Understanding Lipids

• Lipids are organic compounds found in 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.

Classification of Lipids

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

Types of Fat

• Unsaturated fats are considered "good" fats and help lower blood cholesterol.
• Sources of unsaturated fats include fish, nuts, seeds, and oils from plants, avocados, olives, walnuts, and liquid vegetable oils such as soybean, corn, safflower, canola, and sunflower.
• Saturated fats are considered "bad" fats and raise cholesterol.
• Sources of saturated fats include beef, beef fat, veal, lamb, pork, lard, poultry fat, butter, cream, milk, cheeses, and other dairy products made from whole and 2 percent milk.
• Trans fats are considered "very bad" fats, raise cholesterol, and increase the risk of heart disease.
• Sources of trans fats include vegetable shortenings, some margarines, crackers, cookies, and snack foods.

Fats and Oils

• Fats are solid at room temperature (25°C), while oils are liquid at the same temperature.
• The difference of physical state is related to the degree of unsaturation of the constituent fatty acids.
• Lipids from animal sources are usually solids (fats), while oils are generally of plant origin.
• In animal nutrition, both fats and oils are generally referred to as fats.
• Fats and oils are composed of fatty acids "of varying lengths and structures" plus one molecule of glycerol and are referred to as mono-, di-, or triglycerides, depending on the number of fatty acids present.
• Fatty acids consist of chains of carbon atoms ranging from 2-24 or more in length, with a carboxyl group on the end.
• If all available positions on carbon atoms are taken up with hydrogen, the fatty acid is referred to as saturated.
• If one or more double bonds are present, the fatty acid is unsaturated.
• Fats and oils are characterized by their high energy value; like carbohydrates, fats contain the elements C, H, and O, but they are relatively much higher in C and H.
• A 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 in forages, a high percentage of the lipids are diglycerides with a molecule of galactose attached to a glycerol molecule "Galactolipids".
• Branched-chain fatty acids and those with an odd number of carbon atoms are often produced by microorganisms.
• In ruminants, body fats may contain substantial amounts of such fatty acids.
• C 2:0 corresponds to acetic acid, "C 2" meaning that there are two carbon atoms and "0" means that no double bonds are present.
• Some examples of saturated fatty 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).
• Some examples of unsaturated fatty acids listed are palmitoleic (C 16:1), oleic (C 18:1), linoleic (C 18:2), linolenic (C 18:3), and arachidonic (C 20:4).
• Every practical diet for farm animals, particularly poultry, contains a small amount of fats.
• Fats are necessary as a vehicle for transport and absorption of fat-soluble vitamins and also as a source of essential fatty acids for increasing energy concentration of the diets.
• Examples of fats used in animal diets are tallow, soybean oil, and byproducts of edible oil refining.
• Fat supplementation in the diet - Poultry: 5-10%; Adult ruminant: 3-5% ("more in dairy cattle, 6-8%"); Non-ruminant: 10-15%.
• Too much fat in the ration can reduce feed intake, increase the chances of scouring, and render calcium and magnesium insoluble (insoluble soaps), leading to them being passed out in feces.
• Young animals receiving a high level of milk or milk replacer in their diet will get 25-40% fat in their diet on a dry matter basis, which is a desirable level of fat intake for animals in this age.
• Ruminants on dry feed are less tolerant of high fat levels than are monogastrics and should not receive more than 6-8% fat.
• Adverse effects of high lipid intake (above 10%) in ruminants include a reduction in rumen microbe activity and retarded 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).
• The droplets avoid attack by microorganisms during passage through the rumen.
• The fat is released by the acidity of the abomasum and thus becomes available for digestion and absorption from the small intestine, similar to non-ruminants.
• Rumen-active oil kills rumen bacteria, reduces fiber digestion, and produces trans fatty acids - milk fat depression.
• These negative effects on fibre digestion can be reduced using rumen-protected fat supplements.
• These supplements avoid milk fat-reducing trans fat in rumen.
• Major groups of rumen-protected fats are saturated fatty acids like hydrogenated/fractionated fats, which have high melting points.
• Calcium salts of palm oil, like that of Megalac, are also widely used and proven.
• Feeding unsaturated fat in protected form results in a rise in the degree of unsaturation of plasma lipids and the milk and body fat composition.
• The use of rumen-protected unsaturated fats is promoted in the context of prevention of human heart diseases and atherosclerosis.
• The connection between human diseases and saturated fat is considered controversial.
• It is noted that 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), ecosapentaenoic acid (EPA) (C 20:5), and docosahexaenoic acid (DHA) (C 22:6).
• Marine fish have a higher content of omega-3 fatty acids than freshwater fish.
• The content varies with the species, season of the year, water temperature, and the food eaten by the fish.
• Omega-3 fatty acids lower total serum lipids and cholesterol and also modulate the conversion of arachidonic acid to prostaglandins.
• Feeding laying hens with two specific feed ingredients can produce omega-3 fatty acid-enriched eggs, often called "designer eggs".
• Increasing omega-3 products can be achieved by using fish meal or full-fat linseed, with linseed being one of the richest plant sources of omega-3 fatty acids, such as linolenic acid, which are acceptable in taste to consumers and have value in healthful diets for avoiding coronary heart diseases.
• Omega-3 fatty acids are dietary essentials for fish with its deficiency leading to poor growth, liver and heart lesions, and a shock syndrome.
• For dogs, omega-3 and omega-6 fatty acids are especially important as essential fatty acids, preventing and treating dermatitis.

Digestion and Absorption of Fats

• Fat digestion and absorption differ principally from that for carbohydrates or proteins since fats are non-polar and not miscible with water.
• The primary object of lipid digestion is to arrange lipids in a water-miscible form that can be absorbed through the microvilli of the small intestine.
• In simple-stomached animals, fat digestion primarily occurs in the small intestine.
• Digestion occurs through the action of bile, which emulsifies the fat.
• Production of oil-in-water emulsion increases its surface area.
• Further aid is provided by pancreatic lipase, an enzyme hydrolyzing fatty acids from the glycerol molecule.
• The contribution of gastric lipase is considered to be quite low.
• Some diglycerides are absorbed but the majority of absorption is as monoglycerides and fatty acids.
• Longer-chain fatty acids are absorbed by lacteals into the lymph system, which then enters the blood stream just before the vena cava vein enters the heart.
• In ruminants, yields of propionic acid as a result of a rumen process are absorbed like material from carbohydrate fermentation.
• Unlike monogastrics, this process doesn't primarily result in monoglycerides, but rather yields glycerol and free FAs.
• Bacteria and protozoa (rumen microbes) also synthesize FAs for the lipids of their own cells containing branched chains and odd numbers.
• These lipids, containing branched chains and odd numbers, pass from rumen through omasum to the abomasum "where bact. and protozoa disintegrate, releasing their lipids".
• All products then pass to the small intestine, "where any triglyceride is hydrolyzed by pancreatic lipase to monoglyceride and FAs, as in nonruminant" then are absorbed similarly to non-ruminants.
• After, resynthesis of triglycerides and phospholipids occurs in the intestinal mucosa.