Metabolism of dietary fats and blood lipoproteins

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Dietary fats are absorbed in the small intestine. Ingested triglycerides (or triacylglycerols) are hydrolysed by pancreatic lipases into glycerol, fatty acids and some mono-acylglycerol. Absorption of dietary fats is almost complete; 98% or more. Intestinal mucosal cells take up the hydrolysis products from the gut lumen and largely re-esterify these to triglycerides. Short and medium chain fatty acids (C4:0-C10:0), which make up a very small part of SAFAs in the diet, are not re-esterified but directly taken up in the blood and transported to the liver through the portal vein. All other fatty acids are re-esterified and the newly formed triglycerides are excreted in the lymph in particles called chylomicrons, which then enter the peripheral bloodstream.

There are different types of lipids circulating in the blood. Triglycerides and cholesterol are the most abundant ones and these are also most intensively studied because of their link with cardiovascular disease. Because lipids are hydrophobic and blood plasma largely is water, cholesterol and triglycerides are packaged into specific lipoprotein particles for transport in the circulation. The composition of the different lipoprotein fractions in blood varies markedly (Table 1.1). Lipoproteins are categorised according to their density, which varies between 0.9 and 1.1 kg/l. The predominant lipoprotein particles are: chylomicrons, very low-density lipoproteins (VLDL), low-density lipoproteins (LDL) and high-density lipoproteins (HDL) (Table 1.1).

Triglycerides are principally transported in blood in chylomicrons and VLDL. Chylomicrons mainly carry triglycerides derived from the diet through

Table l.l Composition and physical characteristics of plasma lipoproteins

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