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296 A. SCALBERT ET AL. to their isoflavones.260 The restoration of the bone mineral den- sity was not observed when the isoflavones were administered 80 d after the ovariectomy of the rats, suggesting that they may prevent bone loss, but not reverse an established osteopenia.261 The mechanisms responsible for these protective effects are still poorly understood. In vitro, daidzein was shown to inhibit the differentiation of osteoclasts developping on dentine slices and to diminish the dentine resorption.262 On the other hand, the daily subcutanous injection of genistein to ovariectomized rats increased the number of osteoblasts associated to bone formation, but had no effects on bone resorption.263 Structurally related isoflavones may protect bones through different mecha- nisms of action,258 and with different magnitude.259 Evidence of protective effects in humans is based on obser- vational and intervention studies. The consumption of soybeans, a major source of calcium in Japan, was associated to a higher bone mineral density in Japanese women.264 A soy-rich diet followed by postmenopausal women stimulated their bone os- teoblastic activity.265,266 Such effects of soy food consumption could also be explained by the intake of isoflavones, as soy foods are their main dietary sources. However, no correlation between isoflavonoid urinary excretion and rate of bone loss was ob- served in postmenopausal women in the Netherlands.267 Higher intake levels may be required to prevent bone loss. The con- sumption of soy proteins providing 80 mg/d isoflavones during 24 wk was more effective than isoflavone free soy protein to improve bone mineral density and content in perimenopausal women.268 Furthermore, in 3 other studies, isoflavone supple- mentation at a dose of 37–62 mg/d during 4 wk-1 yr significantly improved the urinary excretion level of several biomarkers of bone resorption.269−271 Femur and lumber spine mineral den- sity was also improved in the longer study. Much less is known on the possible impact of other polyphe- nols on osteoporosis. Rutin, a glycoside of quercetin, added to the diet of ovariectomized rats restaured the loss of bone mineral density induced by the ovariectomy and was even more efficient than isoflavones.272,273 It also reduced the urinary excretion of deoxypyridinoline, a marker of bone resorption, and increased osteocalcinemy, a marker of osteoblastic activity. Such an effect of rutin likely explains the inhibition of bone resorption observed in rats fed a diet rich in onions (the main source of quercetin in Western diets), whereas several other vegetables were without effects.274 The consumption of tea has been associated with a higher bone mineral density in a cohort of English older women.275 Catechins abundant in tea could possibly counteract the effects of tea caffeine, known for its adverse effects on bone metabolism. No effect of the consumption of coffee was observed in the same study. A rat experiment also showed no effect of coffee on bone metabolism. Feeding rats during 140 d with a diet supplemented with instant coffee containing both chlorogenic acid and caffeine had no influence on bone histomorphometry, deoxypyridinoline urinary excretion, and osteocalcinemy.276 The effects of cate- chins or chlorogenic acid on bone resorption in animal models so far have not been examined. RISKS ASSOCIATED TO POLYPHENOL CONSUMPTION The scientific data summarized above on the effects of polyphenols on diseases have led to the marketing of new poly- phenol dietary supplements and polyphenol-rich food products. Although no precise claims are attached to these products, dif- ferent polyphenols have been proposed to limit oxidative stress and aging, and isoflavones have been recommended to limit hot flushes and to improve bone health in post-menopausal women. The consumption of such products leads to increase the intake of particular polyphenols beyond common levels of exposure associated to the diet. Cases of acute toxicity have been reported in animals consum- ing plants rich in tannins.277 In humans, no such acute toxicity has ever been reported after the consumption of dietary polyphe- nols. However, a high consumption of polyphenols could in- crease the risk of some diseases. The large majority of published studies were focussed on health benefits, rather than on disease risks. Our knowledge on risks is, therefore, very limited. It is often said that polyphenols consumed in high amounts could have pro-oxidant effects. They can reduce iron (III) to iron (II) and, thus generate hydroxyl radicals through the Fenton reaction. Such pro-oxidant effects have never been demonstrated in vivo. Most dietary polyphenols have catechol groups that can be oxidized to quinone in vivo and generate free radicals through redox cycling. However, quinone reductase, catechol-O-methyltransferase, and other conjugating enzymes limit the formation of such quinones in endogenous tissues. Sim- ilarly, a number of polyphenols, including quercetin, were shown to be mutagenic in cultured cells. A pro-carcinogenic effect of quercetin in rat models of nitrosomethylurea-induced pancre- atic cancer or azoxymethane-induced colon cancer have been reported.278,279 But the majority of the studies carried out with quercetin in rodents showed anticarcinogenic effects. Conjugat- ing enzymes likely plays an essential role to detoxify polyphe- nols and limit their mutagenicity in vivo. Phytoestrogens, acting as estrogen agonists, may stimulate the proliferation of estrogen responding cells and increase the risks of some cancers.280,281 Genistein was shown to stimulate the growth of breast cancer cells implanted in ovariectomized mice.282 The consumption of soy proteins by premenopausal women increased the level of plasma estradiol and stimulated the appearance of hyperplastic epithelial cells in nipple aspirate fluid.283 On the contrary, in other models or conditions, phytoe- strogens were shown to act as antiestrogens and antiproliferative agents and could, therefore, affect reproductive functions. Prob- lems of infertility in sheep grazing on pastures or fodders rich in phytoestrogens were at the origin of their discovery.284 Sup- plementation of the diet of rat dams during the first 21 postnatal d with coumestrol (0.1 g/kg diet) produced a persistant estrus state in female offspring and a deficit in the sexual behavior of male offspring.285 Flavonoids may also influence the thyroid function and have goitrogenic effects. A reduction of thyroid peroxidase activityPDF Image | Dietary Polyphenols and the Prevention of Diseases
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