Fish oil slows some developing cancers.

Fish oil slows some developing cancers

While scientists know little about what causes pancreatic cancer, they have a strong hunch that high-fat diets are a major risk factor for this disease, the fifth-leading cancer killer in the United States. New animal data forcefully support that hunch and suggest that adding a significant amount of fish oil to the diet can slow critical stages in the development of this and other cancers.

Several years ago, researchers from Cornell University in ithaca, N.Y., and Dartmouth Medical School in Hanover, N.H., induced the development of precancerous tumor nodules by injecting two-week-old rats with azaserine--a potent pancreatic carcinogen. After four months on diets containing 20 percent corn oil (by weight), the rats showed a proliferation of growing precancerous lesions. Other rats on diets containing 20 percent menhaden (fish) oil developed only about one-third as many lesions.

Though the fat level in these diets was high--about 45 percent of the calories--it was only 18 percent higher than the level consumed by the average U.S. adult. By lowering the fat in the rats' diets after tumors had begun to develop, the researchers slowed the growth of the tumors, says T. Colin Campbell of Cornell, a nutritional biochemist and coauthor of the study.

In their newest study, described in the June 7 JOURNAL OF THE NATIONAL CANCER INSTITUTE, the same researchers showed that rats started on 20 percent fish oil but switched to corn oil midway through the experiment were hardly better off at the end of four months than those who ate 20 percent corn oil throughout the study. In contrast, rats started on corn oil but switched to fish oil two months later reaped virtually the same benefits in reduced precancer development as those dining on fish oil only. Campbell says these data raise an important question: Would similar benefits result if fish oil were given after the lesions had developed into true cancers?

Recent biochemical data suggest the answer is yes. Working with two types of cancers, human fibrosarcoma and a mouse melanoma, "we are showing that using different [dietary fast], you can affect the progression of a cancer," says Reuven Reich, a biochemist with the National institute of Dental Research in Bethesda, Md.

The body converts linoleic acid--an essential fatty acid that makes up 60 percent of corn oil--into arachidonic acid. Fish oils contain scant linoleic or arachidonic acid but are rich in eicosapentanoic acid. The only difference between arachidonic and eicosapentanoic acid, explains Reich, is that the former has four double bonds and the latter has five. In fact, the same enzymes metabolize both. However, he and his colleagues have recently shown that, given a choice between the two fatty acids, enzymes in mammalian cells preferentially metabolize the eicosapentanoic acid in fish oils.

In the April 28 BIOCHEMICAL AND BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS, Reich and his colleagues present data showing why that's apparently beneficial. Arachidonic acid's metabolites are at least 100 times more biologically active than those of eicosapentanoic acid, they report, and they have demonstrated in mice that this activity relates to the metabolites' ability to foster metastasis--the spawning of new tumors far from the initial cancer.

Arachidonic acid's metabolites probably promote metastasis, Reich's data suggest, by supressing the body's natural killer cells or by promoting the activity of cancer-cell enzymes that can cut through the connective tissue that would otherwise confine a malignancy (SN: 4/15/89, p.228).

"There's no doubt about it; something about fish oil puts it in a separate category from the average oil," says Leonard Cohen at the American Health Foundation in Valhalla, N.Y. He says that's why he and other cancer researchers are increasingly being drawn to it. However, his data also indicate that "you have to have a hefty amount in the diet before you see an [anticancer] effect."