# 6.1 Defining lipids

As defined by Merriam-Webster - noun, concise Encyclopedia:
Any of a diverse group of organic compounds that are grouped together because they do not interact appreciably with water. One of the three large classes of substances in foods and living cells, lipids contain more than twice as much energy (calories) per unit of weight as the other two (proteins and carbohydrates). They include the fats and edible oils (e.g., butter, olive oil, corn oil), which are primarily triglycerides; phospholipids (e.g., lecithin), which are important in cell structure and metabolism; waxes of animal or plant origin; and sphingolipids, complex substances found in various tissues of the brain and nervous system. Since insolubility is the defining characteristic, cholesterol and related steroids, carotenoids (see carotene), prostaglandins, and various other compounds are also classifiable as lipids.
Lipid definition. 2013. In Merriam-Webster.com. Retrieved Jan 27, 2013, from www.merriam-webster.com/dictionary/lipid
Lipids are molecules comprised of carbon, hydrogen and oxygen that are typically hydrophobic (Christie, 2013). These three elements bond together to create fatty acids, which are the simplest of lipids. Sterols, phospholipids and triglycerides are also classified as lipids. Each type of lipid differs in structure. The structure of a fatty acid is a long carbon chain with a carboxyl group (COOH) on one end, while a phospholipid consists of three fatty acids and a phosphate group (PO42-) binding them together. These discrepancies in structure result in different functions for each type of lipid. These lipids not only exist within the human body, but also in plants and other animals. Triglycerides are the most prominent lipid in nature (Whitney, 2013). They contain three fatty acid molecules that are bonded to one glycerol molecule, and are responsible for making up fat and oils (Christie, 2013). Fats can be saturated or unsaturated, the difference between the two originating in the carbon-to-carbon bonds. Saturated fats contain only single bonds between carbon atoms, while unsaturated fats contain one or more double bond. The changes in types of bonds cause differences between saturated and unsaturated fats, such as melting points and rates of oxidation.
Lipids are responsible for many functions throughout the body including the structure and function of cells. In cells, phospholipids are responsible for creating a lipid bilayer that is used as a cell membrane, which not only helps to contain the cells contents but it also helps to regulate the movement of materials into and out of the cell (Aponte-Santamaria 2012). Another major function of lipids is the production of energy. Due to the high ratio of carbon and hydrogen to oxygen, lipids can produce more than twice the energy (per gram) that a carbohydrate can produce (Whitney, 2013). When lipids are not being used they are stored in adipose, or “fat”, cells. These cells are used for insulation from fluctuating temperatures and for the protection of the body's organs. However, because these cells continue to expand and reproduce, there is almost no limit to the amount of fat that can be stored in the body. This becomes dangerous, and is the reason why lipids can be so detrimental to one's health.
The storage of fat results in weight gain, which can lead to obesity. This excess weight further increases the risk for diseases. For example, there is evidence that high body fat may contribute to the development of certain types of cancer. Another disease affected by a high level of lipids in the body is heart disease, which is the leading cause of adult death in the United States (Whitney, 2013). The higher the LDL (low–density lipoprotein) level, the higher the risk for heart disease. In order to avoid health issues such as these, it is important to maintain a level of less than 35% fat in a daily diet and to minimize intake of trans and saturated fats, as well as cholesterol.
Aponte-Santamaria, C., Briones, R., Schenk, A. D., Walz, T., & de Groot, B. L. (2012). Molecular driving forces defining lipid positions around aquaporin-0. Proceedings of the National Academy of Sciences, 109(25), 9887–9892. doi:10.1073/pnas.1121054109
Christie, W. W. (2013). What is a Lipid? Retrieved from http://lipidlibrary.aocs.org/Primer/content.cfm?ItemNumber=39371&navItemNumber=19200
Whitney, E., & Rolfes, S. (2013). Understanding Nutrition (14th ed.). Stamford, CT: Cengage Learning.

