Flavonols are of widespread occurrence in the most common edible fruits, vegetables, and seeds, and their content may reach up to a few hundred mg kg-1 of fresh weight. Berries such as bog whortleberry (Vaccinium uliginosum) and cranberry (Vaccinium oxycoccos) have been found to contain 184 and 263 mg kg-1 fresh weight of total flavonols, respectively. Certain vegetables, however, such as onions (Allium cepa L.) and kale (Brassica oleracea L. cv. Alba DC.), could be considered as exceptional dietary sources, as their content in total flavonols may reach 349 and 311 mg kg-1 fresh weight, respectively, determined as aglycons.
As flavonols constitute an integral part of the human diet, some effort has been expended on the determination of flavonol levels that are consumed daily by humans. Early estimations showed the daily average intake of total flavonoids to be about 1.0 g, with 115 mg being the share of flavonols and flavones. Recently, the “Seven Countries Study” revealed that total daily flavonoid intake may vary from 2.6 to 68.2 mg, with the percentage of quercetin being 39-100%. In another study on 17 volunteers from 14 countries, mean consumption of quercetin and kaempferol was found to be approximately 28 mg/day.
The investigations on the effects of flavonoid-containing plant foods have been based, to a great extent, on a limited number of epidemiological studies. In these studies the flavonoid (mainly flavone and flavonol) content of commonly consumed plant foods and products was used as a crucial factor in estimating the average daily flavonoid intake. However, food composition tables, which are necessary tools for epidemiological and nutritional studies, have been created on the basis of analytical data from raw plant tissues, and thus they actually represent the composition of foodstuffs only in their raw state. Environmental variables and processing may affect to a significant extent the concentrations and biological activities of flavonoids, and these factors have not been taken into consideration. This aspect, nevertheless, is of great importance, considering that only a small amount of fruits and vegetables are consumed in their raw state, whereas most of them need to be processed for safety, quality, and economic reasons.