July 1, 2005
Myths swirl around diets rumored to ward off cancer, from black-walnut hulls to silver. After all, the public finds enormous motivation to stock their pantries with cancer-fighting foods, since one out of two men and one out of three women will develop cancer in their lifetime.
Scientists have explored how a wide range of foods packs a punch against cancer. Headlines on this research are making their way onto the covers of women's magazines and labels of supplements in supermarkets across the country. The National Research Council, Washington, D.C., reports about one-third of all cancer deaths in the nation each year could be attributed to diet. Scientists have associated such dietary concerns as obesity, nutrient-empty foods, low fiber intake, consumption of red and/or processed meats, and the imbalance of omega-3 and omega-6 fatty acids with a higher cancer risk. Research is rapidly sketching out what the perfect picture of a cancer-preventative diet should look like -- and it might have been plucked off the canvas of a 17th century Dutch still-life, as it is painted with plenty of ripe, colorful fruits and vegetables.
Portrait of an anticancer diet
Cancer-fighting benefits of foods might be due to either individual or combined effects of their components, including fiber, micronutrients and phytochemicals, plant compounds that exhibit strong antioxidant activities. It is not clear exactly how phytochemicals act in cancer prevention, as it is difficult to isolate the effects of various phytochemicals in fruits and vegetables. A diverse diet of plant foods can provide over 25,000 different phytochemicals. Antioxidants comprise many food components including vitamins, minerals, cartenoids and polyphenols. Oxidation within the body produces free radicals, which can potentially damage DNA, proteins and lipids, and are implicated in a number of diseases, including cancer. Antioxidants can stabilize free radicals before they cause harm.
The relationship between a high vegetable and fruit intake and a lower cancer risk led the National Cancer Institute (NCI), Bethesda, MD, to search for bioactive food components (those that induce a variety of physiologic functions) to account for this benefit. In the clinical trials, the bioactive food compounds being investigated to reduce breast cancer risk include, indole-3-carbinol, sulforaphanes, phytoestrogen isoflavones, perillyl alcohol and green-tea polyphenols. Prostate-cancer-prevention trials focus on selenium and vitamin E, with other investigations looking into soy isoflavones, green-tea polyphenols and doxercalciferol.
Rating antioxidant power
In the June 2004 issue of the Journal of Agricultural and Food Chemistry, researchers published the most-complete table of antioxidant content in common foods, noting that small red beans contained the highest content of antioxidant capacity per serving size. Cranberries, blueberries and blackberries were ranked highest of the fruits studied; beans and russet potatoes were tops among the vegetables; pecans, walnuts and hazelnuts were top in the nut category; and ground cloves, cinnamon and oregano made the top three antioxidant-rich spices.
Some scientists are quick to point out that new antioxidants await discovery and no antioxidant total, as a nutritional index, exists for food labeling due to the lack of standard quantitation methods. Pantox Laboratories, San Diego, proposed in a 2004 issue of Medical Hypotheses that a phytochemical index, defined as the percent of dietary calories derived from foods rich in phytochemicals, might be an excellent method to evaluate foods for phytochemical and antioxidant content.
Foods and nutrients on the frontline of the battle against cancer include the following:
Dietary fiber, a group of endogenous compounds in plant foods that resist human digestive enzymes, play a role in reducing cancer risk. Even though a recent NCI study showed no reduction of colon cancer with a high-fiber diet, the American Institute for Cancer Research (AICR), Washington, D.C., still continues to recommend a fiber-rich diet, because high-fiber diets are consistently linked with a reduction in colon-cancer risk.
Flaxseed is a dietary source of lignans, which are classified as phytoestrogens because they mimic the action of estrogen in the body. This behavior is linked to reduced cancer risk, according to AICR. Flax is also high in alpha-linolenic acid, an omega-3 fatty acid, which might protect against cancer. In several laboratory studies, flaxseed inhibits the formation of colon, breast, skin and lung tumors and possibly provides a protective effect against breast cancer. An NCI-sponsored randomized clinical trial of flaxseed with men with prostate cancer is underway.
Tomatoes are making the headlines due to their renowned carotenoid, lycopene. This potent antioxidant displays anticancer potential in a variety of studies. A study that tracked a large group of men for six years found that those who ate the most tomato products had a 35%-lower risk of early prostate cancer and a 53%-lower risk of advanced prostate cancer than men who ate the least amount of these foods. A large prospective epidemiologic study reported that increased intake of lycopene and tomato-based foods might be associated with reduced cancer risk.
