Flavonoids are a large group of compounds widely distributed in foods derived from plants. Quercetin is a compound that belongs to the flavonol subclass of flavonoids. Fruit and vegetable-rich diets, which are also rich in flavonoids, have been associated with a reduced risk of many cancers, including colon cancer. Scientific research indicates that quercetin, and other flavonoids have the potential to affect the development and progress of cancers in cultured cells and animal models of colon cancer. The effects of quercetin on colon cancer in human patients are not currently established.
Colon Cancer Recurrence
Cancer that originates in the longest part of the intestine is called colon cancer. Cancers of the colon and rectum, or colorectal cancers, are the second-leading cause of cancer-related deaths in the United States. Recurrent colon cancer is cancer that returns after treatment has been completed. The cancer may recur either locally, near the site of the original tumor, or distally, in another part of the body, such as the liver. Colon cancer that does not recur within five years is considered cured, and early stage colon cancers are considered potentially curable. According to the National Cancer Institute, there is a need for developing preventative strategies, especially in advanced or recurrent colon cancers, since advances in screening and treatments have yielded only modest improvements in patient survival. Diet modification, to increase intake of flavonoid-rich foods, may be one such strategy for prevention of colon cancer recurrence.
Quercetin
Flavonoids are a large group of polyphenolic compounds -- compounds with multiple phenol rings in their chemical structure -- present universally in plants and plant-derived foods. There are over four thousand chemically distinct flavonoids which are placed in subclasses, such as flavonols, anthocyanidins and flavones, based on the features of their chemical structure. Quercetin is the major dietary flavonol. The glycoside, or glucose-modified form, of quercetin, is called rutin. Quercetin is found in apples, onions, tea and red wine. Leafy green vegetables, berries and some herbs, such as gingko, also contain quercetin. Quercetin, like other flavonoids, is a natural antioxidant. The benefits of quercetin to human health are being investigated in many diseases, including allergies, cancer and diabetic complications such as cataracts.
Effects of Quercetin
The effects of quercetin on cancer have been elucidated in studies on cultured cancer cells and rodent models of cancer. Both quercetin and rutin prevent the development of colon cancer in rodent models of cancer in which cancers are induced by mutagenic compounds. For instance, one study published in "Carcinogenesis" in 1991 found that quercetin reduced the incidence of colonic neoplasia -- abnormal growth of cells in the colon -- in rodent models of mutagen-induced colon cancer. Quercetin also inhibits productions of heat shock proteins -- a class of proteins that can improve cancer cell survival -- in many caner cells, including colon cancers. A 2002 population-based study published in "American Journal of Clinical Nutrition" showed a reduced risk of lung cancer in men who consumed the highest amounts of quercetin, mainly from apples.
Applications in Cancer
Preliminary analyses in cancer cells and animal models suggest that quercetin may have anti-cancer potential. A limited number of clinical studies in humans have evaluated the safety and utility of quercetin in human patients. A four-year clinical study, named the Polyp Prevention Trial, led by researchers at the National Cancer Institute, examined the effectiveness of a diet high in fiber, fruits and vegetables and low in fat on the risk of recurrence of colorectal adenomas, benign tumors in the colon that the potential to become cancerous. An analyses of this trial, published in "Cancer Prevention Research" in 2010, concluded that the highest intake of flavonols, including quercetin, were inversely associated with risk of recurrence and likelihood of more advanced colorectal adenomas.
Concerns
Quercetin intake, especially from a diet rich in flavonoids, is generally safe. However, adverse side effects were noted in a few studies with quercetin supplements, or intravenous administration of quercetin. A clinical study, published in "Clinical cancer Research" in 1996, evaluated the safety and effects of intravenously administered quercetin in patients with advanced cancers no longer responsive to chemotherapy. Although molecular changes indicative of anti-cancer effects were indicated in a few patients, and quercetin administration was safe in most patients, toxicity to kidneys were observed in a few patients. Further research is required to assess whether and how quercetin may be applicable in reducing the recurrence of cancer.
References
- "Alternative Medicine Review"; Davis W. Lamson and Matthew S. Brignall; 2000
- American Cancer Society; Colorectal Cancer Detailed Guide
- "Carcinogenesis"; Eleanor Deschner, et al.; July 1991
- "American Journal of Clinical Nutrition'; Paul Knetkt, et al.; September 2002
- "Japanese Journal of Cancer Research'; Mototsugu Koishi et al.; November 1992
- "Clinical Cancer Research'; David R. Ferry, et al.; April 1996
