Problems with lipid metabolism

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Chapter 27 Problems with lipid metabolism

Chapter contents

What happens to lipids after absorption? 203
Atherosclerosis 204
Hyperlipidaemia 206
Antiobesity medication 208

Fatty acids are an important source of energy: potentially, triglycerides can release as much energy per kilogram as carbohydrates or proteins. Lipids are components of membranes in the form of phospholipids (see Chapter 10 ‘Lipids’, p. 73).

Ultimately, the carbohydrates that are not utilized by the body end up as fatty acids, which are stored as triglycerides. This fact is conveniently side-stepped by food manufacturers, who produce low-fat products that often have a very high sugar content.

Fatty acids are usually eaten as triglycerides, which are broken down into free fatty acids and monoglycerides by lipases in the small intestine. This process is assisted by bile salts, which emulsify the fats and increase the efficiency of the enzymes (see Chapter 4 ‘Bonds continued’, p. 25).

Lipids are absorbed largely as fatty acids (although some are absorbed as free glycerol and diglycerides). After absorption, they are packed with protein to create a lipoprotein, which acts as a carrier for the lipid in the bloodstream.

There are four main classes of lipoprotein:

1. Chylomicrons: carry triglycerides from the gut to the liver and adipose (lipid storage) tissue.
2. Very-low-density lipoproteins (VLDL): carry triglycerides made in the liver from the liver to the adipose tissue.
3. Low-density lipoproteins (LDL): carry cholesterol from the liver to the rest of the body. Large amounts of LDLs are undesirable.
4. High-density lipoproteins (HDL): gather cholesterol from the body returning it to the liver. High amounts of HDLs are desirable.

What Happens to Lipids after Absorption?

Figure 27.1 outlines the fate of lipids after absorption. The numbers below correspond to those in the illustration:

(i) Most lipids are absorbed as fatty acids and a small amount as free glycerol and diglycerides.
(ii) In the cells of the gut wall (enterocytes) they reform into triglycerides, and are packaged into chylomicrons along with lipoproteins and other lipids.
(iii) The chylomicrons are then released in the lymph system (sugars and amino acids are absorbed directly into the bloodstream).
(iv) At the thoracic duct, the chylomicrons are released into the sublclavian vein, where they bind to the membranes of adipose cells (v) or pass to the liver (vi) or to the tissues of body to be broken down by lipoprotein lipase in the cell membranes and the resulting fatty acids used as an energy source (vii).
image

Figure 27.1 General metabolism of lipids.

If the lipids are unused, they are stored.

The liver is the key organ for lipid metabolism.

Atherosclerosis

Thrombus Formation

Figure 27.2 illustrates the formation of a thrombus, which is the basis of atherosclerosis. The numbers below relate to those in the illustration:

(i) Lipids are present in the bloodstream under normal conditions.
(ii) High levels of cholesterol in the blood can damage the lining of an artery, causing an inflammatory reaction, which then enables cholesterol and other fats to gather in that area.
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