Lipids (Fats) - Biology Notes PDF

Lipids (Fats) ------------- **Introduction** Fats are an essential part of our body, accounting for a sixth of our body weight. The cells and tissues of our body have fat as an integral part. The vital organs (brain, heart, liver) are protected by a sheath of fat and water, which holds them in pla...

Lipids (Fats) ------------- **Introduction** Fats are an essential part of our body, accounting for a sixth of our body weight. The cells and tissues of our body have fat as an integral part. The vital organs (brain, heart, liver) are protected by a sheath of fat and water, which holds them in place and prevents injury. The nerves are also protected by fat. A layer of fat beneath the skin acts as a insulation against cold. The fat around the joints acts as a lubricant and allows us to move these smoothly. Thus fat is a crucial part of the body composition. **Lipids (Fats):** are organic compounds that are insoluble in water but soluble in organic solvents such as ether and alcohol; they are composed of carbon; hydrogen and oxygen (but have a higher hydrogen concentration). **Lipids** are an essential part of the body structure. Body fat accounts for 15 to 20 per cent of body weight in healthy non-obese men and 18 to 25 per cent in healthy non-obese women. **Fats** may be solid or liquid (oil). **Lipids** are classified as simple, compound, or derived lipids. - **Simple lipids,** which include fats and oils**. Glycerides**, the simple lipids, are the most common form of lipids. They consist of a glycerol molecule with up to three fatty acids attached. - **Compound lipids**, which comprise of phospholipids and lipoproteins. - **Derived lipids** such as fatty acids and sterols. **Triglycerides** (which have three fatty acids) account for more than 90 % of the lipids in food and in the body. **Fatty acids** are the basic structural units of most lipids. **Fatty acids** are described as saturated or unsaturated. **Saturated fatty acids** are those in which all carbon atoms are filled to capacity (i.e. saturated) with hydrogen; an example is butyric acid, found in butter. **An unsaturated fatty acid** is one that could accommodate more hydrogen atoms than it currently does. It has at least two carbon atoms that are not attached to a hydrogen atom; instead, there is a double bond between the two carbon atoms. **Unsaturated fatty acids** with one double bond are called monounsaturated fatty acids; those with more than one double bond (or many carbons not bonded to a hydrogen atom) are polyunsaturated fatty acids. An example of a polyunsaturated fatty acid is linoleic acid, found in vegetable oil. **Triglycerides** Fatty acids combine with glycerol to form a glyceride. When only one fatty acid combines with glycerol, it forms a monoglyceride. Diglycerides have two fatty acids and triglycerides three fatty acids attached to glycerol. Most of the fatty acids in the body as also in foods occur in the form of triglycerides. **Triglycerides** may contain saturated or unsaturated fatty acids. - **Saturated triglycerides** are found in animal products, such as butter, and are usually solid at room temperature. - **Unsaturated triglycerides** are usually liquid at room temperature and are found in plant products, such as olive oil and corn oil. **Cholesterol** is a fat-like substance that is both produced by the body and found in foods of animal origin. Each day our liver makes about 800 mg of cholesterol, which circulates through the blood stream and is used wherever it is needed. It is a precursor of vitamin D, hormones and bile acids. - The hormones made from cholesterol include corticosteroids, estrogens, testosterone and calcitriol (the active vit. D hormone). - Bile acids needed for fat digestion, are formed from cholesterol. - The cholesterol content of heart, liver, kidney and egg (embryonic stage of life) is quite high. - In infancy and toddlers stage, new tissues are formed, especially brain which need cholesterol. Hence fat should not be restricted in the diet of children up to five years of age. **Lipid Digestion** - Although chemical digestion of lipids begins in the stomach, they are digested mainly in the small intestine, primarily by bile, pancreatic lipase, and enteric lipase, an intestinal enzyme. - The end products of lipid digestion are glycerol, fatty acids, and cholesterol. These are immediately reassembled inside the intestinal cells into triglycerides and cholesterol esters (cholesterol with a fatty acid attached to it), which are not water soluble. - For these reassembled products to be transported and used, the small intestine and the liver must convert them into soluble compounds called lipoproteins. **Lipoproteins** Lipoproteins are synthesized in the liver. They are composed of three-fourths to two-thirds lipids and the remainder protein. These serve as the major vehicle for fat transport in the blood stream. The blood is mainly water and fat needs a water soluble cover of protein to permit it to be carried by blood. Four groups of, lipoproteins that have been identified are chylomicrons, low density lipoproteins (LDL), high density lipoproteins (HDL), and very low density lipoproteins (VLDL). - The LDLs carry fat and cholesterol to cells. - The HDLs carry free cholesterol from body tissues to the liver for breakdown and excretion. **Lipid Metabolism** - Converting fat into usable energy occurs through the use of the enzyme hormone- sensitive lipase that breaks down triglycerides in adipose cells, releasing glycerol and fatty acids into the blood. - 0.45 kg of fat provides 3,500 kilocalories. - Fasting persons will obtain most of their calories from fat metabolism, but some amount of carbohydrate or protein must also be used because the brain, nerves, and red blood cells require glucose. Only the glycerol molecules in fat can be converted to glucose. **The function of lipids and essential fatty acids** - Fat is a major nutrient and it has several functions. Oils and fats are concentrated sources of energy. Each, gramme supplies 9 calories. - Food fats are a source of two groups of essential nutrients --- essential fatty acids (EFA) and fat soluble vitamins A, D, E & K and their precursors. - Food fats also aid the transport and absorption of fat-soluble vitamins. - Cholesterol is an essential lipid synthesized in the liver. Some important hormones and bile acids are formed from cholesterol. - Fat forms the fatty center of cell walls, helping to carry nutrient materials across cell membranes. - Fats are used to synthesize phospholipids, which are found in all cells. - Fat stored in various parts of the body is known as adipose tissue. The vital organs in the body are supported and protected by a web-like padding of this tissue. Fats act as a cushion for certain vital organs. Nerve fibers are protected by the fat covering and it aids relay of nerve impulses. - Since fat is a poor conductor of heat, a layer of fat beneath the skin helps to conserve body heat and regulate body temperature. - The flavor, palatability and satiety value of foods is increased by fats. - The slower rate of digestion of fat as compared to carbohydrates results in a feeling of satisfaction. **Essential fatty acids** - The body can make all except two linoleic and linolenic acids. These two acids must be supplied by the diet. **Linoleic acid sources:** Sunflower, safflower, corn and soybean oils. **Linolenic acid sources:** Soybean and canola oils, walnuts and salmon. **Saturated fats and its risks** - Saturated fat is implicated in raising LDL cholesterol - LDL cholesterol raises risk of heart disease **Sources of saturated fats** 1\. Animal sources such as: whole milk, cream, butter, cheese and fatty cuts of beef and pork. 2\. Coconut. 3\. Palm and palm kernel oils and products containing them (pastries, pies, doughnuts and cookies and the like) are also sources of saturated fat. **Trans fat** - The majority of trans fats are formed when liquid oils are made into solid fats like hard margarine. - However, it is found naturally in some animal based foods as well. - Trans fat is made when hydrogen is added to an unsaturated fat such as vegetable oil, in a process called hydrogenation. - Hydrogenation increases the shelf life of products containing these fats. **Sources of Trans fat** 1\. Found in deep fried foods. 2\. Cakes, cookies, margarine, meat and dairy products. 3\. Partially hydrogenated oils are the main dietary source of trans fats. **Trans fat risks:** Trans fats, like saturated fats, can increase the LDL blood cholesterol levels and increase the risk of heart disease. **Unsaturated fats** - It is most effective dietary strategy in preventing heart disease may be replacing saturated fats in the diet with monounsaturated and polyunsaturated fats. **Requirements** **Fat g/day** ------------------------ --------------- 20 20 30 Lactating woman 45 Children (1--9 yrs.) 25 Children (10--18 yrs.) 22