Babcock Institute for International Dairy Research and Development Dairy Essentials University of Wisconsin-Madison 4) LIPID METABOLISM IN DAIRY COWS Michel A. Wattiaux Babcock Institute Ric R. Grummer Department of Dairy Science structure similar to the triglycerides except that one of the three fatty acid has been replaced by a sugar (usually galactose). When one of the fatty acids is replaced by a phosphate bound to another complex structure the lipid is referred to as phospholipid. Phospholipids are minor components in feedstuffs, but they are found in a high concentration in ruminal bacteria. Common fatty acids found in plant lipids range from 14 to 18 carbons (Table 1). The melting point determines whether a lipid is in a liquid or a solid form at room temperature. Melting point is influenced primarily by the degree of saturation and to a lesser extent by the length of the carbon chain. Plant lipids typically contain 70 to 80% unsaturated fatty acids and they tend to remain in the liquid state (oils). On the other hand, animal fats contain 40 to 50% saturated fatty acids and they tend to remain in the solid state (fats). The degree of unsaturation has a marked effect on how well it is digested by an animal and, in the case of ruminants, whether or not it interferes with the fermentation of Triglycerides carbohydrates in the rumen. TYPES OF LIPIDS Usually, the diet eaten by cows contains only 2 to 4% lipids. However, lipids are an important part of the ration of dairy cows because they contribute directly to about 50% of the fat in milk and they are the most concentrated source of energy in feed. Only small amounts of lipids are found in forage and seed. However, some plants (cotton, soybean) have seeds referred to as oilseeds that contain more than 20% lipids. Lipids are usually extracted from oilseeds which may be used unextracted in diets of cows. Lipids are substances which are water insoluble, but are soluble in organic solvents (ether, chloroform, hexane, etc.). Triglycerides are found primarily in cereal grains, oilseeds and animal fats. The basic structure of triglycerides consist of one unit of glycerol (a 3 carbon sugar) and three units of fatty acids (Figure 1). Glycolipids form a second class of lipids found primarily in forage (grasses and legumes). These compounds have a Fatty Acids Glycerol O H2 C OH H C OH H2 C OH + OH C OH C OH C O R1 H2 C O C R2 H C O C R3 H2 C O C O R1 O O R2 O R3 Figure 1: Basic structure of triglycerides. The radicals (R1, R2, and R3) are made of a carbon chain with variable lengths and degrees of saturation. HYDROLYSIS AND SATURATION OF LIPIDS IN THE RUMEN In the rumen, the majority of the lipids are hydrolyzed. The bonds between the glycerol and the fatty acids are broken down to give rise to glycerol and three fatty acids. 240 Agriculture Hall, 1450 Linden Dr., Madison, WI 53706 USA, phone: 608-265-4169, [email protected] 13 Dairy Essentials – Nutrition and Feeding Table 1: Common fatty acids found in the diet of dairy cows Common Name Structure Melting Abbreviation* point (°C) ................................................ Saturated acids ............................................................. Myristic Palmitic Stearic CH3-(CH2 )12-COOH CH3-(CH2 )14-COOH CH3-(CH2 )16-COOH (C14:0) (C16:0) (C18:0) 54 63 70 .............................................. Unsaturated acids .......................................................... Palmitoleic Oleic Linoleic Linolenic CH3-(CH2 )5-CH=CH-(CH2)7-COOH (C16:1) 61 CH3-(CH2 )7-CH=CH-(CH2)7-COOH (C18:1) 13 CH3-(CH2 )4-CH=CH-CH2 -CH=CH-(CH2)7-COOH (C18:2) -5 CH3-CH2 -CH=CH-CH2 -CH=CH-CH2 -CH=CH-(CH2)7-COOH (C18:3) -11 * The first number denotes the total number of carbons and the second number denotes the number of double bonds in the molecule. Glycerol is fermented rapidly into volatile fatty acids (see carbohydrate metabolism). Some fatty acids are used by bacteria for the synthesis of phospholipids that are needed to build cell membranes. Another important action of ruminal microbes is to hydrogenate unsaturated fatty acids. During hydrogenation, a fatty acid becomes saturated because a double bond is replaced by two hydrogen atoms. For example, hydrogenation converts oleic acid into stearic acid (Table 1). Free fatty acids in the rumen tend to attach to feed and microbial particles and impede normal fermentation, especially of fibrous carbohydrates. Excess lipids in the diet (more that 8%) may have a negative effect on milk production and fat percentage in the milk. Unsaturated lipids have a more negative effect than saturated lipids. However, lipids may be "protected" to slow down the rate of hydrolysis and make them more "inert" in the rumen. The seed coat tends to protect lipids within the seeds and make them less rapidly accessible for ruminal hydrolysis compared to free oil. Also, industrial treatments that usually involve the formation of soaps (calcium salts of fatty acids) make the fatty acids insoluble and thus inert in the rumen. Microbial phospholipids make up 10 to 15% of the lipids leaving the rumen, the remaining 85 to 90% are