19  Fatty acid metabolism

19.1 Introduction

Adipose cells store energy as triglyceride. A molecule of triglyceride is composed of three fatty acids on a glycerol backbone like a letter E.  Sometimes the fatty acids pass through pigs and poultry from their feed without much change, and the resulting fat can develop a terrible aroma.  There is no point in producing meat we cannot sell.  Thus, animal science students need to understand how this happens.


19.2 Structure of triglyceride

The long‑chain fatty acids found in animal triglycerides vary in length, as shown by the values of "n" in the general formula for fatty acids. The value for "n" in animal fat usually ranges from 5 to 20. These fatty acids are insoluble in water, like the triglycerides formed from them. Adjacent carbon atoms along a chain may be linked together with an unsaturated double bond. Unsaturated bonds produce a bend in the chain and this lowers the melting point. Unless complicated by the presence of unsaturated bonds, melting points are proportional to chain length.

General structure CH3‑(CH2)n‑COOH

Names for values of "n" - not counting the carbons at the ends of the chain!

2  ‑ butyric

4  ‑ caproic

6  ‑ caprylic

8  ‑ capric

10 ‑ lauric

12 ‑ myristic

14 ‑ palmitic

16 ‑ stearic

18 ‑ arachidic

20 ‑ behenic

The degree of saturation of carcass fat is affected by a number of factors.

19.3 Digestion and deposition of triglyceride

The integration of adipose metabolism with that of the rest of the body is VERY complicated.

Dietary intake

   In the case of a pig with triglyceride in its diet, the triglyceride reaches the small intestine and is hydrolyzed in the presence of pancreatic lipase.

Colipase, a protein secreted by the pancreas, binds to the surface of fat droplets in the presence of bile salts from the liver and facilitates the attachment of lipase. The partially or completely separated fatty acids may pursue a number of different pathways, one of which involves absorption by an intestinal cell and reassembly into a new triglyceride inside the cell. Large numbers of reassembled triglyceride molecules may be wrapped in protein and phospholipid to form a small globule about one micron in diameter called a chylomicron.

The triglycerides contained in the chylomicra then pass to the lacteals. Lacteals are blind‑ending lymphatic capillaries in the axes of the intestinal villi. The lymphatic system conducts chylomicra to the venous system where, after passing into the general circulation, they are made available to adipose cells.

However, chylomicra do not enter directly into adipose cells. Their triglyceride is first hydrolyzed to fatty acids and glycerol by lipoprotein lipase (clearing factor lipase) located on the surfaces of adipose cells or capillary cells. The transfer of the fatty acids to the adipose cell may be facilitated by surface proteoglycans trapping lipoproteins together with lipases, and by the continuity of intercellular connections with intracellular channels. Yet again, triglycerides now are reassembled, this time within the adipose cell. Lipoprotein lipase activity in adipose tissue is linearly related to cell size.

Most of the glycerol released by lipoprotein lipase outside the adipose cell is recycled to the blood stream. Inside the adipose cell, a new carbon backbone for the triglyceride is formed from glycerophosphate derived from glucose. The entry of glucose into a cell is facilitated by insulin. In porcine adipose tissue the enhancement of carbohydrate metabolism by insulin is antagonized by porcine growth hormone (STH). This may explain the leaner carcasses and greater lean growth of pigs treated with growth hormone.

19.4 Lipogenesis

   An alternative carbon input is available to adipose cells from very-low-density lipoproteins (VLDL) from the liver. In VLDL, a central core of triglyceride is packaged by a layer of protein, phospholipid and cholesterol. The leaching of triglyceride by lipoprotein lipase near to adipose cells leads to a reorganization of VLDL components, and a low-density lipoprotein (LDL) is produced. The cholesterol remaining in LDL is released into the blood stream by cells of the reticuloendothelial system. In the liver, cholesterol, triglyceride and circulating high-density lipoproteins (HDL) are assembled into new VLDL.

Meat animals may form new fat by lipogenesis, as shown in the classical experiments of Lawes and Gilbert in Victorian England. In pigs, most of the new triglyceride is synthesized from glucose, and the fatty acids produced are dominated by stearic acid and its desaturated form, oleic acid. In ruminants, acetate is the main substrate, and stearic acid is the dominant fatty acid.

In poultry, lipogenesis occurs in the liver rather than in adipose cells, and triglyceride reaches the adipose cells as VLDL in the blood. Oleic and palmitic acids are dominant when the dietary intake of triglyceride is low.

Further information

Structure and Development of Meat Animals and Poultry.  Pages 116-120.