Digestion and absorption of fats

The major fats in the diet are triacylglycerols and, to a lesser extent, phospholipids. These are hydrophobic molecules and have to be emulsified to very small droplets (micelles section 4.3.2.2) before they can be absorbed. This emulsification is achieved by hydrolysis to monoacyl- and diacylglycerols and free fatty acids, and also by the action of the bile salts (section 4.3.2.1). 

Four groups of compounds that are metabolically important can be considered under the heading of lipids  

They are absorbed in lipid micelles (section 4.3-2.2), and adequate absorption depends on an adequate intake of fat. 

Although it is the fatty acids that are saturated or unsaturated, it is common to discuss saturated and unsaturated fats. Although is not really correct, it is a useful shorthand, reflecting the fact that fats from different sources contain a greater or lesser proportion of saturated and unsaturated fatty acids. 

As shown in Table 4.1, there are three different ways of naming the fatty acids  

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There is no solid evidence that relates human aging and reduction of carotenoid absorption. In some studies, old people have shown a lower (3-carotene absorption than that of young people (Madani and others 1989), whereas the opposite has also been reported by other studies (Sugarman and others 1991). The absorption of lipid-soluble substances, including carotenoids, is affected by any disease related to the digestion and absorption of fats (West and Castenmiller 1998). Inadequate production of lipase and bile as well as an inadequate neutralization of the chyme in the duodenum affect carotenoid bioavailability (Guyton and Hall 2001). 

The digestion and absorption of fat is considerably more complex than that of carbohydrate or protein because it is insoluble in water, whereas almost aU enzymes catalyse reactions in an aqueous medium. In such media, fat can form small droplets, an emulsion, which is stable in this medium. Formation of an emulsion is aided by the presence of detergents these possess hydrophobic and hydrophilic groups, so that they associate with both the fat and the aqueous phases. Such compounds are known as emulsifying agents and those involved in digestion are mainly the bile salts and phospholipids. 

Bile salts consist of taurine linked to bile acids. The salts are essential for digestion and absorption of fat and also of fat-soluble vitamins and cholesterol. 

Bile helps in the digestion and absorption of fats. Its constituent bile acids (BAs) have detergent properties, and some can be carcinogenic. BAs can act as signalling molecules, entering the nuclei and reacting with the nuclear receptors and this could enhance or reduce BA synthesis. In this way, they control their own levels as well as those of their precursor, cholesterol. This controls cholesterol homeostasis and BA and lipid synthesis. 

Abnormalities in protein digestion In individuals with a deficiency in pancreatic secretion (for example, due to chronic pancreatitis, cystic fibrosis, or surgical removal of the pancreas), the digestion and absorption of fat and protein is incomplete. This results in the abnormal appearance of lipids (called steatorrhea, see p. 175) and undigested protein in the feces. 

It is well known that bile acids are produced in the liver of vertebrates for digestion and absorption of fats and fat-soluble vitamins. Starting from isoprene, a series of biochemical reactions yield a key compound, cholesterol, which is converted to primary bile acids, such as cholic acid (CA), deoxycholic acid (DCA), chenodeoxycholic acid (CDCA) and lithocholic acid (LCA). Hereafter the abbreviations of bile acid derivatives can be seen by consulting Table 1 and Figure 1. 

The processes for the digestion and absorption of fat- and water-soluble vitamins are different, due to their solubility properties. Fat-soluble vitamins and their precursors (A, [1-carotene, D, E and K) are digested and absorbed by processes similar to those for dietary fats, mainly in the small intestine. Most water-soluble vitamins require specific enzymes for their conversion from natural forms in feed-stuffs into the forms that are ultimately absorbed. Unlike fat-soluble vitamins that are absorbed mostly by passive diffusion, absorption of water-soluble vitamins involves active carrier systems to allow absorption into the portal blood. 

The drug orlistat (Xenical), which is promoted for the treatment of obesity, acts by inhibiting pancreatic lipase, thereby reducing the digestion and absorption of fat in the smaU intestine. 

Orlistat (Xenical) inhibits the action of the enzyme lipase in the intestine, thereby inhibiting the digestion and absorption of fats. This can lead to significant weight loss, but is often accompanied by flatulence, diarrhoea and oily stools. Its clinical application is described in Chapter 8. 

CAS 81-24-3. C26H45N07S. Occurs as sodium salt in bile. It is formed by the combination of the sulfur-containing amino acid taurine and cholic acid as the sodium salt. It aids in digestion and absorption of fats. 

Ph ology of ASBT. Bile acids mediate the digestion and absorption of fat and fat-soluble vitamins (232). The total bile acid pool in humans, 3-5 g, circulates 6-10 times a day, giving rise to a daily bile acid turnover of... 

Bile salts (made from bile acids) are secreted from the liver, stored in the gallbladder, and passed through the bile duct and into the intestine (Figure 18.3). They are involved in emulsification of fats in the intestine, aid digestion and absorption of fats and fat-soluble vitamins (Figure 18.4). 

Certain foods contain microemulsions naturally, and microemulsions as a functional state of lipids have therefore been used in the preparation of foods since ancient times. Microemulsions can also be formed in the intestine during the digestion and absorption of fat. The possibility to produce microemulsions on purpose and use them as tools (i.e., not only as vehicles) in food production, however, is a neglected field in food technology. 

Cholesterol functions in the body as a precursor for the synthesis of bile acids in liver, which are secreted into the gut to aid in the digestion and absorption of fat in the diet. Cholesterol is also a precursor for the synthesis of steroid hormones such as progesterone and estrogen. 

The digestion and absorption of fats pose considerable problems because of their insolubility in water. The fat splitting enzymes or lipases are water-soluble and consequently can only operate at the interface between lipid droplets and the aqueous phase. For this reason the degree of dispersion or emulsification is very important in ensuring that fat is adequately digested and absorbed. 

Among the most abundant steroids, cholesterol is present in almost all human and animal tissue (Real Life 4-2). Bile acids are produced in the liver as part of a fluid delivered to the duodenum to aid in the emulsification, digestion, and absorption of fats. An example is cholic acid. Cortisone, used extensively in the treatment of rheumatoid inflammations, is one of the adrenocortical hormones produced by the outer part (cortex) of the adrenal glands. These hormones participate in regulating the electrolyte and water balance in the body, as well as in protein and carbohydrate metabolism. 

The salts of the bile acids are the digestion-promoting constituents of bile. They are surface active agents this means that they lower surface tension and thus can emulsify fats. They also activate lipases. The bile acids, for these reasons, play a leading role in the digestion and absorption of fats. 

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