Types of Lipids

Triglycerides

Triglycerides are the most common type of lipid found in foods and in the body. About 95% of the fats we eat and 95% of the fat stored in the body are in the form of triglycerides. 

Structure:

Triglycerides are built from a glycerol backbone and 3 fatty acids. Glycerol has 3 carbons in its chain. A triglyceride forms when each hydroxyl group on the glycerol backbone bonds with the hydrogen atom from the acid (carboxyl) end of a fatty acid. The bond between a fatty acid and glycerol is called an ester bond. One molecule of water forms each time an ester bond forms (this is called esterification), Thus, when a diglyceride (2 fatty acids attached to a glycerol backbone) forms, 2 molecules of water form. Similarly, forming a triglyceride will generate 3 water molecules. Esterification is a condensation reaction. A molecule of water is used when a fatty acid breaks away from a glycerol backbone (this hydrolysis reaction can be thought of as "de-esterification"). Reattaching the fatty acid to a glycerol backbone ( called re-esterification) will produce a water molecule.

Saturation:

Fatty acids can be saturated, monounsaturated, or polyunsaturated. To understand saturation, remember, a carbon atom can form 4 chemical bonds, an oxygen atom can form 2 bonds, and a hydrogen atom can form only 1 bond.

In saturated fatty acid (SFA), every carbon in the chain has formed the maximum of 4 bonds. It is a saturated fatty acid because all the bonds between the carbons are single connections and other carbon bonds are filled with hydrogens. In monounsaturated fatty acid (MUFA), 8 to 9 carbons in the chain are each missing 1 hydrogen, these carbons formed a double bond between each other by each giving up 1 hydrogen. Fatty acids that have 1 double bond in the carbon chain are called monounsaturated fatty acids. They have 1 location in the carbon chain that is not saturated with hydrogen. This monosaturated fatty acid in oleic acid.

A polyunsaturated fatty acid (PUFA) has at least 2 double bonds in its carbon chain. This polyunsaturated fatty acid is called linoleic acid.

Shape:

The shape of the carbon chain varies with saturation. Saturated and trans fatty acids have straight carbon chains, and unsaturated cis fatty acids have bent or linked carbon chains. In cis fatty acids, the hydrogens attached to the double-bonded carbons on the same side of the carbon chain. In trans fatty acids, the hydrogens attached to the double-bonded carbons zigzag back and forth across the carbon chain. The trans fatty acid, which has the hydrogens next to the double bonds on opposite sides of the carbon chain, is straight and resembles a saturated fatty acid.

Most unprocessed unsaturated fatty acids, such as oils freshly pressed from nuts and seeds, are in the cis form. Trans fatty acids are found mostly in the polyunsaturated oils modified by food manufactures using a process called hydrogenation.

Hydrogenation adds hydrogen to the carbon chain of unsaturated fats. As the amount of added hydrogen increases, the unsaturated fat becomes more and more saturated and increasingly solid. For example, corn oil, which is polyunsaturated and liquid at room temperature, can be hydrogenated; a little to make squeeze margarine, some to make tub margarine, and a lot to make stick margarine.

Hydrogenation is like putting children in some of the empty bus seats except, when the children are added to the bus, it changes the shape of the bus. The shape change occurs because hydrogenation creates trans fatty acids, which have a straighter shape than cis fatty acids.

Essential Fatty Acids:

Humans can synthesize a wide variety of fatty acids; but we cannot make 2 PUFAs; alpha-linoleic acid (the major omega-3 fatty acid in food) and linoleic acid (the major omrga-6 fatty acid in food). Alpha-linoleic acid and linoleic acid are essential fatty acids (EFAs) . We must get EFAs from foods because our bodies are unable to synthesize essential fatty acids with a double bond before the 9th carbon in the chain, counting from the omega end.

The location of the double bond closest to the omega carbon of the fatty acid identifies the fatty acid's family. If the first double bond of a polyunsaturated fatty acid occurs after the 3rd carbon from the methyl end, it is called an omega-3 fatty acid. If the first double bond occurs after the 6th carbon on a polyunsaturated fatty acid, it is called an omega-6 fatty acid.

