Quality, labeling accuracy, and cost comparison of purified soy isoflavonoid products

To evaluate the labeling accuracy and product uniformity of available soy isoflavone products in the eastern Washington State region for 13 products and to perform a cost comparison based on the isoflavone content in milligrams for 11 products.

Thirteen (13) isoflavone products (7 tablet and 6 capsule formulations) were randomly obtained from health food, grocery, and pharmacy outlets in eastern Washington State. Four different samples of the same product were extracted using 75% ethanol. Each sample was analyzed for isoflavone content using a gradient high-performance liquid chromatography (HPLC) method. Pure isoflavonoid standards were used to quantify the content of genistein, daidzein, and glycitein and their respective glycosides. The amount of total isoflavonoids per purchased product was then divided into its purchase price in order to make cost comparisons between the products based on a 50 mg/d dose.

The weight variation within each product was generally small (<4%). However, there was significant variability in the composition of the products. Only 4 of the 13 products contained at least 90% of the isoflavone content claimed on the label and 2 of products contained impurities in the HPLC chromatogram that exceeded 40%. There was no difference in the total isoflavonoid contents of the 2 identical products made by the same manufacturer, but the percentages of the main components had changed significantly over time. Based on a 50-mg isoflavone per day dose, the cost of a 30-day supply ranged from $3.20 to $65.88.

There is a lack of uniformity among the products tested and the majority (67%) of the products contained less than 90% of labeled amounts. There is significant variability in the compositions between the products and in the composition of the same product over time. Consumers cannot trust isoflavonoid product labels to represent the product's content accurately or that product pricing is a reflection of isoflavone content.