Flavanols: digestion,absorption and bioactivity

Flavanols, or flavan-3-ols, are a family of bioactive compounds present in cocoa, red wine, green tea, red grapes, berries and apples. With a basic monomer unit of (−)-epicatechin or (+)-catechin, flavanols can be present in foods and beverages as monomers or oligomers (procyanidins). Most, but not all, procyanidins are degraded into monomer or dimer units prior to absorption. The bioavailability of flavanols can be influenced by multiple factors, including food processing, cooking, digestion, and biotransformation. Flavanols are potent antioxidants, scavenging free radicals in vitro and in vivo. While some of the actions of flavanols can be linked to antioxidant activities, other modes of action may also occur, including modulation of intracellular signaling, effects on membrane fluidity and regulation of cytokine release or action. Physiologically, flavanol-rich foods and beverages can affect platelet aggregation, vascular inflammation, endothelial nitric oxide metabolism, and may confer protective effects against neurodegeneration. Epidemiological data suggests that intake of cocoa, a rich source of flavanols, is inversely associated with 15-year cardiovascular and all-cause mortality in older males. (−)-Epicatechin and its metabolite, epicatechin-7-O-glucuronide, have been identified as independent predictors of some of the vascular effects associated with the consumption of a flavanol-rich beverage. Targeted dietary components and nutrition supplements that can influence the vascular system will be of great value in the prevention and treatment of chronic disease.     

Biological activities of (−)-epicatechin and (−)-epicatechin-containing foods: Focus on cardiovascular and neuropsychological health

Recent studies have suggested that certain (?)-epicatechin-containing foods have a blood pressure-lowering capacity. The mechanisms underlying (?)-epicatechin action may help prevent oxidative damage and endothelial dysfunction, which have both been associated with hypertension and certain brain disorders. Moreover, (?)-epicatechin has been shown to modify metabolic profile, blood's rheological properties, and to cross the blood-brain barrier. Thus, (?)-epicatechin causes multiple actions that may provide unique synergy beneficial for cardiovascular and neuropsychological health. This review summarises the current knowledge on the biological actions of (?)-epicatechin, related to cardiovascular and brain functions, which may play a remarkable role in human health and longevity.     

Leaf flavonoid content in Quercus ilex L. resprouts and its seasonal variation

Here, we provide the first report on flavonoid content in holm oak (Quercus ilex L.) leaves, analyzed by HPLC–MS/MS. Flavanols and flavonols were the predominant groups, although proanthocyanidins and many soluble tannins had a relevant presence in all leaf samples. Seasonal variation of flavonoids was determined in extracts from Q. ilex leaves during resprouting after a forest fire in two Mediterranean forests. Similar seasonal trends were observed over 2 years during the two main stress seasons (winter and summer). The most abundant flavonoid was the flavanol epicatechin, which showed similar values during the two seasons. Hexosides of the flavonols, quercetin, kaempferol and rhamnetin showed considerably higher content in winter, especially at the lowest temperatures. These variations in both forests are discussed on the basis of the chlorophyll fluorescence results obtained. Anthocyanins were found practically absent in mature leaves. Nutrient or water availability differences between sites or seasons were not related to changes in leaf flavonol-hexoside content.     

Tea, flavonoids and stroke in man and mouse

Background and purpose

To evaluate the strength of the in vivo evidence of relationships between flavonoids and risk of stroke.

Methods

We reviewed the literature more broadly for flavonoids and stroke and conducted an evidence-based review of original publication experiments on tea or tea components on induced coronary occlusion in animal models and on the observational epidemiology on stroke and either tea or flavonoids in man. Each of the studies was evaluated by two independent reviewers. The evidence in total was compared with the Bradford Hill [1] and Stroke Therapy Academic Industry Roundtable (STAIR)¹ quality-assessment criteria [2].

