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Dietary intake of lipid is mainly in the form of triglycerides which are composed of a glycerol chain and three fatty acids. There are also small quantities of cholesterol, phospholipids and

Figure 6.4 Metabolism and transport of fat into the lymphatic and systemic circulation

cholesterol esters in chyme. Fat is emulsified by the bile salts into small droplets which disperse in water allowing access of digestive enzymes.

Lipase in the pancreatic juice and enteric lipase from the epithelial cells of the small intestine both hydrolyse emulsified triglycerides to monoglycerides and fatty acids. The short and medium chain fatty acids are absorbed passively through the epithelium into the blood. The long chain fatty acids and monoglycerides remain as mixed micelles with the bile salts and are internalized by the epithelium. They are reassembled into triglycerides within the cell and excreted into the lymph as small (0.1 pm) droplets called chylomicra (or chylomicrons) (Figure 6.4).

Iron

Heme iron is absorbed from meat more efficiently than dietary inorganic iron and in a different manner8. Thus, iron deficiency is less frequent in countries where meat constitutes a significant part of the diet. Proteolytic digestion of myoglobin and hemoglobin results in the release of heme, which is maintained in a soluble form by globin degradation products so that it remains available for absorption. Heme enters the small intestinal absorptive cell as an intact metalloporphyrin. This may be facilitated by a vesicular transport system9. In the absorptive cell the porphyrin ring is split by heme oxygenase. The released inorganic iron becomes associated with mobilferrin and paraferritin, which acts as a ferrireductase to make iron available for production of iron-containing end products such as heme proteins. Mucosal transfer of iron into the body occurs competitively with dietary iron that enters the absorptive cell as inorganic iron, because they both share a common pathway within the intestinal cell. Dietary inorganic iron as the ferric iron is solubilized at the acid pH level of the stomach where it chelates mucins and certain dietary constituents to keep them soluble and available for absorption in the more alkaline duodenum.

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