The beneficial health effects of polyphenols from green tea, berries, olive oil, cereals and a wide range of other foods have been heavily linked to a preventive role for the main chronic diseases such as cardiovascular, cancer, and all inflammatory diseases.
The majority of specialists agree that a high consumption of polyphenols can decrease and prevent all these diseases.
However, some concerns have been raised in a new article published in the American Chemical Society's journal Chemical Research in Toxicology.
Scientists from Rutgers, the State University of New Jersey state: "Although consumption of dietary phytochemicals such as flavonoids has been suggested to have beneficial biological effects.
Including the prevention of cancer and heart disease, there is considerable evidence to suggest that such compounds are not without risk of adverse effects.
"The risk of adverse effects is likely increased by the use of pharmacological doses in prevention/treatment and supplement situations… that may increase the bioavailability of test compounds."
The article by Joshua Lambert, Shengmin Sang and Chung Yang has been called paper science with no real world value by Daniel Fabricant, PhD, vice president, scientific and regulatory affairs at US-based trade association The Natural Products Association (NPA).
Lambert and co-workers reviewed in vitro and in vivo studies for a wide range of polyphenols, particularly epigallocatechin gallate (EGCG) and other green tea polyphenols, and genistein from soy.
They quote studies that report an apparent pro-oxidant effect of the polyphenols.
Such an effect would increase oxidative stress at a cellular level and increase the risk of disease.
"Despite several human studies that showed no toxicity of tea polyphenol preparations and that the major adverse effects associated with consumption of high doses of tea preparations are due to gastrointestinal irritation, there have been a number of recent case reports of hepatotoxicity [liver toxicity] related to the consumption of high doses of tea-based dietary supplements (10-29 mg/kg/day)," wrote Lambert.
"Although there have been no reports of toxicity in human volunteers enrolled in intervention studies, careful monitoring of liver and kidney function is required until the risk of toxic events associated with tea catechins is established in humans," he added.
This point was seized on by Dr. Fabricant, who told NutraIngredients.com: "For every one in vivo or in vitro paper on pro-oxidant risk there are thousands on anti-oxidant benefit, more importantly, all the in vitro and in vivo data in the world does not take the place of what we know from clinical research, which is available in droves on polyphenolic extracts, especially green tea and some of the pure compounds referenced in the paper, these extracts and compounds have not demonstrated an increased risk in trials."
Fabricant stressed that, contrary to the authors' claims to the contrary on-going clinical trials do include protocols to assess potential adverse events, (good clinical practice guidelines), and that adverse events are essentially non-existent in trials of green tea, and some of the other compounds and extracts cited in the study.
"The authors lump in vivo and in vitro data together, lump experiments on different animal species together, lump different routes of administration together to draw a broad conclusion about potential toxicity, when they have no substance to do such," he said.
"Solid scientific evaluation of toxicity involves two steps: hazard identification and dose-response evaluation, the authors do neither and should demonstrate some scientific responsibility and refrain from speculating on toxicity."
It would appear that Lambert, Sang and Yang's comment could merely be a storm in a teacup.
Source: Chemical Research in Toxicology 2007, Volume 20, Pages 583-585 "Possible controversy over dietary polyphenols: Benefits vs risks" Authors: J. Lambert, S. Sang and C.S. Yang