High Intake of Vitamin C May Increase Risk of Kidney Stones

SPOKANE, Wash.--According to a study published in the July issue of the Journal of Nutrition (135,7:1677-1677), consuming more than 2,000 mg/d of vitamin C may increase the risk of kidney stone formation.

In a randomized, crossover, controlled design in which subjects consumed a controlled diet in a university metabolic unit, researchers studied whether ascorbic acid (AA) increases risk of kidney stone formation because it converts endogenously to oxalate and appears to increase absorption of dietary oxalate. Subjects were divided into two groups, kidney stone formers (n = 29; SF) and age- and gender-matched non-stone formers (n = 19; NSF). Subjects consumed 1,000 mg AA twice daily for six days (treatment A), and no AA for six days (treatment N) in random order. After five days of adaptation to a low-oxalate diet, participants spent 24 hours in a metabolic unit where they were given 136 mg oxalate, including 18 mg of oxalic acid, two hours before breakfast; they then consumed a controlled, very low-oxalate diet for 24 hours. Of the 48 participants, 19 (12 stone formers, seven non-stone formers) were identified as responders, defined by a 10-percent increase in 24-hour total oxalate excretion after treatment A, compared with treatment N. Responders had a greater 24-hour Tiselius Risk Index (TRI) with AA supplementation due to a 31-percent increase in the percentage of oxalate absorption, and a 39-percent increase in endogenous oxalate synthesis.

Researchers concluded supplementation of 1,000 mg AA twice daily increased urinary oxalate and TRI for calcium oxalate kidney stones in 40 percent of participants (both stone formers and non-stone formers), increasing risk of kidney stone formation.