Ash in pet foods comes primarily from the meat protein ingredients. However, all ingredients contain some ash, including vegetables. Ash is non-combustible, so it is what remains after combusting of the food (digestion is essentially a combustion process). Ash is high in phosphorus, magnesium, and calcium. Because of this, high ash pet foods tend to be high in the above minerals. Much attention is being directed to the three minerals listed and their relationship to various illnesses.
Phosphorus has been implicated as a factor in the progression of renal disease. In patients diagnosed with varying levels of renal compromise, phosphorus has several adverse effects. The mechanism is not very well understood, but various studies have shown a more rapid rate of deterioration of renal function in animals eating a high phosphorus diet. Possible causes for the deterioration are nephrocalcinosis, hyperparathyroidism, and lipoprotein abnormalities. We now know that protein restriction may be detrimental to pets suffering from early renal insufficiency. However, it is hard to restrict phosphorus without restricting protein and we know that we should restrict phosphorus. So, the use of medications to bind excess phosphorus may be an integral part of the treatment of renal insufficiency in pets.
Phosphorus and magnesium in the diet can also be problematic for cats suffering from feline lower urinary tract disease (FLUTD) associated with struvite crystal formation. Many maintenance diets for cats are now formulated to help prevent the formation of struvite crystals in the urinary tract. This is accomplished by controlling the ash (which controls the phosphorus and magnesium) and by adding ingredients to adjust the urine pH. It is important that the urine pH is not too acid, or the problem of calcium oxalate stones becomes more likely to develop.
Excess calcium is most important to growing large breed puppies and pregnant females. Large breed puppies have a greater likelihood to develop orthopedic problems during the growth process. Developmental orthopedic disease has been scientifically proven to be caused primarily to genetics, but environmental factors such as energy and calcium content of the food that the puppy is eating are also involved. Calcium must be less than 2.5% on a dry matter basis by AAFCO standards. This restriction itself has reduced the incidence of developmental orthopedic disease. Calcium is not present in pet foods at high enough levels to be detrimental to pregnant bitches or queens, but supplementation is unnecessary and potentially dangerous.
Ash is present in all pet foods at varying levels. The lower the ash level, the more digestible the diet is and the risk of disease associated with excess minerals is also much lower.