Evidence for health benefits of plant phenols: local or systemic effects?

Authors

  • Peter C H Hollman

    Corresponding author
    1. State Institute for Quality Control of Agricultural Products (RIKILT), Bornsesteeg 45, NL-6708 PD Wageningen, The Netherlands
    • State Institute for Quality Control of Agricultural Products (RIKILT), Bornsesteeg 45, NL-6708 PD Wageningen, The Netherlands
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Abstract

Plant phenols are mostly products of the phenylpropanoid pathway and comprise a large variety of compounds: cinnamic acids, benzoic acids, flavonoids, proanthocyanidins, stilbenes, coumarins, lignans and lignins. They are strong antioxidants and might prevent oxidative damage to biomolecules such as DNA, lipids and proteins which play a role in chronic diseases such as cancer and cardiovascular disease. Plant phenols may interfere with all stages of the cancer process, potentially resulting in a reduction of cancer risk. Only flavonols have been investigated in observational studies. Five out of seven studies showed an inverse association of flavonol intake with subsequent cardiovascular disease (CVD). A protective effect against cancer was only found in one out of four studies. Thus the epidemiological evidence does not yet allow a firm decision on the involvement of flavonols in the aetiology of either CVD or cancer. The epidemiology of flavonols points to a systemic effect. The epidemiology of tea, as a rich source of various phenols, shows inconsistent data for colon cancer, which also does not support a local effect of plant phenols. The absorption and bioavailability of plant phenols have been inadequately studied. Dietary flavonoids were thought to be poorly absorbed because of their presence as β-glycosides (conjugates of sugars). However, conjugation with glucose enhanced human absorption. Flavonoids and other plant phenols are extensively metabolised by colonic bacteria: the ring structure is cleaved, giving a range of phenolic acids which are then absorbed. Human studies showed that only about 1% of a well-absorbed flavonoid was excreted with an intact flavonoid backbone into urine. Major questions to be answered are whether the effective concentrations found in in vitro systems really reflect physiological concentrations.

© 2001 Society of Chemical Industry

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