Cold water fish require adaptive mechanisms to cope with living in often icy cold water. Saturated and short chain lipids in their bodies would become solid at these temperatures and so they instead use longer polyunsaturated lipids. Theses are colloquially referred to as fish oils, although chemically they are a complex mixture of different long chain fatty acids. Two long chain fatty acids in fish oils that may possess beneficial health effects in humans include eicosapentaenoic acid (EPA, C20:5 (n-3)) and docosahexaenoic acid (DHA, C22:6 (n-3)). These fatty acids belong to the omega-3 group and are bioavailable in humans. Both EPA and DHA feed into the omega-3 fatty acid pathway where they allow the synthesis of eicosanoid hormones. These hormones are important regulators of cell function. In addition these omega-3 fatty acids also accumulate in cell membranes, and in doing so make the cell membranes more fluid. In this way they antagonise cholesterol, a substance that decreases fluidity of cell membranes.
Fish oils contain two important omega-3 fatty acids; eicosapentaenoic acid (EPA, C20:5 (n-3)) and docosahexaenoic acid (DHA, C22:6 (n-3)). These fatty acids may decrease triglyceride levels in humans.
Fish oils have some useful properties with regard metabolic syndrome. The most well known effect of fish oils is perhaps their ability to lower triglyceride levels. It is unclear how they achieve this, but it may relate to their ability to increase the oxidation of fatty acids in the liver or to inhibit their synthesis. A number of studies have investigated the effects of fish oils in subjects with diabetes, and these studies have been fairly consistent in their findings. For example, in one meta-analysis of previous studies, researchers showed that fish oil supplements were effective at decreasing triglycerides, particularly in subjects with type 2 diabetes who did not require insulin injections. In subjects who did require insulin injections, fish oil supplements significantly decreased blood sugar levels, and it was shown that this effect was caused by DHA in a dose response manner. Fish oils therefore seem to show benefits to patients with diabetes, particularly in their ability to significantly reduce triglyceride levels in non-insulin dependent subjects.
Cold water fish such as sardines require long chain fatty acids as a sort of antifreeze. Long chain polyunsaturated fats are less likely to become solid at the cold temperatures the fish are exposed to.
Animal studies also suggest that fish oils have the ability to reduce triglyceride levels significantly. Some evidence from studies on healthy humans are also in agreement. For example, in one study, researchers fed healthy human subjects a high fructose diet in order to induce increased de novo lipogenesis (the synthesis of triglycerides) and insulin resistance. Some subjects received the same high fructose diet but with addition of 7.2 grams of fish oils per day. The results of the study showed that, as expected, the fructose diets was able to cause an increase in triglyceride production and a decrease in insulin sensitivity. However, those subjects also receiving the fish oils had significantly lower triglyceride levels compared to the fructose only group. The researchers showed that this was in part due to suppression of de novo lipogenesis by the fish oils. However, the fish oils did not improve the insulin sensitivity when compared to the fructose only group.
Dr Robert Barrington’s Nutritional Recommendation: Fish oils appear to be effective at treating the dyslipidemia caused by the insulin resistant state. Taking fish oils as supplements or eating more fatty fish should be encouraged for general health and for those wishing to lower their triglyceride levels. Some animal studies suggest that fish oils may have insulin sensitising effects, but this view is controversial and the evidence is not strong. There are other dietary strategies that appear more effective in their regard. Fish oils may also lower blood glucose levels in subjects with insulin dependent diabetes, but this effect is not so apparent in those with non-insulin dependent diabetes or in healthy subjects. Eating fatty fish such as trout, salmon, mackerel, sardines or tuna is probably the best way to obtain adequate fish oils, although care should be taken to obtain toxin free fish. Fish oil supplements are also effective and may be more convenient.
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