Ash is mineral content of meals (meat protein ingredients in pet foods). It comes from the bone that is included in the meal. 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 related to the progression of renal disease. In patients diagnosed with varying levels of renal dysfunction, phosphorus has several ill effects. It not only increases the severity of changes to the kidney itself, but also decreases survival time after the diagnosis of kidney disease has been made. Scientific studies have yet to determine the exact way that phosphorus affects the kidneys, but there is proof that it does have an adverse effect. There have not been any studies to determine effects of excess phosphorus on the healthy kidney, but it makes sense that if excess phosphorus hurts a damaged kidney, it could potentially damage a healthy kidney, especially over time.
Phosphorus and magnesium in the diet can also cause problems for cats suffering from another type of urinary tract disease, known as FLUTD (Feline Lower Urinary Tract Disease). Many maintenance cat diets are now formulated to help prevent the formation of struvite crystals in the urinary tract by acidifying the urine and providing lower levels of magnesium and phosphorus.
Excess calcium is most important to large breed puppies and pregnant females. Large breed puppies have a greater likelihood to develop orthopedic problems during the growth process. Developmental orthopedic disease can be affected by energy and calcium content of the food that the puppy is eating. 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 lower the risk of excess mineral content.