Lipids: Classification, Fats, and Oils

• Lipids are organic compounds found in both plants and animals.
• Lipids include substances extractable from biological materials with fat solvents.
• Lipids are insoluble in water but soluble in organic solvents such as benzene, ether, chloroform, carbon tetrachloride, and acetone.

Lipid Classification

• Lipids may be classified as saponifiable and non-saponifiable.
• Saponifiable lipids include simple and compound lipids.
• Simple lipids are fats, oils, and waxes.
• Compound lipids are phospholipids, sphingolipids, glycolipids, and lipoproteins.
• Non-saponifiable lipids are steroids, prostaglandins, fat-soluble vitamins, and trepenes.

Fats and Oils

• Fats are solid at room temperature (25°C), while oils are liquid at the same temperature.
• The difference between fats and oils relates to the degree of unsaturation of the constituent fatty acids.
• Lipids from animal sources are typically solids (fats), whereas those from plants are usually liquids (oils).
• Fats and oils consist of fatty acids and a molecule of glycerol, and are referred to as mono-, di-, or triglycerides.
• Fatty acids are chains of carbon atoms ranging from 2 to 24 or more in length with a carboxyl group on the end.
• A saturated fatty acid has all available positions on carbon atoms taken up by hydrogen.
• An unsaturated fatty acid has one or more double bonds present.

Energy Value of Fats and Oils

• Fats and oils are characterized by their high energy value.
• Fats contain the elements carbon, hydrogen, and oxygen, but are relatively much higher in carbon and hydrogen.
• Fat contains 77% Carbon, 12% hydrogen, and 11% oxygen.
• Starch contains 44% Carbon, 6% hydrogen, and 50% oxygen.
• 1 gram of typical fat yields about 9.45 Kcal of heat when completely combusted, compared with about 4.15 Kcal for a typical carbohydrate.
• Oils found in plant seeds are generally triglycerides, while lipids in forages are high in diglycerides with a galactose molecule attached to glycerol, known as galactolipids.
• Branched-chain fatty acids and those with an odd number of carbon atoms are often produced by microorganisms, and body fats in ruminant animals may contain substantial amounts of such fatty acids.

Fatty Acids

• For the designation C 2:0 of acetic acid, 'C 2' means it contains two carbon atoms and '0' means there are no double bonds present
• Common saturated acids are: Acetic (C 2:0), Propionic (C 3:0), Butyric (C 4:0), Caproic (C 6:0), Myristic (C 14:0), Palmitic (C 16:0), Stearic (C 18:0), Arachidic (C 20:0), and Lignoceric (C 24:0).
• Common unsaturated acids are: Palmitoleic (C 16:1), Oleic (C 18:1), Linoleic (C 18:2), Linolenic (C 18:3), and Arachidonic (C 20:4).

Dietary Fats in Farm Animals

• Every practical diet for farm animals (particularly poultry) contains small amounts of fats.
• Dietary fat acts as a vehicle for the transport and absorption of fat-soluble vitamins.
• Dietary fat is a source of essential fatty acids
• Dietary fat increases energy concentration of diets.
• Examples of fats used are tallow, soybean oil, and byproducts of edible oil refining.
• The percentage of fat supplement in the diet varies among animals: Poultry (5-10%), Adult Ruminant (3-5%, more in dairy cattle, 6-8%), Non-Ruminant (10-15%).
• Too much fat in ration can reduce feed intake, increase the chances of scouring, and render Calcium and Magnesium insoluble potentially leading to excretion in feces.

Fat Intake in Animals

• Young animals receiving a high level of milk or milk replacer in their diet may get 25-40% fat.
• Ruminants on dry feed are less tolerant of high fat levels compared to monogastrics.
• Adverse effects of high lipid intake (above 10%) in ruminants include reduction of rumen microbes activities and fermentation of carbohydrates, which can be overcome by feeding "protected fats".

