The links between cancer and omega-3 fatty acids are less clear but evidence is being found. For instance, researchers at the Karolinska Institute in Sweden have found that omega-3 fatty acids can be protective against the progression of cancers.
When animals are implanted with human tumours and are fed fish oils, the tumours are fewer, smaller and less likely to spread.
A letter to The Lancet in June 2001, reported a link observed in the 30-year Health Professionals Follow-up Study, associating fish consumption with reduced prostate cancer in men in Sweden. This was elaborated in 2003 to show men who ate no fish (or less than twice a month) had a two- to three-fold higher frequency of prostate cancer than those who ate fish more than three times a week. In 2006, a study published in the British Journal of Cancer, also reported that eating oily fish may help prevent the spread of prostate cancer.
The Singapore Chinese prospective health study followed 35,298 women from 1993 to 1998, with a follow up in 2000, and monitored the occurrence of breast cancer. It was 26% lower in the top three quartiles of fish and shellfish consumption (over 44g/day) compared with occurrence in the lowest quartile.
Fish oils may also provide a benefit in lessening cachexia; the severe wasting and weight loss associated with some cancers such as pancreatic cancer. A seven-country study led by the Edinburgh Royal Infirmary, UK, investigated the influence of dietary supplements with and without fish oils on 200 patients with pancreatic cancer. While weight loss stopped in all cases, only the omega-3 fatty-acid-enriched supplement resulted in a net weight gain, lean tissue and improved quality of life. The patients with the omega-3 enriched diets also showed an increase in physical activity.
There is some indication that EPA may reduce the susceptibility of human skin to cancer caused by UV light (photocarcinogenesis). An Australian population-based study compared the diets of 41 women in Brisbane with cutaneous malignant melanoma with those of 297 healthy women from the same community. There was a strong inverse relationship between intakes of polyunsaturated fatty acids and melanoma. The authors concluded the reduction in risk seen among those eating the most fish suggested the effects of marine oils and omega-3 fatty acids deserved specific attention. In 2003, a study in Manchester, UK, reported more detail of a possible link. Forty-two healthy subjects were given 4g/day of purified EPA for three months. This caused an eight-fold increase in bioavailability in the skin. Sunburn sensitivity and other early markers of skin cancer were reduced by 20–50%.
In 2004 researchers at the Karolinska Institute in Sweden published a comprehensive review of current knowledge regarding the role of polyunsaturated acids in carcinogenesis. Their conclusion was that the omega-3 fatty acids are protective against the progression of cancers.