High Intake of Vitamin C May Increase Risk of Kidney Stones 28444

High Intake of Vitamin C May Increase Risk of KidneyStones

SPOKANE, Wash.--According to a study publishedin 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 kidneystone formation.

In a randomized, crossover, controlled design in which subjects consumed acontrolled diet in a university metabolic unit, researchers studied whetherascorbic acid (AA) increases risk of kidney stone formation because it convertsendogenously 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). Subjectsconsumed 1,000 mg AA twice daily for six days (treatment A), and no AA for sixdays (treatment N) in random order. After five days of adaptation to alow-oxalate diet, participants spent 24 hours in a metabolic unit where theywere given 136 mg oxalate, including 18 mg of oxalic acid, two hours beforebreakfast; they then consumed a controlled, very low-oxalate diet for 24 hours.Of the 48 participants, 19 (12 stone formers, seven non-stone formers) wereidentified as responders, defined by a 10-percent increase in 24-hour totaloxalate excretion after treatment A, compared with treatment N. Responders had agreater 24-hour Tiselius Risk Index (TRI) with AA supplementation due to a31-percent increase in the percentage of oxalate absorption, and a 39-percentincrease in endogenous oxalate synthesis.

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