Soy Protein Isolate

Soy protein isolate is the most refined form of soy protein, with most of the fat and carbohydrate removed. It contains no taurine, and the Merck Veterinary Manual says cats need some animal protein or synthetic taurine because plant proteins lack it. It also keeps soy's isoflavones. For a cat with thyroid disease or another medical condition, ask your veterinarian about soy in the diet.

Source: merckvetmanual.com

It is soy protein refined until little else is left, made by removing the fat and most of the carbohydrate from soybeans. Manufacturers add it to raise the protein percentage on the label and, in some recipes, to help bind the kibble. Because it is so concentrated, a small amount can noticeably lift the crude protein figure.

It is easier to digest, but it is still soy. Isolate has most of the soybean's sugars removed, so it causes less gas than soybean meal. It still contains soy isoflavones and no taurine. Pettings scores soy protein isolate and soybean meal the same, 1 out of 5, because both raise label protein with a plant source instead of meat.

Research suggests a small effect. A 2004 study in adult cats without thyroid disease found that a soy-containing diet raised the thyroid hormone T4 slightly compared with a soy-free diet. Researchers have looked at soy isoflavones as a possible reason. Hyperthyroidism is the most common hormone disease of older cats, and soy is one of several dietary factors studied, without being shown to cause it.

Source: pubmed.ncbi.nlm.nih.gov

Pettings gives soy protein isolate 1 out of 5 and lists it under soy. It appears on 13 of the 149 dry cat foods we list. It is a concentrated plant protein with no taurine, and by itself it does not give cats a complete amino acid balance. Each food's page shows how it scores overall.