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Ascorbate increases human oxaluria and kidney stone risk
Linda K Massey1, Michael Liebman, Susan A Kynast-Gales
1Department of Food Science and Human Nutrition, Washington State University, Spokane, WA, USA. massey@wsu.edu
The Journal of Nutrition
|July 1, 2005
Summary
High-dose ascorbic acid (vitamin C) supplementation may increase kidney stone risk. This study found that 2000 mg daily increased urinary oxalate and risk factors for calcium oxalate stones in 40% of participants.
Area of Science:
- Nutrition Science
- Nephrology
- Biochemistry
Background:
- The recommended upper limit for ascorbic acid (AA) intake is 2000 mg/d.
- Ascorbic acid is converted to oxalate and may increase oxalate absorption, potentially raising kidney stone risk.
Purpose of the Study:
- To investigate the effect of high-dose ascorbic acid supplementation on urinary oxalate levels and kidney stone risk factors.
- To identify individuals who are more susceptible to increased oxalate excretion from AA supplementation.
Main Methods:
- A randomized, crossover, controlled study involving 48 participants (29 stoneformers, 19 non-stoneformers).
- Participants received either 1000 mg AA twice daily or no AA for 6 days, in random order, while on a controlled low-oxalate diet.
- Urinary oxalate excretion and the Tiselius Risk Index (TRI) were measured.
Main Results:
- 1000 mg AA twice daily increased 24-h urinary oxalate and TRI in 40% of participants (responders).
- Responders showed a 31% increase in oxalate absorption and a 39% increase in endogenous oxalate synthesis.
- Both stoneformers and non-stoneformers could be responders.
Conclusions:
- High-dose ascorbic acid supplementation (2000 mg/d) can increase urinary oxalate and calcium oxalate kidney stone risk in a subset of individuals.
- Individual responses to AA supplementation vary, with some exhibiting increased oxalate production and absorption.
- Further research is needed to understand the mechanisms and identify predictors of response.