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Lipid

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Lipids, otherwise known as fats, comprise one of the three groups of macronutrients (nutrients that the body needs in large quantities), along with proteins and carbohydrates[1]. There are many different types of lipids, which generally fall under one of three categories: fats (typically triglycerides), phospholipids, and steroids. All lipids, however, share a few common characteristics. First, they all contain primarily carbon and hydrogen atoms. Also, they are hydrophobic (insoluble in water), but they do dissolve in alcohol and other nonpolar solvents[2]. Nowadays, many people want to lose fat and try to do so by cutting fatty foods out of their diets. However, fat serves several vital roles in the body, and no one can survive without some fat. In addition, there are some fats called essential fatty acids that must be ingested because the body cannot manufacture them.

Contents

Functions

Although lipids, more commonly known as fats, are often considered "bad" and unhealthy, they actually perform several essential functions in the body, the most significant of which is energy storage. They contain much more energy per gram than other nutrients; 9 calories per gram, as compared to the 4 calories per gram in proteins and carbohydrates. The body also stores energy in the form of glycogen for quick access, but the supply of glycogen in the muscle is depleted after a few minutes of strenuous activity. In comparison, the energy available from lipids in a 154 lb male could sustain him for 30-40 days without food (given an adequate water supply)[3]. Thus, lipids provide an important energy reserve in the body, providing long-term energy in a much more compact form than other nutrients.

The body stores fats as adipose tissue. This fat surrounds the internal organs to protect and cushion them. Everyone has a relatively large amount of fat around both kidneys, which keeps the kidneys from moving around. In females, a lot of fat tissue protects the mammary glands as well[1]. It also exists as subcutaneous fat (under the skin), where it insulates the body against extreme temperatures and can be broken down to generate heat in cold conditions[3].

Lipids also play a vital role in the cell membrane. The body's cells have an outer membrane (the plasma membrane) composed of a double-layer of phospholipids. This semi-permeable membrane regulates what substances enter and leave the cell, and it also surrounds most of the organelles of the cell.[4]. Phospholipids, each made up of a polar phosphate and a nonpolar fatty acid chain, line up in two rows to form the lipid bilayer of the plasma membrane. The hydrophilic (attracted to water) ends face outwards, forming the inner and outer edges of the membrane, while the hydrophobic (repelled by water) ends face each other [5].

Lipids are now being found to have many highly specific roles in the body. Many fatty acids have important transport roles, others form the precursors for substances like prostaglandins, and some even play a part in gene expression[6]. Lipids can function as hormones as well. Lipid hormones aid in controlling the body's metabolism. Lipids also form the basis for important hormones such as testosterone and other sex hormones[4]. In addition, lipids are needed for the absorption of necessary vitamins, including vitamin A, vitamin D, vitamin K, vitamin E, and carotenoids[7].

With all the functions lipids perform in the body, they are certainly an essential part of a healthy diet. Young children especially need a diet that includes fats to ensure proper growth and development. Fats allow children to absorb the vitamins they need and are vital for the development of myelin sheaths on nerve cells, including brain cells[8].

Processing

The body takes excess fat from the blood and stores it in fat cells until it needs fuel for energy.

Once fats are ingested through food, they travel to the small intestine. There, they stimulate the production of cholecystokinin (CCK), a hormone that tells the pancreas to release digestive enzymes[2]. Bile salts emulsify fats (triglycerides), converting them to smaller fat droplets called micelles. Pancreatic juice further breaks down the micelles into glycerol and fatty acids (the components of triglycerides). These molecules are small enough to be absorbed by the intestinal villi[9]. The intestinal cells reassemble the particles into triglycerides and combine them with a protein to form chylomicrons that can travel through fluids (lipids alone are insoluble in water)[3]. The chylomicrons enter the lymphatic system through lacteals (tiny lymph vessels in the villi) and eventually pass into the circulatory system when the lymph vessels join with blood vessels[9].

The presence of fats in the blood stream stimulates insulin production, which causes, among other things, increased activity of lipoprotein lipases (enzymes that break down fats into fatty acids). These fatty acids can be absorbed by the body cells and stored for later use in adipose or liver cells[2]. Adipose tissue, or fat tissue, can mostly be found around the internal organs and under the skin. Adipose tissue is a type of connective tissue; similar to loose connective tissue, adipose tissue contains elastic fibers that allow it to hold things together while still being flexible[1].

