Despite a wealth of studies reporting beneficial effects for the compounds from chocolate, soy and tea, "for many of the other flavonoids, there was insufficient evidence to draw conclusions about efficacy," reports lead author Lee Hooper from the University of East Anglia in the United Kingdom.
Hooper and co-workers included 133 trials in their meta-analysis.
In a telling note on the state of the science, the researchers state that they could not find any randomised controlled trials looking at flavonoids on morbidity or mortality from cardiovascular disease.
The meta-analysis, described in an independent editorial as showing "the state of the art in flavonoid and cardiovascular research" , is published in this month's American Journal of Clinical Nutrition .
A vast body of epidemiological studies has linked increased dietary intake of antioxidants from fruits, vegetables wine, chocolate, coffee, tea, and other foods to reduced risks of a range of diseases including cancer, cardiovascular disease and diabetes.
Flavonoids can be split into a number of sub-classes, including anthocyanins found in berries, flavonols from a variety of fruit and vegetables, flavones from parsley and thyme, for example, flavanones from citrus, isoflavones from soy, mono- and poly-meric flavonols like the catechins in tea, and proanthocyanidins from berries, wine and chocolate.
The non-flavonoids include phenolic acids, lignans, and stilbenes such as resveratrol.
Meta-analysis The researchers used the data from the 133 studies and found wide variations in effects depending on the nature of the flavonoid and the food source.
Flavonoids from chocolate were associated with a four per cent increase in blood flow, and a reduction in systolic and diastolic blood pressure of 5.88 and 3.30 mmHg, respectively.
Significant blood pressure reductions were also observed for soy protein isolate, but not other soy products or components, said the researchers.
Moreover, LDL cholesterol reductions were also observed.
Pooling the results of trials looking at green tea consumption, Hooper and co-workers found that the flavonoids reduced LDL cholesterol levels, but did not affect blood pressure.
On the other hand, black tea consumption was linked to increases in systolic and diastolic blood pressure of 5.69 and 2.56 mmHg, respectively.
"To date, the effects of flavonoids from soy and cocoa have been the main focus of attention," wrote the researchers.
"Future studies should focus on other commonly consumed subclasses, for example anthocyanins and flavanones, examine dose-response effects, and be of long enough duration to allow assessment of clinically relevant endpoints," they concluded.
Independent comment In an accompanying editorial, Johanna Geleijnse and Peter Hollman from Wageningen University in the Netherlands noted that the contribution of flavonones to a person's antioxidant capacity was significant.
"More than 6000 different flavonoids in plants have been described, and their total intake could amount to 1 g/d, whereas combined intakes of beta-carotene, vitamin C, and vitamin E from food most often are less than 100 mg/d," they said.
While commending the work of Hooper and co-workers, Geleijnse and Hollman commented: "Substantial evidence for a vasoprotective effect of specific flavonoids is, however, still lacking.
Optimal doses of specific flavonoids for cardiovascular protection, one of the aims of the review, are still beyond the horizon.
Flavonoid research has made large progress since the [early days of flavonoid research], but, to really advance the field, the step to individual flavonoids must be made now."
Source: American Journal of Clinical Nutrition 1 July 2008; Volume 88, Number 1, Pages 38-50 "Flavonoids, flavonoid-rich foods, and cardiovascular risk: a meta-analysis of randomized controlled trials" Authors: L. Hooper, P.A. Kroon, E.B. Rimm, J.S. Cohn, I. Harvey, K.A. Le Cornu, J.J. Ryder, W.L. Hall, A. Cassidy Editorial: American Journal of Clinical Nutrition 1 July 2008; Volume 88, Number 1, Pages 12-13 "Flavonoids and cardiovascular health: which compounds, what mechanisms?"
Authors: J.M. Geleijnse, P.C.H. Hollman