The Food and Drug Administration’s investigation of grain-free dog and cat foods, highlights how hard it can be for pet owners to figure out whether they are buying the healthiest products for their beloved four-legged friends.

Consumers used to reading ingredient labels on their own foods might think that checking the corresponding labels on pet products will tell them everything they need to know. But veterinary nutrition experts interviewed by NBC News say those labels won’t provide much enlightenment.

“The food label is not designed to provide the information they are looking for,” said Dr. Jennifer Larsen, a professor of clinical nutrition at the University of California, Davis, School of Veterinary Medicine. “And a lot of the pet food ranking lists available on the internet rely on the label and focus inappropriately on the ingredient list.”

Since 2018, the FDA has been investigating more than 500 reports of canine dilated cardiomyopathy that appears to be linked to dog foods marketed as grain free. But for the majority of dogs, it's not yet clear what is causing the heart disease, experts say.

What’s important is the nutrients in the product, said Dr. Kathryn Michel, a professor of nutrition at the University of Pennsylvania School of Veterinary Medicine. “And those nutrients need to be bio-available, meaning they need to be in a form the pet can utilize.”

A much more informative part of the labeling is the nutrition adequacy statement, Larsen said.

“That has to be on all pet food sold across state lines,” she said, adding that this important part of the label can be hard to locate. “And the fonts are really small. Once you find it, you’re looking to see if it has a ‘complete and balanced claim’ and what species it’s for and what stage of life.”

FDA is providing an update on its ongoing investigation into reports of dilated cardiomyopathy (DCM) in dogs eating certain diets. @FDAanimalhealth is sharing new data, updated stats, & a compilation of adverse reports of DCM submitted to FDA: https://t.co/0ZlqfoPGPf pic.twitter.com/UvVSQRj1wy — U.S. FDA (@US_FDA) June 27, 2019

Of course, on those criteria, many of the grain-free pet foods would have passed muster since they were “complete and balanced,” Michel said.

But the question pet owners ought to have been asking was why they should be avoiding grains in the first place.

That’s where the internet could have helped, Larsen said. While there is a lot of misinformation online, there are some reputable and trustworthy sites that help pet owners learn the facts. Both Lerner and Michel point to the website for the World Small Animal Veterinary Association, which has a page for nutrition guidelines.

The site also has a page that answers common nutrition questions and dashes myths. It’s there that owners can learn that cereal grains are not bad for dogs or cats, that so-called byproducts are not bad for pets, and the potential dangers associated with raw diets and bones.

Another good source of information, experts say, is the Petfoodology blog run by nutritionists at Tufts University. The blog has recently taken up topics such as “red flags” on the ingredient list and terms such as “human grade.”

According to the veterinary nutrition specialists at Tufts, it's not only grain-free foods that have been associated with heart disease, but also foods containing "exotic" ingredients for dogs such as alligator, venison and ostrich.

Cost doesn’t necessarily reflect quality. You don’t always get what you pay for.

Often owners are shopping for pet foods the same way they shop for their own, Michel said.

“Pets are more and more considered members of the family,” she said. “That is one of the reasons why, when we look at trends in pet foods for the last decade or so, they mirror what is seen in the human sector. So, while I don’t know 100 percent where the grain-free idea started from, it started cropping up when people started embracing low-carb diets and gluten-free foods.”

As much as a dog or cat might feel like a member of the family, they often have different needs, Michel said.

In general, it’s probably best to buy pet foods from big companies with long track records, Michel and Larsen said.

That’s because they’ve got lots of scientists and nutritionists on staff with plenty of experience, Larsen said. Beyond that, the bigger companies sell more pet food and if there’s a problem, it’s likely to surface sooner, she added.

And remember, Larsen said, pricey pet foods may not be best ones.

“Cost doesn’t necessarily reflect quality,” she said. “You don’t always get what you pay for.”