Health effects of phenols and polyphenols

Health effects of phenols and polyphenols are the possible health outcomes of consuming polyphenols from polyphenol-rich foods or dietary supplements. Although polyphenols were once considered as antioxidants, this concept is obsolete. The fate and mechanisms of many polyphenols are unknown due to their large structural diversity and extensive metabolism, making their effects difficult to assess in vivo.

Despite considerable research, there are only two authorised health claims in the European Union: 1) "maintenance of normal endothelium-dependent vasodilation" by consuming a high daily dose (200 mg) of flavan-3-ols from cocoa solids; 2) protection of LDL from oxidative damage by consuming 5 mg daily of hydroxytyrosol and its derivatives in olive oil.

Bioavailability

Polyphenols vary considerably in their pharmacokinetic properties. They undergo extensive metabolism, both upon absorption in the small intestine and by the colonic microbiome, and only small amounts of the original compound are present in blood. Bioavailability is therefore low for the parent compound, but larger amounts of metabolites can be present. Bioavailability of polyphenols depends on many factors, including the foods consumed.

Non-nutrient status

Polyphenols are not considered nutrients as they neither provide dietary energy supply, nor are used for growth, survival or reproduction. Therefore, they do not have recommended daily intake levels, as exist for vitamins, minerals, and fiber in the European Union, United Kingdom and United States. Accordingly, in the United States, polyphenols do not appear on food labels with Daily Values for the content present in food or dietary supplement products, as used for vitamins and minerals. Some polyphenols are considered to be bioactive compounds for which the development of dietary recommendation is under consideration.

Toxicity and adverse effects

Adverse effects of polyphenol intake range from mild (e.g., gastrointestinal tract symptoms) to severe (e.g., hemolytic anemia or hepatotoxicity). In 1988, hemolytic anemia following polyphenol consumption was documented, resulting in the withdrawal of a catechin-containing drug.

Metabolism of polyphenols can result in flavonoid-drug interactions, such as in grapefruit–drug interactions, which involves inhibition of the liver enzyme, CYP3A4, likely by grapefruit furanocoumarins, a class of polyphenol. The European Food Safety Authority established upper limits for some polyphenol-containing supplements and additives, such as green tea extract or curcumin. For most polyphenols found in the diet, an adverse affect beyond nutrient-drug interactions is however unlikely.

Research

Cardiovascular diseases

Polyphenols were originally considered to be essential for vascular function and a considerable amount of research has focused on the effect on cardiovascular health. For most polyphenols, there is no evidence for an effect on cardiovascular health, although there are reviews showing a minor effect of a number of polyphenols such as chlorogenic acid or flavan-3-ols on blood pressure.

There are currently two authorised health claims in the European Union, based on opinions by the European Food Safety Authority on the effects of consuming polyphenols in specific conditions: 1) flavanols in cocoa at doses exceeding 200 mg per day may contribute to normal function of the vascular endothelium; 2) olive oil polyphenols (5 mg of hydroxytyrosol and its derivatives (e.g. oleuropein complex and tyrosol) may "contribute to the protection of blood lipids from oxidative damage", if consumed daily.[1]

Cancer

As of 2019, there is little evidence that dietary flavonoids lower the risk of cancer.

Cognitive function

Polyphenols are under preliminary research for possible cognitive effects in healthy adults.

Phytoestrogens

Isoflavones, which are structurally related to 17β-estradiol, are classified as phytoestrogens. There is little scientific evidence that consuming isoflavones has an effect on health or disease. A risk assessment by the European Food Safety Authority found no cause for concern when isoflavones are consumed in a normal diet.

Phlebotonic

Main article: Diosmin § Phlebotonics

Phlebotonics of heterogeneous composition, consisting partly of citrus peel extracts (flavonoids, such as hesperidin) and synthetic compounds, are used to treat chronic venous insufficiency and hemorrhoids. Some are non-prescription dietary supplements, such as Diosmin, while one other – Vasculera (Diosmiplex) – is a prescription medical food intended for treating venous disorders. Their mechanism of action is undefined, and clinical evidence of benefit for using phlebotonics to treat venous diseases is limited.

Gut microbiome

Polyphenols are extensively metabolized by the gut microbiota and are investigated as a potential metabolic factor in function of the gut microbiota.

Antioxidant activity

Most polyphenols are metabolized by catechol-O-methyltransferase, and therefore do not have the chemical structure allowing antioxidant activity in vivo; they may exert biological activity as signaling molecules.

See also

• Polyphenol
• Polyphenol antioxidant
• Flavonoid

Further reading

• Fraga CG, ed. (2010). Plant Phenolics and Human Health: Biochemistry, Nutrition and Pharmacology. Wiley. ISBN 978-0-470-28721-7. Search this book on

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