Excess cholesterol in the blood and heart disease
Cholesterol is present in the blood and being insoluble as such, is transported alongwith other things in different lipoprotein globules.
Cholesterol is an essential constituent of our body, used in its various tissues and activities.
In the brain and nervous tissue, it acts as an excellent insulator so that electrical impulses travel unhindered; without it, electrical impulses traveling in the nerves in our body would get short-circuited.
The human brain contains a heavy concentration of it. Cholesterol is also the source of material for the manufacture of male and female sex hormones. Yolk (yellow) of the egg has a high concentration of it.
An average adult takes about 200 to 800mg. of cholesterol in his diet daily, depending on the amount of intake of animal fats.
Cholesterol is also manufactured in the body itself by the liver from carbohydrates, proteins and fats that we take in our diet; this later amounts to 1.5 to 2.0gm daily.
Absorption of cholesterol is aided by the fats we take and also by the bile in the intestine. The cholesterol is chiefly absorbed by the lymphatics and enters the blood stream where it is transported within the lipoprotein particles.
Cholesterol is oxidized in the body to carbon dioxide by many tissues but most actively by the liver. Some of the cholesterol is converted into steroid hormones and most of it into bile acids which are excreted in the bile.
Bile acids represent the major normal degradation product of cholesterol. The liver is the major organ concerned with cholesterol in the blood which it can manufacture, store and destroy. By these processes, it largely determines and controls the level of blood cholesterol.
The latter is maintained fairly constant by regulatory mechanisms between the absorption and synthesis of cholesterol on the one hand, and the metabolism and excretion on the other.
It has been found that the amount of cholesterol in the food has a significant effect upon the blood cholesterol level.
The addition of dietary cholesterol in the form of the yellow part of the egg in healthy young men causes a significant increase in the concentration of blood cholesterol.
When the yellow part of the egg, is not included in the diet, the level of cholesterol in the blood, decreases to a large extent.
The intake of fat in the diet also exerts a strong influence on blood cholesterol. Short term studies have constantly shown a tendency to a fall in blood cholesterol following a diminution in the intake of fat in the diet.
Populations habitually a subsisting on low fat diets, as in India, certain parts of Africa etc, have a relatively low blood cholesterol levels.
An important finding in regard to fats is that the intake of unsaturated fats prevents the rise of blood cholesterol level which should otherwise occur if more fats are taken in the diet.
This has been proved experimentally by feeding monkeys with more than ordinary quantity of fat, the fat being predominantly of an unsaturated variety.
Clinically also, it has been observed that Eskimos who live largely on fish have lower levels of blood cholesterol; the fish fat and oil being mostly unsaturated – an exception in the case of fats derived from animal sources.
Data collected from studies all over the world indicate that the normal level of cholesterol in the blood should be in the range of 160 to 180 mg. per 100ml.
The average blood cholesterol level in people in the United States and other affluent countries is much higher; it is somewhere between 230 to 260 mg. per 100 ml.
In these affluent countries, the average blood cholesterol level has been known to have arisen during the past few decades. In Holland, it rose from 182 in 1812 to 245 mg. in 1948.
In England, while the average blood cholesterol level now is lower than in the U.S.A has risen over the years; in 1925, it ranged between 150 and 200 mg. per 100 ml.
A high level of cholesterol in the blood, particularly if it is held in combination in the form of LDL globules – which are more unstable so that the cholesterol comes apart easily – leads to its deposition in the walls of the blood vessels.
Although to the naked eye, cholesterol looks waxlike, under the microscope, its real structure, as tiny sharp needles, is visible, which when deposited in the wall of the arteries irritate it severely.
Lesions in blood vessels have been produced experimentally also in rabbits, guinea pigs, hamsters, pigs, chickens and monkeys by feeding them large quantities of cholesterol or animal fat.
A very high blood cholesterol level precedes the development of the lesions.
HDL averages about 25 per cent higher in women than in men. Female sex hormones, the estrogens, tend to raise, and male sex hormones, the androgens, tend to lower HDL.
In women, low HDL, particularly when associated with diabetes and obesity, markedly raises the risk of heart attacks.
Similar correlations of high blood cholesterol levels with increased incidence of subsequent coronary heart disease have also been noted in other long-term studies.
In all, the incidence of coronary heart disease is more particularly low when the blood cholesterol is below 200 mg/100 ml.
In most of the studies, there is a sharp rise in the subsequent development of coronary heart disease in groups of individuals with blood cholesterol above 240 or 260 mg/100 ml.
In populations in which coronary heart disease has been reported to be less common, the average serum cholesterol concentration in men beyond the age of 40 is usually found to be around 180 mg/100 ml.
An interesting observation made among coronary artery disease patients is that the high blood cholesterol, fluctuates widely, whereas in the normal persons the blood cholesterol level is low and it is constant.
Since in the normal person, the level of blood cholesterol is close to its concentration point, deposition of cholesterol follows when there is slight elevation above normal values, resulting in atheromatous lesions.
Coronary artery disease, blood cholesterol level, and the amount of fat taken in the diet usually go hand in hand.
In the United States, the increased frequency of coronary heart disease has been correlated with the increase in fat calories as a per cent of total caloric intake from 31.8 per cent in 1910 to 43 per cent in 1960.
The evidence linking fatty diet, high fat content of the blood and the development of coronary artery disease can be summarized as follows:
Dietary cholesterol intake from 0 to 600 mg/day is closely related to plasma cholesterol levels and dietary saturated fatty acids elevate blood cholesterol levels, whereas polyunsaturated fatty acids reduce them.
Low-cholesterol, low saturated fat diets consistently lower blood cholesterol levels up to 10 to 20 per cent.
Both high cholesterol and high triglycerides in the blood appear to be important risk factors for atherosclerosis.
In adults less than 55 years of age, a cholesterol concentration greater than 200 mg/100 ml or a triglyceride concentration greater than 200 mg/100ml, clearly indicates high fat content of the blood (hyperlipidemia) sufficient to require attention by the doctor.
If hyperlipidemia is not present, the tests need not be repeated for several years in an adult who maintains body weight and does not otherwise change in health or life-style.