Fats as basic fuel for Energy

Fats provide a concentrated storage form of basic fuel for the human energy system. They include substances such as fat, oil and related compounds that are greasy to the touch and insoluble in water. Substances of this class are called lipids. Some basic food fuel forms of fat are easily seen s fat: butter, margarine, oil, salad dressings, bacon, cream. Other food forms of fat are more hidden: egg yolk, meat fats, olives, avocados, nuts, seeds.

Lipids :Group name for organic substances of fatty nature . The lipids include fats, oils, waxes and related compounds.

Dietary Fat :

Many Americans eat a relatively large amount of fat, about 45% of more of the total kilocalories in their diet. However, questions are being raised about the amount and kind of fat we eat in relation to our health.

Food Fats :

• Fuel Source:Food fats supply a basic continuing source of fuel for the body to store and burn as needed for energy. Food fat yields 9 kcal/g.

• Essential nutrient supply:Food fats supply the essential fatty acids, especially linoleic acid, and cholesterol as needed to supplement the body’s endogenous supply.

• Food Satiety :Fats in the diet Supply flavor to food, Which contributes to a feeling of satisfaction that lasts longer after eating than does the feeling of satisfaction after eating carbohydrates. This satiety is enhanced by the fuller texture and body that fat contributes to food mixtures and the slower gastric emptying time it brings.

Linoleic Acid:

Major Essential fatty acid.

It is polyunsaturated.

Body Fats :1. Energy. A Major function of fat in nutrition is to supply an efficient fuel to

all tissues except the central nervous system and brain, which depend on glucose.

2. Thermal Insulation. The layer of fat directly underneath the skin controls body temperature within the range necessary for life.

3. Vital organ protection. A web-like padding of adipose fat surrounds vital organs such as the kidneys, protecting them from mechanical shock and providing a supporting structure.

4. Nerve impulse transmission. Fat layers surrounding nerve fibers provide electrical insulation and transmit nerve impulses.

5. Tissue membrane structure. Fat serves as a vital constituent of the cell membrane structure, helping transport nutrient materials and metabolites across cell membranes.

6. Cell metabolism. Combinations of fat and protein, lipoproteins carry fat in the blood to all cells.

7. Essential precursor substances. Fat supplies necessary components such as fatty acids and cholesterol for synthesis of many materials required for metabolic functions and tissue integrity.

Adipose:Fat present in cells of adipose or fatty tissue.

Lipoproteins:Noncovalent complexes of fat with protein. The lipoproteins probably function as major carriers of lipids in the plasma , since most of the plasma fat is associated with them. Such a combination makes possible the transport of fatty substances in a predominantly aqueous medium such as plasma.

Amount of Fat:

Too much fat in the diet provides excessive kilocalories, more than required for immediate energy needs. The excess is stored as increasing adipose tissue and body weight. How much fat is in your own diet? You might try figuring it out for a day (See the box on P. 72). This increased body weight (More precisely, the increased proportion of body fat making up the total body composition) has been associated with health problems such as diabetes, hypertension, and heart disease. Look for specific relationships in later chapters on these topics.

Kind of Fat :

An axcess of saturated fat and cholesterol in the diet, Which comes from animal sources, has been clearly related by current research to atherosclerosis, The underlying blood vessel disease characterized by fatty plaques on interior vessel walls that can eventually fill the vessel and cut off blood circulation at that point (see Chapter 20). This disease process contributes to heart attacks and strokes ( see issues and answers on P. 84).

Fatty Acid:

The class name for fats and fat-related compounds in lipids. These are compounds that have in common a relationship to the fatty acid. The same basic chemical elements that make up carbohydrate---- carbon, Hydrogen , and oxygen --- also make up the fatty acids and their related fats. Fatty acids are also refined fuel forms of fat that some cells such as heart muscle prefer over glucose.

How Much Fat are you eating:

Calculate the total kilocalories (kcal) and grams of each of the energy nutrient (carbohydrate, fat, and protein) in everything you eat. Multiply the total grams of each energy nutrient by its respective fuel value.

Carbohydrate ____g × 4 = _____ kcal fat ____g × 9 = _____ kcalprotein____g × 4 = _____ kcal

Calculate the percentage of each energy nutrient in your total:

Fat kcal ÷ total kcal × 100 = % fat kcal in diet

Saturation of Fatty Acid:This state of saturation or unsaturation gives fats varying textural characteristics. Saturate fats are harder, less saturated ones are softer, and unsaturated ones are usually liquid oils. This differing state results from the ratio of hydrogen to carbon in the structures of the respective fatty acids that make up a particular fat. If a given fatty acid is filled with as much hydrogen as it can take, the fatty acid is said to be completely saturated with hydrogen. If, however, the fatty acid has less hydrogen it is obviously less saturated. Three terms designate the varying degree of saturation:

1. Saturated . Food fats composed of such saturated fatty acid are called saturated fats. These fats are of animal origin.

