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SECOND INTERNATIONAL SYMPOSIUM
ON THE ROLE OF SOY
IN PREVENTING AND TREATING CHRONIC DISEASESeptember 15-18, 1996
Brussells, Belgium
SCIENTIFIC PROGRAM
Satellite Symposium
Soybean Isoflavones :
Levels in Foods and PharmacokineticsShort Half-Lives and Gender Differences of Excretion of Soya Isoflavones in Humans
Lu, L-J.W. and Anderson, K.E.
Department of Preventive Medicine and Community Health, The University of Texas Medical Branch, Galveston, TX 77555-1110.Soybean consumption may be protective for hormone dependent cancers, possibly due in part to the presence of the isoflavones daidzein and genistein that are weakly estrogenic. The metabolism and disposition of these phytoestrogens was studied in 12 healthy humans on a metabolic unit biweekly during one month of daily soya ingestion (36 oz. of soymilk, providing 80 - 210 mg of each isoflavone daily). Initially, females excreted more of the ingested isoflavones as conjugates in urine than did males: genistein, daidzein, and equol excretion in females was 24%, 66%, and 28% of the amounts ingested respectively, and in males was 15%, 47%, and 15%, respectively. A progressive decrease in urinary excretion of genistein and daidzein was observed in females but not males during 4 weeks of daily soya ingestion. At least 10% of ingested daidzin was excreted in urine as equol conjugate in one of the six males and in one of the six females after the first soya ingestion. During 4 weeks of soya ingestion, three additional females but no additional males developed the ability to produce and excrete large amounts of equol. Absorption half-lives of isoflavones in soymilk were estimated to be 1.5-3.0 hrs. The excretion half-lives of genistein, daidzein, and equol were longer in females (7 hrs, 4 hrs, and 9 hrs, respectively) after the first soya ingestion. The excretion half-lives shortened in females but lengthened in males during 4 weeks of daily soya ingestion. Thus, in this study isoflavone metabolism and disposition were affected by the duration of soya ingestion and by gender.
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