Study questions impact of fish intake on colorectal cancer risk

NEW YORK (Reuters Health) -Increased consumption of fish did not decrease markers of colorectal cancer risk in a six-month randomized trial, researchers report.

The study, conducted in subjects with and without high risk for colorectal cancer, showed that increased intake of oil-rich or lean fish did not favorable alter rates of apoptosis and mitosis in colonic mucosa, the study team notes in the August issue of the American Journal of Clinical Nutrition.

"To date, no intervention trials have examined the beneficial effects of fish intake on colorectal cancer risk," Dr. Elizabeth K. Lund from the Institute of Food Research in Norwich, UK, and colleagues note in the paper.

"Our key aim in this study was to assess whether we could identify underlying mechanisms by which fish consumption may reduce colon cancer, as suggested by recent epidemiological studies," Dr. Lund noted in an email to Reuters Health.

The overall study population included individuals with colorectal polyps, inactive ulcerative colitis, or healthy colons. After randomization, 82 subjects received dietary advice plus instructions to eat 300 g of oil rich fish (salmon) each week, 78 received dietary advice plus instructions to each 300 g of lean fish (cod) weekly, and 82 received dietary advice only. At baseline, participants were eating fish an average of 1.5 times per week.

Apoptosis and mitosis were measured in colonic biopsy samples collected from 213 subjects before and after the intervention. The others did not complete the study, with the most common reasons being unwillingness to have a second sigmoidoscopy or to eat fish.

"The markers of cancer risk reduction we were looking for were lower levels of cell division and an increase in apoptosis," Dr. Lund explained.

"In this population of people already eating between 1 to 2 portions of fish per week, we saw no statistically significant added benefit, in relation to the markers of cancer risk measured, in asking them to eat a further 2 portions of fish per week over a period of 6 months," she reported. "Cell division did fall for both cod and salmon but not significantly."

Moreover, there was no significant decrease in the total number of mitotic cells per crypt with increased salmon (-0.87) or cod consumption (-1.04). There were also no marked changes in the distribution of mitotic cells within the colonic crypt in the fish eaters.

In their report, Dr. Lund and colleagues offer several possible reasons for these findings. It could be that the baseline fish consumption was already high in the study subjects. "Perhaps a more pronounced effect of increasing fish consumption would be expected in a population of nonfish consumers," they suggest.

Also, there was evidence that study subjects did not consume the salmon or cod in addition to their habitual fish consumption, as requested, but partly substituted the fish they would normally eat with the study fish.

Therefore, the intended increase of 2 additional fish servings per week actually only resulted in an increase of 1.3 to 1.4 extra servings per week. Thus, the contrasts between the intervention groups may not have been large enough to observe a beneficial effect of increased fish intake, the researchers note.

They suggest that future intervention studies include nonfish eaters or those with a relative low baseline intake of fish.

Am J Clin Nutr 2009;90:354-361.

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