10. The Biochemistry of Fat John Thompson Learning objectives This lecture examines the biochemical pathways by which fat is digested, absorbed and deposited in both ruminants and monogastrics and the scope for manipulation by either management or genetic means. At the end of this chapter you will: • understand the importance of fat to the meat and food industries. • know the principles of digestion of fat in monogastrics and ruminants • understand the metabolic pathways for fat absorption, synthesis and catabolism in the body • understand the effect of genetic variation on fatty acid composition in ruminants (both between and within breed) • know the importance of environmental effects on fat composition in ruminants (diet, temperature and seasonal effects) 10.1 Introduction Fatty acid composition in meat animals is important because of the impact it has the melting point of fat and therefore presentation in the display cabinet. Perhaps of greater importance is the influence that dietary fat has on human blood lipids and ultimately fatness and perhaps cardiovascular disease in humans. The 5 fatty acids shown below (Figure 10.1) all have the same carboxyl group at the start, but have different chain length and some have double bonds which impacts on the physical properties of the resultant fat depots (ie melting point) 10.2 Importance of fat composition • In addition to being an energy storage depot for the animal, the total amount of fat and its composition is important for the following reasons; • Subcutaneous fat is important to stop dehydration in the carcase. • A good coverage of subcutaneous fat reduces the likelihood of cold-shortening in the carcase. • Fat hardness affects processing efficiency through workplace safety and repetitive strain injury. • Fat is prone to oxidation and therefore responsible for development of rancidity. • Fat quality (colour, hardness and texture) is important in some markets for desirable odour and flavour characteristics. • Approx. 85% of subcutaneous fat tissue is made up of triacylglycerols within fat cells. • The remainder comprises moisture (approx. 12%) and connective tissue (approx. 3%). • The collagen component, which affects the texture hardness, is largely a function of fatty acid composition and the molecular make-up of the triglyceride. • The position of the fatty acids on the triacylglycerol affects hardness. • Much of the characteristic species flavour associated with different types of meat originates from carbonyl compounds concentrated in the adipose tissue. In contrast the flavour from the muscle component is similar between lean meat from different species. MEAT418/518 Meat Technology - 10 - 1 ©2009 The Australian Wool Education Trust licensee for educational activities University of New England Figure 10.1. The chemical formulae for several common classes of fatty acids. Source: Thompson, (2005). 10 - 2 – MEAT418/518 Meat Technology ©2009 The Australian Wool Education Trust licensee for educational activities University of New England 10.3 Digestion and deposition of lipids Triacylglycerols (formerly called triglycerides) are derived from 3 primary sources: the diet; de nova synthesis, particularly in the liver; and storage depots in adipocytes. That is, they are ingested, synthesised in the liver, or mobilised from storage. Triacylglycerols are made up of 3 fatty acids attached to a glycerol. Most triacylglycerols contain a mixture of fatty acids, which may be saturated or unsaturated. Fatty acids are made up of a carboxylate group attached to one end of a hydrocarbon chain. Most of the naturally occurring fatty acids have an even number of carbon atoms. A saturated acid is one in which the carbons are saturated with hydrogen atoms. Many naturally occurring fatty acids are unsaturated – ie they contain one or more double bonds (one or more carbons which are not saturated with hydrogen atoms and therefore have a double bond between consecutive carbons). Nomenclature of fatty acids Each fatty acid has a common name (eg stearic), and a systematic name (eg octadecanoic), and an abbreviation based on their structure (eg 18:.0) where the number before the colon indicates the number of carbons in the hydrocarbon chain, and the number after the colon the number of double bonds. The most common fatty acids present in the adipose tissue of ruminants and non-ruminants, along with their carbon chain length and the number of double bonds, are shown in Table 10.1. The melting points are also given. Note the large decrease in melting point with increased unsaturation of the C18 acids. Table 10.1 Fatty acid composition of beef adipose tissue Source: Thompson, (2005). Name Structure Percent Melting Point (oC) Myristic 14:0 2-4 54 Palmitic 16:0 23-27 63 Stearic 18:0 4-30 70 Oleic 18:1c9 38-45 16 Linoleic 18:2c9,c12 1-2 -5 Digestion of dietary lipids The major problem that animals must cope with in the digestion, absorption and transport of dietary lipids is their insolubility in water. The action of bile salts secreted by the gall bladder is essential to the digestion of