Research has shown that men who ate fish once or twice per week were protected against Coronary Heart Disease (CHD)

The link between fish oil and heart disease has been the subject of thousands of scientific papers in recent years.

The whole story began following the discovery that coronary heart disease, while being one of the biggest killers in some parts of the world, is practically unknown among the Eskimos. Their traditional diet however is high in fat.

Further investigation found that most of the fat in their diet comes from oil-rich fish and marine oils and is high in long chain n-3 fatty acids.

Eskimos also have a reduced tendency to blood clotting and longer bleeding times that when compared to Danish people, who have a low n-3 PUFA intake.

Dutch medical researchers carried out a survey in the town of Zutphen, which showed that men who ate fish once or twice per week were:

• protected against coronary heart disease
• more likely to be alive 20 years later than those who did not eat fish.

Many other studies have shown similar results but three main points should be noted.

1. Increasing fish intake is more effective at preventing CHD in communities that eat little quantities of fish
2. Very large amounts of fish consumption do not appear to be better than moderate amounts in preventing CHD
3. While it is likely that the n-3 fish oils are responsible for the effects of eating fish, some other protective component(s) of fish cannot be excluded.

A study was made in Wales covering 2000 men who had just recovered from their first heart attack. The men were divided into 3 groups, which followed different diets:

1. a low fat diet
2. a high fibre diet
3. iinstructions to eat 2 portions (4 ounces) of oil rich fish per week.

At the end of a 2 year period, it was found that there were 29% fewer CHD-related deaths in the ‘fish’ group compared to the other groups. Although there were fewer fatal heart attackes in the fish eating groups, the total number of heart attacks was not significantly reduced. This showed that eating oil-rich fish regularly seemed to reduce the risk of death after a heart attack rather than preventing a further heart attack.

Mor recently, a French study also looked at a group of patients following their first heart attack. This study compared a Mediterranean-style diet, rich in n-3 alpha-linolenic acid with a prudent diet. It showed a marked reduction in the number of further heart attacks, as well as the number of CHD-related deaths inthe group whose diet was rich in alpha-linolenic acid.

Fish oil is thought to have a number of roles in protecting against heart disease.

Fish make an important contribution to maintaining a healthy diet. 

Fish is a versatile, tasty, easy to prepare food. For a healthier diet include plenty of fresh vegetables and fibre rich foods in your fish dishes. When cooking, use low-fat or polyunsaturated oils  and cut down on the amount of salt used. 

Thrombosis

A heart attack can result when an artery, which has already been narrowed by fatty deposits (atherosclerosis), becomes completely blocked.

This is caused by blood platelets sticking together and forming a clot (thrombus). Platelet aggregation is triggered when the endothelial cells that line the arter walls are damaged in some way. Both the endothelial cells and the platelets produce eicosanoids, which cause the platelets to clump together.

When the diet is rich in fish oil it is thought that different eicosanoids are produced and that these prevent the formation of a clot.

OTHER POSSIBILITIES

An increase in fish oils in the diet results in a marked reduction in blood triglyceride levels. The exact significance of an elevated triglyceride level remains controversial.

One recent Norwegian study has shown that women with high triglyceride levels have up to 5 times the risk of a heart attack compared to women with normal levels. The impact of high triglyceride levels was found to be independent of other risk factors.

Fish oils have no effect on the levels of low density lipoprotein cholesterol (LDL) but they do raise high density lipoprotein (HDL). HDL is a protective type of lipoprotein since it takes excess cholesterol away from the tissues and returns it to the liver. Fish oils normally raise HDL levels by about 10%, however intakes of over 10 g per day may actually decrease HDL levels.

Diseased heart muscle is susceptible to bouts of irregular electrical activity (arrhythmias), which are potentially lethal and often cause sudden cardiac death. There is some evidence from animal studies and very limited evidence from clinical studies that increasing fish oil in the diet helps to reduce these cardiac arrhythmias.

Raised blood pressure is know to be a major risk factor in CHD. Most studies on the effects of fish oil gicen as dietary supplements (supplying 3 g or more per day of n-3 PUFA) have shown modest reductions in blood pressure, especially in people with raised blood pressure. This however is the equivalent of 10 portions of oil-rich fish each week and would be difficult to achieve from diet alone. Combining an increase in fish intake with a low sodium diet might be a more effective means of reducing blood pressure.

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