Gary Williamson

Dietary Intake and Bioavailability of Polyphenols

The main dietary sources of polyphenols are reviewed, and the daily intake is calculated for a given diet containing some common fruits, vegetables and beverages. Phenolic acids account for about one third of the total intake and flavonoids account for the remaining two thirds. The most abundant flavonoids in the diet are flavanols (catechins plus proanthocyanidins), anthocyanins and their oxidation products. The main polyphenol dietary sources are fruit and beverages (fruit juice, wine, tea, coffee, chocolate and beer) and, to a lesser extent vegetables, dry legumes and cereals. The total intake is approximately 1 g/d. Large uncertainties remain due to the lack of comprehensive data on the content of some of the main polyphenol classes in food. Bioavailability studies in humans are discussed. The maximum concentration in plasma rarely exceeds 1 microM after the consumption of 10-100 mg of a single phenolic compound. However, the total plasma phenol concentration is probably higher due to the presence of metabolites formed in the bodys tissues or by the colonic microflora. These metabolites are still largely unknown and not accounted for. Both chemical and biochemical factors that affect the absorption and metabolism of polyphenols are reviewed, with particular emphasis on flavonoid glycosides. A better understanding of these factors is essential to explain the large variations in bioavailability observed among polyphenols and among individuals.

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

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.

Deglycosylation of flavonoid and isoflavonoid glycosides by human small intestine and liver β-glucosidase activity

Flavonoid and isoflavonoid glycosides are common dietary phenolics which may be absorbed from the small intestine of humans. The ability of cell‐free extracts from human small intestine and liver to deglycosylate various (iso)flavonoid glycosides was investigated. Quercetin 4′‐glucoside, naringenin 7‐glucoside, apigenin 7‐glucoside, genistein 7‐glucoside and daidzein 7‐glucoside were rapidly deglycosylated by both tissue extracts, whereas quercetin 3,4′‐diglucoside, quercetin 3‐glucoside, kaempferol 3‐glucoside, quercetin 3‐rhamnoglucoside and naringenin 7‐rhamnoglucoside remained unchanged. The K m for hydrolysis of quercetin 4′‐glucoside and genistein 7‐glucoside was ∼32±12 and ∼14±3 μM in both tissues respectively. The enzymatic activity of the cell‐free extracts exhibits similar properties to the cytosolic broad‐specificity β‐glucosidase previously described in mammals.

Dietary flavonoid and isoflavone glycosides are hydrolysed by the lactase site of lactase phlorizin hydrolase

Lactase phlorizin hydrolase (LPH; EC 3.2.1.62) is a membrane‐bound, family 1 β‐glycosidase found on the brush border of the mammalian small intestine. LPH, purified from sheep small intestine, was capable of hydrolysing a range of flavonol and isoflavone glycosides. The catalytic efficiency (k cat/K m) for the hydrolysis of quercetin‐4′‐glucoside, quercetin‐3‐glucoside, genistein‐7‐glucoside and daidzein‐7‐glucoside was 170, 137, 77 and 14 (mM−1 s−1) respectively. The majority of the activity occurred at the lactase and not phlorizin hydrolase site. The ability of LPH to deglycosylate dietary (iso)flavonoid glycosides suggests a possible role for this enzyme in the metabolism of these biologically active compounds.

Human metabolism of dietary flavonoids: Identification of plasma metabolites of quercetin

The position of conjugation of the flavonoid quercetin dramatically affects biological activity in vitro, therefore it is important to determine the exact nature of the plasma metabolites. In the present study, we have used various methods (HPLC with diode array detection, LCMS, chemical and enzymic synthesis of authentic conjugates and specific enzymic hydrolysis) to show that quercetin glucosides are not present in plasma of human subjects 1.5 h after consumption of onions (a rich source of flavonoid glucosides). All four individuals had similar qualitative profiles of metabolites. The major circulating compounds in the plasma after 1.5 h are identified as quercetin-3-glucuronide, 3′-methyl-quercetin-3-glucuronide and quercetin-3′-sulfate. The existence of substitutions in the B and/or C ring of plasma quercetin metabolites suggests that these conjugates will each have very different biological activities.

A Review of the Health Effects of Green Tea Catechins in In Vivo Animal Models

There is good evidence from in vitro studies that green tea catechins have a role in protection against degenerative diseases. However, the concentrations used in vitro are often higher than those found in animal or human plasma, and so in vivo evidence is required to demonstrate any protective effect of catechins. This article summarizes the most interesting in vivo animal studies on the protective effects of green tea catechins against biomarkers for cancer, cardiovascular disease, and other degenerative diseases. Generally, most studies using animal models show that consumption of green tea (catechins) provides some protection, although most studies have not examined dose response. Tea catechins could act as antitumorigenic agents and as immune modulators in immunodysfunction caused by transplanted tumors or by carcinogen treatment. Green tea has antiproliferative activity in hepatoma cells and hypolipidemic activity in hepatoma-treated rats, and some studies report that it prevents hepatoxicity. It could act as a preventive agent against mammary cancer postinitiation. Nevertheless, the implications of green tea catechins in preventing metastasis have not been clearly established. Long-term feeding of tea catechins could be beneficial for the suppression of high-fat diet-induced obesity by modulating lipid metabolism, could have a beneficial effect against lipid and glucose metabolism disorders implicated in type 2 diabetes, and could also reduce the risk of coronary disease. Further investigations on mechanisms, the nature of the active compounds, and appropriate dose levels are needed.

Use of metabolically competent human hepatoma cells for the detection of mutagens and antimutagens.