When the body does not have a readily available source of energy (such as if a person has not eaten for a while) it must break down stored energy from either fats, proteins, or carbohydrates. To initiate this process, the pancreas, pituitary gland, adrenal gland, and thyroid gland secrete hormones that cause the liver, muscles, and fat tissue to obtain energy from these sources. The body breaks down carbohydrates (including glycogen) first, followed by fats. Through lipolysis, the body converts fats into glycerol and fatty acids. The fatty acids are used for energy or turned into glucose through gluconeogenesis. Lipases within the fat cells can also be activated by hormones to break down fats into glycerol and fatty acids, which are then sent to the liver to be used for energy. Interestingly, fat cells are not destroyed by this process, and neither are more fat cells produced when a person gains weight. The number of fat cells in a person's body tends to remain the same, and the fat cells simply grow larger or smaller depending on how much fat they contain[9].

Fatty Acids

Fatty acids make up triglycerides, the fat molecules typically used by the body to store energy. Fatty acids consist of a hydrocarbon chain with a carboxyl group at one end. The hydrocarbon chain is the hydrophobic portion, and the carboxyl group is hydrophilic. The naturally occurring fatty acids can be divided into saturated and unsaturated fats[10]. Saturated fats and the man-made trans fats are the unhealthy fats which health specialists generally tell people to avoid, but unsaturated fats are necessary in the right amounts. To understand why different kinds of fat are healthy or unhealthy, it is necessary to discuss LDL and HDL. LDL, or low-density lipoproteins, transport fat particles and cholesterol from the liver to the body cells through the bloodstream. Too much LDL in the blood can cause cholesterol deposits to build up on artery walls, blocking the arteries and potentially leading to a heart attack or stroke. HDL, on the other hand, removes cholesterol from the bloodstream and artery walls and takes it back to the liver for elimination. Thus, the lower the levels of LDL and the higher the levels of HDL, the less risk a person will have for heart disease[11].

Saturated Fat

A triglyceride molecule consists of three fatty acids bonded to a glycerol molecule. This triglyceride has two saturated fatty acids and one unsaturated fatty acid.

Saturated fats can be recognized because they are solid at room temperature. Examples include lard and margarine sticks. Saturated fats are solid because of their structure. In the fatty acid chain of a saturated fat molecule, the carbon atoms bond to each other with single bonds, and they bond to as many hydrogen atoms as they can accommodate. This makes a compact, straight fatty acid chain that can be tightly-packed against the fat molecules around it[10].

While people do need some saturated fat, the body can manufacture all that it needs, so it is not necessary to eat saturated fats. These are considered "bad fats" because, while they raise HDL levels, they also raise LDL levels. It is recommended that a person's diet should not include more than 7 percent saturated fat. These fats exist mainly in meat (especially red meat), whole-milk dairy products, seafood, poultry with skin, and some vegetable oils[11].

Unsaturated Fat

Unsaturated fats, such as oils, are liquid at room temperature. Unlike the saturated fats, these fats have some double-bonds in their hydrocarbon chains, meaning that some of the carbon atoms are double-bonded to each other, which leaves less room for hydrogen atoms. The double-bonds cause bends in the chain, so the unsaturated fats cannot be packed as tightly as the saturated fats[10]. These fats occur mostly in plants (vegetable oils), nuts, and seeds[11].

The two types of unsaturated fats are monounsaturated fats and polyunsaturated fats. Monounsaturated fats contain one double-bond, whereas polyunsaturated fats contain two or more double-bonds[12]. Canola oil, peanut oil, olive oil, avocados, almonds, hazelnuts, pecans, pumpkin seeds, and sesame seeds all contain monounsaturated fat. Good sources of polyunsaturated fat include sunflower oil, corn oil, soybean oil, flax seeds, walnuts, and fish[11].

Essential Fatty Acids

Some of the fats needed by the body cannot be manufactured in the body and instead must be obtained through food. These are the omega-6 fatty acids (linoleic acid, arachidonic acid, and gamma linolenic acid) and the omega-3 fatty acids (alpha linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid). Omega-3 and omega-6 fatty acids fall under the category of polyunsaturated fats; the 3 and the 6 tell whether the first double carbon bond occurs 3 or 6 carbons from the end of the chain. Omega-6 acids are found in corn oil, sunflower oil and soybean oil; omega-3 acids occur in flax seeds, pumpkin seeds, walnuts, and fish[13]. These acids are important for the formation of prostaglandins and, along with cholesterol, they help maintain the flexibility of a cell's plasma membrane[1].

Trans Fat

Fast food restaurants often use hydrogenated oils when frying foods such as french fries, causing these foods to contain a lot of trans fat.

