the gastrointestinal tract, bone and kidneys are interdependent and essential for health. Deficiency or excess can have important pathological consequences.
