The food supplement industry's ability to bring high-quality soy isoflavone-containing products to the market is of particular importance for the well-being of postmenopausal women, who utilize these products the most. Although supplement labels commonly contain only the total amount of soy isoflavones, these products' actual isoflavone profiles could influence their biological effects, considering that the isoflavone aglycones glycitein, daidzein, and genistein do not have the same potency for binding to estrogen receptors, a step necessary for the manifestation of their activity.
Twenty-one commercial supplements with soy extract, intended mostly for relief of menopausal symptoms, were subjected to isoflavone profiling (daidzein, glycitein, and genistein, as well as their glucosyl, acetyl, and malonyl glycosides) by high-performance liquid chromatography with diode-array detection. For analytical purposes, a portion equivalent to an average mass of 10 tablets/capsules was measured from the pulverized material and extracted with 80% aq. methanol.
The supplements showed a broad range of total isoflavone content, 0.05-71.0 mg/dose unit expressed as total aglycone equivalents (mean value 20.4 mg/dose unit). The total isoflavone content deviated from the labeled value by less than ± 10 % in two supplements only, while the overall range of deviations was from -94.3 to +18.0%. Regarding isoflavone composition, genistein and daidzein, each with its glycosides, were on average equally abundant, participating with 43% of the total isoflavones, although the former showed greater variations in content (standard deviation 14 vs. 24%), while glycitein and its glycosides accounted for the remaining 14%.
This study showed that the quality of soy-based supplements varies greatly, in terms of both the amount of isoflavones and deviation from the labeled content, as well as in terms of isoflavone profiles. Such findings indicate a need for better control of the production process, including plant extract standardization, as well as of the final products. The increased public interest in health-promoting food supplements makes this issue even more relevant.
