LIPID METABOLISM

UMAR HAYAT

TYPES OF LIPIDS• Lipids are water insoluble, soluble in organic solvents (ether, chloroform,

hexane, etc.). Concentrated source of energy in feed. Usually, the diet of cows contains only 2 to 4% lipids. Important for dairy cows, directly contribute 50% to milk fat.

• Oilseeds contain more than 20% lipids. Oil from oilseeds can be hsed unextracted in diets of cows. Only small amounts of lipids are found in forage and seed.

• 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.

• Glycolipids form a second class of lipids found in forage. Structure similar to the triglycerides except that one of the three fatty acid has been replaced by a sugar (usually galactose).

TYPES OF LIPIDS• When one of the fatty acids is replaced by a phosphate bound

to another complex structure the lipid is referred to as phospholipid. Phospholipids generally found in ruminal bacteria.

• Melting point is influenced 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). Common fatty acids found in plant lipids range from 14 to 18 carbons.

• Animal fats contain 40 to 50% saturated fatty acids and remain in the solid state (fats). The degree of unsaturation affects digestion by animal.

• It interferes with the fermentation of carbohydrates in the rumen.

Common fatty acids in dairy cows diet

Common Name Structure Abbreviation Melting point (°C................................................ Saturated acids ......................................................Myristic CH3-(CH2)12-COOH (C14:0) 54Palmitic CH3-(CH2)14-COOH (C16:0) 63Stearic CH3-(CH2)16-COOH (C18:0) 70.............................................. Unsaturated acids ............................Palmitoleic CH3-(CH2)5-CH=CH-(CH2)7-COOH (C16:1) 61Oleic CH3-(CH2)7-CH=CH-(CH2)7-COOH (C18:1) 13Linoleic CH3-(CH2)4-CH=CH-CH2-CH=CH-(CH2)7-COOH (C18:2) - 5Linolenic 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.

HYDROLYSIS AND SATURATION OF LIPIDS INTHE RUMEN

• Lipids hydrolysis in the rumen: bonds between the glycerol and the fatty acids broken down to produce glycerol and three FAs.

• Glycerol fermentation to VFAs. Some FAs used by bacteria for the synthesis of phospholipids for build cell membranes.

• Ruminal microbes hydrogenate unsaturated fatty acids. Fatty acid becomes saturated.

LIPIDS IN THE RUMEN

• Excess lipids in the diet (> 8%) have negative effect on milk yield and fat % in the milk. Unsaturated have more negative effect than saturated.

• FFAs in rumen attach to feed /microbial particles and impede fibre fermentation.

• Protected Lipids slow down hydrolysis, make them "inert“ . The seed coat protects lipids, less ruminal hydrolysis.

• Industrial treatments involve the formation of soaps (calcium salts) make FAs insoluble and inert in the rumen.

• Microbial phospholipids 10 to 15% of the lipids leaving the rumen, Saturated FAs 85 to 90% (bound to feed and microbial particles).

• Microbial phospholipids in the small intestine make fatty acids pool and mix with bile and pancreatic juice (enzymes +bicarbonate.

INTESTINAL ABSORPTION OF LIPIDS

• 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, fatty acids bound to glycerol to form triglycerides. Triglycerides +free fatty acids+ cholesterol coated with protein form triglyceride-rich lipoproteins (TG-rich LP) called chylomicrons / very low density lipoproteins.

• The TG-rich LP enter lymph vessels and flow to the thoracic duct , 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.

UTILIZATION OF DIETARY LIPIDS BY THE UDDER

• About 50% of milk fat derived from the uptake of fatty acids by the mammary gland. Triglyceride-rich lipoproteins during the intestinal absorption of lipids provide the FAs.

• More long chain fatty acids (LCFAs, C > 16) in diet result in their more secretion in milk, but less synthesis of short- and medium-chain FAs 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 FATMOBILIZATION

o During under feeding or early lactation period, fat mobilized from adipose tissues to obtain energy in addition to dietary fat. Fatty acids from the triglycerides stored in the adipose tissues released into the blood.

o Mobilized fatty acids taken up by the liver , used as energy source or converted to ketones, released in the blood and used as energy source by many tissues.

o 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.

o 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

o Lipids are "cold" nutrients, produce less heat in the body.

o Several potential benefits: – Increase energy density of the ration, in high

forage diet;– Limit need for carbohydrate-rich concentrates

required in early lactation;– Help to reduce the heat stress of a lactating cow.

Animal Response

• Variable feed intake & milk production responses according to the type of lipids.

• Do not feed ore than about 1.5 kg/day of added fat to dairy cows. Milk production is maximized when lipids comprise 5% of the dietary dry matter

• • Negative effects if fed more than 6%. Decreases

milk protein by about 0.1%. In addition, may depress feed intake, milk production and milk fat composition.