Fats and Lipids

FATS AND LIPIDS Lipid Structures and Functions Fats and lipids constitute approximately 34% of the energy in the human diet. Because fat is energy rich and provides 9 kcal/g of energy, humans are able to obtain adequate energy with a reasonable daily consumption of fat-containing foods. Dietary fat is stored in adipose cells. The ability to store and use large amounts of fat enables humans to survive without food for weeks and sometimes months. Some fat deposits are not used effectively during a fast; they are classified as structural fat. Structural fat pads hold the body organs and nerves in position, protecting them against injury and shock. • Fat pads on the palms and buttocks protect the bones from mechanical pressure. A subcutaneous layer of fat insulates the body, preserving body heat and maintaining body temperature. Dietary fat is essential for the digestion, absorption, and transport of the fat-soluble vitamins and phytochemicals such as carotenoids and lycopenes. Dietary fat depresses gastric secretions, slows gastric emptying, and stimulates biliary and pancreatic flow, thereby facilitating digestion. Fat also conveys important textural properties to foods such as ice creams (smoothness) and baked goods (tenderness caused by "shortening" strands of gluten). lists the fat content of some common foods. Unlike carbohydrates, lipids are not polymers; they are small molecules extracted from animal and plant tissues. Lipids are composed of a heterogeneous group of compounds characterized by their insolubility in water; they are classified into three major groups . Fatty Acids Fatty acids are rarely free in nature and almost always are linked to other molecules by their hydrophilic carboxylic acid head group. Fatty acids occur primarily as unbranched hydrocarbon chains with an even number of carbons and are classified according to the number of carbons, the number of double bonds, and the position of the double bonds inthe chain. Chain length and extent of saturation • contribute to the melting temperature of a fat. In general, fats with shorter fatty acid chains or more double bonds are liquid at room temperature. Saturated fats, especially those with long chains, are solid at room temperature. A fat such as coconut oil, which is also highly saturated, is semiliquid at room temperature because the predominant fatty acids are short (8 to 14 carbons). In general, SCFAs are considered to have 4 to 6 carbons, medium-chain fatty acids to have 8 to14, and long-chain fatty acids (LCFAs), to have 16 to 20 or more. In a saturated fatty acid (SFA), all carbon binding sites not linked to another carbon are linked to hydrogen and are therefore saturated. There are no double bonds between carbons. Monounsaturated fatty acids (MFAs) contain only one double bond, and polyunsaturated fatty acids (PUFAs) contain two or more double bonds. In MFAs and PUFAs one or more pairs of hydrogen have been removed, and double bonds form between adjacent carbons. Because fatty acids with double bonds are vulnerable to oxidative damage, humans and other warm-blooded organisms store fat predominantly as saturated palmitic fatty acid (CI6:0) and stearic fatty acid Cell membranes must be stable and flexible. To achieve this requirement, membrane phospholipids contain one SFA and one highly PUFA, the most abundant of which is arachidonic acid (C20:4). Two notation conventions are used to describe the location of the double bonds . In omega notation,a lower case omega (ω) or n is used to refer to the placement of the first double bond counting from the methyl end (the fatty acid's omega number). Thus arachidonic acid (20:4 ω -6 or 20:4 ω -6), the major highly polyunsaturated fat in membranes of land animals, is an omega-6 (ω 6) fatty acid. It has 20 carbons and four double bonds, the first of which is six carbons from the terminal methyl group. Eicosapentaenoic acid (EPA) (20:5 ω -3 or 20:5 ω -3) is found in marine organisms and is an omega-3 (ω-3) fatty acid. It has five double bonds, the first of which is three carbons from the terminal methyl group. Sources of the longer EPA and docosahexaenoic acid (DHA) ω -3 fatty acids are primarily marine, such as cod liver oil, mackerel, salmon, and sardines Essential Fatty Acids and the Omega-6/0mega-3 Ratio Only plants (including