|Tratto da Vegetarian Nutrition & Health Letter Vol.1 n.1, pubblicato dalla Loma Linda University (Traduzione in Italiano).|
DIET AND CANCER
by John Weisburger, PhD
|Dr. John Weisburger has conducted research on cancer prevention and has published over 500 papers. He is a past Director of the Research Institute of the American Health Foundation in Valhalla, New York.|
Over the past 50 years we have learned much about how diet affects cancer risk. If Americans adopted the current dietary guidelines issued by the National Cancer Institute, cancer rates would decrease by as much as 25 to 50%. Plant foods are protective for several reasons.
Fiber Reduces Cancer Risk
Early interest in fiber and cancer came from intriguing observations of the Finnish population . The people in Finland have one of the highest rates of heart attacks, but they have relatively low rates of both colon and breast cancer. Dietary studies showed that the Finns are high consumers of dairy products including milk, which may contribute to their high rate of heart disease. However, they also consume a type of rye bread high in bran fiber. When investigators studied stool samples of Finns and compared them to those of people living in New York they found that stools of Finns weighed two to three times more than those of New Yorkers. Also, the concentration of bile acids in Finnish stools was much lower than that in the American stools. Bile acids are compounds produced by the liver to aid in the digestion of fats but they also appear to promote the growth of cancer cells in the colon.
The larger stool also increases the excretion of estrogen and therefore could result in lower blood estrogen levels in Finnish women. This may protect against breast cancer, since lower estrogen levels are associated with reduced risk for breast cancer.
|In 1978, when the Food and Drug Administration (FDA) initiated a ban on the artificial sweetener saccharin, I noted that the American people would be better off if the FDA banned white bread.|
About the same time that the Finns were being studied, Dr. Denis Burkitt, a distinguished British surgeon stationed in Uganda, noted that colon cancer rates were high among British living in Africa but rare among native Africans . In fact, the Africans rarely had any intestinal diseases such as appendicitis, diverticulosis, and irritable bowel syndrome. Dr. Burkitt found that, compared to the British, the Africans ate more high-fiber cereals and lots of fruits and vegetables-a fiber intake as high as 70 grams a day compared to 15 grams per day which is common intake among westerners. They generated multiple stools every day, weighing as much as 10 to 15 ounces.
Fiber helps to reduce colon cancer in two ways. Bran cereals and high fiber breads contain mostly insoluble fiber, which absorbs fluid and swells up to provide for increased stool bulk. This helps to decrease the concentration of harmful bile acids and other potential carcinogens in the stool. Oat bran and many vegetables contain mostly soluble fiber. This doesn't add much to stool bulk but forms a gel-like matrix that aids in the elimination of bile acids and other potentially harmful components in the colon.
How Vegetarian Diets Protect Against Cancer
Do vegetarians get less cancer?
Well, they do have less cancer than omnivores. However, we don't know how much of this difference is due to diet. It's hard to study cancer because it is a disease that can be decades in the making. And, since many lifestyle factors affect risk, it is always difficult to single out the effects of any one of those factors. For example, vegetarians may have reduced mortality rates because they are less obese, exercise more, smoke less and get more medical check-ups. The lower incidence of smoking is one reason vegetarians have less lung cancer.
In some cases, we have good reason to believe that a vegetarian diet is protective. This is most clear in the case of colon cancer. And, although risk may not be significantly lower for breast and prostate cancer, many studies have found links between animal product intake and risk for these cancers.
Factors in vegetarian diets that appear to be protective against cancer include the following:
Vegetarians eat more fiber.
In fact, vegetarians consume two to three times more fiber than nonvegetarians. The National Cancer Institute (USA) recommends 20 to 35 grams of fiber a day. The average westerner eats just 15 grams a day. Vegetarians typically eat 30 to 45 grams daily.
Vegetarians consume less fat.
Lacto-ovo vegetarians eat diets that are about 10% lower in fat than nonvegetarian diets, and vegan diets are about 20% lower. High fat diets may increase risk for major cancers such as breast, colon, and prostate cancers. There is room for improvement, though, in vegetarian diets. Many vegetarians eat diets that are higher than 30% fat-the upper limit set by the National Cancer Institute. And many experts believe that these limits are still far too high.
Vegetarians don't eat meat.
Red meat in particular can form mutagenic compounds when cooked at high temperatures. Certain groups of people may be at especially high risk if they eat meat because of the way they metabolize these mutagenic compounds.
Vegetarians eat more fruits and vegetables.
Vegetarians consume somewhat more of these foods and may get more of the potential anti-carcinogens in fruits and vegetables. (But everyone, vegetarian or not, needs to eat more of these protective foods.)
Vegetarians consume more phytochemicals.
These are biologically active compounds found only in plant foods. Plants produce them to protect themselves against a host of environmental stressors and they may protect us as well.
Vegetarians have lower body stores of iron.
High iron levels may raise risk for cancer, although this is very speculative. The excess iron may generate free radicals-reactive compounds that wreak havoc upon cells.
Vegetarians consume more antioxidants.
These compounds help to neutralize the effects of free radicals. Some are vitamins, such as beta carotene (the vitamin A precursor), vitamin C, and vitamin E, and some are non-nutrient phytochemicals. Vegetarians consume 50% more vitamin C than nonvegetarians and twice as much vitamin E and beta-carotene.
