Proteins from soy are not necessarily bad for a dog, but it isn’t an ideal source. Here are a couple of reasons why.

First, some dogs are simply allergic to soy. We'd rather not risk using an ingredient that is problematic for some.

Second, many food companies use it as a protein source rather than an animal source like beef, chicken or fish. However, it's an inferior protein. The value of a protein to a body is measured by biological value. For example, an egg has a biological value of 100 (that's very high). Chicken meal has a biological value of about 89 - 90. But soybean meal has a biological value of about 55, which is obviously a lot less.
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