The major lipids in the diet are triacylglycerols and, to a lesser extent, phospholipids. These are hydrophobic molecules and have to be hydrolyzed and emulsified to very small droplets (micelles, 4-6 nm in diameter) before they can be absorbed. The fat-soluble vitamins, A, D, E, and K, and a variety of other lipids (including cholesterol and carotenes) are absorbed dis solved in the lipid micelles. Absorption of carotenes and fat soluble vitamins is impaired on a very low-fat diet.

Hydrolysis of triacylglycerols is initiated by lingual and gastric lipases, which attack the sn-3 ester bond forming 1,2-diacylglycerols and free fatty acids, which act as emulsifying agents. Pancreatic lipase is secreted into the small intestine and requires a further pancreatic protein, colipase, for activity. It is specific for the primary ester links—that is, positions 1 and 3 in triacylglycerols—resulting in 2-monoacylglycerols and free fatty acids as the major end products of luminal triacylglycerol digestion. Inhibitors of pancreatic lipase are used to inhibit triacylglycerol hydrolysis in the treatment of severe obesity. Pancreatic esterase in the intestinal lumen hydrolyzes monoacylglycerols, but they are poor substrates, and only ~25% of ingested triacylglycerol is completely hydrolyzed to glycerol and fatty acids before absorption (Figure 1). Bile salts, formed in the liver and secreted in the bile, permit emulsification of the products of lipid digestion into micelles together with dietary phospholipids and cholesterol secreted in the bile (about 2 g/d) as well as dietary cholesterol (about 0.5 g/d). Micelles are less than 1 μm in diameter, and soluble, so they allow the products of digestion, including the fat-soluble vitamins, being transported through the aqueous environment of the intestinal lumen to come into close contact with the brush border of the mucosal cells, allowing uptake into the epithelium. The bile salts remain in the intestinal lumen, where most are absorbed from the ileum into the enterohepatic circulation.

Fig1. Digestion and absorption of triacylglycerols. The values given for percentage uptake may vary widely but indicate the relative importance of the three routes shown.

Within the intestinal epithelium, 1-monoacyglycerols are hydrolyzed to fatty acids and glycerol and 2-monoacylglycerols are reacylated to triacylglycerols via the monoacylglycerol pathway. Glycerol released in the intestinal lumen is absorbed into the hepatic portal vein; glycerol released within the epithelium is reutilized for triacylglycerol synthesis via the normal phosphatidic acid pathway. Long-chain fatty acids are esterified to triacylglycerol in the mucosal cells and together with the other products of lipid digestion, secreted as chylomicrons into the lymphatics, entering the bloodstream via the thoracic duct. Short- and medium chain fatty acids are mainly absorbed into the hepatic portal vein as free fatty acids.

Cholesterol is absorbed dissolved in lipid micelles and is mainly esterified in the intestinal mucosa before being incorporated into chylomicrons. Plant sterols and stanols (in which the B ring is saturated) compete with cholesterol for esterification, but are poor substrates, so that there is an increased amount of unesterified cholesterol in the mucosal cells. Unesterified cholesterol and other sterols are actively transported out of the mucosal cells into the intestinal lumen. This means that plant sterols and stanols effectively inhibit the absorption of not only dietary cholesterol, but also the larger amount that is secreted in the bile, so lowering the whole body cholesterol content, and hence the plasma cholesterol concentration.

اخر الاخبار

اخبار العتبة العباسية المقدسة

إيذانًا ببدء فعاليات حفل التكليف المركزي.. انطلاق مسير التلميذات في مجمع أبي الفضل العباس (عليه السلام)

بدء توافد التلميذات المشاركات في فعاليات حفل التكليف الشرعي ضمن مشروع الورود الفاطمية

قسم الشؤون الفكرية يقدم محاضرة حول الأخلاق الإدارية في بيئة العمل لوفد من ذي قار

العتبة العباسية المقدسة تقيم مهرجان (وشاح الطهر) لتكريم الطالبات المرتديات للعباءة العراقية في المثنى

المزيد

الآخبار الصحية

عوامل تضعف القدرات الإدراكية لدى الإنسان

طريقة جديدة لتجديد خلايا الدماغ

دراسة بريطانية: الأطعمة النباتية عالية الجودة تقلل خطر فشل القلب

7 أطعمة يومية "تسمم" جسمك بالبلاستيك دون أن تشعر

المزيد

الآخبار التكنلوجية

اكتشاف أثري في أوريغون الأمريكية قد يعيد كتابة تاريخ البشرية

اكتشاف كائن بحري مدرع بلسان "حديدي" في بحر اليابان

نظام كوكبي "مقلوب" يحير علماء الفلك

بسبب الذكاء الاصطناعي.. هذه الوظائف "ستختفي" بعد أشهر قليلة ؟!

المزيد

مواضيع ذات صلة

Most Lipids and Many Proteins Are Laterally Mobile in Biomembranes

Biomembranes Contain Three Principal Classes of Lipids

Platelet Aggregation Requires Outside-In and Inside-Out Transmembrane Signaling

Phagocyte-Derived Proteases can Damage Healthy Cells

Phospholipid Bilayers Form a Sealed Compartment Surrounding an Internal Aqueous Space

Phospholipids Spontaneously Form Bilayers

Phagocytic Leukocytes Generate Reactive Oxygen Species During the Respiratory Burst

Leukocytes are Motile

Platelets

The Biochemical Basis of The ABO System

The Red Blood Cell Membrane

Oxidation of Heme Iron Compromises oxygen Transport