2. Monounsaturated. Food fats composed mainly of fatty acids with one less hydrogen atom creating one double bond are called monounsaturated fats. These fats are mostly form plant sources, for example, olive oil.

3. Polyunsaturated. Food fats composed mainly of unsaturated fatty acids with two or more places unfilled with hydrogen creating double bonds are called polyunsaturated fats. These fats are from plant sources. Notable exceptions are coconut oil, palm oil, and cocoa butter, which are saturated.

Saturation:To cause to unite with the greatest possible amount of another substance through solution, chemical combination, or the like.

Essential Fatty Acids :

The term essential or nonessential is applied to a nutrient according to its relative necessity in the diet. The nutrient is essential if its absence creates a specific disease and the body cannot manufacture it so must obtain it from the diet. If fat makes up only 10% or less of the diet’s daily kilocalories, the body cannot obtain adequate amounts of the essential fatty acids. Three fatty acids ---- Linoleic, linolenic, arachidonic --- are the only ones known to be essential for the complete nutrition of humans. Actually only linoleic acid is a true essential fatty acid (EFA), since the other two may be naturally synthesized from it. These fatty acids, linoleic acid, along with linolenic and arachidonic acids, serve important body functions:

1. Membrane Structure. Linoleic acid strengthens cell membraces, helping prevent a damaging increase in skin and membrane permeability. A linoleic acid deficiency leads to a breakdown in skin integrity l resulting in characteristic eczema and skin lesions. A similar effect also offurs in other tissue membranes throughout the body.

2. Cholesterol transport. Like other fatty acids, linoleic acid combines with cholesterol to form cholesterol esters for transport in the blood.

3. Serum cholesterol. As do other unsaturated fatty acids, Innoleic acid helps lower serum cholesterol levels. It plays a key role in both transport and metabolism of cholesterol.

4. Blood Clotting. With its closely associated metabolic products arachidonic acid and linolenic acid, linoleic acid helps prolong blood clotting time and increase fibrinolytic activity.

5. Local hormone-like effects. Linoleic acid is a major metabolic precursor of a group of physiologically and pharmacologically active compounds known as prostacyclins, prostaglzndins, thromboxanes, and leukotrienes, which are called eicosanoids (from Greek eicosa, twenty) because of their structure, long 20-carbon chain polyunsaturated fatty acids (see the box on p. 74). These eicosanoid compounds have extensive local hormone-like effects. 1 They are synthesized in the body from arachidonic acid , which is derived from essintial linoleic acid. The synthesis of these highly active important compounds is diagrammed in Figure 4-1, which also shows some of their significant physiologic functions and sites.

Essential fatty acid (EFA). Fatty acid that is (1) necessary for body metabolism of function and (2) cannot be manufactured by the body and must thereforebe supplied in the diet. The major essential fatty acid is linoleic acid (C17H34COOH). It is found principally in vegetable oils. Two other fatty acids usually classified as essential are linolenic acid and arachidonic acid.

Prostaglandins:O these groups of eicosanoid compounds related to the omega-3 long-chain fatty acids, perhaps the most familiar is the group of prostaglandins because of their extensive functions. They were first discovered by swedish investigators in their study of reproductive physiology, identified initially in human semen, and named prostaglandins because they were thought to originate in the prostate gland. They are now known to exist in virtually all body tissues, acting as local “ hormones” to direct and coordinate important biologic functions. For example, they have been shown to be powerful modulators of vascular smooth muscle tone and platelet aggregation and hence have a significant relationship to cardiovascular disease. 2

Prostaglandins. Group of naturally occurring long-chain fatty acids having local hormone-like

actions of widely diverse forms.

TriglyceridesStructure :

Fats are glycerides composed of glycerol and fatty acids. When glycerol is combined with one fatty acid it is called a Monoglyceride, with two fatty acids a diglyceride, and with three fatty acids a triglyceride. Whether in food or in the body, Fatty acids combine with glycerol to form glycerides. Most natural fats, whether from animal or plant sources, are triglycerides. These fats the triglycerides, occur in body cells as oily droplets. They circulate in water-based blood serum encased in a covering of water-soluble protein. These fat-protein complexes are called lipoproteins. They serve multiple functions throughout the body.

