"John Ross Mc Master" wrote in message

The cause of the blockage is the cat's body's inabilty to handle ash
(magnesium and the like) in food.

There is no 'ash' in cat food- 'ash' is an analysis of the mineral content-
not an ingredient- that's why its listed in the Guaranteed Analysis and not
in the ingredients list. "Ash" is determined by burning an aliquot of the
diet for 2 hours at 600*C and weighing the residue. "Ash" is actually all
the noncombustible components of the diet- which includes most of the
mineral content- without regard to any specific element. The 'ash' analysis
does not correlate to the magnesium content of the diet.

Also, struvite is a function of urine pH not magnesium. At urine pH of 6.1
or less struvite won't form regardless of the amount of magnesium in the
diet. Magnesium content becomes important only when urine pH rises above
6.2. Urine pH determines *if* struvite will form; the magnesium content
determines *how much* struvite will form.

Phil