This video is about saturated and unsaturated fats, cholesterol, phospholipids, essential fatty acids, and refined oils.
LIPIDS
Function and Solubility
Lipids are an important component of our body and include molecules such as triglycerides (fats), cholesterol, and phospholipids. The body uses fats for many vital functions in every cell. Lipids consist of many carbon-hydrogen bonds, which make them very useful molecules for storing energy. See Table 2.5.
Water molecules are polar because of a slightly unequal sharing of electrons in the covalent bonds. In fatty acids, the carbon and hydrogen covalent bonds have electrons equally distributed between the atoms that make them nonpolar, or hydrophobic. When fats are placed in water they cluster together as droplets in the water. This property is extremely important in the formation of cell membranes.
Types of Lipids
There are three main categories of lipids: triglycerides, cholesterol, and phospholipids. Triglycerides are composed of one glycerol molecule bonded to the carboxylic acid end of three fatty acids (Figure 2.18).
Triglycerides, the body’s main source of stored energy, are found in fat cells (adipocytes). Groups of these cells are called adipose tissue (Chapter 11)—also called subcutaneous fat because it occurs mainly in the subcutaneous layer under the dermis of the skin (Chapter 12). Many people have an excess of this type of fat, and it can only be decreased when the cells use this fat as an energy source. The cells that use the most stored energy to make ATP are the skeletal muscle cells; this explains why exercise helps maintain a healthy body weight.
Did You Know?
Sixty-four percent of North Americans are overweight, an 18% increase since the 1970s. Obesity is closely correlated with an increase in many diseases, such as cardiovascular disease, diabetes, breathing disorders, depression, and cancer.
Saturated and Unsaturated Fats
Fats can be solid or liquid at room temperature. Generally, fats that are liquid at room temperature are unsaturated, and fats that are solid at room temperature are saturated. The reason for this is the chemical structure of the fatty acids. If there are only single bonds between the carbon atoms of the fatty acid chain (Figure 2.19), the structure of the fat molecules will be straight and can pack together easily; this makes the fatty acid a solid. If there are one or more double bonds between the carbons, the fat molecules will be bent and unable to pack closely together; this makes the fatty acid a liquid.
Foods that contain healthy saturated fats are meats, fish, avocados, coconut oil, and dairy products. Foods that contain healthy unsaturated fats—those in the cis formation (Figure 2.20a)—include plant oils, such as olive oil and flaxseeds, nuts, seeds, and vegetables, and also fish oil. Unsaturated fats that have one doublebonded carbon are called monounsaturated. Those that have two or more double bonds in the fatty acid chains are called polyunsaturated.
Trans Fats
Margarine is a vegetable fat that has been converted into a solid by a process called hydrogenation. During this process hydrogen is added to the fatty acids to essentially convert the double-bonded carbon atoms into single-bonded atoms. This changes the fat into a saturated fat that becomes solid at room temperature. The problem is that not every double bond is fully hydrogenated, so margarine is still partially unsaturated. Furthermore, the way the hydrogen atoms are added is not in a chemical structure that can be easily used by the body. These incompletely hydrogenated fats are called trans fats (Figure 2.20b); they are the very worst fats for human health. Some labels claim the margarine is non-hydrogenated, and so it is healthier.
However, the margarine remains semi-solid at room temperature because it contains palm oils, which are saturated fats. This type of margarine is healthier than hydrogenated margarines, but it contains saturated fats, as does butter, except that these are plant-based. Trans fats are found in fast foods and deep-fried foods, because the high cooking temperature converts the unsaturated vegetable oils into trans fats.
Enzymes in our cells that oxidize fatty acids to make energy (Chapter 4), preferentially break down saturated fatty acids and unsaturated fatty acids in the cis conformation, but not those in the trans conformation. Therefore, trans fats cannot be broken down as easily, and they can remain circulating in the bloodstream much longer than other fats. Trans fats can also bind to cell membrane receptors on the endothelial cells that line blood vessels; this results in these fats being taken into the lining of the blood vessels. Trans fats then stimulate the immune response, causing immune cells to also enter the blood vessel wall in an attempt to rid the body of these toxic molecules. The combination of fat and immune cells in vessel walls is the plaque that causes heart disease (Chapter 18). Higher consumption of trans fats has been linked to increases in risk of Alzheimer’s disease, cancers, obesity, liver dysfunction, and mood disorders such as anxiety and depression. Trans fat can be found in packaged and processed foods because they contain refined polyunsaturated oils.
Did You Know?