Nature of Food Fats:Food fats, as well as body fats, are composed of saturated and unsaturated fatty acids, If the food fat is made up mainly of saturated fatty acids, it is called a saturated fat. Foods from animal sources such as meat, milk and eggs contain saturated fats. Conversely, food from plant sources such as the vegetable oils are unsaturated fats. A general saturated – unsaturated spectrum of food fats is shown in Figure 4-2. The animal food fats on the saturated end of the spectrum are solid; those toward the center become somewhat less saturated and are softer.

The plant fats on the unsaturated end are free-flowing oils that do not solidify even at low temperatures. Exceptions are coconut oil, palm oil , and cocoa butter, which are saturated fats. Since these saturated plant fats are used extensively in commercial products because they are usually cheaper oils, it is important to read product labels carefully. More label information about fat composition is needed. This distinction in saturation l as shown in figure 4-2 m is helpful in explaining to persons on modified fat diets the correct choices of food fats. Also, the unsaturated oils can be hardened commercially into products such as margarine and shortening by injection of hydrogen gas to saturate them, a process called Hydrogenation.

Hydrogenation :Process of adding hydrogen to unsaturated fats to produce a solid, saturated fat. This process is used to produce vegetable shortening from vegetable oils.

Glycerides:Group name for fats, any of a group of esters obtained from glycerol by the replacement of one, Two or three hydroxyl (OH) groups with a fatty acid. Glycerides are the principal constituent of adipose tissue and are found in animal and vegetable fats and oils.

Glycerol:Colorless , odorless, syrupy, sweet liquid; a constituent of fats usually obtained by the hydrolysis of fats. Chemically, glycerol is an alcohol; it is esterified with fatty acids to produce fats.

Triglycerides:Compound of three fatty acids esterified to glycerol. A neutral fat, synthesized from carbohydrate, stored in adipose tissue. It releases free fatty acids into the blood after being hydrolyzed by enzymes.

Visible and hidden Food Fat:As indicated, food fats are sometimes called “ visible” if “ Hidden” fats according to how obvious they are in food. In most cases the food fat is quite evident, such as in butter, margarine, oil, salad dressing, bacon, and cream, which account for about 401% of the fat in the American diet. However, less obvious hidden fats in foods such as meat, milk ( unless it is a nonfat form), eggs (only in the yolk; the white is pure protein), nuts, seeds, olives, and avocados contribute more to our fat intake. A large part of this fat comes from our relatively high consumption of meats. Even when all the fat is trimmed off a cut of meat, its lean portion still contains 4% to 12% hidden fat. Higher grades of meat, both prime and choice, have considerable “ marbling”, tiny fat deposits within the muscle tissue. This is especially true of beef, the major meat on the American market for many years. Thus the meat and milk food groups together contribute about half the fat in the American diet. And considering all sources, about 45% of the total kilocalories come from fat, an excessive amount.

yet even with increasing health concerns about their fat intake, Americans still seem ambivalent. They cut down in some areas, but at the same time purchase larger amounts of red meats with considerable nonseparable fat, such as ground beef and hamburgers. This conflicting pattern of meat fat consumption. Termed “ meat fat madness” by one research team, suggests that such contradictory choices are made by an ill-informed public.3 But the meat industry, challenged by health-consumers, is now developing leaner breeds through genetic engineering, the so-called lite or Ultra label for meat containing 25% less fat, and promoting leaner cuts in smaller portions for low-fat cooking. 4

Cholesterol :Structure. Although Cholesterol is often discussed in connection with

dietary fat, it is not a fat (triglyceride) itself. Many people confuse cholesterol with saturated fat. It is a fat-related compound that is quite different from triglycerides in structure. Generally, cholesterol travels in the blood stream attached to long-chain fatty acids, , forming cholesterol esters.

Function. Cholesterol is a vital substance in human metabolism. It belings to a family of substances called steroids, or sterols, and is a precursor to all steroid hormones. A compound in the skin, 7-dehydrocholesterol, a derivative of cholesterol, is irradiated by sunlight’s ultraviolet rays to produce vitamin D hormone. It is also essential in the formation of bile acids, which emulsify fats for enzymatic digestion and then serve as a carrier for fat absorption. Cholesterol is widely distributed in all cells of the body and is found in large amounts in brain and nerve tissue. It is an essential component if cell membranes. It is small wonder therefore that a constant supply of so vital a material for body processes would be made in body tissues, mainly in the liver. If a person consumed no cholesterol at all, the body would still synthesize a needed supply.

