Objective: To determine the absolute and relative soy isoflavone content in commercial cat foods.
Sample population: 14 dry, 6 semimoist, and 22 moist commercial cat foods.
Procedure: Soy isoflavone content of each food was determined by use of acid-methanol hydrolysis and high-pressure liquid chromatography with ultraviolet absorbance detection. Isoflavones were identified and quantified by reference to authentic standards.
Results: Genistein and daidzein were the major soy isoflavones identified in 24 of 42 foods, with concentrations ranging from 1 to 163 microg/g of food. Foods labeled as containing soybean solids (16/42) had isoflavone concentrations > 11 microg/g. More dry (13/14) and semimoist (6/6) foods contained isoflavones than moist foods (5/22). Isoflavone content and food cost were negatively correlated for dry and semimoist foods but not for moist foods. Total amount of isoflavone consumed by cats fed these soy-containing foods as a sole maintenance diet was estimated to be between 0.6 and 4.5 mg/kg of body weight/d, which is comparable to concentrations in humans that result in a measurable although modest effect on serum concentrations of steroid and thyroid hormones.
Conclusions and clinical relevance: Genistein and daidzein are common constituents of commercial cat foods. Predictors of isoflavone content included ingredient labeling, food type, and food cost. Soy isoflavones in some commercial cat foods were detected in amounts predicted to have a biological effect.