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Phytoestrogens: Dietary Sources and Metabolism.Pdf Blackwell Science, LtdOxford, UKNDINutrition & Dietetics1446-6368Journal of the Dieticians Association of Australia, including the Journal of the New Zealand Dietetic Associationxxx 2005624161 164Continuing Education Continuing education Phytoestrogens: Dietary sources and metabolism Continuing education and the APD program Accredited This quiz is an ideal activity for APD members to include in your CPD log, where APD it relates to personal learning goals. Record the time taken, to the nearest hour, to Practising complete the quiz and any associated research. Dietitian ™ This quiz has been prepared by Dr Philippa Lyons-Wall, School of Public Health, Queensland University of Technology and Associate Professor Samir Samman, Human Nutrition Unit, The University of Sydney. Correspondence should be directed to Philippa Lyons-Wall at [email protected] or School of Public Health, Queensland University of Technology, Qld 4059, Australia. Introduction 3. Which of the structures shown in Figure 1 is a phytoestrogen? The phytochemical family includes a diverse array of bio- logically active substances that occur in plant foods such as fruits, vegetables, legumes, cereals, nuts and seeds. Phy- (a) (b) toestrogens are members of the phytochemical family with naturally occurring oestrogenic activity. Particularly high HO O concentrations of phytoestrogens are found in soybeans. A N Me notable feature of the distribution of phytoestrogens in foods is the wide variability in content within the same OH O OH HO CH2 OH item. This reflects genetic variation, as well as the fact that CH2 OH unlike more stable structural components such as proteins, phytoestrogens are part of the plant’s natural response to stress and are synthesised in direct response to microbial or insect damage. (c) (d) OH Phytoestrogens achieved notoriety in the 1940s in Me OH Western Australia when sheep fed large quantities of S O S subterranean clover fodder developed a reproductive S OH H O R S S abnormality called clover disease, which resulted in sub- R stantial losses in productivity—phytoestrogens in the H H H clover were subsequently identified as the bioactive fac- HO OH HO tor responsible for the reproductive abnormality.1 In recent years, phytoestrogens have received increasing public attention in Australia and overseas regarding the Figure 1. Which of the above structures is a phytoestrogen? possible health benefits of moderate quantities in the human diet. The following quiz is designed to enhance your under- 4. What are the distinguishing features of isoflavonoid standing of phytoestrogens from a nutritional science per- metabolism? spective. It will test your knowledge of basic concepts a. Hydrolysis of glycosidic conjugate to produce an including the main types of phytoestrogens and major food aglycone form sources, their absorption, metabolism, excretion and pos- b. Structural transformation by colonic microflora sible toxicity. c. Enterohepatic recycling 1. Besides soy, what are some of the identified food d. All of the above sources of isoflavonoid phytoestrogens? 5. Equol: a. Broccoli, Brussels sprouts a. Is an isoflavonoid metabolite. b. Rye, linseed b. Has a sweeter taste than other isoflavonoids. c. Chickpeas, lentils c. Is an isoflavonoid found in soybeans. d. Onions, leeks d. Is not an isoflavonoid. 2. What are some of the main phytoestrogens in the 6. Which of the following factors has been shown to human diet? specifically affect bioavailability of phytoestrogens? a. Daidzein, genistein, glycitein a. Antibiotics b. Formononetin, biochanin A b. Tannins c. Zearalanone c. Alcohol d. All of the above d. All of the above Nutrition & Dietetics (2005) 62: 4 161 7. What symptoms could arise from overconsumption amounts are found in foods.2 Zearalanone is a of phytoestrogens? phytoestrogen derived from Fusarium moulds that a. Dermatitis flourish in warm moist conditions; it is found as a b. Anaemia contaminant in poorly stored grains and other produce. c. Menstrual cycle irregularity 3. a. d. Bowel irregularity Genistein (Figure 1a) is a plant oestrogen with a similar 8. What is the recommended intake of phytoestrogens molecular weight and structure to the mammalian (mg/day)? oestrogen, oestradiol (Figure 1d). The presence of a a. 1 hydroxyl group (–OH) attached to the phenolic ring is b. 10 a prerequisite for binding to oestrogen receptors, and c. 100 the distance between the two opposing hydroxyl groups d. There is no recommended intake on the phenolic rings of the isoflavonoid is almost 9. Binding of phytoestrogens to oestrogen receptors: identical to the distance between the hydroxyl groups a. Can result in both oestrogenic and antioestrogenic of oestradiol. These structural features permit binding effects. to mammalian oestrogen receptors, but with lower b. Is dependent on circulating levels of endogenous affinity than oestradiol.5 oestrogens. 