Results

The search of epidemiologic publications revealed 7 cohort studies on flavonoid intake and stroke and 7 cohort studies and 3 case control studies on tea and stroke. In studies of tea there was a consistent protective effect. However, the epidemiologic research on flavonoids and stroke was much less consistent. Eleven animal experiments were identified that examined tea or tea components and stroke relevant sequelae, eight of which reported on infarct volume. All studies demonstrated reduced infarct volumes in animals exposed either to tea extracts, theanine or tea catechins prior to or shortly after reperfusion.

Conclusions

Hill’s criteria of causality are largely met in the case of tea and stroke. A high level of consistency across preclinical studies, of the effect of tea components as single agents effective in reducing stroke volume after middle cerebral artery occlusion, is noted in all rodent models (rat, mouse, and gerbil). Reductions in infarct volume are seen with both tea extracts consumed orally and tea components introduced intra-peritoneally. Observational epidemiology supports this finding in man for tea - the studies are consistent across countries and type of tea and the relative risks are moderately strong. That is not the case for the body of evidence on flavonoid intakes and stroke.     

Flavanol binding of nuclei from tree species

Light microscopy was used to examine the nuclei of five tree species with respect to the presence of flavanols. Flavanols develop a blue colouration in the presence of a special p-dimethylaminocinnamaldehyde (DMACA) reagent that enables those nuclei loaded with flavanols to be recognized. Staining of the nuclei was most pronounced in both Tsuga canadensis and Taxus baccata, variable in Metasequoia glyptostroboides, faint in Coffea arabica and minimal in Prunus avium. HPLC analysis showed that the five species contained substantial amounts of different flavanols such as catechin, epicatechin and proanthocyanidins. Quantitatively, total flavanols were quite different among the species. The nuclei themselves, as studied in Tsuga seed wings, were found to contain mainly catechin, much lower amounts of epicatechin and traces of proanthocyanidins. Blue-coloured nuclei located centrally in small cells were often found to maximally occupy up to 90% of a cells radius, and the surrounding small rim of cytoplasm was visibly free of flavanols. A survey of 34 gymnosperm and angiosperm species indicated that the first group has much higher nuclear binding capacities for flavanols than the second group.Abbreviations DMACA p-Dimethylaminocinnamaldehyde Communicated by W. Barz     

Cytotoxic effects of digalloyl dimer procyanidins in human cancer cell lines

Flavanols, a class of polyphenols present in certain plant-based foods, have received increasing attention for their putative anticancer activity. In vitro and in vivo studies, which have compared the effectiveness of various monomer flavanols, indicate that the presence of a galloyl residue on the 3 position on the C-ring enhances the cytotoxicity of these compounds. Procyanidins, oligomerized flavanols, have been reported to be more cytotoxic than monomer flavanols in a variety of human cancer cell lines. Given the above, we evaluated the potential anticancer properties of dimer procyanidins that contain galloyl groups. Specifically, the cytotoxicity of synthetic digalloyl dimer B1 and B2 esters {[3-O-galloyl]-(−)-epicatechin-(4β,8)-(+)-catechin-3-O-gallate (DGB1) and [3-O-galloyl]-(−)-epicatechin-(4β,8)-(+)-epicatechin-3-O-gallate (DGB2), respectively} were tested in a number of in vitro models. DGB1 produced significant cytotoxicity in a number of human cancer cell lines evaluated by three independent methods: ATP content, MTT and MTS assays. For the three most sensitive cell lines, exposure to DGB1 and DGB2 for 24, 48 or 72 h was associated with a reduction in cell number and an inhibition of cell proliferation. Digalloyl dimers exerted significantly higher cytotoxic effects than the structurally related flavanols, (−)-epicatechin, (+)-catechin, (−)-epicatechin gallate, (−)-epigallocatechin gallate, (−)-catechin gallate and dimer B1 and B2. These results support the concept that the incorporation of galloyl groups and the oligomerization of flavanols enhances the cytotoxic effects of typical monomer flavanols. The therapeutic value of these compounds and their derivative forms as anticancer agents merits further investigation in whole animal models.     

