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,"
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.
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.