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Does the all-beef dog food you bought for your family’s pet really contain all beef?

Not necessarily.

In a study published in the journal Food Control, Chapman University in Orange, Calif., researchers used DNA testing to identify the type of meat in 52 pet-food products and found discrepancies between more than a third of the contents and labels.

Of the 52 products tested, 20 were potentially mislabeled, and 16 contained meat that was not indicated on the packaging.

“Although regulations exist for pet foods, increase in international trade and globalization of the food supply have amplified the potential for food fraud to occur,” Rosalee Hellberg, co-author of the study, said in a statement. “With the recent discovery of horse meat in ground-meat products sold for human consumption in several European countries, finding horse meat in U.S. consumer food and pet-food products is a concern, which is one of the reasons we wanted to do this study.”

The researchers did not find any horse meat in the products. They said chicken was the most common meat in the pet-food products. Pork was the second most common, followed by beef, turkey and lamb. The least common was goose.

Of the 20 potentially mislabeled products, the study said 13 were dog food and 7 were cat food. Of those 20, 16 contained meat species that were not included on the product label, with pork being the most common undeclared meat. In three of the cases of potential mislabeling, one or two meat species were substituted for other meat species, the study said.

“Pet-food safety was another area of concern, particularly with pet foods that are specifically formulated to address food allergies in both cats and dogs,” Hellberg said, a professor in Chapman’s Food Science Program.

The researchers said it was not clear whether the mislabeling was accidental or intentional and at which points in the production chain it took place.

The foods developed for pets are regulated by both federal and state entities. The Food and Drug Administration regulates animal feed and pet foods. The Department of Agriculture regulates the interstate transportation and processing of animal products, as well as the inspection of animal-product imports and exports.