Dark-green, leafy vegetables, including spinach, kale, lettuce and greens, are excellent sources of fiber, folate, carotenoids (e.g., lutein and zeaxanthin), saponins and flavonoids. Researchers have found that carotenoids in dark-green, leafy vegetables can inhibit the growth of certain types of breast-cancer cells, skin-cancer cells, lung cancer and stomach cancer. In the Iowa Women's Health Study, an ongoing study since 1985 conducted by University of Minnesota Cancer Center, Minneapolis, women who ate the most green, leafy vegetables were found to have less than half the risk of ovarian cancer compared with subjects who ate the least amount of green, leafy vegetables.
Cruciferous vegetables, such as broccoli, cauliflower, cabbage, Brussels sprouts, bok choy and kale, contain many components linked to lowering cancer risk, including glucosinolates, crambene, indole-3-carbinol and isothiocyanates, according to AICR. Components of these vegetables arrest growth of cancer cells in various cell, tissue and animal models, including tumors of the breast, emdometrium, lung, colon, liver and cervix. Large human studies associate diets high in cruciferous vegetables with lower risk for lung, stomach and colorectal cancers. A study at Johns Hopkins University, Baltimore, revealed that glucoraphanin, a compound in broccoli that produces sulforaphane in the body, might be effective in fighting stomach, breast and colon cancer.
Garlic belongs to the Allium genus, which includes onions, scallions, leeks and chives. These contains compounds currently under investigation for anticancer effects, including allicin, allixin, allyl sulfides, quercetin and organosulfur compounds. Diallyl disulfide, a garlic compound, exerts potent preventive effects against cancers of the skin, colon and lung, according to laboratory research. Components of garlic also show the ability to slow or stop the growth of tumors in prostate, bladder, colon and stomach tissue. The Iowa Women's Health Study found that women who ate garlic consistently had a lower risk for colon cancer.
Beans and legumes have active ingredients, such as saponins, protease inhibitors and phytic acid, that seem to have a role in cancer prevention. Saponins have shown the ability to inhibit cancer-cell reproduction and slow tumor growth in several different tissues. Protease inhibitors have slowed cancer-cell division. Phytic acid has been shown to significantly slow the progression of tumors. In a recent study involving 3,237 men of different ethnic backgrounds, those who consumed the most beans had a 38%-lower risk of prostate cancer than subjects who consumed the least.
Soyfoods have active ingredients, including isoflavones, saponins, phenolic acids, phytic acid, phytosterols and protein kinase inhibitors, that appear to have anticancer effects. "Recent publication of the meta-analysis shows that consumption of soyfood is related to a lower risk of breast cancer in women," says Lin Yan, Ph.D., director of Cancer Research, The Solae Company, St. Louis, and author of the study published in International Journal of Cancer Prevention in April 2005. The meta-analysis reviewed 12 epidemiological studies relating soy consumption and breast cancer. Laboratory studies also demonstrate that soy has protective effects against cancer in the cells and tissues of the bladder, cervix, lung and stomach. Soy might also inhibit the growth of prostate-cancer cells in a variety of lab conditions.
Whole grains contain several substances linked to a lower cancer risk, including fiber, antioxidants, phenols, lignans, phytoestrogens and saponins. Combining data from 40 recent studies, the AICR estimated that the risk for cancer was reduced by 34% for people who ate large amounts of whole grains compared to those who ate small amounts.
Berries are a good sources of vitamin C and fiber and are rich in the phytochemical ellagic acid that has shown the ability to prevent cancers of the skin, bladder, lung, esophagus and breast in laboratory studies. Blueberries contain a family of phenolic compounds called anthocyanosides that seem to be among the most-potent antioxidants. Studies conducted at the Jean Mayer USDA Human Nutrition Center on Aging, Tufts University, Boston, ranked blueberries No. 1 in antioxidant activity when compared with 40 other commercially available fruits and vegetables.
Grapes and grape juice are rich sources of resveratrol, a specific type of polyphenol. Several studies have found resveratrol to possess potent antioxidant and anti-inflammatory properties. Resveratrol can prevent damage known to trigger the cancer process in cell, tissue and animal models. It also slows the growth of cancer cells and inhibits the formation of tumors in lymph, liver, stomach and breast cells.
Calcium is associated with reducing colon-cancer risk. Combined evidence from two clinical trials of calcium supplementation lasting for several years revealed a reduced rate of recurrent colorectal adenoma. In a study of more than 45,000 American women followed for about 8½ years, researchers showed that calcium can cut women's risk of colorectal cancer. The best results came from combining a calcium-rich diet with supplements.
Green tea is causing a stir in the cancer-preventative world, as it is the best food source of catechins, according to the AICR. Catechins prove to be more-powerful antioxidants than vitamins C and E at arresting oxidative damage to cells. In laboratory studies, green tea slows or completely prevents cancer development in colon, liver, breast and prostate cells. Other studies show green tea to have similar effects in tissues of the lung, skin and digestive tract.
