Dear Doctor

Letters to Tufts Veterinarians


Is the percent ash in the diet correct?

Q I have read things online about ash levels of food at or above 9 being harmful to dogs. I have an 18-month-old papillon who I’ve been feeding a grain-free food with an ash level of 9.21 percent, a number given to me by the company when I asked. She seems perfectly okay, but is it safe to continue to feed a food with that level of ash? I can’t get a straight answer from anyone.

Diane DeZalia

Clifton Park, New York

Dear Ms. DeZalia,

AThe ash level is generally so irrelevant for pet owners that it’s not even listed on most dog food packages, which is why you had to ask the company for it. The percent ash is simply the total mineral matter left in the furnace when the diet is burned in a lab, most notably calcium and phosphorus but also magnesium, potassium, and other minerals. Ingredients like protein, fat, and carbohydrates go up in smoke, but minerals don’t burn.

Why do pet food companies even bother to find out the percent ash? It’s used to calculate the percent carbohydrates in the diet, explains Tufts veterinary nutritionist Cailin Heinze, VMD, DACVM. “You measure the protein, fat, fiber, and ash,” she says, “then add them up and subtract from 100. Whatever’s left is assumed to be carbohydrate. It’s quicker and cheaper to measure the minerals in one fell swoop than to figure out their contributions to the food on a percentage basis one by one.”

There are some dogs for whom high levels of particular minerals in the diet could present a problem, Dr. Heinze says. For instance, she points out, in a large-breed puppy (certainly not the case with your papillon, who at 18 months is not a puppy anymore, anyway), high-calcium diets can cause bone growth abnormalities that can result in such problems as crooked legs and increased risk of elbow and hip dysplasia. That’s why, she points out, large-breed puppies should be fed “large-breed puppy food,” a designation that should be on the front of the label in big lettering. Such food, if made by reputable manufacturers, has carefully controlled mineral levels (in addition to carefully controlled calories levels so the dog reaches adult size more gradually, which is desirable from a health standpoint).

Dr. Heinze adds that dogs prone to developing calcium oxalate urinary stones may be at even higher risk for them if they eat a high-calcium food. Then, too, a dog with kidney disease will not fare well on a diet that contains a lot of phosphorus.

But in such cases, she points out, knowing the percent ash would be useless. You’d need to know the actual number of grams of the mineral at issue per 100 calories of the food, or per 1,000 calories. And you’d need to talk to your veterinarian, or better yet, a veterinarian board certified in nutrition, so she could make heads or tails of the number and tell you whether the food is appropriate for your dog. Truth be told, if your dog has a history of calcium oxalate stones or has kidney disease, chances are your vet is already on it and has prescribed the right diet.

The bottom line: don’t worry about the percent ash in your dog’s diet. If your dog is healthy; if you are purchasing food from a reputable manufacturer (link to and choose “Selecting the Best Food for Your Pet”); and if and the label says the food provides complete and balanced nutrition for the maintenance of adult dogs or for dogs “at all life stages,” then your food is likely just fine.


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