Understanding Lipids and Fats

• 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 are soluble in organic solvents like benzene, ether, chloroform, carbon tetrachloride, and acetone.

Lipid Classification

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

Types of Fat

• Unsaturated fat is considered "good fat" and can help to lower blood cholesterol.
• Sources of unsaturated fats include fish, nuts, seeds, plant oils (avocados, olives, walnuts), and liquid vegetable oils (soybean, corn, safflower, canola, and olive oil).
• Saturated fat is considered "bad fat" and can raise cholesterol levels.
• Sources of saturated fats include beef, beef fat, veal, lamb, pork, lard, butter, cream, milk, cheeses, and other dairy products made from whole and 2 percent milk.
• Trans fat is considered "very bad fat," raising cholesterol and increasing the risk of heart disease.
• Sources of trans fats include vegetable shortenings, some margarines, crackers, cookies, and snack foods.

Fats and Oils

• Fats are typically solid at room temperature (25°C), while oils are liquid at the same temperature.
• The difference in melting points reflects the degree of unsaturation of the constituent fatty acids.
• Lipids from animal sources are usually solids (fats), while oils are typically of plant origin. Animal nutrition generally refers to both as fats.
• Fats and oils consist of fatty acids + one molecule of glycerol to form mono-, di-, or triglycerides.
• Fatty acids are chains of carbon atoms (2-24 or more in length) with a carboxyl group at one end.
• Saturated fatty acids have all available positions on carbon atoms taken up by hydrogen.
• Unsaturated fatty acids have one or more double bonds.
• Fats and oils have high energy value because they contain the elements C, H, and O but are higher in C and H than carbohydrates.
• 1 gram of typical fat yields about 9.45 Kcal of heat when completely combusted, versus 4.15 Kcal for a typical carbohydrate.
• Oils in plant seeds are generally triglycerides.
• Lipids in forages are often diglycerides with a galactose molecule attached to glycerol, known as "galactolipids".
• Branched-chain fatty acids (FAs) and those with an odd number of carbon atoms are produced by microorganisms, particularly in ruminant animals.
• Diets for farm animals, especially poultry, require a small amount of fat for transport, absorption of fat-soluble vitamins, as a source of essential fatty acids, and for increasing energy concentration.
• Examples of added fats include tallow, soybean oil, and byproducts of edible oil refining.
• The fat supplement % in the diet are: poultry 5-10%, adult ruminant 3-5% ("more in dairy cattle, 6-8%"), and 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.
• Young animals receiving high levels of milk or milk replacer may get 25-40% fat in their diet (on a dry matter basis).
• Ruminants on dry feed are less tolerant of high fat levels than monogastrics, with a maximum level of 6-8%.
• Adverse effects of high lipid intake (above 10%) in ruminants include reducing rumen microbe activity and retarding carbohydrate fermentation, which can be overcome by feeding "protected fats".
• Young animals can tolerate a higher fat intake (%25-40) in their diet because that's the amount of nutrition they receive from milk.
• Ruminants consuming drier foods can not tolerate high fat levels since the fat reduces fermentation of carbs, limiting microbe activities.

Protected Fats

• Protecting fats involves encapsulating small droplets of oils in a thin layer of formaldehyde-treated protein (casein).
• This prevents attack by rumen microorganisms and then the acidity of the abomasum releases fat for digestion and absorption in the small intestine.
• Rumen-active oil kills rumen bacteria, reduces fiber digestion, and produces trans fatty acids, which depreciate milk fat production.
• Rumen-protected fat supplements avoid negative effects on fiber digestion and milk fat-reducing trans fats in the rumen.
• Major groups of rumen-protected fats: Saturated fatty acids (hydrogenated or fractionated high melting point fats) and calcium salts of palm oil.
• Feeding unsaturated fat in protected form raises the degree of unsaturation of plasma lipids, milk, and body fat.
• This is something that 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
• Unsaturated milk has a poor shelf-life.

Omega-3 and Omega-6 Fatty Acids

• Recent evidence supports the protective effect of fish oils against atherogenic processes in humans due to their high content of polyunsaturated FAs (Omega-3 FAs).
• Examples: linolenic acid (C18:3), ecosapentaenoic acid (EPA) (C20:5), docosahexaenoic acid (DHA) (C22:6).
• Marine fish have a higher content of omega-3 FAs than freshwater fish.
• The content varies with the fish species, season, water temperature, and food eaten.
• Omega-3 FAs lower total serum lipids and cholesterol, and modulate the conversion of arachidonic acid to prostaglandins and other compounds.
• These two ingredients for laying hens can produce omega-3 FAs-enriched eggs "so-called designer eggs."
• Increasing omega-3 products can be done using fish meal or full-fat linseed (flaxseed).
• Flaxseed is a rich plant source of omega-3 FAs, acceptable in taste and beneficial in diets for avoiding coronary heart diseases.
• Omega-3 FAs are dietary essential for fish.
• Deficiency leads to poor growth, liver and heart lesions, and a shock syndrome.
• Omega-3 and omega-6 FAs are important as essential fatty acids for dogs, preventing and treating dermatitis.

Digestion and Absorption of Fats

• Fat digestion and absorption differ principally from that of carbohydrates or proteins because fats are non-polar and not miscible with water.
• The primary goal of lipid digestion is to arrange lipids into a water-miscible form for absorption through the microvilli of the small intestine.
• In simple-stomached animals, the site of digestion is the small intestine.
• Digestion involves actions of bile to emulsify the fat, thus increasing surface area, and pancreatic lipase which hydrolyzes FAs from the glycerol molecule.
• Some diglycerides are absorbed, but the majority of absorption is as monoglycerides and fatty acids.
• The longer-chain FAs are absorbed by lacteals into the lymph system.
• After the lymph system the FAs enter the bloodstream just before the vena cava vein enters the heart.
• In ruminants, galactose is fermented to volatile FAs absorbed through the rumen walls.
• Fat yields mainly propionic acid, which is absorbed similarly to that from carb fermentation.
• Dietary triglycerides are extensively hydrogenated (saturated).
• Hydrolysis of triglycerides doesn't primarily result in monoglycerides (as in nonruminants)
• Hydrolysis of triglycerides yields glycerol and free FAs
• Bacteria and protozoa (rumen microbes) synthesize FAs for the lipids of their own cells, including branched chains and odd numbers of carbons (13, 15, 17), with 15-carbon acids predominating.
• Microbial cells plus hydrolyzed, hydrogenated lipids pass from the rumen through the omasum to the abomasum.
• The abomasum is "where bacteria and protozoa disintegrate, releasing their lipids".
• The small intestine is "where any triglyceride is hydrolyzed by pancreatic lipase to monoglyceride and FAs, as in nonruminants".
• Then the fat is absorbed in the same manner as in nonruminants.
• The fat absorption into the small intestine is followed by resynthesis of triglycerides and phospholipids in the intestinal mucosa.