Although it is clear that diet plays a role in the etiology of many cancers, making dietary recommendations to reduce risk is highly complicated, if possible at all, according to an international cancer prevention expert.
With many cancers traceable to dietary factors, food is as great a concern as a cause of human cancer as is smoking, Michael J. Hill, PhD, told attendees at the second Strang International Cancer Prevention Conference, co-sponsored by Cornell University Medical College. "Unfortunately, we can give up smoking, but we cannot give up eating," said Dr. Hill, who is Chairman of the European Cancer Prevention Organization.
Cancers tied to dietary factors can be divided into two classes, Dr. Hill said: those of overnutrition, such as breast, endometrial, ovarian, prostate, and colorectal cancers; and those of undernutrition, principally stomach, liver, esophageal, and oropharyngeal cancers. Cancers tied to overnutrition are more common in the West, whereas those related to undernutrition are more common in the East.
One problem with making dietary recommendations to reduce cancer risk is the quality of data available on the role of nutrition in the development of cancer. In view of the long latency period of most cancers, the dietary relationship is not well reflected in prospective studies that span only 5 to 10 years, Dr. Hill pointed out. "We're always looking at late-stage events unless there is a very long follow-up, which is what has been the problem with beta carotene studies," he maintained.
Furthermore, population studies necessarily involve other environmental factors that may skew the data. Longer-range prospective studies are impractical, Dr. Hill said, and retrospective studies are extremely vulnerable to recall bias on the part of the study subjects. Animal studies are of limited relevance, he said, and in vitro studies are even more limited, since "mutagens are not necessarily carcinogens." Substances, that may, in isolation, cause DNA damage to cells in vitro may not do so in the company of other substances, as they are found in food. In vivo, they may not cause cancer at all, since the human organism has many effective repair mechanisms, Dr. Hill explained.
Nonetheless, there are some observations that are consistently strong and can be believed, Dr. Hill said. It makes sense to avoid obesity, since many cancers are related to obesity, and none are protected by it. "Avoiding obesity is safe. It won't give you anything else," he said. Second, consuming fruits and vegetables is beneficial. "From the standpoint of epidemiology, it looks like eating fruits and vegetables is protective, and no causal effects have been observed in a wide range of cancers, including gastric, colorectal, esophageal, oropharyngeal, pancreatic, breast, prostate, bladder, and liver cancers," he said. It is also apparent, he asserted, that cereals and the dietary fiber that they provide are protective against colorectal and probably breast cancer.
The role of fat and meat, however, is less clear, according to Dr. Hill. The recommendation that fat should represent less than 30% of the total daily caloric intake caused a furor, he pointed out, because those European countries with the highest fat intake also have the lowest rate of fat-related cancers. Also, a study in Japan showed meat to be protective against gastric cancer. "What is not clear is whether fat and/or meat intake is a genuine effect or a surrogate marker of a good diet," he said.