Omega-3 fatty acids are found in fatty fish, flaxseed oil, canola oil and walnuts. Epidemiological studies and animal experiments indicate that omega-3 fatty acids exert protective effects against some common cancers, especially cancers of the breast, colon and prostate.
Prebiotics and probiotics show a glimmer of hope, as a recent study suggests taking a symbiotic (combination of prebiotics and probiotics) each day, could significantly reduce the damage to cell DNA by 60%. In addition, synbiotics were found to protect the cells from potentially damaging substances, such as cytotoxic and genotoxic agents, that occur in the lower digestive tract and increase the risk of some cancers.
Vitamins and micronutrients such as vitamin C, vitamin E, vitamin A, calcium, folate, beta-carotene and selenium found in foods have demonstrated cancer-protective relationships. Epidemiological studies suggest that high plasma levels of selenium, carotenoids and ascorbic acid protect against cancer. Much is yet to be learned about how micronutrients play a role in cancer protection, illustrated by the recent study that showed an increase in lung cancer with beta-carotene.
Cancer-fighting food claims
In addition, the leading natural lycopene manufacturer, LycoRed Natural Products Industries, Ltd., Be'er-Sheva, Israel, and several of the largest tomato-industry participants have submitted a petition to FDA providing strong scientific support for a qualified health claim about the role of lycopene in the prevention of prostate cancer, according to Paul Flowerman, president, P. L. Thomas & Co., Morristown, NJ.
According to Thomas Payne, food industry marketing consultant, for U.S. Highbush Blueberry Council, Folsom, CA. "65% of consumers are aware that blueberries have antioxidants. There's a health halo for blueberries. Every time there's a positive story on blueberries, it plays very well. The blueberry market will jump 10% to 15% overnight. From a marketing standpoint, blueberries as a food ingredient are an explosive growth category, with 40% to 50% growth."
Though the biggest gain in blueberry sales has been in fresh, other forms, such as blueberry juice, purées, and freeze-dried berries have found markets.
Heggie reports that in the recent 2004 HealthFocus survey, conducted by HealthFocus International, St. Petersburg, FL, 35% of consumers said that they had heard a lot or some about soy's role in cancer prevention. And lycopene is so hot that manufacturers need only list its presence on tomato products to help sell products.
Bioactive compounds are finding their way into an increasingly large number of products. Flowerman reports that lycopene is being added to breakfast cereal, beverages, breads, tortillas and pizza. Consumers can easily spy tea extracts and beta-carotene in many popular foods and beverages. P. L. Thomas provides a number of natural ingredients derived from plants, from grapeseed extract to onion extract. Examples of bioactive compounds available to savvy customers looking to fight off cancer for another day include the Swiss manufacturer, Linnea's HMRlignan(TM), from the Norway spruce; OptiBerry®, a standardized anthocyanin extract from InterHealth Nutraceuticals, Bencia, CA; and Synergy(TM) a pomegranate smoothie that contains pomegranate juice with a nutritional premix of antioxidants and nutrients from The Wright Group, Crowley, LA.
But some scientists say that it's still too early to pump foods with bioactive compounds to fight cancer. The USDA Agricultural Research Service, Grand Forks, ND, reports in the Annals of Botany 2005 that the evidence of efficacy is weak for certain carotenoids and polyphenols, the evidence is stronger for glucosinolates and lycopene, but the production of enriched foods is still premature. The functional-food industry has produced and marketed foods enriched with bioactive compounds, but there are no universally accepted criteria for judging efficacy of the compounds or enriched foods, and inadequate chemical identification of compounds and inconsistencies in hypotheses and methodologies leave many gaps in our understanding of bioactive compounds.
"People want the fast and easy way of doing things," says Dyer, who finds convenience products such as prepared stir-fry mixes, salads and fresh produce a beautiful way of marketing cancer-fighting foods. She reports that consumers often think that the way to fight off cancer is to down a whole bottle of pomegranate juice. She challenges food scientists to take advantage of the synergistic health benefits of whole foods by using them as ingredients, such as bars made with only dried fruits and nuts on the ingredient list. "The Chinese and Japanese have low cancer rates because their diet is composed of mixtures of foods, like stir-fries with 10 different ingredients," she adds.
As research continues to bring new evidence of the powerful cancer-preventative ability of thousands of bioactive compounds, it looks like the food industry will be very busy finding unique ways of delivering them to people.
Sharon Palmer is a registered dietitian with 16 years of experience in health-care and foodservice management. She writes on food and nutrition for newspapers, magazines, websites and books. Palmer makes her home in Southern California.
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