What is the evidence?
Although some research supports the anticancer claims made for omega-3 fatty acids, far more investigation is needed before researchers reach firm conclusions. The strongest evidence for the health benefits of fatty acids from fish is in the area of heart disease and its risk factors. The relationship between omega-3 fatty acids, cancer, and other diseases is not as well known.
The evidence from clinical studies published in peer-reviewed medical journals is mixed. A limited study conducted at Harvard Medical Center suggested that one of the omega-3 fatty acids limited the recurrence of colon cancer. A preliminary study of omega-3s published in the Journal of the National Cancer Institute said that even though there appears to be a biological reason for omega-3s to fight cancer, clinical findings didn혪t prove that fish oil containing omega-3 fatty acids prevented cancer or its recurrence.
A clinical study published recently in the journal Cancer concluded that omega-3 fatty acids seemed to prolong the survival of people with cancer who were also severely malnourished. A small study recently looked at patients with advanced pancreatic cancer and severe weight loss. It compared EPA (one of the fish oil omega-3s) mixed with a high-protein, high-calorie supplement to the protein-calorie supplement without EPA. After 8 weeks, the study found that EPA did not help the group gain weight.
In a 14-year observational study of nearly 50,000 US men, high intake of ALA (the plant-based omega-3 fatty acid) was not linked to overall prostate cancer risk. However, men who were diagnosed with prostate cancer and had high ALA intakes were more likely to have advanced prostate cancer. In that same study, men with a higher intake of omega-3 fatty acids from fish oil (EPA and DHA) seemed to have a lower risk of both early and advanced prostate cancer. This study also looked at omega-6 fatty acids (see below) and found no relationship between their intake and prostate cancer risk.
A different group recently published an analysis of 5 observational studies of men and prostate cancer risk, looking at their intake of alpha-linolenic acid, and found that it appeared to be linked to an increased overall risk of prostate cancer. Carefully controlled studies need to be undertaken to explain the conflicting information and find out whether the omega-3 fatty acids accounted for any differences, and whether ALA has a different effect than the fish oil omega-3s.
Research is also focusing on the role of omega-3 fatty acids in relation to omega-6 fatty acids. Omega-6 is another essential fatty acid that is found in many vegetable oils (corn, safflower, and sunflower), cereals, snack foods, and baked goods. Some researchers believe one of the reasons why Americans suffer high rates of cardiovascular disease and certain cancers may be due to an imbalance in the ratio of omega-3 to omega-6 fatty acids. Ideally, the ratio of omega-3 to omega-6 fatty acids in the human body is 1-to-1. However, because the typical US diet is low in omega-3s and high in omega-6s, many people have 10 to 20 times more omega-6 fatty acids than omega-3 fatty acids in their systems. Studies show that women with breast cancer have 2 to 5 times more omega-6 fatty acids than omega-3 fatty acids in their systems.
A large study of over 34,000 women followed them from 1980 to 1998, observing intake of fish and the ratio of fish fatty acids to omega 6 fatty acids to find out how this affected their colorectal cancer risk. Women who took in more omega 3 fatty acids did not have a lower colorectal cancer risk, but they had fewer large adenomas (a type of colorectal tumor). This study suggests that omega-3 fatty acids may not reduce colorectal cancer risk, but may slow its growth. More research is needed to find out if this holds true.
Laboratory experiments suggest that omega-3 fatty acids may delay or reduce tumor development in animals. Other laboratory evidence shows that the balance between omega-3 and omega-6 fatty acid levels plays a role in the formation of breast cancer. To further muddy the waters, one laboratory experiment suggested that both omega-3 and omega-6 fatty acids may promote the spread of colon cancer. However, at least one London study observed no effect on deaths from breast cancer in women who took in higher amounts of fish and fish oils compared to women who didn혪t. Animal and laboratory studies may show that a substance has certain effects, but carefully controlled scientific studies are necessary to find out if the results apply to humans.
It can be confusing to learn about omega-3 fatty acids, in part because alpha linolenic acid may have different effects than the fish oils. Some studies did not distinguish between these types of fatty acids. Others looked only at the fish oil supplements. This may account for some conflicting results of older studies.
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