Links Between Cancer, Dietary Fat Increase
Americans need only look at the Chinese, Japanese and Southern Italians to see we eat far too much fat, say authors of several recent studies pointing to the strong connection between dietary fat and particular types of cancers.
Dietary fat, fondly found in such goodies as ice cream and avocados, is a well-known contributor to heart disease, diabetes and high blood pressure. Now evidence is accumulating that it contributes mightily to some cancers, particularly breast and colorectal cancer, and, possibly, prostate, pancreatic and ovarian cancer.
While physicians know dietary fat increases deposits in blood vessels that cause heart disease, much less is known about how fat contributes to cancer.
But the mystery hasn’t held back health experts from strong recommendations that Americans reduce dietary fat--perhaps even below current recommendation of 30% of total daily calories.
“The fat story is real,” says Clifford Welsch, a pharmacology and toxicology professor at Michigan State University. “There is overwhelming evidence in experimental animals that the amount and type of dietary fat markedly affects this (cancer) process.”
But strong evidence also exists outside the laboratory, health experts note. And it’s the comparison of diets in other countries that convinces many health experts that Americans eat far too much fat.
* A new study cites high-fat diets and lack of exercise for higher rates of colorectal cancer in Chinese-Americans compared with Chinese living in China.
* A study in Italy shows two-fold increases in breast cancer rates in Northern Italy, where diets are much higher in fat, compared with Southern Italy.
* Studies show that Japanese women, who consume less fat, have lower breast cancer rates and better breast-cancer survival rates.
“Maybe it comes back to what we ate a million years ago when, probably, we only ate 10% of our calories in fat and ate 50 to 80 grams of fiber a day and when we exercised a lot,” says Dr. Ernst L. Wynder, president of the American Health Foundation, a private research group.
The foundation estimates that half of all cancers in women and a third of all cancers in men are related to dietary fat.
“I believe the major cause of disease is excessive exposure” to fat, Wynder says. “When it comes to fat, I believe our system was not developed to handle 100 grams of fat a day.”
Most Americans consume about 40% of their daily calories from fat. Health experts generally agree that fat should be limited to 30% of total daily calories. The health foundation, however, has taken a more extreme view, suggesting that fat should make up 25% or less of the daily diet.
The emerging studies are important in part because the cancers that appear to be influenced by dietary fat are major problems in the United States.
Other than common skin cancers, the incidence of colorectal cancer ranks second only to lung cancer in the United States. The American Cancer Society estimates that 155,000 new cases will be reported this year. Breast cancer affects about one in every 10 women.
“I think people are well aware of the role of dietary fat and heart disease, but the data haven’t been as strong with dietary fat and cancer,” says Stanford epidemiologist Alice Whittemore, co-author of a five-year study of Chinese populations. “I think that’s changing, though. The data are beginning to come in quite conclusively, on large bowel cancers, that fat is an important factor. I think this is an important public-health message.”
A rate of colorectal cancer four to five times higher among Chinese-Americans than among Chinese living in China can be strongly linked to a diet high in saturated fats and a sedentary lifestyle, Whittemore’s group at Stanford and researchers at the University of Southern California reported in a study published this month in the Journal of the National Cancer Institute.
Exercise is thought to help reduce incidence of cancer by speeding by the movement of food through the intestinal tract.
“The two factors--high saturated-fat diet and lack of exercise--are working together to be harmful,” Whittemore says. “I believe that this is a message everyone needs to hear, not just the Chinese.”
Studies comparing the rate of breast cancer in other countries are equally compelling, Wynder says.
Besides the comparison of breast cancer rates in Italy, other studies show that Japanese women, who consume less fat, have lower rates of breast cancer.
The link between dietary fat and breast cancer is strong enough that researchers now are studying whether breast cancer patients might have better survival rates if they adopt lower-fat diets.
Wynder says studies have shown that Japanese women who do develop breast cancer have much better survival rates.
A leading theory about breast-cancer development holds that increases in the hormone estrogen contributes to cancer. Dietary fat raises estrogen levels. Hormones are thought to play a role in other cancers as well.
“A new approach to cancer treatment is ‘Can you treat cancer with a low-fat diet?’ ” Wynder says. “In my opinion, dietary fat not only plays a role in the promotion of breast cancer but also in terms of metastasis.”
A study, funded by the National Cancer Institute, is under way at seven institutions to explore whether post-menopausal women who have been treated for breast cancer have better survival rates when they consume a diet of less than 20% fat.
“This is certainly a worthwhile trial,” Wynder says. “And one thing you can say about nutrition: It’s not toxic. It’s not costly at all. And almost anyone can do it. The women who are on the trial are complying with the diet very well. My own hunch is it will not only work for cancer in the breast but also work for other hormone-related cancers, particularly cancer of the prostrate and possibly ovary and endometrian. It certainly doesn’t relate to all cancers.”
Preliminary results from 20 patients in the study show a 60% drop in a particular type of estrogen.
Wynder, one of the strongest advocates for nutritional changes to reduce cancer rates, says it was tough convincing his peers that such a study was of value: “When it comes to nutrition to treat breast cancer, people says it’s hokey. I’ve had a difficult time convincing my colleagues that nutrition could be part of treatment.”
But others caution that it’s too soon to implicate diet as a major factor in the development of some cancers. One of the most comprehensive recent studies on diet and breast cancer in nurses, for example, showed no relationship. The study has been criticized, however, because the nurses were consuming diets of 32% to 44% fat, thought by some experts to be too high to make a difference in breast cancer rates.
Others suggest that while diets may help prevent cancer, many factors contribute to cancer development and survival rates.
“I think there is a difference in the way an individual responds,” says Paul Newberne of the Mallory Institute of Pathology in Boston. “Overall, I think nutrition is going to help us prevent cancer . . . rather than cure it. And I don’t think we should try to oversell something to the public that we can’t really deliver.”
Experts agree they must learn more about what is in fat that could contribute to cancer. Some researchers suggest saturated fat is the culprit; others say calories in fat are responsible.
“There is a whole array of ideas as to how fat might enhance (cancer) processes, but we don’t know definitively which of these are the most important,” says Michigan State’s Welsch. “I feel that the major impact from dietary fat happens to be the calories derived from fat.”
Studies have shown that those with colorectal cancer generally consume more calories than healthy people.
But the Stanford-USC study was among the first to strongly implicate saturated fat as a contributor to colorectal cancer. Because of variations in Chinese diets, researchers were able to link saturated fat to higher rates of colorectal cancer.
“This is the strongest association we’ve seen that unequivocally pulled out saturated fat as the culprit,” Whittemore says. Other studies soon to be released will support the relationship, she said.
Whittemore also suggests that the new evidence might lead health officials to recommend further reductions in dietary fat, under 30% of total calories.
“We found that those with the lowest risk among the Chinese had only consumed about 10 grams of saturated fat per day, about 100 calories,” Whittemore says. “It could well be that we should cut down even more.”
REDUCING DIETARY FAT
* Choose lean meat, fish, poultry and dry beans and peas as protein sources.
* Use skim or low-fat milk and milk products.
* Moderate use of egg yolks and organ meats.
* Limit intake of fats and oils, especially those high in saturated fat, such as butter, cream, lard, heavily hydrogenated fats (some margarines), shortenings and foods containing palm and coconut oils.
* Trim fat off meats.
* Broil, bake or boil rather than fry.
* Moderate use of foods that contain fat, such as breaded and deep-fried foods.
* Read labels carefully to determine both amount and type of fat present in foods.
Source: U.S. Department of Health and Human Services