The flavonoid paradox: conjugation and deconjugation as key steps for the biological activity of flavonoids

J Sci Food Agric. 2012 Jul;92(9):1822-5. doi: 10.1002/jsfa.5697. Epub 2012 May 4.


Flavonoids have been proposed to exert beneficial effects in the prevention of a large number of diseases, including cancer, cardiovascular disease, and neurodegenerative disorders. Paradoxically, despite the most representative flavonoid--quercetin--exerting biologically demonstrable systemic effects, it is not found in plasma after oral administration and its circulating metabolites show weak activity in vitro. The current available evidence indicates that quercetin is extensively metabolized into methylated and glucurono- and sulfo-conjugated metabolites, which are the plasma circulating forms; and glucurono-, but not sulfo-conjugates, can be hydrolyzed at the vascular level, yielding the parent aglycone which accumulates in tissues. Thus conjugation is a reversible process and, at least regarding the vasodilator and antihypertensive effects, the conjugation-deconjugation cycle appears to be an absolute requirement. Glucuronidated derivatives transport quercetin and its methylated form, and deliver to the tissues the free aglycone, which is the final effector.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Antihypertensive Agents / metabolism
  • Antineoplastic Agents, Phytogenic / metabolism
  • Biological Transport
  • Blood Vessels / drug effects
  • Blood Vessels / metabolism
  • Humans
  • Hydrolysis
  • Metabolic Detoxication, Phase II*
  • Methylation
  • Neuroprotective Agents / metabolism
  • Phytotherapy*
  • Plant Extracts / metabolism*
  • Plant Extracts / pharmacology
  • Quercetin / metabolism*
  • Quercetin / pharmacology
  • Vasodilator Agents / metabolism


  • Antihypertensive Agents
  • Antineoplastic Agents, Phytogenic
  • Neuroprotective Agents
  • Plant Extracts
  • Vasodilator Agents
  • Quercetin