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Yes, there has been a link between omega-3 fatty acids and prostate cancer found in a couple of different studies. The most recent study found a 70% increase in the risk of high-grade prostate cancer in risk in men that have higher levels of omega-3 fatty acids indicating a greater intake, although the study did not differentiate whether the levels were related to the diet or an omega-3 supplement. There also was a 40% increase in low-grade prostate cancer risk.
There also was found to be an increase in prostate cancer risk in men taking a vitamin E supplement. It is the vitamin E that is frequently taken as an anti-oxidant; the omega-3 fatty acids are thought to provide their benefit because of the anti-inflammatory effect of these fatty acids.
However, these studies are relatively recent, and the reason for the increase in risk is not yet known. Therefore, whether there will be a similar increase in risk found with any other type of cancer cannot yet be stated. From the perspective of breast cancer, there is some evidence of a decrease in breast cancer risk associated with a diet that is high in foods that contain omega-3 fatty acids. Therefore, the current evidence does not say whether other cancers may also be affected by intake of omega-3 fatty acids, but the risk for breast cancer is apparently not adversely affected.
From the perspective of which anti-oxidant would be preferred, the answer would be vitamin C. There are studies that show that eating a diet rich in foods that are high in vitamin C is associated with a lower risk of cancer. This effect has not yet been able to be duplicated by using a vitamin C supplement alone, but there also was no increased risk identified with using a vitamin C supplement. Since the reduced risk is more associated with the diet rich in vitamin C, there may be many factors that interact in decreasing the cancer risk, but this also is not yet clarified.
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