Cholesterol. Fat-related compound, a sterol (C27H45 OH). It is a normal constituent of bile and a principal constituent of gallstones. In body metabolism, cholesterol is important as a precursor of various steroid hormones, such as sex hormones and adrenal corticoids.

Ester. A compound produced by the reaction between an acid and an alcohol with elimination of a molecule of water. For example, a triglyceride is an ester.

Steroids. Any of a large group of fat-related organic compounds, including sterols, bile acids, sex hormones of the adrenal cortex, and D vitamins.

Food Sources:Cholesterol occurs naturally in all animal foods. There is none in plant foods. Its main food sources are egg yolks and organ meats such as liver and kidneys. In fact, cholesterol occurs only in animal fats and animal tissues, not in plant fats or tissues. Therefore vegetable oils do not contain cholesterol. Plant oils may very in degree of saturation, but none of them contain cholesterol.

Health Concerns:Cholesterol has now bgeen strongly implicated in vascular disease as a large risk factor in the development of atherosclerosis, the underlying pathology in coronary heart disease, in which cholesterol-containing fatty plaques build up in blood vessel walls. Current research has strengthened the association of this process with elevated serum cholesterol levels. 5,6 As a result , the U.S. Department of Health and Human Services Through its National Institutes of health is currently conducting a National Cholesterol Education Program to help physicians screen and treat persons with high blood cholesterol levels, working first with nutritionists to improve food habits and adding drug therapy as needed. 7,8 The various U.S. dietary guidelines (See Chapter 1) recommend that Americans reduce their dietary cholesterol intake to about 300 mg/day. An increase in soluble types of dietary fiber , described in the previous chapter, is also recommended because these fibers bind bile acids and dietary cholesterol, helping to eliminate excess cholesterol from the body.

LipoproteinsFunction :

The lipoproteins are important combinations of fat with protein and other fat-related components that are highly significant in human nutrition. They are complexes of lipids and apoproteins that serve as the major vehicle for fat transport in the blood stream.

Fat Transport :Fat is insoluble in water. This simple fact poses a problem in carrying fat to cells in a water-based circulatory system. The body has solved this problem through the development of the lipoproteins, packages of fat wrapped in water-soluble protein. These plasma lipoproteins contain fatty acids, triglycerides, cholesterol, phospholipids, and traces of other materials such as fat-soluble vitamins and steroid hormones. The high or low density of the lipoprotein is determined by its relative loads of fat and protein. The higher the protein ratio, the higher the density:

Apoprotein:Protein part of a compound, as of a lipoprotein. For example, apoprotein C II, an apoprotein of HDL and VLDL that functions to activate the enzyme, lipoprotein lipase.

Phospholipids:Any of a class of fat-related substances that contain phosphorus, fatty acids, and a nitrogenous base. The phospholipids are essential elements in every cell.

1. Chylomicrons, formed in the intestinal wall folloeing a meal and carryin a high ratio of fat (90%) with a small amount of protein, have the lowest density. They deliver diet fat to liver cells for initial conversion to other transport lipoproteins.

2. Very Low-density lipoproteins (VLDLs), deliver endogenous triglycerides to tissue cells.

3. Intermediate low-density lipoproteins (ILDLs), continue the delivery of endogenous triglycerides to tissue cells.

4. Low-density lipoproteins (LDLs), deliver cholesterol to the peripheral tissue cells.

5. High-density lipoproteins (HDLs), transfer free cholesterol from tissues to the liver for catabolism and excretion.

Cholecystokinin:Hormone that is secreted by the mucosa of the duodenum in response to the presence of fat. It causes the gallbladder to contract, which propels bile into the duodenum, where it is needed to emulsify the fat.

Bile:Greenish yellow to golden brown alkaline fluid secreted by the liver and concentrated in the gallbladder. Made of bile salts, cholesterol, phospholipid, bilirubin diglucuronide, and electrolytes.

Emulsifier:An agent that breaks down large fat globules to smaller, uniformly distributed particles.

Mouth. No Chemical fat breakdown takes place in the mouth. In this first portion of the gastrointestinal tract, fat is simply broken up into smaller particles through chewing and moistened for passage into the stomach with the general food mass.

Stomach. Little if any chemical fat digestion takes place in the stomach. General peristalsis continues the mechanical mixing of fats with the stomach contents. No significant amount of enzymes specific for fats is present in the gastric secretion except a gastric lepase (tributyrinase), which acts on emulsified butterfat. As the main gastric enzymes act on other specific nutrients in the food mix, fat is separated from them and made readily accessible to its own specific chemical breakdown in the small intestine.

