Wine is a complex mixture of phenolic compounds which have not yet been fully characterised. Plant phenolics are good in vivo antioxidants and also though to be able to modulate gene expression. These effects confer health benefits to those who consume wine, as has been evidence anecdotally for centuries and confirmed in recent epidemiological studies. Although plant phenols are acknowledged to have health benefits, recent evidence suggests that the biological effects of phenolics comes from their metabolites and not the parent compounds present in the wine. Phenolics are extensively metabolised by enterocytes in the gut during absorption, by the liver during first pass metabolism and also by bacterial colonies within the colon. Parent compounds tend not to be present in high concentrations within the plasma or urine following ingestion in humans or animals.
Stilbenes are a group of chemicals, the most common of which is resveratrol. Stilbene concentrations in red grape skins ranges from 50 to 100 mg.kg-1 and roughly 20 mg/L in red wine. Resveratrol is present in grapes as both cis-resveratrol and trans-resveratrol, both of which are extensively metabolised by enterocytes of the gut and then absorbed in humans. Resveratrol has been extensively researched and data shows that it is metabolised at a lower rate than many other phenolic compounds such as the catechin and epicatechin (flavan-3-ols). Resveratrol is detected in urine unmetabolised, but urine contains mainly sulphated and glucuronidated conjugates. The beneficial effects of resveratrol may relate to its ability to inhibit the oxidation of LDL cholesterol. Both cis- and trans-resveratrol glucuronide metabolites are detectable in LDL cholesterol particles after ingestion of red wine.
Phenolic acids are an important group of plant phenolics and are present in the pulp of grapes and also in both red and white wine. Phenolic acids may confer anticancer effects. Phenolic acids are divided into hydroxybenzoic acid (e.g. p-hydroxybenzioc acid, gallic acid and ellagic acid) and hydroxycinnamic acid (e.g. p‑coumaric acid, caffeic acid, caftaric acid and ferulic acid) compounds. The major phenolic acids found in grapes are caftaric and gallic acid, but in wines the major phenolic acids are caffeic and gallic acids. Phenolic acids are extensively metabolised by bacterial colonies within the colon, but their absorption and metabolism is not fully understood. Metabolites of phenolic acids are known to be absorbed and transported to the liver where they are further metabolised to hippuric acid and excreted. The intestinal metabolism of phenolic acids can result in the production of both glucuronidated and sulphated forms.
Anthocyanins (e.g. cyanidin-3-glucoside and malvidin-3-glucoside) are also present in wine with concentrations reaching around 500 mg/L in red wines. Anthocyanins are thought to provide protection from cancer and cardiovascular disease. Those populations that consume large quantities of wine tend to have higher intakes of anthocyanins because wine is an important source. Evidence suggests anthocyanin absorption is low in humans, but the glycoside forms of anthocyanins are absorbed intact. As well as the glycoside forms, humans studies have shown that humans are able to metabolise anthocyanins. Anthocyanin sulphates and glucuronides are detected in human plasma and some studies suggest that anthocyanins are methylated. Conjugates of anthocyanins are detected in the urine of human subjects. Anthocyanins can also be metabolised by gut bacteria to form phenolic acids which are then absorbed and further metabolised in the liver.
Flavonols are an important class of phenolic compounds that have been researched in relation to their health benefits in cardiovascular disease and cancer. Flavonol levels in wine can be around 50 mg/L in red wine, but are almost absent from white wine. Flavonols found in wine include glycosides of quercetin, myricetin, syringetin, laricetin, kaempferol and isorhamnetin. Quercetin appears to be absorbed in larger quantities when ethanol is present, such as with red wine. Flavonols are extensively metabolised by gut enterocytes during absorption and are present in plasma and urine as glucuronide and sulphate conjugates. Small amounts of unmetabolised flavonols are also detected in urine after ingestion of flavonol rich foods. Quercetin can be methylated to form isorhamnetin and tamarixetin and this metabolism occurs in the liver. Some evidence suggests that the glycoside forms of flavonols are absorbed in low amounts.
Flavan-3-ols (e.g. catechin and epicatechin) represent a group phenolic compounds that belong to the flavonoid sub-category. Flavan-3-ols are found in the skins and seeds of grapes and as such, white wine is a poor source. Flavan-3-ols have been researched in relation to their ability to prevent cardiovascular disease, cancer and help maintain appropriate body weight. Catechin is present in red wine at roughly 100 mg/L and epicatechin is present at roughly 75 mg/L. Consumption of red wine increases plasma levels of flavon-3-ols, which may protect plasma lipoproteins from oxidation. Flavan‑3‑ols are extensively metabolised in the enterocytes of the small intestine and sulphate and glucuronide conjugates are detected in plasma. Flavan-3-ols and conjugates from the enterocyte metabolism can also be further metabolised by colonic micro flora to phenolic acids which are then absorbed from the large intestine.
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