Editing Stones: Epidemiology and Pathogenesis (section)

== Mineral metabolism ==

=== Calcium ===
* '''<span style="color:#ff0000">30-40% of dietary calcium is absorbed from the intestine, with most being absorbed in the small intestine and only ≈10% absorbed in the colon</span>'''
* '''Absorption of calcium varies with calcium intake by a process of intestinal adaptation''' - at times of low calcium intake, fractional calcium absorption is enhanced; during high calcium intake, fractional calcium absorption is reduced
* '''<span style="color:#ff0000">Substances that complex with calcium, such as oxalate, citrate, phosphate, sulfate, and fatty acids, reduce the availability of calcium for absorption</span>'''
* '''<span style="color:#ff0000">Calcium homeostasis</span>'''
** '''<span style="color:#ff0000">PTH</span>'''
*** '''<span style="color:#ff0000">Secreted in response to low serum calcium</span>'''
*** '''Functions (3):'''
***# '''Increases renal reabsorption of calcium and reduces reabsorption of phosphate from distal tubule (primary effect)'''
***# '''Stimulates the enzyme 1α-hydroxylase in the proximal renal tubule which converts 25-hydroxyvitamin D3 (calcifediol/calcidiol) to 1,25(OH)2D3 (calcitriol)'''
***#* This enzyme is also stimulated by hypophosphatemia
***#* The first step to synthesis of calcitriol involves the conversion of 7-dehydrocholesterol in the skin to previtamin D3 and is promoted by sunlight. Previtamin D3 is hydroxylated in the liver, which is further hydroxylated in the proximal renal tubule to 1,25(OH)2D3.
***# '''Increases calcium release from bone'''
*** A population-based study found that serum PTH levels were positively correlated with serum uric acid levels§
** '''Calcitriol (1,25(OH)2D3)'''
*** '''Active form of Vitamin D'''
*** '''Functions (4):'''
***# '''Most potent stimulator of intestinal calcium absorption'''
***#* '''Note PTH does not target intestine'''
***# '''Increases renal reabsorption of calcium and phosphate'''
***#* '''Note PTH increases phosphate excretion'''
***#* Similar to PTH, vitamin D induces calcium reabsorption in the distal tubule
***# '''Increases calcium release from bone'''
***# '''Inhibits release of PTH'''

=== Oxalate ===
* '''Only 6-14% of ingested oxalate is absorbed (compared to 30-40% of ingested calcium is absorbed)'''
** '''Oxalate absorption occurs throughout the intestinal tract, with about half or more occurring in the small intestine and half in the colon'''
** '''Factors that influence oxalate absorption (2):'''
**# '''Oxalate-degrading bacteria'''
**#* '''Oxalobacter formigenes use oxalate as an energy source and consequently reduce intestinal oxalate absorption'''
**# '''Presence of oxalate-binding cations such as calcium or magnesium'''
**#* '''Co-ingestion of calcium, magnesium, and oxalate-containing foods leads to formation of calcium oxalate complexes, which limits the availability of free oxalate ion for absorption'''
* '''Absorbed oxalate is nearly completely excreted in the urine'''
** '''Urinary oxalate is derived from both endogenous production in the liver''' (from ascorbic acid and glycine) '''and dietary sources.'''
*** '''On average, half of urinary oxalate is derived from the diet, with the precise amount depending on the relative amount of ingested calcium and oxalate'''
** It is estimated that between 86-98% of oxalate is ultrafilterable. However, '''renal tubular handling of oxalate has not been clearly defined, although both secretion and reabsorption have been suspected.'''