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After examining how people buy food in an aisle-by-aisle guide called What to Eat, Marion Nestle picked up a can of pet food to see how its ingredient label compared with the ones on human chow. Nestle, a professor of nutrition, food studies and public health at New York University, found the pet-food ingredient label incomprehensible. This led to her latest book, Feed Your Pet Right: The Authoritative Guide to Feeding Your Dog and Cat (Free Press, 2010), written with Malden C. Nesheim, professor emeritus of nutritional sciences at Cornell University. Nestle spoke with TIME about navigating the maze of pet-food products in what has become an $18 billion industry in the U.S.

What most surprised you in writing this book?
We were surprised by the lack of research on important questions. We were very eager to compare the longevity of pets based on their diets, whether they were fed raw, vegetarian, premium or complete and balanced pet-food products. We were unable to find any research that compared pet foods in this way. It was very frustrating. Of course, pet-food companies don't conduct this type of research!

Mainstream pet-food products often get a bad rap.
If you feed your pet a complete and balanced product, it will meet your pet's nutritional needs. These products are all the same, from a nutritional standpoint. And if you're doing that, your pets do fine. That's the great contribution of the pet-food industry — they've given consumers a cheap, convenient product that keeps their pets alive. We found no evidence that complete and balanced pet food is harmful to pets, except where mistakes have been made in the course of production.

What do terms like natural, human-grade and premium mean when describing pet food?
Here you run into exactly the same problems you have with human food, where there's no official regulatory definition [for terms like natural and premium]. In pet food, human-grade means all the ingredients meet the standards you set for human-food production. We visited a facility which was used to manufacture human food on some days and pet food on others. Very few pet foods are allowed to use this term.

What's the most important thing to look for on a label?
Ingredients. If you're dealing with a complete and balanced pet food, the nutrients in it will be adequate, because they meet a standard. So the only difference among these products is what the ingredients are. In the book we recommend that pet-food labels be changed to resemble human-food labels so that they're easier to understand.

Why does some pet food contain ash?
It's not like the stuff you take out of fireplaces! The ash in pet food refers to minerals.

What ingredients, besides chocolate, should pets not eat?
Onions, garlic and macadamia nuts are the main ones, but little bits are unlikely to be harmful.

Is there concern that BPA and other chemicals used in sealing canned foods are harmful to pets' health?
There's certainly a great deal of concern, but there's no research on it whatsoever.

Can cats be vegetarian?
Cats are carnivorous, but there's plenty of research that shows they're perfectly capable of digesting grains, as long as they also get the amino acids, vitamins and minerals they need. Dogs are fine with grains too. In fact, most complete and balanced pet-food products contain grains.

Should you let your cat drink out of the toilet bowl?
I wouldn't advise it for cats or dogs. Toilet water is likely to be contaminated. Even though dogs and cats don't get sick from salmonella the same way humans do, you still don't want to expose your pets to unnecessary germs.

Do you have any pets?
I did when my children were young, but not at the moment, in part because I travel so much. I do I have grandpets, though.

What did you feed them back then?
I was quite thoughtless about it. It was just something that came out of a can.

• Coeli Carr
• After examining how people buy food in an aisle-by-aisle guide called 'What to Eat,' Marion Nestle picked up a can of pet food to see how its ingredient label compared with the ones on human chow