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Dietary Flavonoid Quercetin and Associated Health Benefits—An Overview

Dietary Flavonoid Quercetin and Associated Health Benefits—An Overview
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   PLEASE SCROLL DOWN FOR ARTICLE This article was downloaded by: [Jan, Arif Tasleem]  On: 19 June 2011 Access details: Access Details: [subscription number 922821804]  Publisher Taylor & Francis  Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Food Reviews International Publication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t713597252 Dietary Flavonoid Quercetin and Associated Health Benefits—AnOverview Arif Tasleem Jan a ; Majid Rasool Kamli a ; Imtiyaz Murtaza b ; Jang Bahadur Singh c ; Arif Ali a ; Q. M. R.Haq aa  Department of Biosciences, Jamia Millia Islamia, New Delhi, India b  Biochemistry and MolecularBiotechnology Lab., Division of PHT, SKUAST-Kashmir, India c  Department of Pathology, Govt.Medical College, Srinagar, IndiaAccepted uncorrected manuscript posted online: 19 April 2010Online publication date: 19 April 2010 To cite this Article  Jan, Arif Tasleem , Kamli, Majid Rasool , Murtaza, Imtiyaz , Singh, Jang Bahadur , Ali, Arif and Haq,Q. M. R.(2010) 'Dietary Flavonoid Quercetin and Associated Health Benefits—An Overview', Food Reviews International,26: 3, 302 — 317, doi: 10.1080/87559129.2010.484285, First posted on: 19 April 2010 (iFirst) To link to this Article DOI 10.1080/87559129.2010.484285 URL http://dx.doi.org/10.1080/87559129.2010.484285 Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdfThis article may be used for research, teaching and private study purposes. Any substantial orsystematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply ordistribution in any form to anyone is expressly forbidden.The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae and drug dosesshould be independently verified with primary sources. The publisher shall not be liable for any loss,actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.  302 Food Reviews International , 26:302–317, 2010 Copyright © Taylor & Francis Group, LLC ISSN: 8755-9129 print / 1525-6103 online DOI: 10.1080/87559129.2010.484285 LFRI8755-91291525-6103FoodReviews International,Vol.26,No.3, May 2010:pp.0–0FoodReviews International Dietary Flavonoid Quercetin and Associated Health Benefits—An Overview Dietary FlavonoidQuercetin andAssociated Health Benefits—AnOverviewJanet al. ARIF TASLEEM JAN 1 , MAJID RASOOL KAMLI 1 , IMTIYAZ MURTAZA 2 , JANG BAHADUR SINGH 3 , ARIF ALI 1 , AND Q.M.R. HAQ 1 1 Department of Biosciences, Jamia Millia Islamia, New Delhi, India 2 Biochemistry and Molecular Biotechnology Lab., Division of PHT, SKUAST-Kashmir, India 3 Department of Pathology, Govt. Medical College, Srinagar, India Phytochemicals have received a considerable attention in the present day world. Epidemiological studies have established that phytochemicals contribute more qualita-tively to the total antioxidant activity of foods than nutrient antioxidants like vitamin C and vitamin E. Among polyphenols, quercetin constitutes the main flavonoid in our daily diet being particularly abundant in onions and apples. Since the realization that many folk medicines in use contain flavonoids, interest in this class of compounds hasintensified. Quercetin acts as a strong reducing agent, which together with other dietary reductants such as vitamin C, vitamin E, and carotenoids protect body tissueagainst oxidative stress. Recent reports suggest that quercetin as antioxidant improvesnormal cell survival and as pro-oxidant induces apoptosis in cancerous cells whereby prevents tumor proliferation. Among other important properties like modulation of genes related to cell cycle, signal transduction, and xenobiotic metabolism, quercetinhas also been attributed with antiviral, anti-inflammatory, antibacterial, and muscle-relaxing properties. In the literature, only a few in vivo studies have been carried out;therefore, before making any authentic health claim about this compound, it is essen-tial to know its nature and its dietary srcin. Furthermore, it is important to know theamount present in different diets and its bioavailability, followed by clinical trials and investigations, if researchers are to use it as an chemo-preventive and chemotherapeuticagent against various deleterious degenerative diseases. Keywords phytochemicals, quercetin, antioxidant Introduction Naturally