Very healthy unsaturated fats such as olive oil can be transformed into trans fats if they are exposed to high temperatures during cooking. This happens because the double bonds between the carbons are delicate and can be damaged by light and heat. Saturated fats are best for cooking because they cannot be converted into trans fats: for example, coconut oil (which is saturated even though it is called an oil). Some oils, such as peanut oil, have a higher “smoking” point and can be cooked at higher temperatures. If the oil produces smoke when you cook, your food will contain trans fats.
Refined Oils
Refined oils, such as canola, corn, and vegetable oils, go through several processes that can damage the double bonds in unsaturated fats. The Canola Council of Canada describes the general process used in refining oils:
- Cleaning—to remove weeds, stems, and other materials
- Flaking—rolling the seeds to break them open
- Cooking—heating to approximately 90 to 120°C to open the oil cells in the crushed seeds, denature hydrolysis enzymes, and volatize sulphur compounds that can cause odours
- Pressing—to extract the oils
- Solvent extraction—using hexane to extract the remaining 40 to 50% of the oils left in the seeds after heating and pressing
- Desolventizing and toasting—to remove the solvent by heating to 95 to 115°C for up to 90 minutes.
- Refining—putting the oil through a series of processes— to increase shelf life—by using water or organic acids to remove compounds such as phospholipids, free fatty acids, and colour pigments
- Bleaching—filtering the oil through clay to remove any colour compounds
- Deodorizing—by steam distilling the oil to remove any compounds that could cause any unpleasant odour or taste
- Further processing—such as using hydrogenation to solidify canola oil to produce margarine or shortening, or mixing with other oils that are more solid, such as palm kernel oil.
Did You Know?
Cold-pressed olive oil is mechanically extracted without the use of solvents, and it is not heated beyond 28°C because heat damages the oil and alters the flavour. “Pure” and “light” olive oils are heat treated, and so they no longer contain beneficial antioxidants. Heat and light can damage the double bonds in vegetable oil, so buy oil that is in dark glass bottles and has a pressed date of less than one year.
Essential Fatty Acids
We need essential fatty acids in our diet because the body’s cells cannot produce them from other sources. There are three essential fatty acids: alpha-linolenic acid, which is omega 3; linoleic acid, which is omega 6; and oleic acid, which is omega 9 (Figure 2.21). These fatty acids are essential for immune-cell regulation and for the production of mood hormones; deficiency in essential fatty acids can cause immune disorders and depression.
Alpha-linolenic acid is an 18-carbon fatty acid chain with three double bonds. This makes it a polyunsaturated fatty acid: having its first double bond at the third carbon gives the term omega 3. Linoleic acid is also an 18-carbon fatty acid chain, but it has two double bonds, is polyunsaturated, and the first double bond is at the sixth carbon, making it omega 6. Dietary sources of essential fatty acids include fish, shellfish, olive oil, peanuts, flaxseed, leafy green vegetables, walnuts, and eggs.
Cholesterol
Most cholesterol in our body is produced by the liver (Figure 2.22). Cholesterol plays very important roles in human physiology: (1) it is a major constituent of cell membranes; (2) it is used by the liver to make bile, which helps in the digestion of fats; (3) it is used to produce low-density lipoproteins (LDLs) that transport fats through the bloodstream; and (4) it is used as the starting material for the production of steroid hormones, such as estrogen, progesterone, testosterone, and aldosterone.
Did You Know?
“Bad” blood cholesterol levels (which are LDLs and not just cholesterol) increase more by eating trans fats and sugar than by eating cholesterol. This occurs because eating trans fats and sugar causes the liver to produce more LDLs that contain cholesterol and other fats so that the trans fats—which are not easily taken up by cells—can be transported. This results in the trans fats staying longer in the circulatory system and therefore playing a significant role in the development of cardiovascular disease.
Phospholipids
The last category of lipids is phospholipids, the primary fats found in cell membranes. Phospholipids are unique because they have two nonpolar fatty acid chains, called tails, and a polar head that contains a phosphate group. Because they contain both polar and nonpolar regions, phospholipids are amphipathic. This important feature of these fats explains why they can produce cell membranes. All cell membranes consist of a bilayer of phospholipids: the polar heads are directed toward the water-filled cell cytoplasm and the exterior of the cell, the interstitial fluid; and the tails are directed toward the interior of the membrane (Figures 2.23 and 2.24).
Since phospholipids are mostly unsaturated, the cells are liquid. In addition to the phospholipids, most animal cell membranes contain cholesterol, which helps the membrane remain flexible.