marine phytoplankton) can synthesize ω -6 and ω -3 fatty acids. Humans and other animals can only place double bonds as low as the ω -9 carbon and cannot produce their own ω -6 and ω -3 fatty acids. But humans can desaturate and elongate linoleic acid (18:2 n-6) to arachidonic acid (20:4 n-6) and alpha-linoleic acid (ALA) (CI8:3 ro-3) to EPA (C20:5 ω -3) and DHA (C22:6 ω -3). Therefore both linoleic (18:2 n-6) and ALA (CI8:3 ω -3) acids are essential in the diet. They can alter the size and permeability of the blood vessels, alter the activity of platelets and contribute to blood clotting, and modify the processes of inflammation. Derivatives of n-3 fatty acids from dietary sources or fish oil can have beneficial effects in a number of disease, including improved brain function during aging. Roles for ω -3 fatty acids are discussed related to cardiovascular disease, arthritis and inflammatory conditions, and neurologic disorders. An imbalance between dietary ω -3 and ω -6 fatty acids contributes to a wide range of disease states. Excess amounts of ω -6 fatty acids in the diet prevents conversion of ALA into EPA and DHA. The optimal ω-6/ω-3 ratio has been estimated to be 2: 1 to 3: 1, four times lower than the current intake; therefore it is recommended that humans consume more ω -3 fatty acids from vegetable and marine sources. ALA is found in flaxseed (57%), canola oil (8%), soybean oil (7%), and green leaves in a few plants such as purslane. Trans-Fatty Acids In natural unsaturated fatty acids, the two carbons participating in a double bond each bind a hydrogen on the same side of the bond (the cis-isomer form), causing the fatty acid to bend. The more double bonds per fatty acid, the more bends in the molecule. Hydrogenation of unsaturated fatty acids is a chemical process that adds hydrogen to oils to form a stable, solid fat such as margarine. Membrane function depends on the three-dimensional configuration of membrane fatty acids found in phospholipids. The cis double bonds in the membrane bend, allowing the fatty acids to pack loosely, thus making the membrane fluid. Because proteins embedded in a membrane float or sink, depending on the membrane's fluidity, membrane viscosity is important for membrane protein function. Major sources of trans-fatty acids in the American diet are chemically hydrogenated margarine,shortening,commercial frying fats, high-fat baked goods, and salty snacks containing these fats. Butter and animal fat can also contain trans-fatty acids from bacterial fermentation in the rumen of cows and sheep. Higher intakes of trans-fatty acids are associated with increased risk for coronary heart disease, cancer, type 2 diabetes, and allergies, possibly because of their potential to influence membrane fluidity. Triglycerides The body forms triglycerides (triacylglycerols TAG) by joining three fatty acids to a glycerol side chain, thereby neutralizing reactive fatty acids and making triglycerides water insoluble (hydrophobic). Storage triglycerides from land animals are predominately saturated because SFAs are relatively inert and not susceptible to oxidative damage during storage. Cold-water creatures must maintain their fatty acids in liquid form even at low temperatures; therefore triglycerides in fish oils and marinederived fats contain longer (C20 and C22), highly unsaturated fatty acids Phospholipids Phospholipids are derivatives of phosphatidic acid, a triglyceride modified to contain a phosphate group at the third position . Phosphatidic acid is esterified into a nitrogen-containing molecule, usually choline, serine, inositol, or ethanolamine, and named for its nitrogenous base (e.g., phosphatidylcholine, phosphatidylserine). Membrane phospholipids usually contain one SFA (C16 to C18) at C-I and a highly PUFA (C16 to C20) at C-2, usually one of the essential fatty acids. ALA (CI8:3 ω-3), arachidonic acid (C20:4 ω-o) , and ω -3 substitutes can be cleaved from the lipid bilayer to provide substrate for synthesis of prostaglandins and other mediators of cell activity. Lecithin (phosphatidylcholine) is a major phospholipid, and it is the primary component of lipid in the membrane lipid bilayer. Lecithin is made by the body with arachidonic acid. Because all cells contain lecithin as a lipid bilayer component, animal products, especially