Max Hauser said:
"Mark Thorson" in news:[email hidden]:
| ...
| Megadoses of C raise the risk of kidney stones.
| I've already had one stone, which is enough.That's a good point (for instance, urinary oxalate is a major metabolic
byproduct of the vitamin C not excreted directly, according to "Vitamin C"
chapter in Goodman and Gilman, the standard medical pharmo text).
J Nutr. 2005 Jul;135(7):1673-7.
Ascorbate increases human oxaluria and kidney stone risk.
Massey LK, Liebman M, Kynast-Gales SA.
Department of Food Science and Human Nutrition,
Washington State University, Spokane,WA.
Currently, the recommended upper limit for
ascorbic acid (AA) intake is 2000 mg/d. However,
because AA is endogenously converted to oxalate
and appears to increase the absorption of
dietary oxalate, supplementation may increase
the risk of kidney stones. The effect of AA
supplementation on urinary oxalate was studied
in a randomized, crossover, controlled design in
which subjects consumed a controlled diet in a
university metabolic unit. Stoneformers (n = 29;
SF) and age- and gender-matched non-stoneformers
(n = 19; NSF) consumed 1000 mg AA
twice each day with each morning and evening
meal for 6 d (treatment A), and no AA for 6 d
(treatment N) in random order. After 5 d of
adaptation to a low-oxalate diet, participants lived
for 24 h in a metabolic unit, during which they
were given 136 mg oxalate, including 18 mg
(13)C(2) oxalic acid, 2 h before breakfast;
they then consumed a controlled very low-oxalate
diet for 24 h. Of the 48 participants, 19
(12 stoneformers, 7 non-stoneformers) were identified
as responders, defined by an increase in 24-h
total oxalate excretion > 10% after treatment A
compared with N. Responders had a greater 24-h
Tiselius Risk Index (TRI) with AA
supplementation (1.10 +/- 0.66 treatment A vs.
0.76 +/- 0.42 treatment N) because of a 31%
increase in the percentage of oxalate absorption
(10.5 +/- 3.2% treatment A vs. 8.0 +/- 2.4%
treatment N) and a 39% increase in endogenous
oxalate synthesis with treatment A than during
treatment N (544 +/- 131 A vs. 391 +/- 71 umol/d N).
The 1000 mg AA twice each day
increased urinary oxalate and TRI for calcium
oxalate kidney stones in 40% of participants,
both stoneformers and non-stoneformers.