4. d. c. Is tissue-specific. d. All of the above. In the original plant, isoflavonoids and lignans occur as conjugates to a range of sugars that renders them stable, water-soluble and inactive for storage in the plant. Answers When consumed in the diet, conjugates are readily hydrolysed, mainly by bacterial β-glucosidases in the 1. c. proximal colon, to the unconjugated (or aglycone) form 6 The three classes of phytoestrogens of importance in that can be absorbed. Hydrolysis is essential for the human diet are the isoflavonoids, lignans and absorption. coumestans. Isoflavonoids are widely distributed in the A distinctive feature of dietary isoflavonoids and legume family, with concentrations approximately 50- lignans is their capacity to undergo structural fold higher in soybeans (37.3–131 mg/100 g) than transformations, by colonic microflora in the proximal other legumes such as chickpeas, peanuts and lentils; colon, to a range of hormone-like compounds. These one striking exception is Kudzu root (Japanese compounds can be absorbed and have the ability to bind arrowroot), in which isoflavonoid concentrations with low affinity to oestrogen receptors and exert weak exceed those in the soybean (197 mg/100 g).2 Lignans oestrogenic activity.6 As the metabolites differ in their are the basic monomer units for lignin, a polyphenolic biological activity, individual differences in these constituent of dietary fibre. Therefore, they are found transformations could have important consequences for widely in plant foods in small amounts, including nuts, potential health benefits. seeds, legumes, cereals, vegetables and fruits (0.01– After absorption, isoflavonoids and lignans are 0.9 mg/100 g). An analysis of 19 tea samples reported transported to liver where they are conjugated with relatively high concentrations in tea infusions (up to glucuronic acid and sulfate by hepatic phase II 2.9 mg/100 g); uniquely high concentrations of lignans enzymes. These conjugates can enter the systemic are reported in linseed (67–369 mg/100 g).2 circulation and be finally excreted in urine; or be Coumestans are found in highest amounts in sprouted resecreted with bile, deconjugated once again and beans, including alfalfa and clover, with only trace reabsorbed to undergo enterohepatic circulation. amounts in non-sprouted legumes. Unabsorbed conjugates are lost in faeces.6 2. a. 5. a. Soybeans and soy products provide the main sources of The isoflavonoid daidzein can be transformed phytoestrogens in the human diet. Soy phytoestrogens by colonic bacteria to at least three metabolites: belong to the isoflavonoid class and comprise mainly dihydrodaidzein, O-desmethylangolensin (O-DMA) genistein (about 60%) and daidzein (30%), with and equol. Interest has focused on equol as this smaller quantities of glycitein (10%); glycitein is found metabolite has greater oestrogenic potency than either in relatively high concentrations in the hypocotyls of daidzein or O-DMA and, therefore, excretor status the soybean. Isoflavonoids are stable to heating and could have physiological implications. Equol excretion processing, but are lost with certain extraction in women has been associated with a hormonal procedures, for example, leaching with alcohol in the profile of reduced risk for breast cancer.7 However, preparation of soy protein concentrates removes only about one in three individuals are able to isoflavonoids into the extraction solvent, whereas the synthesise equol.8–10 It has been proposed that diet original isoflavonoids are retained with aqueous could facilitate equol synthesis by selectively extraction procedures. Dehulling or defatting soybeans modifying the growth of intestinal bacteria involved in has little effect on concentrations because relatively the transformation, by influencing either intestinal pH small amounts are found in the outer hull and there are or availability of substrates for the bacteria to utilise. 3,4 no isoflavonoids associated with the lipid moiety. However, results of studies are conflicting. Intervention Formononetin and biochanin A are methylated studies with a soy protein beverage or textured soy precursors of daidzein and genistein, respectively, protein have reported that the ability to excrete equol found in high concentrations in phytoestogen is associated with significantly more carbohydrate and supplements derived from red clover; only small dietary fibre,8 or less fat and more carbohydrate,9 162 Nutrition & Dietetics (2005) 62: 4 consumed in the habitual diet; whereas an 9. d. observational study reported that equol excretors had One of the defining characteristics of phytoestrogens is 10 a significantly lower intake of protein. their ability
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