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Role of Diet in Cancer Hard to Study, Expert Says

Role of Diet in Cancer Hard to Study, Expert Says

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, indeed, it is possible at all, according to an international cancer prevention expert. With 30% to 50% of all cancers traceable to dietary factors, food is as important as smoking as a cause of human cancer, Michael J. Hill, phd, told attendees of 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. Worldwide, those tied to overnutrition are more common in the West, whereas those related to undernutrition are more common in the East.

Pitfalls of Making Dietary Recommendations

The problem with making dietary recommendations to reduce cancer risk is two-fold, according to Dr. Hill. First, altering diet to avoid one class of cancer may put one at risk for the other class. He gave as an example the so-called Mediterranean diet, which may reduce the risk of colorectal cancer while increasing the risk for gastric cancer. "Given this inverse relationship, it is hard to say what to recommend dietarily, since there's no use preventing one cancer if you're giving yourself another one. Obviously, we have to find a middle course," he said.

The second problem has to do with 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 besides diet 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, and in vivo, they may not cause cancers at all, since the human organism has many effective repair mechanisms, Dr. Hill explained.

Noncontroversial Recommendations

Nonetheless, there are some observations that are so strong and consistent that they can be believed, Dr. Hill said. It makes sense to avoid becoming overweight, since there are many cancers related to obesity, and none for which it is protective. "Avoiding overweight is safe. It won't give you anything else," he said.

Second, it seems clear that fruit and vegetable intake is a good thing. "From the standpoint of epidemiology, it looks like it's 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 apparent, he asserted, that cereals are protective against colorectal and probably breast cancers, and it is fairly certain that dietary fiber, not crude fiber, is responsible for the benefit.

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 European countries with the highest fat intake also have the lowest rate of fat-related cancers, and 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.

Dr. Hill concluded by saying that dietary intervention is probably the best tool for studying the issue, using a high-risk population with short follow-up. Nonetheless, such studies are really asking the question of whether dietary factors can save someone in whom the process of carcinogenesis is already well underway, and clearly the answer is that they cannot.

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