Keep in mind that vitamins & minerals are important for kitty, but what's most important is that you get them in the right ratios. Calcium and phosphorus without enough vitamin D, for example, will produce bone abnormalities.
Magnesium is probably the mineral of the most concern in cat nutrition for owners, and especially to owners of male cats because of its role in the formation of struvite uroliths (magnesium ammonium phosphate). However, magnesium is not a "bad guy". Magnesium is a macromineral, its amount in the body is much lower than that of calcium and phosphorus. Approximately 60% to 70% of the magnesium found in the body exists in the form of phosphates and carbonates in bone. Most of the remaining magnesium is found within cells, and a very small portion is present in the extracellular fluid. In addition to its role in providing structure to the skeleton, magnesium functions in a number of metabolic reactions; a magnesium ATP complex is often the form of ATP that is used as a substrate in many of these processes. As a cation in the intracellular fluid, magnesium is essential for the cellular metabolism of protein. Protein synthesis also requires the presence of ionized magnesium. Balanced in the extracellular fluids with calcium, sodium, and potassium, magnesium allows muscle contraction and proper transmission of nerve impulses.
A commercial cat food should not be selected only on the basis of its magnesium content. The quantity of magnesium required to saturate urine with struvite at alkaline pH is very small. As pH decreases below 6.4, the amount of magnesium required to saturate the urine with struvite increases exponentially. Conversely, as urine pH increases above 6.9, the amount of struvite that forms in the urine increases markedly. When urine pH is alkaline, the amount of struvite formed in urine is proportional to the dietary magnesium concentration. At urine pH values less than 6.1, struvite does not form regardless of the magnesium concentration of the diet. Thus, the tendency of struvite to form is a function of urine pH. The magnesium content of the diet only becomes important when urine pH is greater than 6.1.
Well...it's more the overall amount in the cat's diet, than it is the amount in the food your buying. Technically, it's the caloric density (digestibility) of the food that's more important (the more moderate the better).
Basically, it's sulpher/salt content you're more concerned with (depending on kitty's ph balance). High sulpher, phospholipids and phosphoamino acids content makes for a more acidic kitty. Salt alkanizes it.
This is why you're generally better off with a higher quality cat food. The caloric density is less, making for less magnesium intake. Generally a cheaper cat food will tack on "low magnesium"...and while it's true, it isn't exactly totally helpful. It is some, but overall the higher density makes for processing more magnesium into the diet, which, can cause kidney stones.
...but this is where you get into the whole vitamin/mineral balancing thing as well. The magnesium amount can cause the stones, but it's more reliant on the proper balance of the other mineral/vitamins than whether there's too much magnesium.
Ummm...did I answer the question? I'm not sure now. :)
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