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By Mark Anthony, Ph.D. | 08/01/2006
According to the American Cancer Society, the best diet advice for preventing cancer is, essentially, to eat lots of fruits and vegetables and avoid obesity. Although we’d love for the advice to be more specific – more formulaic – the truth is the diet-cancer relationship is hard to pin down because of the inherent confounding variables. The variety and action of reputed anti-cancer compounds in food are matched only by the complexity and unpredictable nature of the disease itself.
Cancer is varied and complex because it is not one disease. As defined by the American Cancer Society, “Cancer is a group of diseases characterized by uncontrolled growth and spread of abnormal cells.”
Normal cells have a programmed life span, but cancer cells remain perpetually youthful, highly energetic rebels that endlessly proliferate. They restlessly migrate using the blood or lymphatic system as their means of transportation. To support their wanderlust, they give off chemicals that increase the blood supply and eat away at the membranes of the tissues they randomly invade. This “metastasis” is what makes cancer so insidious.
In whatever new location the deviant cells set up shop, they continue their incessant replication, displacing healthy, well-behaving cells and impairing vital function.
Damage to genes that determine cellular life span converts healthy cells into mutant ones. This damage may happen as a result of exposure to radiation, pollutants and dangerous chemicals that cause oxidative damage, or even viral infections. Mutations also may happen spontaneously. All these factors can take advantage of a genetic predisposition to cancer.
This first stage of cancer is called “initiation” or “transformation.” Transformed cells do not automatically become malignant tumors; they still need to avoid detection by the immune system, which normally destroys aberrant cells. The cells then go through a “promotion” stage where conditions in the body must favor their tendency to reproduce. This phase may occur over a few months or it may take many years.
For diet to head off cancer, it must make an impact at the initiation and promotion phases, where nutrients and phytochemicals may prevent the DNA damage while enhancing the power of the immune system to pick off mutant cells before they enter the promotion phase. Certain nutritional components also may help slow the promotion phase to prevent cancer from entering its final stage. The final stage, progression, accelerates as cancer cells quickly multiply, disrupting body functions and leading to death.
|The list of potential anti-cancer agents that occur naturally in fruits, vegetables, spices and herbs is impressive and growing, as scientists learn more about these phytochemicals. They are categorized by structure and include polyphenols, terpenes, alkaloids, flavonoids, phenolics and many others.|
On the other hand, a deficiency of nutrients critical to normal DNA synthesis and repair could also increase cancer risk. For example, the B vitamin folate provides the single-carbon units for the synthesis of nucleic acids – a critical step in DNA synthesis (see “To B or Not to B”, also from the August 2006 issue of Wellness Foods).
Poor folate status is associated with cancers of the brain, breast, cervix, lung, esophagus, pancreas and lower GI tract. A folate deficiency can lead to breaks in the DNA strand, which increases the rate of mutations and hampers the synthesis and repair of DNA.
Folate is abundant in green vegetables, orange juice and many legumes (the name comes from “foliage”). It’s also in liver. Still, many people rely on fortification of processed foods to supply their needs.
Factors other than intake and absorption are at work in folate metabolism. A deficiency of vitamin B12 can weaken DNA, and without B12, folate remains trapped in a form that cannot be used for DNA synthesis.
Reducing cancer risk by preventing oxidation is a strategy that grew out of the observation that fruits and vegetables contain numerous natural antioxidants, such as vitamin C. The connection between C and cancer prevention is unclear.
Several studies show a lowered risk of various cancers with a vitamin C intake of 80 to 200 mg per day – greater than the current RDA of 60 mg, but certainly not a megadose. However, low intake of vitamin C can also indicate low intake of fruits and vegetables, a risk factor for health in general.
A placebo-controlled intervention study designed to look at the effect of another antioxidant vitamin, alpha-tocopherol (vitamin E) supplementation on lung cancer in smokers showed the vitamin E group had a 34 percent lower rate of prostate cancer. It also prompted interest in how vitamin E may act in conjunction with selenium, an antioxidant mineral.
