Cold water fish can be a good source of the long chain polyunsaturated fatty acids eicosapentanoic acid (EPA, C20:5 (n-3)) and docosahexanoic acid (DHA, C22:6 (n-3)) which are required by the body for the synthesis of the series 3 eicosanoids. Eicosanoids are important signal molecules that can influence cellular function, and through their ability to limit inflammation, the series 3 eicosanoids have been shown to be protective of cardiovascular disease. Epidemiological data shows an inverse association between high intakes of marine cold water fish and the risk of cardiovascular disease. The protection offered by cold water oily fish against type 2 diabetes is still unclear however, with the epidemiological evidence being less conclusive. The Japanese are known to have high intakes of fish in their traditional diets, and this makes then a good population to study with regard the health benefits of fish consumption.
Recent epidemiological evidence suggests that the high fish intakes associated with the Japanese may be protective of diabetes, but only in men. Researchers1 used a prospective study design to investigate the nutritional intake of 22,921 men and 29,759 women using a food frequency questionnaire. The data was part of the Japan Public Health Centre-based Prospective Study and all subjects were free of type 2 diabetes at the time of the enrolment for the study. During the 5 year follow-up, the number of subjects reported to have developed type 2 diabetes was 971 (572 men and 399 women). Fish intake was significantly associated with a reduction in the risk of type 2 diabetes and comparison of the highest quartile to the lowest quartile gave an odds ratio of 0.73 for total fish (including seafood) and 0.68 for small and medium fish (including mackerel, pilchards, saury and eel).
Interestingly, the association between fish intake and diabetes was not significant for women, however the reason for this is not clear. Analysis of the fat content of the fish also did not reveal any sub-categorisations of associations for fatty acids and the risk of diabetes. Because fish is also a source of vitamin D, and vitamin D deficiency has been shown to be implicated in the development of insulin resistance, metabolic syndrome and type 1 and type 2 diabetes, it is very possible that fish consumption is protective of type 2 diabetes because of the vitamin D content. That vitamin D is the beneficial component of fish is highlighted in studies that show benefits for white non-oily fish. In addition, adjustment for the vitamin D content of the diet has attenuated the association between fish and diabetes in some studies. However, it could be possible that the polyunsaturated fatty acids and vitamin D work synergistically to provide protection.
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