While both omega-3 and omega-6 fats are essential for health, the typical U.S. diet contains an abundance of omega-6 fatty acids, whereas consumers have to make a special effort to obtain omega-3s (found in fatty fish, walnuts, flaxseed oil, and canola oil). A low dietary omega-6 to omega-3 fatty acid ratio is associated with reduced breast cancer risk. Now a new study has reported that the high omega-6 content of vegetable oils may interfere with the potentially beneficial effects of the most common vegetable omega-3 fat.

Omega-6 to omega-3 ratio and breast cancer risk

It is the ratio of omega-6 to omega-3 fats in the diet that is important, rather than their absolute amounts. For example, one Chinese study found that women with both the lowest intake level of marine omega-3 fats (from fish) and the highest intake of omega-6 fats had double the risk of breast cancer of women who had both the highest marine omega-3 fat intake and the lowest omega-6 fat intake.

Some observers have suggested that human beings evolved on a diet with an omega-6 to omega-3 ratio of approximately 1-to-1. This was in part because meat from wild or grass-fed animals has a much higher omega-3 content than that of corn- and soy-fed animals. In addition, people consumed far less vegetable oils. Today, the typical U.S. diet has an omega-6 to omega-3 ratio estimated to be at least 15-to-1. The ideal dietary omega-6 to omega-3 ratio for cancer prevention has not been determined, but it appears to be 3-to-1 or lower.

Sources of omega-3 and omega-6 polyunsaturated fatty acids (PUFAs)

While beneficial, the trend away from animal fats (such as lard) and partially hydrogenated oils in processed foods and fast food meals has left the false impression that high omega-6 fats such as soybean oil and corn oil are "healthy." Below are some dietary sources omega-3 fats that have also been reported to be associated with reduced breast cancer risk, as well as some common sources of omega-6 fats. Reducing consumption of omega-6 fats can be an important part of a strategy to improve your dietary omega-6 to omega-3 fat ratio.

Dietary sources of omega-3 and omega-6 fats

Canola oil
Flaxseed oil
Lake trout
Salmon, wild
Walnuts & walnut oil
Corn oil
Grape seed oil
Safflower oil
Soybean oil
Sunflower oil

Omega-6 content of common oils can interfere with benefits of vegetable omega-3

The omega-3 fatty acid alpha-linolenic acid found in plant-based foods such as walnuts is converted in the body to the omega-3 fatty acids eicosapentaenoic acid and docosahexaenoic acid. These are the omega-3 fats found in abundance in fatty fish and which appear to protect against breast cancer. However, note that dietary alpha-linolenic acid above a certain threshold level results in no additional increases in circulating docosahexaenoic acid levels in both animals and humans. The study referenced at the beginning of this article was designed to investigate the influence of the omega-6 fatty acid linoleic acid on the conversion of alpha-linolenic acid to eicosapentaenoic acid and docosahexaenoic acid. Linolenic acid is found in abundance in the omega-6 oils listed above.

To conduct the study, the authors evaluated various combinations of dietary alpha-linolenic acid and linoleic acid in rats. The rats were fed one of 54 diets for three weeks. The diets varied in the percentage of energy (calories) provided by linoleic acid (0.07-17.1%) and alpha-linolenic acid (0.02-12.1%) by manipulating both the fat content and ratio of vegetable oils. The peak circulating docosahexaenoic acid (>8% of total fatty acids) was produced by a narrow dietary range of 1 to 3% of calories from alpha-linolenic acid and 1-2% calories from linoleic acid. Circulating docosahexaenoic acid was suppressed to basal levels (approximately 2% of total fatty acids) at dietary intakes of total polyunsaturated fatty acids (alpha-linolenic acid plus linoleic acid) at levels above 3% of total calories. The authors conclude that it is possible to enhance the docosahexaenoic acid status of rats fed diets containing alpha-linolenic acid as the only source of omega-3 fatty acids but only when the level of dietary polyunsaturated fatty acids is low.