“Selenium, especially in organic form, is one of the most versatile antioxidants,” says Vladimir Badmaev, Ph.D., vice president of medical and scientific affairs at Sabinsa Corp. (www.sabinsa.com), Piscataway, N.J. He adds, “Selenium is an essential trace element in nutrition for the prevention of disease in humans. Epidemiological studies indicate an association between low nutritional selenium and increased risks of cardiomyopathy, cardiovascular disease and carcinogenesis in various sites in the body.”
Sabinsa currently supplies its Selenium Select for the National Cancer Institute’s Selenium and Vitamin E Cancer Prevention Trial. The study, which began in 2001 on 35,000 men from the U.S., Canada and Puerto Rico, is examining how selenium and vitamin E, both separately and together, can protect against prostate cancer.
A wealth of other antioxidants, such as vitamin A and related carotenoids, anthocyanins and other phytochemicals, have been and are still being studied for their abilities to protect cells against damage that influences cancer development and growth.
|The same phytochemical that gives these peppers their fire – capsaicin – has a protective, anti-inflammatory effect on human cells. Photo courtesy of wikipedia.com.|
For example, most peppers contain capsaicin, a fat-soluble chemical responsible for the strong burn of hot peppers. Most mammals (except for enlightened chili freaks) find this unpleasant. Both the heat and bright colors (anthocyanins) turn out to be protective.
These compounds can act in a number of ways, neutralizing free radicals or suppressing inflammatory processes that lead to transformation and the proliferation of cancer cells. They may also interfere with cell-signaling pathways that direct cancer’s rapid and uncontrolled growth. Some may even slow metastasis by making it harder for the growing cancer to obtain its blood supply.
Another example of how plant chemicals may work against cancer is their effect on a substance called nuclear factor-kappa B (NF-KB). According to Bharat Aggarwal, Ph.D., of the University of Texas M.D. Anderson Cancer Center, Houston, NF-KB describes a family of proteins that can bind to a specific DNA site.
Once activated by free radicals, inflammatory stimuli, carcinogens, UV light or X-rays, NF-KB migrates to the cell nucleus. It can then switch on more than 200 genes shown to suppress normal cell death, thus transforming the cells into potentially aggressive (pre)cancerous cells.
Many proteins regulate the cell cycle. Loss of this regulation virtually defines cancer. Isothiocyanates (found in cherries and berries), resveratrol (grapes), genistein (soybeans, chickpeas), apigenin (seeds and many vegetables), and silibinin (artichokes) have been shown to prevent aberrations of the cell cycle.
P53 is a protein that protects genes from damage. But damage to the gene that makes p53 removes a natural tumor suppressor, increasing the risk for many different cancers.
Phytochemicals such as epigallocatechin gallate (EGCG) from green tea, indole-3-carbinol and silibinin have been shown to affect the activity of this critical protein. EGCG has also displayed many favorable effects on the proteins that govern cell signaling and inflammation.
The list of potential anti-cancer agents in fruits, vegetables, spices and herbs is impressive and growing. They are categorized by structure and include polyphenols, terpenes, alkaloids, flavonoids, phenolics and many others. In addition to their potential to prevent cell transformation and promotion, some may slow metastasis by hindering angiogenesis, the proliferation of blood vessels that aid cancer cells in their invasion of healthy tissues.
Evidence that diet may prevent a large number of cancer deaths is encouraging. “Hippocrates’ quote ‘let food be thy medicine’ is now being more widely adopted and the benefits of natural and organic foods continue to be recognized,” says Gerry Amantea, Ph.D., vice president of technical service for Melville, N.Y.-based The Hain Celestial Group Inc.
But the diversity of phytochemicals and the complexity of their interactions means that no single diet, individual food, or ingredient emerges as a superhero all of the time. There are just too many complicating factors.
The USDA may have just come around to whole grains in its latest version of the Food Guide Pyramid, but Nature’s Path Foods Inc., of Blaine, Wash., has been banking on this healthy tradition for more than two decades. The company provides a complete line of organic whole-grain cold and hot cereals, breads, pastas, baking mixes and energy bars.