Trans fat, the man-made fat, is the least healthy fat and can lead to serious health problems. It became widely used because of its longer shelf-life, convenience, and low cost; it can be easily transported, which makes it very appealing to manufacturers of baked goods and processed foods. However, researchers now know that trans fats can be even more detrimental than saturated fats to a person's health because they lower HDL and raise LDL. Too much trans fat in the diet (the optimum amount per day is under 2 grams) can drastically increase the risk of heart disease and has also been shown to cause inflammation[11].

Trans fat is made by hydrogenating vegetable oils. In this process, the vegetable oil is heated to break down the double carbon bonds, and hydrogen is added with a catalyst[13]. Partially hydrogenated oil, often used in frying, can stand reheating several times without breaking down. Fully hydrogenated oil comes in a solid form, offering easy transportation at a lower cost. It is found in many snack foods, commercial baked goods, margarine, and fried foods such as french fries. However, U.S. companies are now required to list the amount of trans fat with the nutrition information on their products; the words "hydrogenated" or "partially hydrogenated" in the ingredients list also indicate the presence of trans fat[11].

Triglycerides

Triglycerides contain three fatty acid molecules bonded to a glycerol molecule. One fatty acid bonds to each of three carbons on the glycerol molecule[10]. These are the units of energy storage for the body; a fat cell is simply a cell with many droplets of these triglycerides in its cytoplasm[14]. Triglycerides compose the body's adipose tissue, and aside from energy storage, they also insulate the body and cushion the organs. They can be broken down through hydrolysis to release energy[6]. Triglycerides can contain three identical fatty acid chains (simple triglycerides) or three different chains (mixed triglycerides). They are categorized as either saturated or unsaturated fats depending on whether the fatty acids in them are saturated or unsaturated[15].

Waxes

Waxes are esters of fatty acids with long alcohol chains, and they are found throughout nature. Plants produce these lipids for protection. A waxy coating on leaves and fruit provides water-proofing and protection from many insects[15]. A layer of wax also prevents too much water from escaping through the leaves, thereby reducing dehydration. In some marine animals, wax is used for storage. Birds have special glands that secrete wax, which they can then spread over their feathers to waterproof them. Bees also use wax in making honeycombs[6].

Phospholipids

Phospholipids form the lipid bilayer of cell plasma membranes. These membranes also contain the lipid cholesterol and certain proteins.

Phospholipids are very important in the body because they make up cell membranes. Their composition is very near to that of triglycerides. In a phospholipid, one glycerol molecule bonds to two fatty acids and a phosphate group. These molecules make excellent membranes because of a unique property; the glycerol molecules are polar, and the fatty acid portions are non-polar. The polar ends are attracted to water, while the non-polar ends are repelled by water. In the presence of water, then, phospholipids line up in bilayers, with the polar ends facing outwards towards the water, and the non-polar ends facing inwards towards each other. Hydrogen bonds form between the glycogen molecules and the water, and weak bonds between the fatty acid molecules hold the phospholipids together in the membrane, forming a flexible, semi-permeable membrane that sets the boundaries of each cell[10].

Many different kinds of phospholipids comprise the plasma membrane of the cell. The membrane also contains other molecules, including cholesterol to help maintain the structure, and proteins to aid in transporting substances across the membrane, signal recognition, etc. The individual phospholipids can move about and rotate freely within their layer, as long as they maintain the correct orientation, but they cannot move to the other side of the bilayer because of the energy between the two layers. A group of phospholipids called the sphingomyelins play another vital role in the body. They form the myelin sheaths around nerve cells, which insulate the nerves and allow action potentials to be conducted[15].

Phospholipid membranes exist within cells as well as around them. Nuclei, mitochondria, and other organelles are surrounded by the same lipid bilayers that make up the outer plasma membrane of the cell[10].

Steroids

A cholesterol molecule has four fused rings at its center.

Like phospholipids, steroids form part of cell plasma membranes. While steroids are lipids, they do not contain any fatty acids. Cholesterol is one of the more well known steroids, but some hormones and vitamins are steroids as well[14]. Structurally speaking, all steroids share a common four fused rings, and each steroid has different components attached to the four central rings[16].

Cholesterol has several important functions in the body. It plays a role in cell membranes along with the phospholipids. Cholesterol helps maintain the fluidity and flexibility of the plasma membranes, especially when they are subjected to lower-than-normal temperatures[17]. Additionally, cholesterol aids in vitamin D absorption and is a precursor for the sex hormones (estrogen and testosterone) and aldosterone[1].

However, too much cholesterol can lead to serious health problems such as heart disease. Due to its hydrophobic nature, cholesterol does not dissolve in the blood. It must be transported by lipoproteins (LDL and HDL). High levels of LDL lead to an excess of cholesterol in the blood, some of which will accumulate on the artery walls as plaque, restricting blood flow and potentially resulting in a blood clot, heart attack, or stroke[10].

