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CHAPTER 3 Atherosclerosis, Cholesterol and Dietary Fats

People with hypertension need to pay particular attention to the fats in their diet. By eating more of certain beneficial fats and less of potentially harmful ones, you can reduce your risk of future complications such as atherosclerosis.

Atherosclerosis

Atherosclerosis is the medical term for hardening, furring up and narrowing of the arteries. This process starts early in life, usually in the teens, and is triggered by normal wear-and-tear damage to your artery walls. Once the damage occurs, small cell fragments in the bloodstream – known as platelets – stick to the damaged area and form a tiny clot. These platelets release chemical signals to stimulate healing of the damaged area. Under normal circumstances, this would lead to healing, but if excessive damage continues – as a result of high blood pressure, raised cholesterol levels, poorly controlled diabetes or lack of antioxidants in the diet – the damaged area becomes infiltrated with a porridge-like substance that builds up to form a fatty plaque known as atheroma.

At the same time as the fatty plaques are developing, the underlying middle layer of the artery wall is affected and starts to degenerate, become fibrous and less compliant. Whereas the walls of healthy arteries are elastic and help to even out the surges of blood pressure produced every time the heart beats, the walls of arteries that have started to harden become more rigid. As a result, blood-pressure surges caused by the heartbeat are not evened out, and systolic blood pressure shoots up higher when the heart contracts. A vicious cycle then sets up, for just as atherosclerosis leads to high blood pressure, untreated hypertension can also lead to atherosclerosis by damaging artery linings and hastening the hardening and furring-up process.

If atherosclerosis is widespread throughout the body, it narrows the circulation so the diastolic BP – the pressure in the system when the heart is resting between beats – also becomes raised. Atherosclerosis can therefore raise both diastolic and systolic blood pressure. If left untreated, the raised BP in turn causes damage to the arterial system, hastening the development of atherosclerotic plaques and causing blood pressure to rise even further.

As a result, the heart has to pump blood out into a circulation whose vessels are narrowed and have lost their elasticity. This increases the workload of the heart – which has to pump blood out into the high-pressure system – and its need for oxygen increases at a time when its blood supply is often already compromised due to atherosclerosis of the coronary arteries. As the heart muscle beats over 100,000 times per day, lack of oxygen rapidly leads to muscle cramping, making angina and a heart attack more likely. In some people, two-thirds or more of a coronary artery may be furred up and blocked without causing symptoms. In others, angina may be triggered even though only a small plaque is present and the coronary artery is narrowed only slightly. It all depends on:

the exact site where the atheroma and narrowing have developed – the most common is within 3cm of where a coronary artery originates from the aorta, so the effects of ischaemia (lack of blood supply) are likely to be more widespread and serious

how well the two main coronary arteries join up to share the load of supplying blood

how good the blood supply from the other coronary artery is

the type of coronary arteries you have inherited – whether they are the vascular equivalent of motorways or winding country lanes.

Cholesterol Levels

Fats from your food are processed in the small intestines to form fatty globules (chylomicrons) bound to carrier proteins, which together form substances known as lipoproteins. After a fatty meal, there may be so many of these fatty particles in the circulation that blood takes on a milky-white appearance. These fatty globules are cleared from your bloodstream by the action of an enzyme (lipoprotein lipase) found in the walls of blood capillaries. Some of the fat released in this way is taken up into cells, while some remains in the circulation and is transported to the liver. In the liver, the fats are processed, packaged to different types of carrier proteins and passed out into the circulation again for further distribution around your body.

There are two main types of circulating cholesterol:

low-density lipoprotein (LDL) cholesterol, which is linked with hardening and furring up of artery walls, high blood pressure and coronary heart disease

high-density lipoprotein (HDL) cholesterol, which protects against atherosclerosis and CHD by transporting LDL-cholesterol away from the arteries for metabolism.

Research shows that for every 1 per cent rise in beneficial HDL cholesterol, there is a corresponding fall in the risk of CHD of as much as 2 per cent. This seems to be due to reversed cholesterol transport in which HDL moves LDL cholesterol away from the tissues and back towards the liver.

It is, therefore, not so much your total blood cholesterol level that is important when it comes to atherosclerosis but the ratio between beneficial HDL cholesterol and harmful LDL cholesterol. If you are told you have a raised blood cholesterol level, it is important to know whether your LDL or HDL cholesterol is high:

if your blood fats consist mainly of HDL-cholesterol, your risk of CHD is significantly reduced

if most of the lipids are in the form of LDL-cholesterol, with low HDL levels, your risk of CHD is significantly increased. Ideally, total cholesterol level should be less than 5mmol/l, with LDL cholesterol less than 3mmol/l.

Where LDL cholesterol levels are raised, it is estimated that reducing the average total blood cholesterol level by 10 per cent could prevent over a quarter of all deaths due to coronary heart disease. Unfortunately, attempts to reduce dietary cholesterol for improved cardiovascular health often have the opposite effect. Rather than just lowering the potentially harmful LDL form of cholesterol, dietary interventions often reduce levels of beneficial HDL-cholesterol as well. This is because the types of fat in your diet are also important, and people often cut out the good fats as well as the less desirable ones. If you ate all your fat in the form of essential fatty acids, monounsaturated fats (e.g. olive oil) and fish oils, for example, your risk of CHD would be low as most circulating fats would be in the form of beneficial HDL-cholesterol.