Fish Oil - Evidence From Epidemiological Studies

Fish oils are unique because they contain high levels of n-3 PUFAs that are relatively low in vegetable oils and animal fats. n-3 PUFAs that are present in fish oil include docosahexaenoic acid (C22:6, DHA), eicosapentaenoic acid (C20:5, EPA), and docosapentaenoic acid (C22:5, DPA).
Vegetable oils including corn oil and safflower oil contain high levels of n-6 PUFAs including linoleic acid (C18:2, LA). International epidemiological studies have provided evidence that dietary factors are important determinants of large bowel cancer in different populations worldwide.
Cancer statistics in Japan for 2001 published by the Foundation for Promotion of Cancer Research indicate that there is an upward trend in age-adjusted mortality rates for colon cancer from 1955 to 1999.3 According to this report, the death rates due to colon cancer in Japanese men and women in 1955 were 2.9 and 3.0, respectively, whereas they increased to 14.7 and 9.8, respectively, in men and women in 1999.
Interestingly, animal fat consumption in 1960 was about 25 g/day (per capita), whereas in 1999 it increased to about 58 g/day. Meat intake was increased from 19 to 78 g/day (per capita), whereas grain consumption decreased from 453 g to 245 g/day during these years.
In fact, meat consumption has been found to be associated with an increased risk for the development of colon adenomas in Japan. The change in dietary habits in Japan may, in part, explain the time trend in colon cancer mortality. The importance of types of dietary fat differing in fatty acid composition rather than total fat cannot be discounted.
A recent report by an expert panel assembled by the American Institute for Cancer Research and the World Cancer Research Fund came to a scientific consensus that evidence for an association between the intake of saturated fat and/or animal fat and colon cancer risk is very strong.
Continuing population studies reveal that mortality data for colorectal cancer in 22 European countries, the U.S., and Canada correlate with the consumption of animal fat. Eating a diet rich in n-3 PUFAs, as present fish and fish oil, may decrease the risk of colorectal cancer.
Caygill et al. reported an inverse correlation between fish and fish oil consumption and colorectal cancer. This inverse relationship was significant for both male and female patients with colorectal cancer, whether the intakes were in the current period or 10 years or 23 years before cancer mortality.
It is noteworthy that these effects were only observed in countries with a high (>85 g/day) animal fat intake. Also, the Mediterranean diet, which is rich in olive oil and fish, is associated with a low risk of colorectal cancer. The results of these epidemiological studies suggest that diets high in saturated fats increase the risk of colorectal cancer, whereas diets high in n-3 PUFAs reduce the risk.