Good review articles on flavonoid metabolism are available and can be useful. Some examples are:

- C. Rice-Evans (2001) Flavonoid antioxidants. Curr. Med. Chem. 8, 797-807.

- J. Ross and C.M. Kasum (2002) Dietary flavonoids: bioavailability, metabolic effects and safety. Ann. Rev. Nutr. 22, 19-34.

- T. Walle (2004) Absorption and metabolism of flavonoids. Free Radical in Biology and Medicine 36, 829-837.

- J. Alvarez-Suarez, F. Giampieri and M. Battino (2013) Honey as a Source of Dietary Antioxidants: Structures, Bioavailability and Evidence of Protective Effects Against Human Chronic Diseases. Current Medicinal Chemistry 20, 621-638.

And for bioavailability and some pharmacokinetic data in humans:

- C. Manach et al. (2005) Bioavailability and bioefficacy of polyphenols in humans. I. Review of 97 bioavailability studies, American Journal of Clinical Nutrition 81, 230S-242S.

- G. Williamson and C. Manach (2005) Bioavailability and bioefficacy of polyphenols in humans. II. Review of 93 intervention studies, American Journal of Clinical Nutrition 81, 243S-255S.

- Samantha Ellam and Gary Williamson (2013) Cocoa and Human Health, Annual Review of Nutrition 33, 105-128.

- Surangi Thilakarathna and H. Vasantha Rupasinghe (2013) Flavonoid Bioavailability and Attempts for Bioavailability Enhancement, Nutrients 5, 3367-3387.

- Daniele Del Rio, Ana Rodriguez-Mateos, Jeremy P.E. Spencer, Massimiliano Tognolini, Gina Borges, and Alan Crozier (2013) Dietary (Poly)phenolics in Human Health: Structures, Bioavailability, and Evidence of Protective Effects Against Chronic Diseases, Antioxidants & Redox Signaling 18, 1818-1892 (open access at http://online.liebertpub.com/doi/abs/10.1089/ars.2012.4581).

Some metabolism and bioavailability issues more specifically related to neuroprotection, including access to the brain, can be found in:

- C. Gutierrez-Merino et al. (2011) Neuroprotective Actions of Flavonoids, Current Medicinal Chemistry 18, 1195-1212.

Flavonoids are a large assortment of compounds and the biological properties, such as metabolic fate and bioactivity, can greatly differ between different groups and individual compounds.

Many thanks Ricardo Lagoa, for the great number of references, and resource material. Hope I should have access to full text article from the list.

Dear Prof Bhagwat:

With due respect , the term, “bioflavonoid”, originally was applied to the vitamin P aspect of citrus flavones by the Nobel Laureate, Albert Szent Gyorgyii, is not being used now, even though some authors use it interchangeably with “flavonoids”.

The Nobel Prize in Physiology or Medicine 1937

Albert Szent-Györgyi

http://www.nobelprize.org/nobel_prizes/medicine/laureates/1937/szent-gyorgyi-lecture.pdf

 You may want to peruse this:

 The Effects of Plant Flavonoids on Mammalian Cells: Implications for Inflammation, Heart Disease, and Cancer

Elliott Middleton, Jr., Chithan Kandaswami, and Theoharis C. Theoharides

Pharmacol Rev December, 2000 52:673-751

 An original paper:

DasNP (1971) Studies on flavonoid metabolism: Absorption and metabolism of (+)-catechin in man. Biochem Pharmacol 20:3435–3445.

An early evaluation of the absorption and metabolism of (+)-catechin in humans was presented by Das in 1971.  Oral administration (83 mg/kg) resulted in rapid absorption, metabolism, and excretion of the flavonoid within 24 h. Eleven metabolites were detected in urine. 

Many thanks Prof Chithan C Kandaswami for the response to the question. Thanks also to the additional information and the imp links related to the topic. Lot more area are untouched by researchers in this field.

Like other phenols, flavonoids are initially conjugated with sulphate, a process which has high affinity but low capacity. When this process is exhausted, we have to rely on glucuronidation, which has low affinity, but high capacity.

People with good production of sulphate and high activity of phenolsulphotransferase (PST) enzymes tolerate flavonoids well, and for them they are useful antioxidants. Those with poor sulphate production, or poor PST activity, react badly to them. These are a minority of the population, and susceptible to autism, migraine, MS, Parkinson's, chronic fatigue syndrome, depression and auto-immune disease. One man's meat is not another man's poison. Different members of families may manifest poor sulphation in different ways. Maybe uncle has rheumatoid arthritis, mother has migraine, and the child has autism. They are seen by a rheumatologist, neurologist and paediatrician, and the doctors may not connect the conditions.

Sulphate production may be enhanced with molybdenum, magnesium, zinc, vitamins B2, B5 and B6 and omega three fatty acids. Avoiding spinach, radish and orange, which are potent PST inhibitors, may also help.

Many thanks Margaret Moss, for the very useful piece of information to my question. This aspect is most often overlooked by almost all physicians and clinicians. I certainly agree with you, it is so important that clinicians should always have the basic opinion and consultation of expert Nutritionist while treating their patients with obscure etiology.

If you have any references on PST, especially  assay methods which can be used to assess the tolerance capacity of an individual, I will be most grateful to you sir.

Thanks again for your valuable comment.

Have a look at

Moss M, Waring RH. The Plasma Cysteine/Sulphate Ratio: a Possible Clinical Biomarker. J Nutr Env Med 2003; 13(4): 215-229, and the references we listed there.