Polyphenols and health: Update and perspectives

Current issues in research on health effects by polyphenols are addressed. As to the cardiovascular system, flow-mediated dilation (FMD), a functional biomarker, can be used as surrogate marker for cardiovascular risk. Acute short-term effects peaking at 2 h after ingestion of polyphenol-rich food items are distinguished from longer-term effects over days and weeks. The role of polyphenol metabolites as bioactives is presented, underlining that specific target enzymes such as NADPH oxidases or lipoxygenases provide a basis for molecular action of polyphenols, rather than unspecific direct antioxidant effects. Cautionary words are given for the use of non-compositional assays of ‘total antioxidant capacity’ (TAC) in blood plasma. Enhanced interest emerges for polyphenols in the gastrointestinal tract. Recommendations for health professionals and the public are summarized, as well as prospects and challenges for future research.     

Cocoa flavanols lower vascular arginase activity in human endothelial cells in vitro and in erythrocytes in vivo

The availability of l-arginine can be a rate-limiting factor for cellular NO production by nitric oxide synthases (NOS). Arginase competes with NOS for l-arginine as the common substrate. Increased arginase activity has been linked to low NO levels, and an inhibition of arginase activity has been reported to improve endothelium-dependent vasorelaxation. Based on the above, we hypothesized that an increase in the circulating NO pool following flavanol consumption could be correlated with decreased arginase activity. To test this hypothesis we (a) investigated the effects of (−)-epicatechin and its structurally related metabolites on endothelial arginase expression and activity in vitro; (b) evaluated the effects of dietary flavanol-rich cocoa on kidney arginase activity in vivo; and (c) assessed human erythrocyte arginase activity following flavanol-rich cocoa beverage consumption in a double-blind intervention study with cross-over design. The results demonstrate that cocoa flavanols lower arginase-2 mRNA expression and activity in HUVEC. Dietary intervention with flavanol-rich cocoa caused diminished arginase activity in rat kidney and, erythrocyte arginase activity was lowered in healthy humans following consumption of a high flavanol beverage in vivo.     

Effect of perinatally supplemented flavonoids on brain structure,circulation, cognition,and metabolism in C57BL/6J mice

Evidence suggests that flavanol consumption can beneficially affect cognition in adults, but little is known about the effect of flavanol intake early in life. The present study aims to assess the effect of dietary flavanol intake during the gestational and postnatal period on brain structure, cerebral blood flow (CBF), cognition, and brain metabolism in C57BL/6J mice.Female wild-type C57BL/6J mice were randomly assigned to either a flavanol supplemented diet or a control diet at gestational day 0. Male offspring remained on the corresponding diets throughout life and performed cognitive and behavioral tests during puberty and adulthood assessing locomotion and exploration (Phenotyper and open field), sensorimotor integration (Rotarod and prepulse inhibition), and spatial learning and memory (Morris water maze, MWM). Magnetic resonance spectroscopy and imaging at 11.7T measured brain metabolism, CBF, and white and gray matter integrity in adult mice. Biochemical and immunohistochemical analyses evaluated inflammation, synaptic plasticity, neurogenesis, and vascular density.Cognitive and behavioral tests demonstrated increased locomotion in Phenotypers during puberty after flavanol supplementation (p = 0.041) but not in adulthood. Rotarod and prepulse inhibition demonstrated no differences in sensorimotor integration. Flavanols altered spatial learning in the MWM in adulthood (p = 0.039), while spatial memory remained unaffected. Additionally, flavanols increased diffusion coherence in the visual cortex (p = 0.014) and possibly the corpus callosum (p = 0.066) in adulthood. Mean diffusion remained unaffected, a finding that corresponds with our immunohistochemical data showing no effect on neurogenesis, synaptic plasticity, and vascular density. However, flavanols decreased CBF in the cortex (p = 0.001) and thalamus (p = 0.009) in adulthood. Brain metabolite levels and neuroinflammation remained unaffected by flavanols.These data suggest that dietary flavanols results in subtle alterations in brain structure, locomotor activity and spatial learning. Comparison of these data to published findings in aging or neurodegeneration suggests that benefits of dietary flavanols may increase with advancing age and in disease.