Numerous case-control studies link a lower risk of more than 20 different types of cancers with increased consumption of whole grains. Known mostly for their fiber contribution to the diet, whole grains contain compounds called lignans.
Lignans are similar to isoflavones in that they have weak estrogenic and antioxidant activity. But unlike isoflavones, lignans must be activated. Lignan precursors in the plant fiber are converted to the animal lignans enterolactone and enterodiol by bacteria in the colon.
|Wild-caught salmon is among the best sources of healthful omega-3 fatty acids.|
People who ate the most red and processed meats had a higher risk of colon cancer, compared with those who ate the least. Fish eaters’ risks were the opposite. Cancer was lower among people who ate the most fish compared with those who ate the least. No link was apparent between poultry consumption and colon cancer risk.
Unlike meat, fish is not generally associated with the “Western Dietary Pattern” that includes low intake of fruits and vegetables, and high consumption of desserts and fried foods. This could account for some of the differences in cancer risk between the two groups.
Another factor is the presence of omega-3 fatty acids. Fish, especially the wild, deep-ocean varieties, are high in omega-3s.
“Cold-water fatty fish, such as wild salmon, sardines, albacore tuna and sablefish are among the very best sources of omega-3s,” says Randy Hartnell, president of Vital Choice Seafood (www.vitalchoice.com), Anacortes, Wash. Omega-3 fatty acids are powerful anti-inflammatory agents, but their effect on cancer is unclear.
High meat consumption was also linked to gastric cancer, but the risk was greatest if the subjects also were infected with Helicobacter pylori, a bacterium responsible for most cases of ulcers and chronic gastritis. H. pylori can weaken the protective lining of the stomach and small intestine, allowing corrosive digestive acids and enzymes to irritate the sensitive gut lining.
In 1994, the World Health Organization’s International Agency for Research on Cancer declared H. pylori a "definite carcinogen": Half of the world’s population may be infected with H. pylori, which thrives in crowded, unsanitary conditions. Yet many persons have the organism in the gut without any ill effects. Also, in the study meat eaters were more likely to be former smokers, and tended to eat less fruits and vegetables – both confounding risk factors.
Many healthy foods and food components are associated with a reduced risk of cancer, and while it’s tempting to read a study and declare that green tea, soy, blueberries, tomatoes or any one food or food fraction prevents cancer, we’d be missing the point: It’s the whole diet.
It also is clear that lowering our risk of cancer requires pursuing total health. “Manufacturers of nutrient-rich foods and beverages need to meet consumers’ taste, packaging and branding expectations,” says Seth Goldman, co-founder of Honest Tea (www.honesttea.com), Bethesda, Md. “If our products don’t taste great and look great or are not easy to understand, consumers will pass us by. They have too many choices, and processed food folks have already set the bar quite high.”
The difficulty in pinning down the relationship between diet and cancer drives the highly stringent FDA guidelines regarding health claims on food labels. The FDA requires “convincing evidence” that a particular food or food ingredient is associated with a decreased risk of cancer before any claims to that effect can be made on the label.
For example, omega-3 fatty acids are essential to health and are known anti-inflammatory agents. Some studies link them to a lowered risk of cancer, too. However, the evidence is not sufficiently convincing to claim this – or even imply it – on a food label.
The FDA allows only a general statement regarding fats and cancer; “Development of cancer depends on many factors. A diet low in total fat may reduce the risk of some cancers,” is considered a responsible claim.
Claims regarding fiber-containing fruits, vegetables, and whole grains are similarly restricted. The food must be low in fat; a good source of dietary fiber (without fortification) and the type of fiber may not be specified. “Low-fat diets rich in fiber-containing grain products, fruits, and vegetables may reduce the risk of some types of cancer, a disease associated with many factors,” is a model claim permitted by the FDA.
This conservative approach protects consumers and manufactures. But it also makes it tricky to promote foods with added nutraceuticals as helping to prevent cancer. For more information on FDA claims, visit www.fda.gov.
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