occurring substances that afford protection from toxins and potential carcinogenshave recently come to the forefront of scientific attention. Associative evidences fromvarious epidemiological studies have revealed that an increased intake of fruits and vege-tables is associated with a low risk of various degenerative diseases. (1)  Scientific literaturehas opened new avenues for a better understanding of medicinal value of plants as theyproduce thousands of bioactive compounds like phytochemicals. Plant phytochemicalssuch as polyphenols (abundant micronutrients in our diet) are of great interest due to theirwide range of biological activities such as antioxidant potential. Antioxidant potential of a Address correspondence to Q.M.R. Haq, Department of Biosciences, Jamia Millia Islamia,New Delhi, 110025, India. E-mail:qmrhaq@yahoo.co.in  D o w nl o ad ed  B y : [ J a n ,  A rif  T a sl e e m]  A t : 13 :35 19  J u n e 2011   Dietary Flavonoid Quercetin and Associated Health Benefits—An Overview303 particular food commodity depends on how well it can quench free radicals generatedthrough various life processes thus protecting macromolecules from being being oxi-dized. (2)  Keeping in view the strong antioxidant potential, recommended dietary intake of polyphenols is believed to be as high as 1 g/d, much higher than all other classes of dietaryantioxidants i.e., ∼ 10 times higher than intake of vitamin C and as much as 100 timeshigher than that of vitamin E and carotenoids. (3) Polyphenols especially flavonoids and their corresponding glycosides are importantconstituents of fruits, vegetables, and beverages. (3,4)  Flavonoids are plant pigments thatare synthesized from amino acids, phenylalanine and tyrosine (Fig. 1). Polyphenolsshow a great diversity of structures ranging from simple molecules (monomers andoligomers) to polymers with different properties. However, basic structure of flavonoidsconsists of a heterocyclic ring fused to an aromatic ring, with a third ring systemattached at C3 or C4 position (Fig. 2). Quercetin - a ubiquitous flavanoid belongs to fla-vonol class of flavonoids. It is found abundant in onion and apple, besides being presentin other fruits and vegetables as well. (5,6)  Among various flavonoids, quercetin acts as astrong reducing agent, which together with other dietary reductants such as vitamin C,vitamin E, and carotenoids protects body tissues against oxidative stress. Commonlyreferred to as strong antioxidant, it prevents organisms from various diseases associatedwith oxidative stress such as cancer, cardiovascular diseases, inflammation, and otherdegenerative diseases. (7) Figure 1. Pathway showing synthesis of flavonoids in plants.  D o w nl o ad ed  B y : [ J a n ,  A rif  T a sl e e m]  A t : 13 :35 19  J u n e 2011  304Jan et al. Enthusiasm for quercetin in cancer prevention stems from its number of separate andindependent mechanisms of antitumor action, especially through inhibiting tyrosinekinase in phase I clinical trials. (8)  Similarly, high concentrations of quercetin have beenfound to be associated with the inhibition of growth of the malignant cells by arresting thecell cycle in the late-G1-phase or causing apoptosis of malignant cells. (9,10)  In order tounderstand the impact of quercetin on human health, it is essential to know the nature of quercetin, its dietary srcin, amount present in different diets, their bioavailability andother factors controlling their bioavailability and molecular mechanisms involved in actingas antidegenerative agent. Keeping in view beneficial effects of flavonoids especiallyquercetin and its frequent occurrence among common dietary items as because of which itis easily available and consumed regularly, this review has been written in a way in orderto evaluate its safety and provide an overall view of quercetin in relation to diseaseprevention. Flavonoids, Disease Prevention and Nutrigenomics A number of epidemiological studies have investigated the association between flavonoidintake and the incidence of a variety of cancers and other degenerative diseases. (11,12) Through years of research, researchers have affirmed that flavonoids are effective inreducing the leading causes of death that occurs due to cancer and other degenerative dis-eases on the basis that flavonoids have been found to posses both antioxidant as well aspro-oxidant effects with contrasting effects on cell physiological processes. As an antioxi-dant, they improve normal cell survival and as pro-oxidant induces apoptosis in cancerous Figure 2. Structure of flavonoid Quercetin.  