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Absorption:

The mechanism of the intestinal fat absorption has been studied with C14 labeled fat (combinations of corn oil and palmitic acid) in rats with the intestinal lymph duct cannulated (Borgström, 1951). The recoveries in the lymph of the fat fed varied widely. Diarrhoea occurred in some animals especially after feeding hydrolysed corn oil. In all three groups of experiments maximum recoveries were found after 24 hours, i.e. 80.9, 85.0 and 87.5 % of the activity given. These results indicate that most of the absorbed fat is transported via the lymphatic channels to the systemic circulation whether fed as glycerides or free fatty acids. The proportions of neutral fat and phospholipids in the lymph were in all three cases about the same. 90% of the fatty acids were present in the neutral fat and the remaining 10 % in phospholipids. The neutral fat consisted chiefly of triglycerides; cholesterol and cholesterol esters representing only a minor part of this fraction. No free fatty acids or soaps appeared in the lymph. The results indicated that glycerides might be completely hydrolysed in the intestinal lumen of the rat and then resynthesized in the intestinal wall.

In another study with soybean oil the oral absorption in rats when fed at 17% of the diet was found to be 95 -98% (Nolen, 1972).

The distribution of the fatty acids in the triglycerides of the lymph was determined upon oral administration of triglycerides of known structure to rat (Mattson and Volpenhein, 1961).The extent of absorption of palmitic acid depended on the form in which it was fed (rates between 52 an 96 %). Absorption was greatest when palmitic acid was fed as β-palmitoyl diolein, and least when it was fed as the free acid.

Metabolism:

Fatty Acid Glycerides (mono-, di-, and tri-esters of carboxylic acids with glycerol) have a common metabolic fate that involves stepwise hydrolysis to the carboxylic (e.g. fatty) acids and glycerol. Carboxylic acids and glycerol feed into physiological pathways like the citric acid cycle, sugar synthesis, and lipid synthesis. Fatty Acid Glycerides constitute a large part of the human diet. Triglyceride fats are a major source of calories in the human diet.

Matulka (2009) summarized that the metabolism of Medium chain triglycerides in the canine is a process whereby lipases from the buccal cavity and pancreas release the fatty acids in the gastrointestinal tract where they are absorbed. Unlike long chain triglycerides (LCT), where long chain fatty acids (LCFA) form micelles and are absorbed via the thoracic lymph duct, MCFA are most often transported directly to the liver through the portal vein and do not necessarily form micelles. Also, MCFA do not re-esterify into MCT across the intestinal mucosa. MCFA are transported into the hepatocytes through a carnitine-independent mechanism, and are metabolized into carbon dioxide, acetate, and ketones through b-oxidation, and the citric acid cycle.

Adolph (1999) summarized that lipids are not only structural building blocks of cells and tissues but at the same time suppliers of C- atoms for a number of biosynthetic pathways as well as carriers of essential fatty acids and fat-soluble vitamins. In addition, fatty acids are precursors of prostaglandins and other eicosanoids and therefore have important metabolic functions. Fatty acids can be divided into three groups, saturated, monounsaturated, and polyunsaturated fatty acids.

Each class of fatty acids has a preferential specific role. Saturated fatty acids (medium or long-chain) are more devoted to energy supply, but one should not forget their specific structural role. The polyunsaturated fatty acids of the n–3 and n–6 families have very important structural and functional roles and ideally should not be utilized for energy purposes.

 

Excretion (Lipolysis):

Typical dietary lipids from vegetable oils, termed long-chain triacylglycerols (LCT), are degraded by salivary, intestinal and pancreatic lipases into two fatty acids and a monoacyl glycerol; whereas, MCT are degraded by the same enzymes into three fatty acids and the simple glycerol backbone. Medium-chain fatty acids (MCFA) are readily absorbed from the small intestine directly into the bloodstream and transported to the liver for hepatic metabolism, while long-chain fatty acids (LCFA) are incorporated into chylomicrons and enter the lymphatic system. MCFA are readily broken down to carbon dioxide and two-carbon fragments, while LCFA are re-esterified to triacylglycerols and either metabolized for energy or stored in adipose tissue.

 

Metabolism upon parenteral application in humans:

In a human volunteer study on the use of Medium and Long Chain Triglycerides for parenteral nutrition different types of lipid emulsions were infused to male volunteers (see 7.10.5, Schulz, 2002). Biomonitoring of blood triglycerid levels upon single bolus, short time infusion, 12 -hour low concentration infusion and 8 -hour higher concentration infusion revealed that intravenous application of triglycerides in human subjects at concentrations of 100 mg/kg bw/h were well tolerated resulting in a dynamic equilibrium.