saturated free fatty acids found primarily in the form of 14 palmitic and stearic acids bound to feed and microbial particles. INTESTINAL ABSORPTION OF LIPIDS Microbial phospholipids are digested in the small intestine and contribute to the pool of fatty acids that are processed and absorbed through the intestinal wall. The bile secreted by the liver and the pancreatic juice (rich in enzymes and bicarbonate) are mixed with the contents of the small intestine. These secretions are essential to prepare the lipids for absorption by forming water miscible particles called micelles that can enter the intestinal cells. In the intestinal cells, a major portion of fatty acids are bound to glycerol (coming from blood glucose) to form triglycerides. Triglycerides, some free fatty acids, cholesterol and other lipid-like substances are coated with protein to form triglyceride-rich lipoproteins (TG-rich LP) also called chylomicrons or very low density lipoproteins. The TG-rich LP enter lymph vessels and flow to the thoracic duct (the junction of the lymphatic system with the blood system) where they enter the blood system. In contrast to most nutrients absorbed from the gastro intestinal tract, the absorbed lipids enter the general circulation directly and are used by all body tissues without a preliminary processing by the liver. The Babcock Institute 4 - Lipid Metabolism in Dairy Cows DIET RUMEN Triglycerides Phospholipids Glycolipids Grains INTESTINE FECES Undigested bacterial Lipids Bacterial Phospholipids Bacterial Phospholipids Saturated Free Fatty acids Saturated Free Fatty acids ;;; ;;;; ;;; ;;;; ;;; ;;;; ;;;;;;; ;; ;;; ; ;;;;;;; ;; ;;;;;;; Forage Sugar RUMINAL WALL VFA INTESTINAL CELLS Triglyceride rich Lipoprotein (TG-rich LP) Free Fatty acids TG-r ich L LYMPH P VESSEL PORTAL BLOOD (To the liver) carbohydrate metabolism Glucose LIVER Ketones Glycerol Fatty Acids Energy Triglyceride storage TG-rich LP TG-rich LP BLOOD (General circulation) Gly Tissue Mobilisation cer Fatty Acids TG-rich LP Glycerol ol Glucose Fatty Acids Energy Glycerol Glycerol Triglycerides ADIPOSE TISSUES Figure 2: Overview of lipid metabolism in dairy cows University of Wisconsin-Madison Glycerol TG-rich LP Glucose Fatty Acids Energy Glycerol Milk Fat (Long chains) MAMMARY GLAND 15 Dairy Essentials – Nutrition and Feeding UTILIZATION OF DIETARY LIPIDS BY THE UDDER About half the fat in the milk is derived from the uptake of fatty acids by the mammary gland. These fatty acids come primarily from the triglyceride-rich lipoproteins formed during the intestinal absorption of lipids. An increase in long chain fatty acids (i.e., acids made of more than 16 carbons) in the diet increases their secretion in milk, but it also inhibits the synthesis of short- and medium-chain fatty acids in the mammary tissue. Thus, the marked depression in fat secretion when cows are fed low fiber diets can be compensated only partially by increasing fat in the diet. THE ROLE OF LIVER AND FAT MOBILIZATION During periods of under feeding or in early lactation, cows meet their energy demand by mobilizing fat from adipose tissues to obtain energy in addition to that provided by the diet. Fatty acids coming from the triglycerides stored in the adipose tissues (located primarily beneath the hide, in the abdomen and over the kidneys) are released into the blood. Mobilized fatty acids are taken up by the liver where they can be used as an energy source or be converted to ketones that may be released in the blood and used as an energy source by many tissues. The liver does not have a high capacity to form and to export TG-rich LP and the excess mobilized fatty acids are stored as triglycerides within the liver cells. 16 The fat deposited in the liver contributes to development of metabolic disorders (e.g., ketosis and fatty liver) in early lactation. ADDED LIPIDS IN DAIRY RATIONS Lipids contain about 2.25 times more energy than carbohydrates. Also, lipids are sometimes referred to as a "cold" nutrients because during digestion and utilization by the body they produce less heat than carbohydrates and proteins. Thus, increasing lipids in dairy cow rations may have several potential benefits: • Increase the caloric (energy) density of the ration, especially when intake may be limited as in high forage diet; • Limit the need for carbohydrate-rich concentrates which are usually required in early lactation when a cow is in negative energy balance; • In hot weather, lipids may help to reduce the heat stress of a lactating cow. Feed intake and milk production responses vary greatly according to the type of lipids added in a diet. Cows should not be fed more than about 1.5 kg/day of lipids in addition to the lipids present in feedstuffs. This amount of lipids translates into a total of about 6 to 8% lipids in the diet before negative effects become evident. Milk production is maximized when lipids comprise 5% of the dietary dry matter. Added dietary fat usually decreases milk protein by about 0.1%. In addition, excess lipids may depress feed intake, milk production and milk fat composition. The Babcock Institute