## 6.1.1 Fatty acids

Saturation Fatty Acids (FA), being one category of lipids, has even further subcategories of how they are saturated/unsaturated FA's. This stems from how all fatty acids have a chain of carbon and hydrogen atoms in their structure. These chains of carbon and hydrogen will vary in length, but saturated fatty acids (SFA) essentially have the maximum number of hydrogen atoms in that chain and contain ONLY single bonds between its carbon atoms. In real life we see that these SFA are solid at room temperature (like butter) and occur a lot in animal fat, palm oil, and coconut oil. Conversely there are unsaturated fatty acids (UFA) that will have carbons with a double bond, wherever a double bond occurs is a point of unsaturation. The double bond happens because there is an absence of two hydrogens, therefore forcing the carbons in the chain to fulfill their requirement of four bonds by double bonding to the nearby carbon atom. You can visibly see that UFA's are liquid at room temperature. There are also different types of UFA's, one is mono unsaturated fatty acids (MUFA) that lacks only two hydrogen atoms and therefore only has one double bond between carbon atoms. Typically MUFA's are found in olive oil, peanut oil, and canola oil. The other type of UFA is poly unsaturated fatty acids (PUFA's) which has four or even more hydrogen atoms that forces the UFA to have two or more carbon double bonds. Typically PUFA's are found in corn oil, canola oil, sunflower oil, fish oils, and in nuts/seeds.
Fatty Acid Nomenclature The naming of fatty acids follows a certain formatting of ∆x of which the template looks like Figure 1 (Jakubowski, 2013):
Monounsaturated Fatty Acid Naming
Monounsaturated fats follow a standard chemical naming system where number starts at the Carboxyl (COO-) terminal and proceeds to the Methyl (CH3) terminal. In this example, there is an eighteen carbon chain with one unsaturated bond at carbon number nine, in the cis formation. Therefore, its naming is, 18:1(∆9) cis-9-octadecenoic acid. (Appendix, 1976) See Figure 2.
Polyunsaturated Fatty Acid Naming Polyunsaturated fats can follow this same systematic nomenclature, or also an alternative method which is used to name our two essential fatty acids, omega-3 and omega-6 fatty acids.
See Figure 3. The naming of this fatty acid could follow earlier taught principles and name this fatty acid 18:3(∆9,12,15) Alpha-linolenic acid. Or it could be named alternatively, with the Methyl group (CH3) end is named “ω” (omega) and number toward the carboxyl terminal, with the first unsaturated bond receiving the number three. This give us an omega-3 fatty acid. Both omega-3 and omega-6 fatty acids are essential fatty acids. (IUPAC, 2001)
References Appendixes A-C. (1976). Retrieved November 15, 2015, from http://www.chem.qmul.ac.uk/iupac/lipid/appABC.html Jakubowski, D. (2013, November 20). BioWiki: The Dynamic Biology Hypertext. Retrieved November 15, 2015, from http://biowiki.ucdavis.edu/Biochemistry/Lipids/LIPID_Structure Pure Applied Chemistry (4th ed., Vol. 73, p. 702). (2001). IUPAC.
Fatty Acid- is composed of a carbon and hydrogen chain, a methyl group (CH2) at one end, and an acid group (COOH) at the other end Monounsaturated Fatty Acid- a fatty acid that is lacking only two hydrogen atoms, therefore having only one double bond between carbons Polyunsaturated Fatty Acid- a fatty acid that is lacking more than two hydrogen atoms, therefore having at least two or more double bonded carbon atoms. Saturated Fatty Acid- a fatty acid that has the most possible number of hydrogen atoms in its carbon and hydrogen chain structure Unsaturated Fatty Acid- a fatty acid that has at least one double bond between its carbon atoms due to its lack in at least two hydrogen atoms in its structure
References 1)White M.D., B (2009, August 15). Dietary Fatty Acids. Retrieved December 1, 2015, from http://www.aafp.org/afp/2009/0815/p345.html 2)Whitney, E., & Rolfes, S. (2015) Chapter 5: The Lipids: The Triglycerides, Phospholipids, and Sterols. In Understanding Nutrition (14th ed). Stamford, CT: Cengage Learning 3)Fats and Fatty Acids in Human Nutrition Report of an Expert consulation. (2008, November 14). Retrieved December 1, 2015, from http://www.fao.org/3/a-i1953e.pdf

