Phytonutrients - Quercetin

Question 1

Quercetin

Answer 1

Quercetin is the most abundant of the flavonoids. It is also a building block for other flavonoids (e.g. kaempferol):
* Quercetin has a fast excretion rate so needs to be consumed regularly for therapeutic use.
* Therapeutic dosage range: 150‒1500 mg / day.

Question 2

Food sources:

Answer 2

• Onions (especially red), apples (especially the peel), grapes, capers, asparagus, tomatoes, kale, brassica vegetables and berries (blueberries, bilberries, blackberries, elderberry, cranberry and goji.

Question 3

Functions and Therapeutic uses:

Answer 3

• Cardiovascular health
• Anti-cancer
• Immune system
• Xanthine oxidase:

Question 4

Cardiovascular health:

Answer 4

Functions:
* Quercetin is a powerful antioxidant protecting the vascular endothelium. It inhibits LDL oxidation.
* Shown to have vasodilatory effects (via nitric oxide release), reduces platelet aggregation and lowers blood pressure (in those hypertensive) via ACE-inhibiting properties.
* Supports athletic performance (endurance) by ↑ blood circulation.
* Dosage: 150 mg‒1000 mg per day.
Therapeutic uses:
* Cardiovascular diseases (e.g. coronary artery disease) —incl. prevention.
* Hypertension.
* Hyperlipidaemia.
* Previous myocardial infarction.
* Exercise performance.
ACE = ‘angiotensin converting enzyme’. Its activity ↑ blood pressure

Question 5

Anti-cancer:

Answer 5

Therapeutic uses:
* Its anti-tumour functions occur by a variety of mechanisms:
o Shown to induce cell cycle arrest (e.g. in leukaemic, prostate and ovarian cancer cells) —induces apoptosis.
o It can inhibit blood vessel growth. In prostate and breast cancers, quercetin targets the VEGFR-2-mediated angiogenesis pathway.
o Evidence of anti-metastatic properties.
* Shown to reduce head / neck skin damage during radiotherapy.
* Shown to increase the efficacy of some types of chemotherapy including: Cisplatin, busulfan, topotecan, cyclophosphamide, gemcitabine
VEGFR = vascular endothelial growth factor
Apoptosis= cell death

Question 6

Immune system:

Answer 6

Functions:
* Anti-inflammatory—quercetin inhibits LOX and COX, and is also a potent down-regulator of NF-κB.
* Anti-histamine—quercetin inhibits histamine release by mast cells.
* Reduces inflammation in the brain (and is hence protective against Alzheimer’s and Parkinson’s, of which inflammation is an exacerbating factor).
Therapeutic uses:
* Hay fever.
* Histamine reactions e.g. urticaria, hives.
* Asthma.
* Prostatitis.
* IBD.
* Arthritis (e.g. RA, osteoarthritis).
NF-κB= a transcription factor that drives inflammation

Question 7

Xanthine oxidase:

Answer 7

Functions:
* Quercetin can inhibit xanthine oxidase (normally converts purines uric acid).
Therapeutic uses:
* Gout.

Question 8

Absorption:

Answer 8

• Only a small percentage of ingested quercetin is absorbed in the blood:
  o Quercetin occurs in most food as a glycone so it has to split from the sugar portion to be absorbed and become active.
  o Onion and shallots contain quercetin in its unbound (aglycone) form, which may make absorption easier.
  o Quercetin is hydrophobic, so is best absorbed with fats, emulsifiers or apple pectin.
  Glycone = attached to a sugar molecule

Question 9

Safety:

Answer 9

• No side effects noted in doses of a few grams a day.

Question 10

Interactions:

Answer 10

• Some antibiotics e.g. fluoroquinolones and some types of chemotherapy e.g. paclitaxel.