Unsaturated Fat - experts tell us we should eat more unsaturated and less saturated fat. For humans it appears that our diet has become unbalanced in terms of the make-up of fat. This issue seems to be extremely complex mainly due to the long and complex names given to the fats. Fat is a confusing word and it is difficult to understand why eating more fat should be a good thing when so many sources are telling us to eat less fat. The secret is in the type of fat that we eat. It is not that we are eating too much fat per se or that any particular type of fat is bad for us, but more that the balance of the types of fat that we consume has become upset.

There are a number of different types of fat. Saturated fat in our diet mainly comes from land animals and different unsaturated fats come from either plants or fish. Fatty acids compose the majority of fat. However it is two different types of unsaturated fatty acids, EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), which come from oily fish and which are particularly important for our health. EPA and DHA are known as polyunsaturated fats and contain the important omega-3 fatty acids.

There is wealth of evidence that the fat in fish, and particularly these omega-3 fatty acids, are an important factor in the diet of man and contribute to good health. It is thought that it is the balance of omega-3 fatty acids with other unsaturated omega-6 fatty acids which is important. In early human diets, which are thought to have been very healthy, the ratio of omega-6 to omega-3 fatty acids was 1:1, but in recent times the amount of omega-3 fatty acids has declined and that of omega-6 fatty acids has increased, so that the ratio is now around 10:1. A ratio of omega-6:omega-3 fatty acid of around 5:1 is now regarded as optimal, particularly where the omega-3 fatty acids are EPA and DHA. The effectiveness is thought to reduce if it goes beyond 5:1. Restoring this balance can be achieved by eating more fish.

The human body cannot make the precursors of omega-3 fatty acids and omega-6 fatty acids and they cannot be inter-converted; they are known as "essential fatty acids": in other words they must be eaten in the diet. Whilst more derivations of omega-6 fatty acids can be manufactured by humans, plankton are the only organisms than can efficiently make omega-3s. Omega-3 fatty acids such as linolenic acid are found in plant oils but are much less effective at promoting good health. However marine plankton is very efficient in turning this linolenic acid into EPA and DHA. When fish eat this plankton they themselves become rich in EHA and DHA.

Research has shown that when fatty acids in human diets are balanced, specifically with increased omega-3 from fish, there may be a reduction in the incidence of the following diseases and conditions in humans:-

• Impaired development of brain and visual acuity
  - reduced intellectual capacity in infants
  - aggression and depression
• Neurological dysfunction, including reduced retinal function
• Coronary heart disease:
  - restenosiscardiac arrhythmias (irregular electrical activity)
  - high blood pressure
• Mild hypertension (high blood pressure)
• Inflammatory and auto-immune disorders:
  - rheumatoid arthritis
  - psoriasis
  - ulcerative colitis
  - asthma
  

That fish is 'brain food' has a scientific basis. The major component of the fat in the brain is DHA. An unborn child needs an adequate supply of DHA if the grey matter in its brain and nerve tissue and cell membranes of the retina are to develop fully and properly. There is now extensive epidemiological data showing that fish eating populations with high omega-3 fatty acid intake in the form of EPA and DHA, such as Greenland Eskimos and the Japanese, have a lower incidence of cardiovascular disease and inflammatory diseases such as asthma and psoriasis. However, in the last two decades the Japanese have been changing to more Western diets containing not only more saturated fat and less omega-3 but also more omega-6 fatty acids. The incidence of these diseases is now increasing amongst them. There have been numerous clinical trials investigating fish/fish oil consumption and diseases. In these, researchers have cited evidence that shows potential health benefits of fish oil which supplies the most effective source of EPA and DHA.The cause of the beneficial effects of omega-3s is quite well understood. Both EHA and DHA are converted by our bodies into hormone-like substances called eicosanoids by a series of enzyme reactions. Prostoglandins and leukotrienes are the two main types of eicosanoids and they are import to us because they may harm a range of bodily functions such as regulation of smooth muscle e.g. of the heart, blood clotting, and proper functioning of the immune system.EHA and DHA both use the same enzymes to manufacture eicosanoids and they compete with each other for entry to those enzymes. Those that are derived from omega-3 fatty acids are generally much less harmful than those derived from omega-6s. So, by keeping the level of omega-3 oils high, 1:5 ratio with omega-6s (above), we can reduce the harmful effects of the eicosanoids derived from omega-6s. It is thought that there are other beneficial effects of eating fish oils but these are not so well understood yet. For example, fish oil may have a function in the treatment of a number of immune related disorders. EPA in particular has an effect as an anti-inflamatory agent and therefore fish oil has a role in the treatment of a number of immune related disorders.

Ulcerative colitis involves the inflammation of the mucosa of the large intestine. Its cause is unknown but in Europe the incidence is much higher northern countries than in the southern ones and its incidence may be linked to diet and may reflect fish consumption. Patients suffering from ulcerative colitis have high levels of LTB4 (Leukotriene B4) but when they are given high levels of EPA the levels of LTB4 fall.

It is thought that gallstones are produced when the wall of the gall bladder is damaged. This injury is associated with elevated levels of Leukotrienes but when the diet is high in fish oil a different kind, LTB5, is produced instead of LTB4. It is thought that LTB5 prevents the formation of gallstones. Indeed when laboratory animals are fed a diet high in fish oil they do not form gallstones.

There is some evidence that a diet high in fish oil may alleviate some of the symptoms of arthritis by altering the levels of tiredness, swollen and tender joints, grip strength, and mobility.

based on the documentation of "Nutritional Aspects of Fish" prepared by the Irish Sea Fisheries Board (AN BORD IASCAIGH MHARA)