## 6.1.2 Triglycerides

Triglycerides are a type of fat found not only in food, but in the body as well. They can be found in all types of fats, whether they are considered healthy or not, such as saturated fats, polyunsaturated and monounsaturated fats, and trans fats ("A Roadmap for managing your triglycerides and protecting your heart," n.d). Triglycerides have a number of health benefits which include providing your body with energy, they help absorb vitamins and nutrients, and they contain essential fatty acids ("A Roadmap for managing your triglycerides and protecting your heart," n.d); which cannot be synthesized by the body and must be consumed via the diet. Triglycerides are also utilized by the body to build tissues, and they are carried through the blood on lipoproteins (transporters), including chylomicrons, HDL, LDL, and VLDL ("A Roadmap for managing your triglycerides and protecting your heart," n.d). The structure of triglycerides is formed by combining a glycerol molecule and three fatty acid tails (Khan Academy, 2014), which makes it insoluble in blood and water, hence the lipoproteins assisting in carrying them through the blood.
For as many benefits as they have, triglycerides also have their share of consequences when not consumed properly. Triglycerides should be consumed in moderation like most other foods, and when consumed in excess they can cause issues with weight and heart health ("Triglycerides: Why do they matter?" 2015). To avoid consuming too many unhealthy triglycerides, refrain from eating foods that contain trans fats such as hydrogenated oils, cokes, butterfat, and cookies, while limiting foods that contain saturated fats such as lard, cocoa butter, meat, and poultry ("A Roadmap for managing your triglycerides and protecting your heart," n.d). If triglyceride levels become too high in the blood, it can cause a condition known as hypertriglyceridemia. This condition may present with no onset of symptoms, and the only definitive way to know what the body's blood triglyceride levels are is to have routine blood tests ("A Roadmap for managing your triglycerides and protecting your heart," n.d). Another consequence of high triglyceride levels is coronary heart disease, which is a condition where blood vessels in the heart narrow due to fatty deposits and are unable to supply enough blood and oxygen ("A Roadmap for managing your triglycerides and protecting your heart," n.d). These fatty deposits can harden (known as plaque) and reduce elasticity of the vessels and arteries, causing high blood pressure. Large plaque deposits and tight vessels can cause chest tightness, called angina, which can lead to myocardial infarction, or a heart attack ("Triglycerides: Why do they matter?" 2015).
High levels of triglycerides in the blood can be caused by other factors besides the diet that can be uncontrollable, such as genetic predisposition, certain medical conditions, and medications ("A Roadmap for managing your triglycerides and protecting your heart," n.d). There are a few methods to lower triglyceride levels in the body other than avoiding certain fatty foods. It is recommended to increase physical activity and exercise to more than thirty minutes every day, eat less calories, and reduce alcohol intake. Simple ways to adjust triglyceride intake in the diet include substituting canola oil instead of butter and fish instead of red meat ("A Roadmap for managing your triglycerides and protecting your heart," n.d). Overall, triglycerides are an essential fat that we need in our diet to receive essential amino acids and be healthy, but if not consumed in moderation, can have very adverse effects on the body such as obesity, heart disease, or even arterial plaque accumulation. References
Molecular Structure of Triglycerides (fats). (2014, July 14). In Khan Academy. Retrieved from https://www.khanacademy.org/science/biology/macromolecules/lipids/v/molecular-structure-of-triglycerides-fats
A Roadmap for Managing your Triglycerides and Protecting your Heart . (n.d.). In American Academy of Nurse Practitioners. Retrieved from https://www.aanp.org/images/documents/education/ManagingYourTriglycerides.pdf
Triglycerides: Why do they matter? . (2015, August 15). In Mayo Clinic. Retrieved December 2, 2015, from http://www.mayoclinic.org/diseases-conditions/high-blood-cholesterol/in-depth/triglycerides/art-20048186

## 6.1.3 Phospholipids and sterols

Phospholipids resemble triglycerides in many ways, but instead of having 3 chains of fatty acids like a triglyceride, phospholipids have 2 chains of fatty acids and a chain composed of a phosphate group and a compound that contains nitrogen. In Lecithin, for instance, there are two fatty acids and then a phosphate group and a choline molecule in the third spot. Choline is a molecule the body makes from the amino acid methionine, but choline can also be obtained from food sources such as milk, eggs, peanuts, soybeans, wheat germ, and liver. The phosphate group in phospholipids is hydrophilic so it can dissolve in water, and the fatty acids in phospholipids can dissolve in fat. This makes phospholipids great emulsifiers (emulsifiers can blend water and fats).
Sterols are 4-ring carbon compounds with side chains. Sterols are critical substances in the human body. Hormones that influence sex (estrogen, testosterone, etc.) and the adrenal glands (such as aldosterone), Vitamin D, cholesterol, and bile acids are all sterols. Both plant and animal foods have sterols, but plant foods do not have cholesterol. Cholesterol has a 4-ring structure typical of sterols, but has carbon side chains. It can help build the cell membrane, as most of the body's cholesterol is found there in the cell and cell membrane. Cholesterol is ideal for forming the cell membrane because of its hydrophobic and hydrophilic properties, which allow the more fluid structure of the cell membrane. Additionally, cholesterol can assist in forming hormones and bile acids. The liver produces cholesterol, so not much of it needs to be consumed through the diet. Eggs, seafood, meat, and dairy foods all have cholesterol. To keep cholesterol from being absorbed in the body, plant sterols that interfere with absorption can be added to the diet. If there is an excess build-up of cholesterol, plaque can grow within the arteries, and the body can be at a threat for atherosclerosis- a disease of the arteries that can lead to strokes and heart attack, among other health concerns. The Daily Value for cholesterol is 300 mg per day, because the liver is producing roughly 800 to 1500 mg of cholesterol per day. Additional consumption of cholesterol can cause a buildup of unwanted plaque. We describe cholesterol that is consumed from our diet as exogenous, and cholesterol that is manufactured by the liver as endogenous.
While they are critically important to the human body, phospholipids and sterols constitute only a small portion of the fats in our diet- around 5%.
References Berg, J., Tymoczko, J., & Stryer, L. (2002). Section 12.4, Phospholipids and Glycolipids Readily Form Bimolecular Sheets in Aqueous Media. In Biochemistry (5th ed.). New York: W H Freeman. Dietary Guidelines for Americans. (2010, December 1). Retrieved November 20, 2015, from http://www.cnpp.usda.gov/DGAs2010-DGACReport.htm Whitney, E., & Rolfes, S. (2015). Chapter 5: The Lipids: Triglycerides, Phospholipids, and Sterols. In Understanding Nutrition (14th ed.). Stamford, CT: Cengage Learning.