The human hepatoma line (Hep G2) has retained the activities of various phase I and phase II enzymes which play a crucial role in the activation/detoxification of genotoxic procarcinogens and reflect the metabolism of such compounds in vivo better than experimental models with metabolically incompetent cells and exogenous activation mixtures. In the last years, methodologies have been developed which enable the detection of genotoxic effects in Hep G2 cells. Appropriate endpoints are the induction of 6-TGr mutants, of micronuclei and of comets (single cell gel electrophoresis assay). It has been demonstrated that various classes of environmental carcinogens such as nitrosamines, aflatoxins, aromatic and heterocyclic amines and polycyclic aromatic hydrocarbons can be detected in genotoxicity assays with Hep G2 cells. Furthermore, it has been shown that these assays can distinguish between structurally related carcinogens and non-carcinogens, and positive results have been obtained with rodent carcinogens (such as safrole and hexamethylphosphoramide) which give false negative results in conventional in vitro assays with rat liver homogenates. Hep G2 cells have also been used in antimutagenicity studies and can identify mechanisms not detected in conventional in vitro systems such as induction of detoxifying enzymes, inactivation of endogenously formed DNA-reactive metabolites and intracellular inhibition of activating enzymes.

A critical review of the bioavailability of glucosinolates and related compounds

Glucosinolates (GLSs) are relatively inert (Z)-N-hydroximinosulfate esters, possessing a sulfur-linked beta-D-glucopyranose moiety and a variable side chain, found almost exclusively in cruciferous vegetables. Following cell disruption, they are hydrolysed by plant myrosinases, forming a group of chemically reactive and biologically active compounds. There is considerable evidence that these breakdown products, when consumed in the diet, may affect the risk of developing chronic diseases. However, in order for any compound to exert an activity in vivo, it is necessary to reach the site of action in an appropriate form and sufficient concentration. Deleterious and toxic effects may be observed at high concentrations: hence, bioavailability is a key factor defining the physiological, beneficial dose window of GLS hydrolysis products (GLS-HPs). For some GLS-HPs, this window can be rather narrow, and therefore is a critical parameter to be considered. In this review we critically evaluate the present state of knowledge on all factors that affect bioavailability of GLS-HPs. This includes liberation from the plant material, absorption from the digestive system, distribution around the body, metabolism and excretion.

Metabolite Profiling of Hydroxycinnamate Derivatives in Plasma and Urine after the Ingestion of Coffee by Humans: Identification of Biomarkers of Coffee Consumption

Human subjects drank coffee containing 412 μmol of chlorogenic acids, and plasma and urine were collected 0 to 24 h after ingestion and were analyzed by high-performance liquid chromatography-mass spectrometry. Within 1 h, some of the components in the coffee reached nanomole peak plasma concentrations (Cmax), whereas chlorogenic acid metabolites, including caffeic acid-3-O-sulfate and ferulic acid-4-O-sulfate and sulfates of 3- and 4-caffeoylquinic acid lactones, had higher Cmax values. The short time to reach Cmax (Tmax) indicates absorption of these compounds in the small intestine. In contrast, dihydroferulic acid, its 4-O-sulfate, and dihydrocaffeic acid-3-O-sulfate exhibited much higher Cmax values (145–385 nM) with Tmax values in excess of 4 h, indicating absorption in the large intestine and the probable involvement of catabolism by colonic bacteria. These three compounds, along with ferulic acid-4-O-sulfate and dihydroferulic acid-4-O-glucuronide, were also major components to be excreted in urine (8.4–37.1 μmol) after coffee intake. Feruloylglycine, which is not detected in plasma, was also a major urinary component (20.7 μmol excreted). Other compounds, not accumulating in plasma but excreted in smaller quantities, included the 3-O-sulfate and 3-O-glucuronide of isoferulic acid, dihydro(iso)ferulic acid-3-O-glucuronide, and dihydrocaffeic acid-3-O-glucuronide. Overall, the 119.9 μmol excretion of the chlorogenic acid metabolites corresponded to 29.1% of intake, indicating that as well as being subject to extensive metabolism, chlorogenic acids in coffee are well absorbed. Pathways for the formation of the various metabolites within the body are proposed. Urinary dihydrocaffeic acid-3-O-sulfate and feruloylglycine are potentially very sensitive biomarkers for the consumption of relatively small amounts of coffee.

Nutrigenomics and nutrigenetics: the emerging faces of nutrition

The recognition that nutrients have the ability to interact and modulate molecular mechanisms underlying an organisms physiological functions has prompted a revolution in the field of nutrition. Performing population‐scaled epidemiological studies in the absence of genetic knowledge may result in erroneous scientific conclusions and misinformed nutritional recommendations. To circumvent such issues and more comprehensively probe the relationship between genes and diet, the field of nutrition has begun to capitalize on both the technologies and supporting analytical software brought forth in the post‐genomic era. The creation of nutrigenomics and nutrigenetics, two fields with distinct approaches to elucidate the interaction between diet and genes but with a common ultimate goal to optimize health through the personalization of diet, provide powerful approaches to unravel the complex relationship between nutritional molecules, genetic polymorphisms, and the biological system as a whole. Reluctance to embrace these new fields exists primarily due to the fear that producing overwhelming quantities of biological data within the confines of a single study will submerge the original query; however, the current review aims to position nutrigenomics and nutrigenetics as the emerging faces of nutrition that, when considered with more classical approaches, will provide the necessary stepping stones to achieve the ambitious goal of optimizing an individuals health via nutritional intervention. Mutch, D. M., Wahli, W., Williamson, G. Nutrigenomics and nutrigenetics: the emerging faces of nutrition. FASEB J. 19, 1602–1616 (2005)