lipids and their absorption through the intestinal mucosa. The complexing of lipids with proteins to form lipoproteins enables transport through the blood and lymph. Monogastric Pancreatic juices provide lipase, which hydrolyses triacylglycerols in the small intestine to form two free fatty acids and a monoglyceride, which are soluble in bile salts. The fatty acids and monoglyceride combine with bile salts form a micelle, which passes across the intestinal wall. The triglyceride is then resynthesised and combines with proteins to form lipoproteins (chylomicrons). This process solubilises the lipids and permits their transport through blood and lymph, and it is in this form that dietary fat is transported from the intestine to peripheral tissues, eg for storage in adipocytes. MEAT418/518 Meat Technology - 10 - 3 ©2009 The Australian Wool Education Trust licensee for educational activities University of New England Ruminants Figure 10.2 summarises the processes of digestion, absorption and synthesis of fat in the ruminant. Figure 10.2. A diagrammatic representation of the pathways for digestion, absorption and synthesis of fat in the ruminant. Source: Kelly (1999). The high intake of unsaturated fatty acids from plant materials is to a large degree hydrogenated in the rumen. Dietary lipids are hydrolysed by lipases produced by the rumen bacteria and the released free fatty acids are then hydrogenated (ie. hydrogen is incorporated into the double – unsaturated – bonds to produce more saturated fatty acids). Linoleic (C18:2) and linolenic (C18:3) acid, which are very common in plant material, are hydrogenated to form stearic acid (C18:0). The hydrogenation is not always complete. As a result, appreciable amounts of geometrical and positional isomers of octadecenoic and octadecadienoic acids are formed. There is commonly more lipid in fluid that reaches the abomasum than that in the diet. This is mostly due to lipid synthesis by rumen bacteria and protozoa. Elongation of dietary fatty acids is the main mode of synthesis by bacteria and protozoans, as the process of building fatty acids from acetate is energetically expensive. Rumen bacteria and protozoans build more odd chain length fatty acids than are formed by plants and animals. There is little alteration of fatty acids between the rumen and the small intestine. The physiological mechanisms of digestion in the intestines are similar for ruminants and non-ruminants. Fatty acids arrive into the small intestine attached to insoluble particulate matter. Any esterified fats that “ escaped” the rumen are lipolysed in the duodenum. Free fatty acids are then emulsified by the action of bile to form micelles. These micelles are absorbed through the epithelial cells of the small intestine. Although the lipid content of ruminant digesta is generally low, the alimentary system has the capability to absorb considerable quantities of lipid. Absorption into the plasma is highly efficient with 82-92% of C16:0 and C18:0 absorbed respectively in cattle. Although the absorption is high there is some preferential absorption of oleic (C18:1) palmitic (C16:0) and stearic (C18:0). However, discrimination against the absorption of stearic acid may be hardly noticeable, since this is the predominant component of the digesta. 10 - 4 – MEAT418/518 Meat Technology ©2009 The Australian Wool Education Trust licensee for educational activities University of New England Lipoprotein lipase is responsible for lipid uptake by adipose tissue. Exogenous free fatty acids are taken up readily. Triglycerides of plasma lipoproteins (VLDL and chylomicron) are the main source of exogenous fatty acids for adipose tissue. The rate of uptake by lipoprotein lipase is dependant on age, diet, pregnancy and lactation. Exogenous fatty acids are rapidly incorporated into lipids. However the absorption of fatty acids may be differential. Studies have reported that palmitic (C16:0) was the preferred fatty acid for absorption, followed by oleic (C18:1c), linolenic (C18:3c), linoleic (C18:2c) then stearic (C18:0). As the amount of VLDL in the plasma is low, the lipoprotein lipase should incorporate any chain length into the adipocyte. In the plasma the major fatty acid is C18:0 which can constitute up to 47% of the total amount of lipid available for absorption The fatty acid composition of ruminant fat is characterised by a high stearic content, the presence of positional and geometric isomers of unsaturated fatty acids and the existence of appreciable amounts of branched chain and odd numbered fatty acids. The effect of the rumen on modification of the fatty acids is shown in Table 10.2. Major fatty acids in the diet and lymph triglycerides for pigs, sheep fed a hay and oats diet and sheep infused with maize oil via a duodenal fistula. Table 10.2. where there is little relationship between the fatty acid profile of dietary components and the lymph in the sheep. However in an animal fed through a duodenal fistula, or a monogastric, the fatty acid profile in the lymph largely reflects the profile in the dietary components.