Physiology of Finfish and Shellfish (2+1)

4.3.2.10 Fat (Lipid) Metabolism

Fats contain mostly carbon and hydrogen, some oxygen and sometimes other atoms. Dietary lipids are a complex mixture of the neutral lipids - glycerides (principally triacylglycerol [triglyceride], the form in which fat is stored for fuel), the phospholipids, the sterols (principally cholesterol) and other complexlipids. Fats provide 9 kilocalories per gram (kcal/g), compared with 4 kcal/g for carbohydrate and protein. Triacylglycerol, whether in the form of chylomicrons (microscopic lipid particles) or other lipoproteins, is not taken up directly by any tissue, but must be hydrolyzed outside the cell to fattyacids and glycerol, which can then enter the cell.

Fattyacids come from the diet, adipocytes (fat cells), carbohydrate and some amino acids. After digestion, most of the fats are carried in the blood as chylomicrons. The main pathways of lipid metabolism are lipolysis, betaoxidation, ketosis and lipogenesis. Lipolysis (fat breakdown) and beta-oxidation occurs in the mitochondria. It is a cyclical process in which two carbons are removed from the fatty acid per cycle in the form of acetyl CoA, which proceeds through the Krebs cycle to produce ATP, CO₂ and water. Ketosis occurs, when the rate of formation of ketones by the liver is greater than the ability of tissues to oxidize them. It occurs during prolonged starvation and when large amounts of fat are eaten in the absence of carbohydrate. Lipogenesis occurs in the cytosol. The main sites of triglyceride synthesis are the liver,adipose tissue and intestinal mucosa. The fatty acids are derived from the hydrolysis of fats, as well as from the synthesis of acetyl CoA through the oxidation of fats, glucose and some amino acids. Lipogenesis from acetyl CoA also occurs in steps of two carbon atoms. NADPH produced by the pentose-phosphate shunt is required for this process. Phospholipids form the interior and exterior cell membranes and are essential for cell regulatory signals.

(A stage in fat metabolism, which is stimulated primarily when the liver is low on glycogen (stored glucose). The breakdown of fats in this process results in three different molecules called “ketones” or “ketone bodies”, two of which (acetoacetate and ß-hydroxybutyrate) can be used as fuel, by most tissues of the body. Some tissues, such as the heart muscle, prefer ketones to glucose.The third ketone, acetone, cannot be used by the body and is excreted in the urine and breath. In organic chemistry, a ketone (pronounced /ˈkiːtoʊn/ ) is a compound with the structure RC(=O)R', where Rand R' can be a variety of atoms and groups of atoms. It features a carbonyl group (C=O) bonded to two other carbon atoms. Acetone is the simplest example of a ketone, and in fact the word ketone derives its name from Aketon, an old German word for acetone.

Ketones differ from aldehydes , in that the carbonyl is placed between two carbons rather than at the end of a carbon skeleton . They are also distinct from other functional groups , such as carboxylic acids , esters and amides , which have a carbonyl group bonded to a heteroatom ).

Fats are readily catabolised as a source of daily maintenance and movement energy needs. If there is too much fat in the diet or fish are overfed for a long period of time, fat deposition will occur. Fats can be synthesized from amino acids and glucose as well as from glycerols and free fatty acids - it is thus easy for fish to accumulate fat from their diet. Fat is also the most economical way to store energy as it has inherent higher energy latency.