Protected Fats

• Protecting fats involves encapsulating small droplets of oils in a thin layer of formaldehyde-treated protein (casein).
• This protects the droplets from attack by microorganisms during passage through the rumen.
• The fat is released by the acidity of the abomasum, making it available for digestion and absorption from the small intestine.
• Rumen-active oil kills rumen bacteria, reduces fiber digestion, and produces trans fatty acids, leading to milk fat depression.

Rumen Protected Fats

• Rumen-protected fat supplements avoid negative effects on fiber digestion in the rumen and avoid milk fat-reducing trans fats in the rumen.
• Major groups of rumen-protected fats include saturated fatty acids like hydrogenated/fractionated ones, and calcium salts of palm oil (Megalac).
• Feeding unsaturated fat in protected form results in a prompt rise in the degree of unsaturation of plasma lipids, milk, and body fat.
• Feeding unsaturated fat in protected form has been promoted in the context of prevention of human heart diseases and atherosclerosis.
• The connection between human diseases and saturated fat intake is controversial, and the shelf life of unsaturated milk is poor.

Omega-3 and Omega-6 Fatty Acids

• Recent evidence shows a protective effect of fish oils against atherogenic processes in humans due to their high content of polyunsaturated fatty acids (Omega-3 FAs)
• Examples of omega-3 fatty acids are linolenic acid (C 18:3), Eicosapentaenoic acid (EPA) (C 20:5), and Docosahexaenoic acid (DHA) (C 22:6).
• Marine fish have a higher content of omega-3 FAs than fresh water fish.
• The content varies with species, season of the year, water temperature, and the food eaten by the fish.
• Omega-3 FAs lower total serum lipids and cholesterol, and also modulate the conversion of arachidonic acid to prostaglandins and other compounds.
• Feeding laying hens with specific feed ingredients can produce omega-3 FAs-enriched eggs ("so-called designer eggs").
• Fish meal or full-fat linseed can increase omega-3 products content, as linseed is one of the richest plant sources of omega-3 FAs, such as linolenic acid.
• Omega-3 FAs are dietary essential for fish.
• Omega-3 deficiency in fish can leads to poor growth, liver and heart lesions, and a shock syndrome.
• For dogs, omega-3 and omega-6 FAs are important as essential fatty acids that prevent and treat dermatitis.

Digestion and Absorption of Fats

• Fat digestion and absorption differ from that for carbohydrates or protein because fats are non-polar and not miscible with water.
• The primary goal of lipid digestion is to arrange the lipid in a form that is water miscible and can be absorbed through the microvilli of the small intestine.

Fat Digestion in Simple-Stomached Animals :

• Takes place in the small intestine.
• Digestion occurs through the action of bile, which emulsifies the fat, increasing the surface area.
• Digestion occurs through pancreatic lipase, an enzyme which hydrolyzes FAs from the glycerol molecule.
• Gastric lipase activity is low.
• Some diglycerides are absorbed, but the majority of absorption is as monoglycerides and fatty acids.
• The majority of the longer-chain FAs are absorbed by lacteals into the lymph system and enter the bloodstream just before the vena cava vein enters the heart.

Fat Digestion in Ruminants

• The digestion of lipids yields mainly propionic acid, which is absorbed similar to carbohydrates.
• Lipids are extensively hydrogenated (saturated) in the rumen.
• This does not primarily result in monoglycerides (as in non-ruminants), but rather yields glycerol and free FAs.
• Bacteria and protozoa (rumen microbes) synthesize FAs for the lipids of their own cells, including acids with branched chains and odd numbers such as 13, 15, and 17 carbons.
• Microbial cells, combined with hydrolyzed, hydrogenated lipids, pass from the rumen through the omasum to the abomasum.
• In the abomasum, bacteria and protozoa disintegrate, releasing their lipids.
• All these products pass to the small intestine where any triglyceride is hydrolyzed by pancreatic lipase to monoglyceride and FAs.
• Digestion occurs in same manner as in nonruminants then absorbed.
• In the intestinal mucosa of ruminants, resynthesis of triglycerides and phospholipids occurs.