Dietary fat and inflammation- implications for heart disease and stroke
Surprising though it may sound, our immune system, which normally protects us from infections and repairs injuries, is also guilty of the silent narrowing and hardening of the arteries that puts us at risk of a heart attack or stroke
The immune system normally protects us from infections and repairs injuries.
White blood cells, which include macrophages, mast cells, granulocytes and natural killer cells, form a first line of defence against an invading organism. They migrate through the bloodstream to sites of damage or infection and either engulf the invading organism by a process called phagocytosis, or they release chemicals which are toxic to the invading organism. Occasionally the invader proves too aggressive and we succumb to an infection. While the immune system, on the whole, serves to protect us, some of its actions are undesirable. When macrophages release toxic chemicals to kill an invading organism, they can also do some damage to the host. However, this damage is normally repaired very quickly. This is not the case for the long-term damage that occurs in our arteries from an early age and which involves macrophages as a key component.
Arteries build-up fatty deposits, macrophages and fibrous material.
Narrowing of the arteries is a result of the build up of fatty material, macrophages and fibrous deposits, collectively termed plaques, which begins as early as 14 years of age. Plaques form mainly at arterial branchpoints, which are subject to stress. They are thought to be initiated by macrophages, which enter the artery wall to repair damage arising from wear and tear. They also remove cholesterol, which can accumulate here. However, while macrophages are performing these important functions, they become trapped and, failing to leave the site, become embedded in the artery wall. They continue to take up large amounts of cholesterol, eventually becoming ‘foam cells’ because of their foamy appearance. They release toxic chemicals, which cause inflammation within the artery wall. Finally, they die, depositing their cholesterol load. Gradually, as this process continues, the artery wall begins to thicken. Plaques can obstruct blood flow in arteries and, if they rupture, they can trigger the formation of a blood clot.
Plaque rupture leads to a heart attack or stroke.
Plaques remain a silent threat until they rupture. When a plaque ruptures in the coronary arteries, a blood clot- or thrombosis- forms, which obstructs blood supply to the heart, causing a heart attack. When a plaque ruptures in the carotid arteries (in the neck) the brain is deprived of its blood supply, causing a stroke. The severity of the stroke depends on the degree to which brain tissue dies while the blood supply is blocked. Since the process is so simliar to what happens during a heart attack, stroke is sometimes referred to as a ‘brain attack’.
Stable and unstable plaques.
The morphology of a plaque can indicate its susceptibility to rupture. Plaques which have accumulated a large amount of cholesterol in the lipid core, have a thin fibrous cap and a large infiltration of macrophages are described as vulnerable and likely to rupture. In contrast, plaques with a small lipid core, a thick fibrous cap and a lower infiltration of macrophages are thought to be relatively stable, even if the restriction in blood flow is more severe.
Can we do anything to prevent plaque rupture?
Plaque formation appears to be an inevitable consequence of having an efficient immune system, but we may be able to modify plaque stability. There are certain risk factors that increase an individual’s likelihood of suffering a heart attack or stroke. Some risk factors, such as age, gender, family history and ethnic group are not modifiable. However, there are some risk factors that you can do something about. The main ones are your weight and level of activity, blood pressure, blood cholesterol level, smoking and diet. Not surprisingly, the recommended diet includes at least 5 portions of fruit and vegetables per day, fibre, unrefined carbohydrates, such as wholegrain cereals and root vegetables, and avoidance of fatty and sugary foods.
Good fats and bad fats….
The type of fat you consume may be important in determining your risk for a heart attack or stroke. Saturated fats, found mainly in animal products, dairy products and processed foods are ‘bad’ fats because they contribute to raised blood cholesterol levels. ‘Good’ fats include some vegetable oils, particularly olive oil, and the omega-3 fatty acids, which are found in oily fish, such as salmon, herring and mackerel and can be taken in the form of fish oil capsules.
The Eskimo connection….
In the 1970s it was noted that the prevalence of heart disease was extremely low in Greenland Eskimos, despite their consumption of large amounts of fat. The Eskimo diet is very rich in omega-3 fatty acids. It was suggested that these fats may protect against heart disease. Several European studies have subsequently shown that consumption of oily fish or fish oil reduces mortality in men who have already survived a first heart attack.
Fish oil and plaque stability- the Southampton Study
Plaques located in the carotid artery (putting an individual at risk of stroke) can be detected by ultrasound. They can then be removed by surgery in an operation known as a carotid endarterectomy. We conducted a study to investigate the effect of fish oil supplementation, prior to surgery, on the morphological characteristics of the plaques. Patients consumed 6 capsules per day of either placebo or fish oil- 50 patients per group completed the study. At surgery, their plaques were removed and retained for analysis. We assessed the effects of the fish oil treatment on plaque stability and on the degree of infiltration of macrophages.
Fish oil Placebo
% stable plaques66.054.4
% vulnerable plaques15.122.8
% macrophage-poor plaques38.113.2
% macrophage-rich plaques61.984.2
Table 1. Effect of fish oil supplementation on morphology of carotid plaques removed at surgery (Thies et al., 2003, Lancet 361, 477-485)
The results demonstrate that fish oil supplementation can significantly increase the stability of carotid plaques and decrease the extent of macrophage infiltration. Fish oils may therefore protect against a stroke or heart attack and could be useful as a preventative measure. Evidence such as this could be important in the formulation of dietary guidelines for patients at risk of heart attack or stroke and also for the general population.