liver and egg yolks, are rich sources of lecithin. Plant products such as soybeans, peanuts, legumes, spinach, and wheat germ are also rich sources. Lecithin is widely distributed in the food supply and is added to food products such as margarine, ice cream, snack crackers, and confections as a stabilizer. Sphingolipids, Alcohols, Waxes, Isoprenoids, and Steroids Sphingomyelin includes the nitrogenous base choline and makes up more than 25% of the myelin sheath, the lipid-rich structure that protects and insulates cells of the central nervous system. Sphingomyelin includes the nitrogenous base choline and makes up more than 25% of the myelin sheath, the lipid-rich structure that protects and insulates cells of the central nervous system. In addition to phosphatidylcholine, sphingomyelin is found in all membranes. Isoprenoids, activated derivatives of isoprene, are an extraordinarily large and diverse group of lipids built from one or more five-carbon units. Isoprene contains alternating single and double (conjugated) bonds, an arrangement that can quench free radicals by accepting or donating electrons. Terpene is a generic term for all compounds synthesized from isoprene precursors and include essential oils of plants (e.g.,turpentine from trees and limonene from lemons). Plant pigments that transfer electrons in photosynthesis are also isoprenoids and include lycopene (the red pigment in tomatoes), carotenoids (the yellow and orange pigments in squash and carrots), and the yellow and green chlorophyll group. Fat-soluble vitamins A, D, E, and K and the electron transducer coenzyme Q have isoprenoid structures. Vitamin E, lycopene, and β-carotene are effective antioxidants; nonnutritive phytochemicals with antioxidant function also have an isoprenoid structure. Steroids constitute a class of lipids derived from a four membered saturated ring. Cholesterol is the basis for all steroid derivatives made in the body, including glucocorticoids (cortisone) and mineralocorticoids (aldosterone), which are made in the adrenal gland; androgens (testosterone) and estrogens (estradiol), which are made in the testes and ovaries, respectively; and bile acids, which are made in the liver. Vitamin D hormone is made when ultraviolet rays from the sun cleave cholesterol in subcutaneous fat to form cholecalciferol (D3). Synthetic vitamin D is made by irradiating the plant steroid ergosterol to form ergocalciferol (D2). Structurally both compounds are components of nerve tissue and cell membranes, where they playa role in lipid transport. Medium-chain triglycerides (MCTs) are SFAs with a chain length of between 6 and 12 carbons. Although MCTs occur naturally in milk fat, coconut oil, and palm kernel oil, they are also produced commercially as a by-product of margarine production. MCT oils provide 8.25 kcal/g and are of value in a number of clinical situations because they are short enough to be water soluble, require less bile salt for solubilization, are not reesterified in the enterocyte, and are transported as free fatty acids, bound to albumin, through the portal system. Because the portal blood flow rate is approximately 250 times faster than the lymph flow, MCTs are digested quickly and not likely to be affected by intestinal factors that inhibit fat absorption. They are not stored in adipose tissue but are oxidized to acetic acid. Fat replacers are structurally different from fats and do not provide readily absorbable nutrients. Their commercial importance is that they imitate the texture and other sensations of fat, especially in the mouth. Fat replacers differ in their macronutrient base and the extent to which they mimic the characteristics of fat. The caloric value of these substitutes varies between 5 kcal/g (e.g., caprenin) and 0 kcal/g (e.g., olestra, carrageenan). Most fat replacers are derived from plant polysaccharides such as gums, cellulose, dextrins, fiber, maltodextrins, starches, and polydextrose. The product has the physical properties of natural dietary fats. Because they are nonabsorbable and sucrose polyesters do not contribute calories to the diet. Protein-based fat replacers alter the texture of a product in various ways. However, under most circumstances they appear to be safe, effective, and feasible alternatives for controlling fat and energy in diets.