Fiber is extremely important in reducing risk for colon cancer. In 1978, when the Food and Drug Administration (FDA) initiated a ban on the artificial sweetener saccharin, I noted that the American people would be better off if the FDA banned white bread.
In view of what we know about fiber and cancer risk, it is indeed important to consume a diet high in cereals, whole grain breads, fruits and vegetables.
Phytochemicals and Antioxidants: The Power of Fruits and Vegetables.
International comparisons and comparisons within countries show that people who eat green and yellow vegetables and fruits have a lower incidence of major chronic disease, including many types of cancer. More than 200 studies show the benefits of fruits and vegetables. The immediate thought that comes to mind is that vegetables and fruits are good sources of vitamins and minerals. .
However, vegetables and fruits contribute much more to good health than just vitamins. They are sources of phytochemicals-or plant chemicals-which are not nutrients, but are biologically active compounds. Just as humans produce antibodies to ward off infection, phytochemicals offer protection to plants. They may also be protective for us [4, 5].
There are many different types of phytochemicals in plant foods. These include indoles in vegetables of the cabbage family, sulfur compounds in garlic and leeks, isoflavones (plant estrogens) in soybeans, lignans in flax seed, and carotenoids in many vegetables. Phytochemicals appear to exert physiological effects in many different ways . They may stimulate the immune system, help to detoxify harmful chemicals, affect hormone levels, and control cell growth. But of particular importance is the action of some phytochemicals as antioxidants.
Why is this significant? Living cells require oxygen because the enzymes that generate the energy essential to cell functioning require oxygen. But during the process of oxygen use, cells can also generate abnormal reactive chemicals. Some of these reactive oxygen components are potentially dangerous and may lead to heart disease or initiate cancer. Nature has provided for neutralization of these potentially harmful components by supplying antioxidants in food.
Some Fats Raise Cancer Risk
As long ago as the 1940s, a few early pioneers suggested that dietary fat played a role in enhancing cancer risk. However, their research was not considered "fashionable" by mainstream cancer researchers and was largely ignored until the mid-1960s. At that time, Canadian researcher Kenneth Carroll showed that laboratory animals fed a 40% fat diet (similar to what westerners eat) had a much higher rate of breast cancer compared to those fed a 10% fat diet . Later research at the American Health Foundation (AHF) in New York confirmed these findings.
The studies at the AHF were inspired by the dramatic differences in breast cancer rates between women in the United States, whose fat intake averaged about 40%, and those in Japan, whose fat intake was closer to 10%. But women in Italy also had relatively lower cancer rates despite a fairly high fat intake. This gave rise to questions about type of fat as well as amount of fat in the diet .
Further studies showed that high intakes of animal fat and of polyunsaturated oils like corn or safflower oil increased the number of breast and colon tumors, compared to low-fat intake. A high dose of corn oil, for example, led to increased synthesis of bile acids, the compounds that raise risk for colon cancer.
Other types of fat had different effects, however. The omega-3 fatty acids found in fish oils and some plant foods actually reduced the incidence of breast and colon tumors, while monounsaturated oils such as olive oil were neutral. Although it hasn't been tested, canola oil, which is high in monounsaturated oil and contains some omega-3s, is likely a lower risk oil than other vegetable oils. The same may be true of flax seed oil, which is high in omega-3 fats. Both monounsaturated and omega-3 fats may also protect against heart disease. This would help to explain the lower rates of both cancer and heart disease in southern Italy and Greece . Even though these diets are relatively high in total fat, much of the fat in the cuisines of these countries is monounsaturated or rich in omega-3 fatty acids.
|Here at the American Health Foundation, our research goal is to "allow us to die young as late as possible."|
Other Food Components May Raise Cancer Risk
Cooking meat to the well-done stage involves the generation of reactive chemicals called heterocyclic amines. These chemicals, formed during browning of meat, may increase the risk of cancer of the colon, breast, prostate, and pancreas. Red meat appears to be particularly harmful .
The use of salt and pickled foods, common in many types of Asian cuisine, raises risk for stomach and esophageal cancer .
Early humans consumed foods that were freely available from nature-wild fruits, berries, greens, seeds, and roots. They may have used some milk from lactating animals but ate little meat. In modern times, many cultures have adopted a meat-and-potatoes diet with excessive amounts of salt.
Scientific research has produced a sound basis of knowledge for developing guidelines for a healthy lifestyle in order to reduce risk for cancer [12, 13]. Western eating habits are based on the wrong foods-too much of the wrong kinds of fat, too much meat with dangerous heterocyclic amines, too many salted foods, too little dietary fiber, and too few fruits and vegetables. Healthier eating habits should include generous use of fruits and vegetables-5 to 10 servings a day should be a goal, and foods such as cooked tomatoes and soy products should be used frequently. Foods high in fiber such as cereals, whole grain breads, and legumes should also be a regular part of the diet. And, choosing sources of monounsaturated fat and omega-3 fatty acids over animal fats and other plant fats should be a goal. Here at the American Health Foundation, our research goal is to "allow us to die young as late as possible.".
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