The typical Western diet contains too much omega-6 fat, and this causes an imbalance in the dietary and cellular omega-6 to omega-3 fatty acid ratio. The correct ratio of omega-6 to omega-3 fatty acids in the human diet is estimated to be around 3 to 1, but the typical Western diet may contain an omega-6 to omega-3 ratio if upwards of 10 to 1. The ratio is important because too much omega-6 fat increases cell membrane levels of arachidonic acid (AA, C20:4 (n-6)) which then increases production of the pro-inflammatory series 2 eicosanoids. Balancing the high intakes of omega-6 fats in the typical Western diet can be achieved through consumption of increased amounts of the essential fatty acid alpha linolenic acid (ALA, C18:3 (n-3)). However, this is often not an effective strategy as many individuals of Western European descent may have inefficient conversion of ALA into the anti-inflammatory series 3 eicosanoids and this conversion is blocked further by the high omega-6 fat content of the diet.
A more effective way of increasing the anti-inflammatory series 3 eicosanoids is to consume the oils of fatty fish. These oils contain high concentrations of omega-3 fatty acids including eicosapentaenoic acid (EPA, C20:5 (n-3)) and docosahexaenoic acid (DHA, C22:6 (n-3)) which are converted to the anti-inflammatory eicosanoids of series 3. Recommendations are therefore to consume more fish. However, fish has been shown to contain high concentrations of methylmercury, which can have serious health consequences through its effects as a neurotoxin and as a cardiotoxin. The effects of methylmercury from fish may now be so serious that pregnant women are advised to avoid all fish completely and the adult population is recommended to limit consumption of contaminated fish. As more is understood about the bioaccumulation of methylmercury, studies have investigated the associations between fish consumption and methylmercury blood levels in healthy individuals to assess the relationships.
One study investigated the seafood consumption of over 10,000 adults1. The authors then assessed the data to find associations between seafood blood methylmercury levels. The results of the study showed that 80 % of the subjects in the study consumed seafood. Of the adults consuming seafood, the methylmercury concentration of the blood increased as the amount of seafood increased. Of the subjects, 4.6 % showed blood methylmercury concentrations above 5.8 μg/L, and this was associated with consumption of tuna, swordfish, shark and salmon, but not crab or shrimp. The reason that the methylmercury blood levels rise with fish, but not shellfish is likely due to the fact that the larger predatory cold water fish are higher up the food chain, which increases the risk of them bioaccumulating toxins. Limiting intakes of larger coldwater fish is therefore recommended, with a compensatory increase in consumption of smaller cold water fish and fish oil supplements that have been treated to remove methylmercury.
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