Fatty fish are a rich source of docosahexaenoic acid (DHA, C22:6 (n-3)) and eicosapentaenoic acid (EPA, C20:5 (n-3)). These fatty acids are produced by algae, and bioaccumulate in fish, becoming more concentrated in the larger fish at the top of the food chain. When humans eat these larger fish both EPA and DHA have beneficial effects and are thought to confer protection from cardiovascular disease. Fish oils are beneficial to the health because they provide a source of omega 3 fatty acids that are needed for correct metabolic function. In this regard, ingestion of both EPA and DHA are beneficial because they correct deficiencies of omega 3 fatty acids that are caused by consumption of the Western diet, a poor source of omega 3 fats. In humans, DHA is converted to EPA which is subsequently converted to the series 3 eicosanoids, hormone like substances that regulate cell function. The series 3 eicosanoids can interfere with production of pro-inflammatory series 2 eicosanoids and in this way produce an anti-inflammatory effect.
Inflammation is now thought to be pivotal in the aetiology of cardiovascular disease. Systemic inflammation induces immune reactions and this in turn increases oxidative stress. The elasticity of arteries is dependent on the production of nitric oxide in the endothelial lining, as nitric oxide acts locally to cause relaxation of smooth muscle, and this allows the correct contraction and relaxation cycle in response to blood flow. Oxidative stress can interrupt this cycle because nitric oxide synthase, the enzyme that produces nitric oxide, is inhibited by oxidative stress. Inhibition of nitric oxide synthesis by oxidative stress is now thought to lead to endothelial dysfunction which is a primary driver of increased blood pressure. As well as inhibiting inflammation, fish oils, as with all polyunsaturated fatty acids, increase β-oxidation of fatty acids in the liver. This decreases the amount of fatty acids available for packaging into the very low density lipoproteins, which in turn limits the efflux of fatty acids from the liver to circulation and lowers plasma triglyceride levels.
Researchers have assessed the effects of fish oil supplements on the arterial stiffness in subjects following a weight loss diet1. Subjects followed a 12 week hypocaloric diet followed by a 4 week maintenance phase and half the subjects were supplemented with either 4 grams per day of omega 3 fish oils in the ethyl ester form (46 % EPA and 38 % DHA). The calorie deficit during the weight loss period was 450 kcal per day, which was roughly 25 % of the total energy needs of the subjects. All subjects had reductions in body weight, waist circumference and improvements in systolic and diastolic blood pressure. There were also improvements in insulin resistance and plasma triglyceride levels. However, in the fish oil group, there were also improvements in arterial stiffness, and this improvement was reflected in greater reduction in blood pressure. Large arterial stiffness was improved by 20 % and small artery stiffness was improved by 22 %, suggesting improvements in endothelial function.
The subjects in this study were obese and it is therefore likely that they had some form of metabolic dysfunction caused by non-alcoholic fatty liver disease and abdominal obesity. This was reflected in the high plasma triglyceride levels (~185 mg/mL) at baseline and the slight insulin resistance as detected in the HOMA score. Addition of polyunsaturated fatty acids in the form of fish oils was effective at reducing plasma triglyceride levels beyond that seen in the weight loss only group, and this has previously been investigated and been found to be due to increased β-oxidation in the liver. Only the group taking the fish oil showed improvements in arterial stiffness, and this may relate to the anti-inflammatory, and therefore anti-oxidative effects of fish oils. Fish oils have been shown to improve endothelial function and allow increased production of nitric oxide, thus increasing arterial elasticity. It is interesting to note that weight loss alone caused significant improvements in a number of risk factors for cardiovascular disease as has been shown previously.