A Simplified Method to Quantify Isoflavones in Commercial Soybean Diets and Human Urine after Legume Consumption

Reliable and economical quantification of micronutrients in diets and humans is a critical component of successful epidemiological studies to establish relationships between dietary constituents and chronic disease. Legumes are one of the major dietary components consumed by populations worldwide. Consumption of legumes is thought to play a major role in lowering breast and prostate cancer risk. In this study, a simplified method that uses solid-phase extraction and gas chromatography was developed to measure isoflavones at levels down to 10 fig/5 ml. With the use of this method, 12.5 g miso (a soybean paste), 12 ounces Isomil, and 12 ounces soymilk had daidzin/daidzein levels of 2, 5, and 12.4 mg, respectively, and genistin/genistein levels of 3, 6.5, and mg, respectively. In these products, most of the isoflavones were present as glucosides. With the same method, urinary levels of isoflavones in six 15-17-year-old subjects were determined after soymilk ingestion. Each subject was placed on unrestricted nonsoya diets, and three 12-ounce portions of soymilk were given at 12-h intervals. Males exereted 15.02 ± 2.74 (SD) mg of daidzein glucuronides/sulfates [mean recovery, 40.4 ± 7.4% (SD)] by 24 h after the third soymilk ingestion, whereas females exereted 25.56 ± 5.10 mg (68.7 ± 13.7%) of daidzein conjugates, which was more than males (P = 0.02). Males and females exereted 7.73 ± 1.95 mg and 9.11 ± 0.84 mg of genistein glucuronides/sulfates (20% recovery of genistin intake), respectively, in the urine. Most of the isoflavones were exereted within 24 h after ingestion. The relative urinary levels of daidzein to genistein exereted were signifleantly (P < 0.05) higher in females than males after the third ingestion. The observed sex difference requires more study since two of the females are siblings. Thus, the method described can be used to measure isoflavones in soya products and urinary exeretion after soya ingestion.