The basic units of fats are fatty acids and glycerol. There are over forty different types of fatty acids. Fatty acids can be divided into three groups:
1. Saturated fatty acids
2. Monounsaturated fatty acids
3. Polyunsaturated fatty acids
Fatty acids are long chains of carbons with hydrogens attached. In addition, fatty acids can be saturated or unsaturated. A saturated fatty acid has all the carbons in the chain filled with hydrogens. An unsaturated fatty acid is missing hydrogens, and as a result the carbons bond to each other twice, forming what is called a double bond. If there is one place in the fatty acid missing hydrogens (or one double bond) the fatty acid is monounsaturated. If there are two or more places in the fatty acid missing hydrogens (two or more double bonds) the fatty acid is polyunsaturated.
The body can make all but two fatty acids. The body can make saturated fatty acids, and the body can convert some saturated fatty acids into monounsaturated fatty acids if the body needs them. However, our body is not able to make two polyunsaturated fatty acids, linoleic acid (omega-6) and linolenic acid (omega-3). These two polyunsaturated fatty acids are therefore essential and must be provided from the diet. A good diet needs to provide about 3% of total calories in the form of these essential polyunsaturated fatty acids. From these two polyunsaturated fatty acids the body can make other fatty acids the body needs that are important in the maintaining the structure of cell membranes, make hormone like compounds that help regulate blood pressure, blood clotting, blood lipids, and immune response to injury and infection.
Fats are made up of two substances, fatty acids and glycerol. Glycerol is a water soluble compound that can bind to three fatty acids. When one fatty acid is on the glycerol the compound is called a monoglyceride. When two fatty acids are on the glycerol the compound is called a diglyceride. And when there are three fats on the glycerol, the compound is called a triglyceride. Most fats in foods and in our bodies are in the form of triglycerides.
Foods contain combinations of the three types of fatty acids. Completely saturated fats and completely polyunsaturated fats are very rare in nature. Whether a fat is solid or liquid depends of the relative amounts of the different types of fatty acids and the temperature. Unsaturated fats tend to be liquid at room temperature and contain more unsaturated fatty acids. Most saturated fats are solid at room temperature and contain more saturated fatty acids. Exceptions are tropical oils, such as palm oil, palm kernel oil, coconut butter and coconut oil. Even though these fats are liquid at room temperature, they are very saturated.
In food processing polyunsaturated fats may be modified because unsaturated fats are less pliable and tend to go rancid. When oils have been hydrogenated, they become more solid. Margarine is an example of liquid polyunsaturated oil that is hydrogenated to become more solid. In the process of hydrogenation, hydrogens are added to the unsaturated carbons in a double bond making the fatty acid more saturated. In addition, some fatty acids that remain unsaturated after hydrogenation change shape from a cis to a trans configuration. The relationship between trans-fatty acids and heart disease has been the subject of much recent research.
The dominate fatty acid in the fat determines whether the fat is refereed to as saturated or unsaturated. The following table shows the fatty acids composition of some common fats, oils, and foods. Lard, for example, contains over one-third saturated fatty acids. Even though the other fatty acids are monounsaturated or polyunsaturated, it is refereed to as a saturated fat. Corn oil contains significant amounts of both saturated and monounsaturated fatty acid. However, since about half of the fatty acids in corn oil are polyunsaturated, it is refereed to as a polyunsaturated fat
Percentage of Different Fatty Acids in Some Common Fats
Common Fats/Oils
Saturated Fatty Acids (%)
Monounsaturated Fatty Acids (%)
Polyunsaturated Fatty Acids (%)
Corn
13
25
55
Olive
17
71
10
Palm
52
38
10
Rapeseed (Canola oil)
7
53
22
Soybean
15
23
51
Butter
65
26
1-3
Lard
42
46
6-8
Tallow
53
42
2
Dietary fats are necessary for a healthy body. But just like any good thing, fats can be overdone. Fats have many functions in our bodies. Fats along with protein are a basic part of cell membranes, and are components for many body compounds. Body fat cushions internal organs and protects them from being damaged. The fat layer below the skin insulates the body from heat loss. Fats also are important sources of the fat-soluble vitamins, A, D, E and K. Fats are a concentrated source of energy. One gram of fat provides 9 calories. Our bodies use this energy or store it as fat for later use. The reason that we store energy as fat instead of carbohydrate is because fats are calorie dense. Almost half the weight of fat compared to carbohydrate is needed to provide the same amount of calories. In addition, water is also stored with carbohydrates; further increasing stored carbohydrate weight. This means the body has less weight to carry. Plants don’t move around and so the weight of stored energy is not a problem. Plants can easily store energy as the bulkier carbohydrate. Excess fat, carbohydrate and protein, beyond what our body needs are converted into fats and stored for later use.