Small Intestine. Not until fat reaches the small intestine do the chemical changes necessary for fat digestion occur, with agents from three major sources: a preparation agent through the biliary tract (liver and gallbladder) and specific enzymes from the pancreas and the small intestine itself.

Bile from the Liver and Gallbladder.The presence of fats in the duodenum stimulates the secretion of cholecystokinin, a local hormone from glands in the intestinal walls. In turn, cholecystokinin causes contraction of the gallbladder, relaxation of the sphincter muscle, and subsequent secretion of bile into the intestine by way of the common bile duct. The liver produces a large amount of dilute bile, then the gallbladder concentrates and stores it, ready for use with fat as needed. Its function is that of as emulsifier. Emulsification is not a chemical digestive process itself, but is an important first preparation step for fat’s chemical digestion by its specific enzymes. This preparation process accomplishes two important tasks: (1) it breaks the fat into small particles, or globules, which greatly enlarges the total surface area available for action of the enzyme, and (2) it lowers the surface tension of the finely dispersed and suspended fat globules, which allows the enzymes to penetrate more easily. This process is similar to the wetting action of detergents. The bile also provides an alkaline medium for the action of the fat enzyme lipase.

Enzymes from the Pancreas:Pancreatic juice contains an enzyme for fat and one for cholesterol. First , pancreatic Lipase, a powerful fat enzyme, breaks off one fatty acid at a time from the glycerol base of fats. One fatty acid plus a diglyceride, then another fatty acid plus a monoglyceride, are produced in turn. Each succeeding step of this breakdown occurs with increasing difficulty. In fact, separation of the final fatty acid from the remaining monoglyceride is such a slow process that less than one third of the total fat present actually reaches complete breakdown. The final products of fat digestion to be absorbed are fatty acids, diglycerides, monoglycerides, and glycerol. Some remaining fat may pass into the large intestine for fecal elimination. Second, the enzyme cholesterol enterase acts on free cholesterol to form cholesterol esters by combining free cholesterol and fatty acids in preparation for absorption.

Enzymes from the small intestine:The small intestine secretes an enzyme in the intestinal juice called lecithinase. As its name indicates, it acts on lecithin, a phospholipid, to break it down into its components for absorption.A summary of fat digestion in the successive parts of the gastrointestinal tract is given in Table 4-1 for review.

Stage 1: Initial Fat AbsorptionIn the small intestine, bile combines with products of fat digestion in a micellar bile-fat complex. This unique carrier system, shown in Figure 4-3. then takes fat along its initial passage into the intestinal wall.

Stage 2: Absorption within the Intestinal WallOnce inside the wall of the small intestine, the bile separates from the fat complex and returns in circulation to accomplish its task over and over again. Two important actions on the fat products occur inside the intestinal wall: (1) enteric lipase action—an enteric lipase within the cells of the intestinal wall completes the digestion of the remaining glycerides, and (2) triglyceride synthesis– with the resulting fatty acids and glycerol, new human triglycerides are formed as body fats, ready now for final absorption and circulation.

Micellar bile-fat complexA particle formed by the combination of bile salts with fat substances (fatty acids and glycerides) to achieve the absorption of fat across the intestinal mucosa. Bile salt micelles act as detergents to prepare lipids for digestion and absorption.

Summary of Fat Digestion

organ Enzyme Activity

Mouth None Mechanical, mastication

Stomach No major enzyme Mechanical separation of fats as

protein and starch digested out

Small amount of gastric Tributyrin (butterfat) to fatty lipase tributyrinase acids and glycerol

Small intestine Gallbladder bile salts (emulsifier) Emulsifies fatsPancreatic lipase (steapsin) Triglycerides to diglycerides

and monoglycerides in turn, then fatty acids and glycerol

Stage 3: Final Absorption and Transport of FatThese newly formed human fats--- triglycerides--- and other fat materials present are combined with a small amount of protein covering to form lipoproteins called chylomicrons. These packages of fat, in a milk-like liquid called chyle, Cross the cell membrane intact into the lymphatic system and then into the portal blood. Here a final fat-clearing enzyme, liplprotein lipase, helps clear the large meal load of dietary fat from circulation. In the liver the fat is converted to other lipoproteins for transport to the body cells for energy and other structural functions.In the body cells, fatty acids are “burned” as concentrated fuel to produce energy. These derived units of fats have about twice the energy value of glucose products. As indicated in Chapter 2, cell metabolism of fat is closely interrelated with that of the other nutrients.

ChylomicronsParticles of fat—lopoproteins --- appearing in the lymph and blood after a meal rich in fat.