Genetic Predisposition Also a Factor : The Risk of Colon Cancer May Vary With Amount of Fiber, Fat in Diet
Had President Reagan adopted a diet high in fiber, would recurrence of cancerous polyps have been prevented?
The question was posed to Dr. David Heber of the UCLA Division of Clinical Nutrition, which conducts both prevention and treatment programs for cancer patients.
“The question has to take into account President Reagan’s family history of cancer of the colon,” Heber said. “The President had a genetic predisposition to polyp development. However, there is a lot of evidence from population studies suggesting that diets high in fat, which is a typical American diet, help promote cancer.”
Heber pointed out that epidemiologic studies show that when Japanese migrate to Hawaii there are increased incidences of colon cancer when they adopt the Hawaiian diet. Similar studies show that Japanese living in California for one generation developed as much of a risk of colon cancer as American-born Californians, probably because of the change in diet.
Animal experiments support the high-fat-diet theory. Several studies (including an American Health Foundation study) show that more animals given chemical agents that induced cancer of the intestine developed intestinal tumors when a high-fat diet was fed to them along with the carcinogen than animals fed a low-fat diet along with carcinogens.
“But cancer has many risk factors, and some groups of people are more predisposed than others,” Heber said.
Heber pointed out that another high-risk group for colon cancer are patients who have ulcerated colitis. Other high-risk groups are the obese (people who weigh more than 20% of their normal weight) and those whose intake of fiber is low and fat intake is high. High intake of animal protein also is thought to be a risk factor.
Studies suggest that cancer of the colon, which is one of the most common cancers in the Western world, is associated with a mortality rate of 50%. Mortality may be associated with a diet too low in fiber. Epidemiologic evidence shows that fiber has a protective role in colon cancer. Dietary recommendations to increase fiber and decrease fat intake have been advocated by the National Cancer Institute to lower colon cancer risk.
Finns Have Less Colon Cancer
In spite of a diet high in fat, natives of Finland have low incidences of colon cancer (about one-fourth to one-third that of North Americans). Rural Finns consume about 30 grams of dietary fiber per day, whereas average Americans consume 10 to 15 grams.
Other laboratory studies with animals have shown that fiber in the diet can affect conversion of precursors of carcinogens or their promoters simply by changing bacterial action, decreasing time of fecal transit or decreasing frequency of bowel movement.
Dr. E. E. Ho of the Cancer Center at the University of Arizona thinks that intake of only two ounces of fiber in the form of 100% wheat cereal per day can markedly reduce incidences of colon cancer in many people.
Ho and associates are planning a large-scale, long-term trial with fiber supplementation to prevent recurrences of polyps, as announced at the Society for Nutrition Education annual meeting in Los Angeles recently.
“We are now in the process of assessing people’s attitudes toward general health maintenance and cancer prevention,” Ho said . The Arizona university team is currently analyzing the data. A pilot study of 30 individuals showed that most of the subjects were willing to change their diet to prevent cancer.
“We find that fiber intake produces less incidences of colon cancer. Mormons and Seventh-day Adventists have lower cancer incidences probably because of a diet high in fiber and low in fat,” Ho said.
Decreasing the Risk
“Ideally, we’d like to see everyone moving in that direction. What has excited me is that by increasing fiber to only two ounces of a fiber cereal, without changing anything else we can decrease the risk of cancer of the colon,” Ho said. “If we change other things, such as increase intake of complex carbohydrates, decrease fats from 40% to 30%, minimize intake of salt, cured or smoked foods, and drink alcoholic beverages in moderation, the risk of colon cancer would decrease even further.”
“What we need now is to have a well-controlled clinical trial with humans to prove that if we intervene, we can change the process of colon cancer by preventing polyps from developing or from reoccur ring,” Ho said.
The essence of dietary change is different from screening (a test performed for signs of polyp development). Ho pointed out that screening is important but it is a secondary prevention. “That’s when you’ve already formed polyps. If we start (screening) at an early age, cancer of the colon can probably be prevented. That’s the unique point of dietary preventing,” Ho said. The best screening program for colon cancer can potentially reduce mortality by 20%, Ho thinks.
There is no Recommended Dietary Allowance for fiber, and it has been estimated that Americans consume less than required for good health--25 to 50 grams is considered adequate. The U.S. Dietary Goals of the Senate Select Committee on Nutrition, as well as the American Cancer Society, advise an increase in consumption of fruits, vegetables and whole grains, which are rich in fiber.
There are two major categories of fiber--water-insoluble and water-soluble. Water-insoluble fiber is called cellulose, hemicellulose, lignin and cuticle and is found in whole-wheat products, wheat bran, fruit and vegetable skins. Pectins, found in fruits and vegetables and gums from tropical plants, are the major water-soluble dietary fibers. Most plants contain a combination of soluble and insoluble fibers.
Special Report on Fiber
According to a chart on the content of high-fiber foods printed in a special report on fiber in Tufts University Diet & Nutrition Letter, July 1985, there are about nine grams of fiber per serving of cooked legumes (about one-half cup of kidney, lima or pinto beans or one cup of lentils).
Cooked vegetables contain about five grams of fiber per serving (about one-half to three-quarters cup). Cooking, incidentally, does not appear to decrease fiber in most foods.
Fruit contains about three grams of fiber per medium fruit, and cereal about three grams per serving (two tablespoons wheat bran, oat bran, 100% bran, two slices bread, two-thirds cup brown rice or three cups popcorn).
Nuts contain about five grams of fiber per one to 2 1/2 ounces of nuts, depending on the type.
Consumers are warned to increase, but not to overdo, fiber in the diet. Too much fiber, it seems, may bind minerals such as calcium, iron and zinc. Too much fiber, without adequate amounts of liquid, can also cause the intestines to become blocked. However, reasonable, moderate intakes of fiber should cause no problems with mineral metabolism.
For those who have had cancer surgery or those at risk of breast, uteral, colon, prostate or other forms of cancer, UCLA Medical Center, Division of Clinical Nutrition, offers an individualized outpatient program. According to Heber, the program’s director, dietary and behavioral changes can prevent cancer in 60% of women and 40% of men.
“When patients discuss nutrition with their doctor, they (physicians) usually discount the importance or don’t pay too much attention to the nutritional approach,” Heber said. “That’s because physicians are not trained in nutrition in medical schools.”
No. 2 Killer Cancer
The importance of low-fat diets is stressed for cancer patients. High-fat intake is correlated with breast cancer, the No. 2 killer cancer (lung cancer is No. 1), as well as other cancers, such as uteral, colon and prostate cancer.
“We are starting some trials using women within two months of breast surgery and (who are) considering chemotherapy,” Heber said. “We are doing the study in the absence of chemotherapy and are looking for women ages 51 to 60 who will derive little benefit from chemotherapy because they are post-menopausal.”
The general outpatient program is geared toward prevention as well as treatment. For information about the study as well as the outpatient program, call the reference center at (213) 825-4713.