Other steroids include estrogen, progesterone, corticosteroids, aldosterone, testosterone, and vitamin D[17].

Fat in the Diet

Nuts, a good source of healthy unsaturated fat, provide plenty of protein as well.

Due to the critical functions fat has in the body, no one should completely remove fat from their diet. Fat not only plays important roles in the body, it helps people feel more satisfied for a longer period of time, which helps them eat less and curb their appetites. People trying to lose weight and burn fat often think that they need to stop eating fat altogether, but this is not really the case. According to a recent article published by the Harvard School of Public Health, while low-fat diets do help people lose weight, so do high-fat low-carb diets, high-protein low-carb diets, and really any diet that decreases the total amount of calories a person consumes[11].

Typical recommendations for a healthy amount of fat in a person's diet are to decrease the "bad fats" (trans fat and saturated fat) and get the right amount of the "good fats" (unsaturated fats). Ideally, a person should keep trans fat consumption under 2 grams per day. Saturated fat should make up no more than 7 percent of daily food intake, and total fat should make up about 30 percent of one's daily diet[18]. Total fat should primarily come from unsaturated fat, especially monounsaturated fat. Omega-3 and omega-6 fatty acids are also necessary for a healthy diet, and many health experts recommend eating fish once to twice a week for the essential fatty acids it contains.

There are many ways to get the proper amount of healthy fats while avoiding trans fat and saturated fat. The American Heart Association recommends eating limited amounts of whole-milk dairy products, fatty meats, tropical oils (coconut and palm oil, for example), and hydrogenated oils, because these foods are high in unhealthy fats. Instead, choose fruits and vegetables (naturally fat free), grains, fat-free or low-fat dairy products, beans and legumes, lean meats, and nuts. When cooking, choose liquid oils (like vegetable, canola, or olive oils) over butter or margarine[18]. Also, avoid frying foods; try grilling, broiling, baking, braising, steaming, poaching, slow-cooking, or microwaving instead[19].

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References

  1. 1.0 1.1 1.2 1.3 1.4 The Human Body: Fearfully and Wonderfully Made! by Dr. Jay L. Wile and Marilyn M. Shannon, M.A. Apologia Educational Ministries, Inc. Indiana: 2001. pp. 47, 49, 381, 407, 454.
  2. 2.0 2.1 2.2 Fats by Jeffrey Radecki and Susan Kim, faqs.org, Nutrition and Well-Being A to Z.
  3. 3.0 3.1 3.2 Overview of Lipid Function by Charles E. Ophardt, Elmhurst College. Virtual Chembook, 2003.
  4. 4.0 4.1 The Function of Lipids: An In-depth Explanation of These Fats by Lain, Associated Content, April 26, 2007.
  5. Biomolecules: The Lipids by Barbara Liang, WISC-Online, General Anatomy & Physiology.
  6. 6.0 6.1 6.2 What Lipids Do: Their Biological Functions by William W. Christie, The Lipid Library, Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland, May 5, 2010.
  7. Fats: The Good, the Bad, and the Dangerous by Suzanne Bennet, Associated Content, February 5, 2009.
  8. Learning About Fats reviewed by Mary L. Gavin, February 2008, KidsHealth from Nemours.
  9. 9.0 9.1 9.2 How Fat Cells Work by Craig Freudenrich, HowStuffWorks.
  10. 10.0 10.1 10.2 10.3 10.4 10.5 10.6 WKU BIO 113--Lipids
  11. 11.0 11.1 11.2 11.3 11.4 11.5 11.6 Fats and Cholesterol: Out with the Bad, In with the Good by Harvard School of Public Health.
  12. Fats by John Kimball, 15 November 2002.
  13. 13.0 13.1 How Fats Work by Marshall Brain, TLC Cooking, Discovery Communications, 2009.
  14. 14.0 14.1 Lipids: Their Structure and Function by Joanne Stafferton, Suite101.com, 11 August 2007.
  15. 15.0 15.1 15.2 Lipids by William Reusch, 1999.
  16. What Is a Lipid Molecule? Organic Chemistry of Fats, Phospholipids, Waxes & Steroids by Tami Port, Suite101.com, 15 October 2007.
  17. 17.0 17.1 Lipids: Fats, Oils, Waxes, etc. by J. Stein Carter, 2 November 2004.
  18. 18.0 18.1 Fat by the American Heart Association.
  19. 20 Ways to Cut Fat in Cooking: Easy Ways to Cut Fat in Cooking by Fiona Haynes, About.com Guide, The New York Times Company.

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