*Effect of Ascorbic Acid on the Kidneys DOI: http://dx.doi.org/10.5772/intechopen.111913*

this and other studies with similar findings have concluded that dietary vitamin C intake should not be restricted [21].

Another large study looked at the renal effects of vitamin C consumption in the diet and as a dietary supplement. This study looked at how men and women formed stones differently. For nearly 12 years, the effect of vitamin C consumption in diet and tablet form on kidney stone formation has been studied by approximately 157,000 healthcare professionals of various ages. This study was evaluated alongside Taylor et al.'s 14-year study on men. The findings of this study, in which dietary variations were recorded on a regular basis, suggest that consuming vitamin C, either in diet or as a supplement, does not cause kidney stones in women. Supplemental vitamin C consumption, on the other hand, is thought to be a significant cause of stone formation in men. The reason for the difference between men and women has not yet been clearly elucidated [12, 21, 28].

For 11 years, the last large cohort study in men looked at the incidence of kidney stone formation with vitamin C under dietary control. Participants in this study reported taking ascorbic acid as a food supplement. In this study of approximately 50,000 people, 436 people were found to have kidney stones for the first time. Stone samples taken from 3176 people were found to be calcium oxalate stones among the reported stone cases. This corresponds to 90% of all cases [29].

A comparison was made between healthy subjects and patients with the stone formation in a study that investigated the effect of daily consumption of 2 g ascorbic acid on urine pH and urinary oxalate formation. Each group was randomly divided into two groups for ascorbic acid and placebo administration. Urine pH was measured in both the placebo and ascorbic acid groups of participants in the study. No significant difference was found in terms of urine pH. However, a significant increase in urinary oxalate was observed in both healthy subjects who consumed ascorbic acid and individuals who developed calcium oxalate stones. Similarly, the urinary calcium oxalate saturation ratio was found to be significantly higher in both ascorbic acid and placebo groups. This diet-controlled study with similar age, gender, race, and body mass index distribution suggests that ascorbic acid consumption promotes the formation of calcium oxalate stones. In this study, urine samples were placed in acid medium to prevent the conversion of ascorbic acid to oxalate in vitro. When compared to previous studies, it has higher reliability for urinary oxalate determination [23]. Baxmann et al. reported in a similar study that vitamin C consumption increased urinary oxalate at a higher rate in patients with a history of kidney stones than in healthy subjects [30].

The half-life of vitamin C after parenteral administration is approximately one hour. After this time, urinary elimination begins [31]. There have been studies that show the benefits of high doses of vitamin C given intravenously, particularly in certain types of cancer, major burns, and sepsis [32–34]. However, according to a case report, a 74-year-old patient with endometrial cancer who was given 100 g of intravenous vitamin C weekly without medical supervision presented to the hospital 1.5 months later with acute renal failure. Serum creatine levels were found to be extremely elevated, and a renal biopsy revealed the presence of calcium oxalate crystals. It has been reported that the patient developed nephrotoxicity to the point where permanent renal replacement was required. This case shows that the use of vitamin C above therapeutic doses may have nephrotoxic effects on the kidney as well as stone formation [35].

Despite the fact that some of the studies involve large cohorts, there is no agreement on the effect of ascorbic acid on calcium oxalate stone formation due to the

short duration of the majority of the studies, the lack of dietary control, the small number of individuals studied, and, most importantly, the lack of reliability in the sample measurement methods. However, the number of studies that do not support stone formation is considerably less than the number of studies that support it. The following are the main findings of these studies:

The theory that oxalate forms as a result of ascorbic acid metabolism and causes calcium oxalate stone formation is rejected by a center that regularly administers parenteral vitamin C to its patients. If this theory is correct, researchers believe that epidemic kidney stone formation should occur as herbivores can produce their own vitamin C and frequently have alkaline urine, which is one of the most common causes of kidney stones. The lack of such a situation, as well as the low rate of kidney stones reported despite parenteral vitamin C administration to their patients for over 20 years, is the basis for their rejection of this theory [12].

In one study, men and women were given equal amounts of parenteral vitamin C for 12 months. A total of 8% of the subjects had a history of kidney stones. Despite the fact that patients were given parenteral vitamin C in doses ranging from 1 g to 119 g, no results were obtained that provided clear evidence for kidney stone formation. There was no statistically significant change in glomerular filtration rate and serum creatine levels in these individuals [36].

The effect of ascorbic acid on kidney stone formation is still being debated today. The effect of urinary oxalate content on calcium oxalate stone formation is generally accepted. However, long-term maintenance of urinary oxalate concentrations is required for stone formation [2]. As a result, considering the duration and dose amount used therapeutically, we believe that the effect of vitamin C on stone formation may be limited. In individuals with normal renal function, parenteral administration and oral consumption at doses not exceeding 1000 mg daily are not objectionable due to its beneficial effects. However, in individuals with a history of kidney stones, the dose of vitamin C taken as a supplement should be adjusted. There is no need to restrict dietary vitamin C consumption in these people [14].
