Vitamin C (Ascorbic Acid)
Cats synthesize sufficient vitamin C (ascorbic acid) from glucose in their small intestine, and the cat's natural prey diet is devoid of a pure source of this vitamin. Additional supplementation can be harmful, because excessive ascorbic acid is excreted in the urine as oxalates. A high concentration of oxalate in the urine can contribute to the formation of calcium oxalate stones in the urinary tract, including the kidneys. A controlled study has also shown that large doses of injectable ascorbic acid has no beneficial effects for preventing or reducing the severity of feline respiratory disease. In addition, an overdose on vitamin C can enhance iron absorption and cause a resulting overdose of this mineral. The cat's resistance to disease is much less dependant on vitamin C, as is the case for humans. Instead, it is much more vital that sufficient amounts of vitamin A are available to the cat through her diet, which promotes disease resistance and proper immune function.
"Cats normally synthesize vitamin C (ascorbic acid) from glucose, so there is no need to supplement the diet unless there is a high metabolic need or inadequate synthesis. Excessive supplementation may actually be harmful, because excessive ascorbic acid is excreted in the urine as oxalate. A high concentration of oxalate in the urine has the potential to contribute to the formation of calcium oxalate stones in the urinary tract."
The Cornell Book of Cats, 2nd. edition, page 82
There are many molecular substances in foods that offer no nutritional benefit, and must be processed and excreted. Oxalic acid, for example, is excreted in the urine, and its crystals are commonly found in microscopic urinalysis. Too much oxalic acid in the urine will result in kidney or bladder stones. Calcium combines with oxalic acid to form the less soluble salt, calcium oxalate, which is also found in kidney stones. Plant leaves, especially rhubarb, cabbage, spinach, and beet tops, contain oxalic acid. Oxalic acid is also found in potatoes and peas. Vitamin C is metabolized to oxalic acid; it contributes to over-saturation of the urine with crystals and possibly to stone formation."
Excerpt from: Food Chemistry: Chemical Stressors and Toxins in Plant Foods and Herbs.
Stephen Gislason, M.D.
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