Linoleic acid and linolenic acid are two essential fatty acids. From these two polyunsaturated fatty acids the body can make other fatty acids the body needs that are important in the maintaining the structure of cell membranes, make hormone like compounds that help regulate blood pressure, blood clotting, blood lipids, and immune response to injury and infection.
Although fat is important in the diet, many adults eat more fat than they should. High fat diets are a risk factor for heart disease, cancer, and obesity. By increasing the risk of obesity, high fat diets may indirectly increase risk of diabetes and high blood pressure. The American Heart Association recommends that 30 percent or less of total calories come from fat. Lower fat intake by choosing lean meats and low-fat dairy products, and using low-fat preparation methods. Limit the use of added or hidden sources of fat.
Sterols
Sterols are compounds that have a multiple-ring structure. Cholesterol is one of the best known sterols. Both plant and animal foods contain sterols, but only animal foods contain cholesterol.
The body needs cholesterol every day. Cholesterol serves as the starting material for many important body compounds including bile acids, s*x hormones, adrenal hormones, and vitamin D. Cholesterol is also need to form cell membranes and the myelin sheath around nerve fiber. The liver makes about 800 to 1500 mg of cholesterol each day, much more that the cholesterol provided in the diet out of fragments from fats, proteins, and carbohydrates. Cholesterol is made in many cells, but liver cells make the majority of cholesterol. In a healthy body, if we eat more dietary cholesterol, the body makes less cholesterol. If we eat less dietary cholesterol, the body makes more cholesterol.
Cholesterol is carried in the blood to body tissues where it is used. Our blood contains only seven percent of total cholesterol. However, high blood cholesterol levels are an important risk factor for coronary heart disease. Dietary fats can affect blood cholesterol levels. Saturated fats increase blood cholesterol levels in responsive people. Dietary cholesterol has a fairly small effect on blood cholesterol level.
Phospholipids
Phospholipids are compounds that have both a water-soluble group (phosphate group) and a fat-soluble group (fatty acid). This allows phospholipids to dissolve in both water and fat. Lecithin is one of the best known phospholipids. Lecithin is similar to a triglyceride. Lecithin is composed of glycerol with two fatty acids and a phosphate group attached. Lecithin is used as an emulsifier in many food products to keep fat particles dispersed in water, such as salad dressings and mayonnaise. Lecithin also occurs naturally in many foods that have a high fat content such as eggs, soybeans, and peanuts. In the body phospholipids are an important structural part of cell membranes. Because phospholipids can dissolve in both fat and water they can help fats and fat like compounds, such as fat soluble vitamins and hormones, to move across cell membranes. Phospholipids also help keep fat and fat like compounds dispersed in the blood which is water based. The body can make lecithin in the liver.
Digestion, Absorption and Transport
Little digestion of lipids occurs in the mouth and stomach. Lipids tend to separate from water, whereas the digestive enzymes are soluble in water. This makes digestion of lipids more complicated. When fat enters the small intestine, bile is released which is an emulsifier. Bile helps disperse fat into small particles in the watery fluids so that the fat digestive enzymes can work. The fat digestive enzymes remove the fatty acids from triglycerides, leaving monoglycerides, free fatty acids and free glycerol. The fat digestive enzymes also remove the fatty acids from phospholipids. Serols are absorbed unchanged.
The products of lipid digestion are absorbed by intestinal cells. From the intestine glycerol, short-chain fatty acids and medium chain fatty acids are released into the bloodstream. Larger monoglycerides and long-chain fatty acids are not soluble in the water based blood, and once inside the intestinal wall they reform triglycerides. Phosholipids also reform inside the intestinal wall. Within the intestinal wall, the newly formed triglycerides, phospholipids and cholesterol are packaged into a carrier called a chylomicron. Chylomicrons are released into the lymph system which enters the bloodstream near the heart. The blood can then carry these lipids to the rest of the body. Chylomicrons carry lipids from the diet to body cells where lipids are removed.
Liver cells use fatty acids from the blood to make new triglycerides, cholesterol, and phospholidis. The liver is the most active site of lipid synthesis. In the liver newly formed lipids are packaged into a carrier called a very-low-density-lipoprotein (VLDL) and released into the bloodstream. As the VLDL is carried through the blood body cells remove triglycerides, causing the VLDL to get smaller. The VLDL also picks up cholesterol from other lipoproteins, eventually becoming a low-density lipoprotein (LDL). The LDL has a high cholesterol content and low triglyceride content. As the LDL is carried through the blood body cells remove cholesterol and phospholipids. LDL receptors on the liver play an important role in removing LDL from circulation. The liver also makes another carrier called high-density-lipoprotein (HDL). The HDL is released from the liver and circulates in the blood picking up cholesterol and phospholipids from body cells and returning them to the liver to be removed from the body. LDL carries cholesterol out to body cells and is associated with increased risk of heart disease. HDL carries cholesterol away from body cells back to the liver to be removed and is associated with decreased risk of heart disease.
