By Diana Price, Medically Reviewed by Dr. C.H. Weaver M.D. 7/2022
The next time you sit down to enjoy a steaming cup of fragrant green tea, top your taco with avocado slices or add a handful of dried cranberries to your granola, you will be happy to know that you are taking an active step to boost your immune system and help your body ward off cancer. This is because all three of these foods contain naturally occurring plant chemicals, or phytochemicals, which are increasingly being studied for their impact in cancer prevention and treatment.
Phytochemicals have in fact been studied for decades, but only recently in humans. Research in this area is becoming so specific that individual phytochemicals are being applied to particular types of cancer cells, and the studies are producing exciting results. The findings show that these plant chemicals that occur naturally in whole foods have a profound effect on cancer prevention and treatment.
Research indicates that phytochemicals inhibit every stage of cancer development: they stop the formation of new cancer cells, inhibit the growth of existing cancer cells, help remove toxins that can lead to cancer, influence the activity of genes that are involved in cancer, suppress hormones that can stimulate some cancers to grow and cause cell death. In fact, bioactive chemicals interact with and support every level of the immune system.
The immune system is a complex defense system that equips us with barriers designed to protect the body from foreign substances, such as bacteria, viruses, parasites, chemicals, allergens and cancer. When most people think of immune defenses, they think of white blood cells and antibodies that actively fight off infection. But the body also uses other techniques, such as physical barriers (like the skin and the lining of the gastrointestinal tract), chemical barriers (such as stomach acid) and microbial barriers (such as beneficial microflora [bacteria]) that exist in the gastrointestinal tract).
While much research has been conducted on the ways in which the immune system can be suppressed—including chemical and biochemical imbalances and nutrient deficiencies—scientists have only recently begun to learn how to actively support and strengthen the system. The direct and indirect impact of phytochemicals on the immune system’s ability to respond to invaders has been one aspect of this recent research.
One branch of this research has focused on flavonoids, a group of bioactive chemicals that have been shown to slow and often halt the progression of cancers by, among their many mechanisms, inhibiting the division of cancer cells, altering gene expression, inhibiting tumor blood supply and reducing damage caused by free radicals and inflammation.1
One frequently studied flavonoid is quercetin, which is found in green tea. Researchers discovered that green tea extract inhibited the growth of breast cancer and lung cancer by 50 percent.2 Researchers believe that the flavonoid causes this anticancer effect by blocking the production of growth factors, hormone receptors and other components that promote cancer growth. Other studies have shown that quercetin’s potential as an anticancer agent includes cell cycle regeneration, interacting cell binding sites and tyrosine kinase inhibition. 3,4,5,6
Quercetin has also been shown to increase the activity of some anticancer drugs. The addition of quercetin increases the anticancer effect of the chemotherapy drug Camptosar® (topotecan) in the treatment of breast cancer cells. 7 In addition, quercetin appears to increase the sensitivity of cervical cancer cells to cisplatin.8
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Several other noteworthy examples of foods containing phytochemicals proven to have anticancer properties include cranberries, avocados and pomegranates. The phytochemicals in cranberries include flavonol glycosides, polyphenols, anthocyanins, proanthocyanidins and phenolic acids. The phenol compounds alone block the action of tumor promoters, thus reducing the load on the immune system.1,9
The bioactive substances in avocados have been shown to inhibit prostate cancer cell growth, and they are a richer source of carotenoids, (Lutein, zeaxanthin, a-carotene, and ß-carotene) than any other commonly eaten fruit in the U.S. Researchers noted that “because the avocado also contains a significant amount of monounsaturated fat, these bioactive carotenoids, or fat-soluble substances, are likely to be absorbed into the bloodstream, where in combination with other diet-derived phytochemicals they may contribute to the significant cancer risk reduction associated with a diet of fruits and vegetables.”10
Pomegranate products have created a recent craze in supermarkets, with products such as bottled juice popping up with increasing frequency. This is largely due to studies that found potent antioxidant properties associated with the polyphenols ellagic acid and hydrolyzable ellagitannins, both found in pomegranates.11
Additional studies of phytochemicals have shown similarly beneficial anticancer properties in garlic and black tea. The organoselenium metabolites in garlic have been found to penetrate avascular tumor tissue and inhibit angiogenesis, and a study verified that green and black teas are rich sources of polyphenols such as epigallocatechin-3-gallate, which are proven anticancer agents.12
These studies provide exciting data, and the global scientific community continues to discover new phytochemicals and their impact on specific cancers at a rapid pace, with thousands of phytochemicals discovered to date. Yet dietary studies have found that Americans eat such a narrow variety of plant foods that we are largely missing out on these powerful healing agents. In an effort to encourage a change in these habits, the National Cancer Institute encourages Americans to eat a minimum of five servings of fruit and vegetables per day, and nutritionists and our nation’s health organizations have now boosted this recommendation to eight servings per day.13
These recommendations are supported by a study that found that the intake of 400 to 600 grams per day (or 8 to 12 servings) of fruits and vegetables is associated with reduced incidence of many common forms of cancer. The study points to the fact that many phytochemicals are colorful and, for this reason, recommends a diet consisting of a wide variety of colorful fruits and vegetables.
