Quercetin

Flavonoids are common in plants, and are responsible for the various colours of many fruit and vegetables. The human diet usually contains about one gram per day of flavonoids, with tea and red wine containing especially large amounts. More than 4000 flavonoids have been discovered so far, but few have been properly identified and tested in human studies. The exception to this is the most common flavonoids, quercetin, which has been relatively well tested. Quercetin is the most active of the flavonoids and it is estimated that we eat about 25 mg per day.

Quercetin has been shown to inhibit tumour formation consistently. It doesn’t attack tumours directly, but works by keeping cancer cells from dividing. For example, in breast cancer, the presence of mutant p45 (a common cancer mutation) allows the cancer cells to grow out of control. Quercetin is able to suppress the production of mutant p45 to nearly undetectable levels. Quercetin also inhibits tyrosine kinases, a class of enzymes present in the cell membrane that control cell division, that is often over-produced in many cancers. Quercetin has been shown to be able to inhibit tyrosine kinase in cancer patients, and thus reduce the rate of cancer cell growth. This effect can be seen within 16 hours after administration. Since it inhibits cancer cell growth but does not actually kill cells, quercetin has no known side effects.

As a possible treatment for breast cancer, quercetin appears to work in a unique way, causing the cancer cells to produce type II oestrogen receptors (ER II) and then binds (attaches) to these receptors. By blocking these receptors in this way, quercetin stops growth signals from being produced, thereby stopping the cell from dividing. The more ER II the cell produces in response to quercetin, the more sites there are for quercetin to attach itself to, and the greater the degree of tumour suppression. It is thought that this mechanism also translates to other oestrogen-sensitive cancers such as ovarian and uterine cancers.

Quercetin has demonstrated antitumour activity against many cancers in experimental models, including those of the breast, lung, skin, ovaries, colon, rectum and brain, and also enhances the effects of chemotherapy while reducing its side effects. In humans, ER II is found in all the cancers mentioned as well as in leukaemias.

The recommended dosage of quercetin from experimental models is between 200 to 400 mg one to three times a day. It is probably best to start at a low dose and to gradually increase the dose over a couple of weeks. It is suggested that quercetin is taken concurrently with proteolytic enzymes to increase absorption in the gut.