Lipid metabolism from digestion and synthesis to storage

The most important lipid components of our diet are triacylglycerols (TAG, about 100 g/day), followed by phospholipids (about 5 g/day) and small amounts of glycerolipids, sterols and fat-soluble vitamins (Mu and Hoy 2004). The efficiency of the organism in digesting and absorbing TAG is very high (about 95%). TAG consist of a glycerol molecule acylated with three fatty acids. The positions are numbered by the stereochemical numbering system, i.e. fatty acids may be designated sn1-, sn2- and sn3-. In human diets the fatty acids vary in chain length from C2 to C24, and from saturated to unsaturated fatty acids with up to six double bonds (Mu and Porsgaard 2005). Quantitatively relevant fat sources in the diet are oils - such as olive oil, soybean oil and fish oil - milk fat and adipose tissue of certain animals including marine species (Mu and Hoy 2004).

Fat digestion occurs in the stomach and intestine. After chewing, a food bolus is formed and transported to the stomach, where a partial hydrolysis of TAG into diacylglycerols and free fatty acids (FFA) takes place (Mu and Hoy 2004). In humans the lipases in the stomach, are derived from the tongue (lingual lipase) or from the stomach, with gastric lipase being the predominant enzyme (Denigris et al. 1988; Hamosh 1990). Gastric predigestion accounts for about 15% of fat digestion and facilitates the digestion process in the small intestine. Pancreatic lipase is the major contributor to TAG hydrolysis (Lowe 1997). The appearance of TAG degradation products in the proximal intestine causes gall bladder emptying, pancreatic lipase secretion and cholecystokinin release (Meyer and Jones 1974; Watanabe et al. 1988). TAG are emulsified by bile acids, which are strong detergents, markedly increasing the available surface for pancreatic lipase binding, hence promoting TAG digestion. The degradation process is region-specific and ideally results in the formation of sn2-monoacylglycer-ols and FFA, which are then absorbed by enterocytes (Mu and Hoy 2004). In the smooth endoplasmic reticulum of the enterocytes, new TAG are synthesized and lipoproteins are formed, which are subsequently secreted into the lymph. Two major lipoproteins are secreted by the intestine: chy-lomicrons (CM) and very low density lipoproteins (VLDL). CMs are TAG-rich lipoproteins synthesized in the small intestine after a meal to transport lipids, whereas VLDL are formed during fasting when the level of exogenous lipids is too low to drive CM formation. Intestinal lipoproteins are secreted into tiny lymph vessels inside each of the intestinal villi, and then enter the circulation in the subclavian vein via the thoracic duct (Mu and Hoy 2004; Williams et al. 2004). Nevertheless, not all lipids are transported via lymphatics, and some can also be transported directly to the liver via the hepatic portal vein. The most important factor affecting the portal-lymph distribution is the chain length of fatty acids: short- and medium-chain fatty acids are mainly transported via the portal vein, whereas long-chain fatty acids are transported via the lymphatic system (St Onge and Jones 2002). Lipoprotein lipase located at or close to the capillary endothelial wall in extra-hepatic tissues, such as heart, skeletal muscle and adipose tissue, rapidly hydrolyzes circulating CM TAG. The resulting CM remnants are recognized and removed by the liver. The action of lipoprotein lipase provides FFA and 2-monoacylglycerols for tissue utilization. In white adipose tissue, FFA are re-esterified with glucose-derived glycerol-3-phosphate for the storage of energy as TAG, whereas in other tissues FFA are mainly oxidized to drive cell metabolism or thermogenesis (Mead et al. 2002).

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