## 1 Introduction  
Polyphenols existing in fruits and vegetables represent a group of chemical substances that are characterized by the presence of one or more phenol rings per molecule. The most common and important low molecular weight phenolic compounds are simple phenolic derivatives (phenolic acids e.g., caffeic acid, coumaric acid, gallic acid, hydroxycinnamic acids) and flavonoids (e.g., quercetin, catechin, cyanidin, pelargonin). Phenolic acids account for about one-third of the total dietary intake of polyphenols: flavonoids account for the remaining two-thirds [1]. They have various biological properties, including anti-inflammatory, anti-cancer and anti-oxidant activities [1; 2; 3; 4]. Epidemiologicalstudies revealed inverse relationship between the risk of cardiovascular diseases, malignant neoplasm, diabetes, osteoporosis and the consumption of a polyphenic rich diet [3; 4]. Moreover, numerous observational and interventional studies showed positive effect of increased polyphenols dietary intake on cognitive performance in older subjects [2]. For instance, in the group of 2031 subjects aged 70-74 years, consumers of chocolate, wine or tea (products rich in plant polyphenols) had higher mean cognitive test scores than non-consumers [5]. Low flavonoid consumers without dementia revealed higher decline in cognitive performance over 10 years' follow up [6]. Supplementation of daily diet with complex antioxidant blend composed of plant extracts, including grape seed, ginkgo biloba and gotu cola, for 4 months in seniors without dementia resulted in the significant improvement of memory [7]. An improvement of spatial working memory was also observed in older adults after treatment with flavonoid antioxidant Pycnogenol in a daily dose 150 mg for 3 months [8]. One of the mechanisms which can explain benefits of high dietary intake of polyphenols for brain function could be a direct effect of this plant compound on neurons, involving the suppression of neuro-inflammation, decrease in stress signal transduction pathways, or expression of genes encoding neuro-protective and neurotrophic proteins [9]. To evoke such action, circulating polyphenols and their metabolites have to cross the blood-brain barrier (BBB) or blood-cerebrospinal fluid (CSF) barrier [10; 11], be present in the close neighborhood of neurons and interact with membrane or intracellular receptors. These suggest that polyphenols could be detected in the brain tissue and CSF fluid samples. Dietary supplementation with blueberry or blackberry extracts resulted in the appearance of anthocyanins in various regions of the brain in rats [12; 13]. Additionally, other polyphenols, such as naringenin, epicatechin and epigallocatechin, were found in the brain of laboratory animals after intravenous or oral administration [14; 15; 16]. Moreover, green tea polyphenols were able to reduce the elevated BBB permeability, probably by inhibition of caveolin-1 expression in the rat model of experimental cerebral ischemia caused by middle cerebral artery occlusion [17]. Ferulic acid was detected in the CSF of rats and mice [18; 19]. Detectable concentrations of vanillic acid, homovanillic acid (HVA), vanillylmandelic acid and 3,4-dihydroxyphenylacetic acid were reported in human CSF samples [20; 21; 22; 23; 24] as markers of brain catecholamines (adrenaline, noradrenaline, dopamine) metabolism, but not as indicators of neurons exposition to dietary phenolics. On the other hand, catechin and epicatechin were reported to cross through the model of BBB composed of immortalized human cerebral micro-vessel cell monolayer in vitro. In addition, BBB cells were able to conjugate these polyphenols with glucuronic acid [25]. Recently, the transportation of quercetin-glucosidase, kaempfer, myricetin and myricetin glucosides via a similar in vitro BBB model was also described [26]. These results suggest that various phenolics, and not only those related to catecholamine metabolism, could be present in human CSF.