Soy Flour

Soy flour is ground soybeans, used in a few dry foods for protein and starch. The Merck Veterinary Manual notes that plant protein sources contain no taurine, so cats need some animal protein or added taurine. Soy flour also keeps soy's isoflavones, plant compounds with weak estrogen-like activity. For a cat with thyroid disease or another medical condition, ask your veterinarian about soy in the diet.

Source: merckvetmanual.com

Soy flour is a low-cost way to raise the protein percentage on the label while also adding starch that helps bind the kibble. It is made by grinding soybeans, often after the oil has been removed. Because it is less refined than soy protein isolate or concentrate, it carries more of the soybean's fiber and sugars. Pettings found it on only a few dry foods.

They are made from soybeans at different levels of refinement. Soy flour is ground soybeans and keeps most of the bean's carbohydrate. Soy protein concentrate has much of the soluble sugar removed. Soy protein isolate is refined further, until it is mostly protein. Pettings scores all of them 1 out of 5 because each raises label protein with a plant source.

Soy does not contain the hormone estrogen, but it does contain isoflavones, plant compounds that can act like weak estrogen in the body. Soy flour, soybean meal and the refined soy proteins all keep these compounds. A 2004 Tufts study found that healthy cats fed a soy diet for three months had small increases in thyroid hormone (T4) levels. Pettings treats the isoflavone content as one reason soy is a less suitable protein source for cats.

Source: pubmed.ncbi.nlm.nih.gov

Pettings gives soy flour 1 out of 5 and lists it under soy. It appears on 3 of the 149 dry cat foods we list. Soy flour raises label protein at low cost, but it contains no taurine and does not give cats a complete amino acid balance on its own. Each food's page shows how it scores overall.