D o w nl o ad ed  B y : [ J a n ,  A rif  T a sl e e m]  A t : 13 :35 19  J u n e 2011   Dietary Flavonoid Quercetin and Associated Health Benefits—An Overview305 cells, where by prevents tumor proliferation. Structure-antioxidant activity of flavonoidshas identified a number of structural elements in flavonoids that contribute to their antiox-idant activity. It include a C 2 =C 3  double bond, a C 4 -keto group, a C 3 -hydroxyl group andan ortho-diphenolic structure also called catechol group, in the B-ring. (13)  Quercetinpossesses all the above-mentioned structural elements as such posses the ability to scav-enge reactive oxygen and singlet oxygen species, in addition to radicals of different srcinin various in vitro  systems. (14,15) Biochemical activities of flavonoids and their metabolites depend on their chemicalstructure and relative orientation of various moieties in the molecule. Both antioxidantand pro-oxidant potential of quercetin are related to the number and position of freehydroxyl groups in the molecule. (16)  A number of studies have revealed that the 3 ′ - and4 ′ -hydroxyl groups of quercetin (catechol hydroxyl groups) are important targets of conjugation such as methylation, sulfation, and glucuronidation. As biological activityof flavonoids is dependent on the number and position of free hydroxyl groups, the regi-oselectivity of phase II metabolism of quercetin is expected to be an important determi-nant of the ultimate biological effects of quercetin in vivo , as during and after absorptionin the intestine, quercetin is extensively metabolized by the phase II biotransformationsystem. (17,18) Quercetin has been ascribed as health promoting and disease-preventing dietary sup-plement as it exert multiple pharmacological effects on mammalian cells and tissues. (19) Many claims on the beneficial health effects of quercetin have been stated, including pro-tection against various forms of cancer, cardiovascular diseases, and neurodegenerativediseases. (20)  Quercetin has also been attributed with anti-inflammatory, antibacterial, andmuscle-relaxing activities besides being found to possess antiviral activity as well. (21,22) Mechanism of its antiviral activity is related with its ability to bind with viral proteinswhereby it interferes with the viral nucleic acid synthesis; for instance methyl-quercetin,which was found to block polio virus replication by interfering with single strand RNAreplicative intermediate in association with block of cellular protein synthesis. Thesehealth claims are supported by in vitro  evidences related to the strong antioxidant activityand ability of quercetin to modulate the activity of numerous enzymes involved in signaltransduction, cell growth, and biotransformation. (10,24)  Because of the various healthclaims, the use of flavonoids like quercetin as functional food ingredient or food supplementhave lead to a rapid increase in the daily consumption of flavonoids especially quercetinby several folds.Nutrients have long been considered merely as a fuel or cofactor. (25)  However, itwas appreciated that they also participates in the regulation of metabolic pathways aswell. Interaction between human body and nutrition is an extremely complex processinvolving multiorgan physiology with molecular mechanisms on all levels of regulation,i.e., gene expression, proteins as well as at metabolic level. (26)  Importance of theinteraction between nutrition, metabolism and gene expression for homeostasis hasemerged with a new field of science referred to as “Nutrigenomics.” Nutrigenomics isdefined as the application of genomic tools to study the integrated effects of plant basednutrients and non-nutrients on gene regulation in the whole organism in health anddisease. (27,28)  Nutrigenomics offers a powerful and exciting approach to unravel theeffects of diet on health, and thus, increase our fundamental knowledge about the inter-action between life processes with our daily diet. This can lead to the development of novel functional foods to improve the health status of the general population and to thepersonalized diet to prevent the onset of nutrition related disorders in genetically predis-posed individuals. (29)  D o w nl o ad ed  B y : [ J a n ,  A rif  T a sl e e m]  A t : 13 :35 19  J u n e 2011
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