Red foods like tomatoes, for example, contain lycopene, which is thought to be involved in maintaining prostate health. Green foods, including broccoli, brussels sprouts and kale, contain glucosinolates which have also been associated with a decreased risk of cancer. Garlic and onions contain allyl sulphides, which may inhibit cancer cell growth. The researchers recommend including one serving of each of the seven color groups daily, which is consistent with the United States National Cancer Institute and American Institute for Cancer Research guidelines of five to nine servings per day.14,15
Following these recommendations may provide benefit not only from the anticancer effects found in these recent studies, but also from the increased intake of vitamins, minerals and fiber needed to support overall healthy immune function.
- Boik J. Cancer & Natural Medicine—A Textbook of Basic Science and Clinical Research . Oregon Medical Press; 1995.
- Roomi MW, Ivanov V, Kalinovsky T, Niedzwiecki A, Rath M. In vitro and in vivo antitumorigenic activity of a mixture of lysine, proline, ascorbic acid, and green tea extract on human breast cancer lines MDA-MB-231 and MCF-7. Med Oncol. 2005;22(2):129-38.
- Roomi MW, Ivanov V, Kalinovsky T, Niedzwiecki A, Rath M.Seely D, Mills EJ, Wu P, Verma S, Guyatt GH. The effects of green tea consumption on incidence of breast cancer and recurrence of breast cancer: a systematic review and meta-analysis. Integr Cancer Ther . 2005 Jun;4(2):144-55.
- Baliga MS, Meleth S, Katiyar SK. Growth inhibitory and antimetastatic effect of green tea polyphenols on metastasis-specific mouse mammary carcinoma 4T1 cells in vitro and in vivo systems. Clin Cancer Res. 2005 Mar 1;11(5):1918-27.
- Yang CS, Liao J, Yang GY, Lu G. Inhibition of lung tumorigenesis by tea. Exp Lung Res. 2005 Jan-Feb;31(1):135-44.
- Laurie SA, Miller VA, Grant SC, Kris MG, Ng KK. phase I study of green tea extract in patients with advanced lung cancer. Cancer Chemother Pharmacol . 2005 Jan;55(1):33-8. Epub 2004 Aug 7.
- Akbas SH, Timur M, Ozben T. The effect of quercetin on topotecan cytotoxicity in MCF-7 and MDA-MB 231 human breast cancer cells. J Surg Res. 2005 May 1;125(1):49-55
- Jakubowicz-Gil J, Paduch R, Piersiak T, et al. The effect of quercetin on pro-apoptotic activity of cisplatin in HeLa cells. Biochem Pharmacol. 2005 May 1;69(9):1343-50.
- Seeram NP, Adams LS, Hardy ML, Heber D. Total cranberry extract versus its phytochemical constituents:antiproliferative and synergistic effects against human tumor cell lines. J Agric Food Chem. 2004 May 5;52(9):2512-7.
- Lu QY, Artega JR, Zhang Q, Huerta S, Go VL, Heber D. Inhibition of prostate cancer cell growth by an avocado extract: role of lipid-soluble bioactive substances. J Nutr Biochem . 2005 Jan;16(1):23-30.
- Seeram NP, Lee R, Heber D. Bioavailability of ellagic acid in human plasma after consumption of ellagitannins from pomegranate (Punica granatum L.) juice. Clin Chim Acta. 2004 Oct; 348(1-2):63-8.
- Lu QY, Jin YS, Pantuck A, Zhang ZF, Heber D, Belldegrun A, Brooks M, Figlin R. Rao green tea extract modulates actin remodeling via Rho activity in an in vitro multistep carcinogenic model. J Clin Cancer Res. 2005 Feb 15; 11(4):1675-83.
- Heber D. Nutrition and cancer prevention: new insights into the role of phytochemicals. Am J Clin Nutr. 2002 Jul;76(1):259
- Henning SM, Niu Y, Lee NH, Thames GD, Minutti RR, Wang H, Go VL, Heber D. Bioavailability and antioxidant activity of tea flavanols after consumption of green tea, black tea, or a green tea extract supplement. *Am J Clin Nutr.*2004 Dec;80(6):1558-64.
- The USDA and The New Food Pyramid. Dietary Guidelines.