Kidney regulates the plasma levels of vitamin C. The feared formation of oxalate stones due to high vitamin C is unfound. In fact vitamin C helps in the dissolution of other types of kidney stones.
Renal resorption threshold and excretion of vitamin C by kidneyThe optimum renal resorption threshold levels of vitamin C in healthy men and women are 1.5 mg/dl and 1.3 mg/dl respectively. The body pool levels of 1500-3000 mg are considered tissue saturation levels of ascorbic acid. Once the plasma levels of vitamin C exceeds this level it is not further resorbed by the kidney and gets excreted in urine.
Further the water soluble properties of vitamin C leads to urinary excretion as ascorbic acid in case of elevated levels. In higher plasma levels the half-life of vitamin C is about 30 minutes. When the concentrations are less than the renal resorption threshold the vitamin C is resorbed actively and the half-life is greatly extended in depleted conditions.
Ascorbic acid has an average half-life of about 15-20 days. The half-life is inversely decreases or increases with its intake. The metabolic products of vitamin C, like 2-ketoascorbitol, 2-O-methyl ascorbate, dehydroascorbate and oxalic acid are excreted by the kidney.
Does vitamin C cause Oxalate stones in kidney?There are various types of kidney stones made up of urate, calcium phosphate, struvite, cystine. Oxalate and urate are the main deposits in renal stones. A section of the medical community believes that ascorbic acid in high doses can cause kidney stones. As one of the by-product of vitamin C metabolism is oxalic acid, it is being concluded that, these ions on combining with calcium, form calcium salt and get deposited in kidney and form stones.
There are conflicting reports of studies conducted on these deposits formation. One study conducted on 45,250 men for 6 years and another on 85,557 women for 14 years, had given the conclusion that daily consumption of more than one gram of ascorbic acid did not result in oxalate deposit formation in the subjects. Another recent study on 45,619 men for 14 years had given a conflicting report that there is 41% higher risk in people consuming more than one gram of vitamin C in developing oxalate stones.
However this study had also concluded that persons consuming less than 250 mg/ day of ascorbic acid were also under high risk. The acidic vitamin C in urine may bind to calcium and reduce its availability for getting bound to oxalate. The possibility of deficiency of vitamin B6 and magnesium rather than the excess of ascorbic acid for the formation of calculi must be probed.
Though we may not fear for the risk of oxalate stones on high dose ascorbic acid consumption, it is prudent to evaluate the status of the individual prone to oxalate deposits and its recurrence, before embarking on high-dose vitamin C consumption. A person prone to calculi may also avoid excessive use of oxalate rich spinach, chocolate and rhubarb.
Vitamin C in the role of removing other types of kidney stonesVitamin C has a diuretic action when consumed in high doses and this helps in flushing out the possible deposits, including calcium oxalate. Many calcium oxalate and other deposits usually form around a nucleus of infection. Ascorbic acid due to its pH lowering action, appears to retard, kill and flush out the causative agents like bacteria and nanobacteria.
Calcium phosphate stones, unlike calcium oxalate stones are alkaline in nature and get dissolved by the ascorbic acid in the urine. Struvite kidney stones ( formed by magnesium ammonium phosphate) also differ in nature and are easily soluble in urine due to high ascorbic acid.
Vitamin C depletion due to maintenance hemodialysis in kidney impaired patientsIn maintenance hemodialysis, the water soluble ascorbic acid is depleted. The food intake of ascorbic acid must be monitored, so as avoid development of deficiency. If necessary supplements in regulated doses must be given.
Doses in excess of 100-200 mg must be avoided in chronic renal disease as oxalates can accumulate in the body causing oxalosis. This can even cause acute renal failure. Oxalosis due to excess vitamin C in kidney transplant recipients can affect renal transplants and cause graft failure.
Vitamin C and oxalate stones in kidney