Vitamin supplements, beta-carotene and Vitamin A have been linked with cancer prevention but there now are doubts about their protective effect, according to James Marshall, member of the cancer epidemiology research team at State University of New York in Buffalo.
"Studies do not support the idea of self-prescribing vitamin and mineral supplements to prevent cancer," he said, speaking of human diet and cancer studies based on food consumption.
However, certain foods, particularly those containing Vitamin A and carotene, which is converted in the body to Vitamin A (found in high amounts in fruits and vegetables and in lesser amounts in animal products such as milk, meat and eggs), have been identified as having a protective effect in the diet. "We don't know what is doing the work, but something is happening," Marshall said.
Pills Versus Food
At a recent seminar on pills versus food sponsored by the California Dietetic Assn. and the Dairy Council of California, Marshall told dietitians that vitamin and mineral supplements can fail to have any effect because pills do not duplicate the combination of nutrients found in foods.
"While recommendations regarding diet and cancer are probably the best we can come up with at this time, they aren't gospel," Marshall said. "It is quite possible that nutrients relative to cancer have yet to be identified. It's also possible that nutrients found to inhibit certain growths could induce others."
So far, the link between diet and cancer is not fully understood. According to Marshall, diet may be responsible for 20% to 35% of cancers, although some experts extend the range to 70%. Tobacco, a known cause of cancer, takes the blame for 30% of the risk.
Marshall pointed out that studies on diet and cancer cover only information derived from laboratory animal and ecological studies, with most of the attention focused on fiber, fats and vitamins A and D.
Added Cancer Risk
"There is no evidence that any dietary element, whether in the form of food or vitamin supplements is as powerful as cigarettes in the causes of cancer," Marshall said.
A combination of high fat and low fiber intake places an added risk of cancer. "Many studies, however, failed to show that effect. We still have a long way to go before we understand the effects of fiber and cancer," Marshall said.
According to some studies, fats may increase the risk of cancer, but these studies have not been completely convincing. "We still don't have evidence that meat consumption is associated with cancer risk. A few studies have shown that beef has a protective effect on both the colon and gastrointestinal tract. A direct link is still to be proven," Marshall said.
Many studies related to Vitamin A deal with beta carotene, which has received the bulk of attention. Beta carotene is but one of many carotenes found in fruits and vegetables, such as carrots, spinach, winter squash and tomatoes. It is also found in cruciferous vegetables (from the cabbage family) such as broccoli, Brussels sprouts and cabbage. Fruits high in Vitamin A are often high in Vitamin C, too, and include cantaloupe, watermelon, grapefruit and strawberries.
Controlled studies performed by Marshall and his colleagues have shown an increased incidence of cancers of the mouth, esophagus, cervix, bladder, larynx and lung among people with a low intake of these carotene-rich foods.
'A Lot to Learn'
However, thinks Marshall, it would be foolish to single out specific nutrients, such as vitamins A or C in pill form as preventions against cancer. "Consuming them in pill form to prevent cancer may not only be a waste of money, but also dangerous," Marshall said.
"We still have a lot to learn. A well-balanced diet is the best we can offer at this time. It's just wishful thinking that vitamins and minerals will prevent cancer," he said.
The American Cancer Society's advice for cancer prevention is to reduce the proportion of calories derived from fat; increase intake of complex carbohydrates such as fruits, vegetables and grains that are rich in vitamins A and C as well as more cabbage-family vegetable and cauliflower; eat only moderate amounts of salt-cured, smoked and nitrite-cured foods.
Adding exercise to the daily routine is advised. The cancer society also advises moderation in the consumption of alcoholic beverages. Heavy drinkers, especially those who also smoke cigarettes or chew tobacco, are at unusually high risk for cancer of the mouth, larynx and esophagus. Alcohol abuse can result in cirrhosis, which sometimes leads to liver cancer.
Cancer prevention guidelines, with few exceptions, follow the Dietary Guidelines by the U.S. Department of Health and Human Services: eat a variety of foods, maintain desirable weight, avoid too much fat, saturated fat and cholesterol, eat foods with adequate starch and fiber, avoid too much sugar, avoid too much sodium and drink alcoholic beverages in moderation.