Bioavailability and bioefficacy of polyphenols in humans. II. Review of 93 intervention studies

Abstract

For some classes of dietary polyphenols, there are now sufficient intervention studies to indicate the type and magnitude of effects among humans in vivo, on the basis of short-term changes in biomarkers. Isoflavones (genistein and daidzein, found in soy) have significant effects on bone health among postmenopausal women, together with some weak hormonal effects. Monomeric catechins (found at especially high concentrations in tea) have effects on plasma antioxidant biomarkers and energy metabolism. Procyanidins (oligomeric catechins found at high concentrations in red wine, grapes, cocoa, cranberries, apples, and some supplements such as Pycnogenol) have pronounced effects on the vascular system, including but not limited to plasma antioxidant activity. Quercetin (the main representative of the flavonol class, found at high concentrations in onions, apples, red wine, broccoli, tea, and Ginkgo biloba) influences some carcinogenesis markers and has small effects on plasma antioxidant biomarkers in vivo, although some studies failed to find this effect. Compared with the effects of polyphenols in vitro, the effects in vivo, although significant, are more limited. The reasons for this are 1) lack of validated in vivo biomarkers, especially in the area of carcinogenesis; 2) lack of long-term studies; and 3) lack of understanding or consideration of bioavailability in the in vitro studies, which are subsequently used for the design of in vivo experiments. It is time to rethink the design of in vitro and in vivo studies, so that these issues are carefully considered. The length of human intervention studies should be increased, to more closely reflect the long-term dietary consumption of polyphenols.

Enriched cereal bars are more effective in increasing plasma quercetin compared with quercetin from powder-filled hard capsules

“…In accordance with previous human studies, we observed a relatively high inter-individual variance in plasma concentration -time curves of quercetin, isorhamnetin and tamarixetin following supplementation with quercetin (15 -18) , although our non-smoking female study population was very homogenous with respect to age, baseline plasma quercetin concentrations, body weight, BMI, fat mass and fat-free mass as well as lifestyle and physical activity. This variability in plasma kinetic profiles may be attributed to inter-individual differences in digestive transit time, preferential site of absorption and enteric bacterial metabolism of quercetin as reported in the literature for quercetin and other polyphenols (3,19,20) as well as to the intrinsic solubilisation capacity of each individual.…”

Associations between dietary flavonoid intakes and bone health in a scottish population

“…The validity study did not use biomarkers because correlations between these and dietary records of flavonoid intakes are not good (r ¼ 0.35) (33) owing to their bioavailability and metabolism in the gut. (34)(35)(36) Limitations of this study include those associated with using an FFQ because FFQs are known to overestimate the true overall frequency of consumption of fruit and vegetables. (37,38) There are issues that affect the flavonoid content of foods, namely, analytical and environmental factors, processing, storage, and species differences, and these cannot be detected using a FFQ because it is a crude measure of diet.…”

Dietary genistein stimulates mammary hyperplasia in gilts

“…Studies in humans have shown promising, yet variable results. Consumption of isoflavones for up to a year increased expression of beneficial bone biomarkers, such as markers for bone mineral density (Williamson and Manach, 2005). On the other hand, feeding 1 mg of genistein to humans for a year did not alter bone density or bone resorption but increased markers of bone formation (Atkinson et al, 2004).…”