Natural source of antioxidants
A "natural source of antioxidants" in dog nutrition refers to foods or ingredients that are naturally rich in antioxidants. Antioxidants are substances that help neutralize harmful free radicals in the body. Free radicals are unstable molecules that can cause cell damage and are associated with various health problems, such as aging and certain diseases.
Natural sources of antioxidants can be various fruits, vegetables, herbs and other botanicals. Examples of such natural antioxidants are vitamin C (ascorbic acid), vitamin E (tocopherols), beta-carotene and certain polyphenols. These substances support your dog's immune system, promote general health and can help reduce inflammation.
So when you read in the ingredients list of a dog food that it contains a "natural source of antioxidants", it means that there are ingredients in that food that naturally provide antioxidants to support your dog's health.