Lipids: Classification and Types of Fat

• Lipids are organic compounds found in plants and animals
• Lipids include all substances extractable from biological materials with typical fat solvents
• Lipids are insoluble in water but 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 and compound lipids
• Simple lipids include fats & 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 "good" fats that help lower blood cholesterol
• Sources of unsaturated fats include fish, nuts, seeds, and oils from plants, avocados, olives, walnuts, and liquid vegetable oils like soybean, corn, and sunflower
• Saturated fats are "bad" fats that raise cholesterol
• Sources of saturated fats include beef, beef fat, veal, lamb, pork, lard, poultry fat, butter, cream, milk, cheeses, and other dairy products
• Trans fats are "very bad" fats that 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
• Lipids from animal sources are usually solids (fats), and those from plant origins are usually liquids (oils)
• In animal nutrition, both fats and oils are generally referred to as fats
• Fats and oils are composed of fatty acids and one molecule of glycerol (mono-, di-, triglycerides)
• Fatty acids consist of chains of carbon atoms (2-24 or more) with a carboxyl group on the end
• If all available positions on carbon atoms are occupied by hydrogen, it is a saturated fatty acid; otherwise, it is unsaturated
• Fats contain the elements carbon, hydrogen, and oxygen, but are much higher in carbon and hydrogen
• 1 gram of typical fat yields about 9.45 Kcal of heat when completely combusted, compared to about 4.15 Kcal for a typical carbohydrate
• Oils found in plant seeds are generally triglycerides, while forages contain a high percentage of diglycerides with a galactose molecule attached to glycerol (galactolipids)
• Branched-chain fatty acids and those with an odd number of carbon atoms are often produced by microorganisms
• Ruminant animals' body fats may contain substantial amounts of such fatty acids
• Abbreviated designation for fatty acids indicates carbon atoms and double bonds
• C 2:0 means contains 2 Carbon atoms and no double bonds
• Examples of 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), lignoceric (C 24:0)
• Examples of unsaturated acids are palmitoleic (C 16:1), oleic (C 18:1), linoleic (C 18:2), linolenic (C 18:3), arachidonic (C 20:4)
• Practical diets for farm animals, particularly poultry, need small amounts of fats for transport and fat-soluble vitamin absorption; source of essential fatty acids; increasing energy concentration
• Common fat sources are tallow, soybean oil, and byproducts of edible oil refining.

Fat Supplement in Diet

• Fat supplement percentages in animal diets vary
• Poultry: 5-10%
• Adult ruminant: 3-5% (6-8% for dairy cattle)
• Non-ruminant: 10-15%
• Too much fat can reduce feed intake, increase scouring, and render calcium and magnesium insoluble, leading to excretion in feces
• Young animals on high milk diets get 25-40% fat (desirable for their age)
• Ruminants on dry feed are less tolerant of high fat (not more than 6-8%)
• Above 10% lipid intake reduces rumen microbes' activities and retards carbohydrate fermentation and can be overcome by feeding protected fats

Protected Fats

• Fats are protected via encapsulation of small oil droplets in a thin layer of formaldehyde-treated protein (casein)
• Protection avoids microorganism attacks in the rumen
• Acidity in the abomasum releases the fat, which is available for digestion and absorption in the small intestine

Rumen-Active Oil vs Rumen-Protected Fat Supplements

• Rumen-active oil kills rumen bacteria, reduces fiber digestion, and produces trans-fatty acids which leads to milk fat depression
• 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 are saturated fatty acids (e.g. hydrogenated/fractionated) with high melting points
• Major groups of rumen-protected fats are calcium salts of palm oil (Megalac)
• Feeding unsaturated fat in protected form raises unsaturation in plasma lipids, milk, and body fat
• This is beneficial in preventing heart diseases and atherosclerosis
• The connection between saturated fat intake and human diseases is controversial; unsaturated milk has a poor shelf-life

Omega-3 and Omega-6 Fatty Acids

• Fish oils have a protective effect against atherogenesis due to polyunsaturated fatty acids (Omega-3 FAs) such as linolenic acid (C 18:3), eicosapentaenoic acid (EPA) (C 20:5), and docosahexaenoic acid (DHA) (C 22:6)
• Marine fish have higher omega-3 content than freshwater fish
• The content varies with species, season, water temperature, and the fish's diet
• Omega-3 FAs lower total serum lipids and cholesterol while modulating arachidonic acid conversion to prostaglandins
• Feeding laying hens two feed ingredients can produce omega-3 FAs-enriched eggs ("designer eggs")
• Omega-3 products can be increased using fish meal or full-fat linseed, a primary plant source of omega-3 FAs such as linolenic acid acceptable to consumers and healthy for avoiding disease
• Omega-3 FAs are dietary essentials for fish
• Omega-3 deficiency in fish leads to poor growth, liver and heart lesions, and shock syndrome
• For dogs, omega-3 and omega-6 FAs are essential for preventing and treating dermatitis

Digestion and Absorption of Fats

• Fat digestion and absorption differ from carbs or protein due to the non-polar, water-immiscible nature of fats
• Lipid digestion arranges lipids into a water-miscible form for absorption through small intestine microvilli

Fat Digestion in Simple-Stomached Animals

• Site: small intestine
• Digestion occurs through the action of bile (emulsifies fat, increasing surface area) and pancreatic lipase (hydrolyzes FAs from glycerol)
• Gastric lipase exists, but it is rather low
• Some diglycerides are absorbed, but the majority of absorption as monoglycerides and fatty acids
• The majority of long-chain FAs are absorbed by lacteals into the lymph system, entering the bloodstream near the vena cava vein

Fat Digestion in Ruminants

• Galactose is fermented to volatile FAs (absorbed through rumen walls)
• Fats hydrogenated (saturated)
• Yields mainly propionic acid, which is absorbed
• Glycerol and free FAs are yielded
• Bacteria and protozoa (rumen microbes) synthesize FAs for their lipids using acids with branched chains and odd numbers (13, 15, and 17 carbons)
• Microbial cells + hydrolyzed, hydrogenated lipids
• Lipids pass from rumen to abomasum, and protozoa disintegrate
• In the small intestine, triglycerides are hydrolyzed by pancreatic lipase, and monoglycerides and FAs are absorbed as in non-ruminants
• Triglycerides and phospholipids are resynthesized in the intestinal mucosa