A daily dose of quercetin or (-)-epicatechin led to improvements in endothelial function, a key marker of cardiovascular health, according to results of a small randomized, placebo-controlled, crossover trial published in the American Journal of Clinical Nutrition.
“Numerous studies have shown that acute and repetitive consumption of flavonoid-rich foods for up to four weeks can improve endothelial function in both subjects with coronary artery disease and healthy volunteers,” wrote the researchers, led by Professor Kevin Croft fro the University of Western Australia.
“However, to date, there is little direct evidence that flavonoids are the bioactive molecules responsible. We have shown that oral administration of pure dietary flavonoids, quercetin, and (-)-epicatechin augment NO status in healthy men,” they added.
Nitric oxide (NO) is a molecule used by the endothelium to signal surrounding muscle to relax.
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.
Study details
Croft and his co-workers recruited 12 healthy men with an average age of 43.2, and an average BMI of 25.1 kg/m2, and randomly assigned them to receive a 200 mg dose of quercetin, (-)-epicatechin, or epigallocatechin gallate.
“These amounts were chosen as they represent a reasonable dose that could be achieved by eating flavonoid-rich foods such as chocolate, onions, and tea,” they explained.
The participants’ response to these doses, in terms of changes to vasodilators such as NO, and vasoconstrictors, such as endothelin-1 (ET-1), was measured. Oxidative stress was also assessed by measuring levels of compounds called F2-isoprostanes in the blood and urine.
The researchers report a significant increase in NO when the subjects consumed quercetin and (-)-epicatechin, while they did not note any changes to NO as a result of the epigallocatechin gallate dose.
Moreover, both quercetin and (-)-epicatechin doses were associated with significant reductions in ET-1 blood levels, while only quercetin was associated with significant decreases in urinary ET-1.
No effects from any flavonoid dose were observed on blood or urinary levels of F2-isoprostane, added the researchers.
Bioavailability questions
Importantly, Croft and his co-workers did record significant increases in blood levels of all three flavonoids. This result indicated that the flavonoids were absorbed by the subjects.
“There is a growing body of evidence from controlled human trials casting doubt as to whether flavonoids can act as antioxidants in vivo,” wrote the researchers.
“Because oxidative stress is implicated in the development of cardiovascular diseases, one of the main properties thought to explain the effects of flavonoids is their antioxidant property. However, recent studies carried out in this area should be interpreted with caution because the native unmodified forms of flavonoids found in the diet were used in in vitro experiments instead of the metabolites found in vivo.
“We recently showed that structural modification of flavonoids, such as quercetin, by metabolic transformation, is likely to have a profound effect on biological activity,” they said.
Ongoing study
Prof Croft told NutraIngredients.com that research in the area if ongoing. “[New studies] will include some longer term follow-up studies where we intend to measure vascular function and BP as well as NO release,” he said.
“We are also conducting a long term intervention study of the effects of black tea polyphenols on BP in a human population.”
Source: American Journal of Clinical NutritionVolume 88, pages 1018-1025“Pure dietary flavonoids quercetin and (-)-epicatechin augment nitric oxide products and reduce endothelin-1 acutely in healthy men”Authors: W.M. Loke, J.M. Hodgson, J.M. Proudfoot, A.J. McKinley, I.B. Puddey, K.D. Croft.