Different eicosanoids are produced from dihomo-gamma-linoleic acid, arachidonic acid and eicosapentaenoic acid. Eicosanoids are hormonelike compounds, such as prostaglandins, prostacyclins, thromboxanes, leukotrienes, and lipoxins, that affect the body in the region where they are produced. ( They are called local hormones because unlike typical hormones, they are made and use in the same area of the body).

Food Sources of Triglycerides:

Most foods provide at least some triglycerides. Certain foods, such as animal fat and  vegetable oils, are primarily triglycerides. Bakery items, snack foods, and dairy desserts also contain significant amounts of fat. In contrast, fat-free milk and fat-free yogurt, as well as many breakfast cereals and yeast breads, contain little or no fat. Other than coconuts and avocados, fruits and vegetables are low in fat.

Fats from animal sources and tropical oils (coconut, palm, palm kernel) are rich in saturated fatty acids. Omega-3 fatty acid sources include cold-water fish (salmon, tuna, sardines, mackerel), walnuts, and flaxseed. Fish oil and flaxseed oil supplements are another source of omega-3 fatty acids.

Butter contains saturated, monounsaturated, and polyunsaturated fatty acids. Saturated fats are the predominant fatty acid in butter; so it is referred to as a saturated fat. Similarly, olive oil contains saturated, monosaturated, and polyunsaturated fatty acids, but monounsaturated fat predominates. Thus, it is referred to as a monounsaturated fat.

Functions of Triglycerides:

Triglycerides are essential for optimal health. They provide a concentrated source of energy, insulate and cushion vital organs, and help transport essential nutrients in the bloodstream. However, high intakes, especially of saturated and trans fats, and imbalances of EFAs can present health challenges.

Provide Energy:

Triglycerides in food and body fat cells are a rich source of energy, with each gram providing about 9 calories. Triglycerides are the main fuel source for all body cells, except the nervous system and red blood cells. When you are resting or engaging in light physical activity, triglycerides provide 30 to 70% of the energy you burn. 

Provide Compact Energy Storage:

Triglycerides are the body's main storage form of energy. Excess calories from carbohydrate, fat, protein, and alcohol can be converted to fatty acids and then to triglycerides. They make an excellent energy "saving account" because they are stable and calorie dense. Fat cells contain about 80% lipid and only 20% water and protein. Muscle cells also contain fat and protein but are 37% water. This difference means that the lipid rich fat cells can deliver much more energy than the water rich muscle cells. Another reason triglycerides make an excellent storage form of  energy is that they can expand to an average of  2 to 3 times their normal size.

Aid Fat-Soluble Vitamin Absorption & Transport:

Fats found in food carry fat-soluble vitamins (vitamin A, D,E and K) to the small intestine. Once there, dietary fat assists in the absorption of these vitamins. Fat-soluble vitamins transported in the bloodstream in the same manner as dietary fat. Those who eat an extremely low fat diet, use mineral oil as a laxative, take certain medications, or have diseases that affect fat absorption may be unable to absorb enough amount of fat-soluble vitamins.

Insulate & Protect the Body:

The insulating layer of fat just beneath the skin (called subcutaneous fat) is made mostly of triglycerides. By insulating the body, subcutaneous fat helps keep body temperature at a constant level.  We usually don't observe the insulating function of subcutaneous fat because we wear clothes and add more when needed. However, people who are starving lose most of their body fat and as a result, feel chilled even when the environment is warm.

Essential Fatty Acid Functions:

Essential fatty acid along with phospholipids and cholesterol, are important structural components of cell membranes. They also keep the cell membrane fluid and flexible so that substances can flow into and out of the cell membrane. The omega-3 fatty acid docosahexaenoic acid is needed during fetal lie and infancy for normal development and function of the retina. Starting in the first week of embryonic life, DHA is vital for normal development and maturation of the nervous system. Throughout life, DHA helps regulate nerve transmission and communication.

Eicosanoids, which are made from essential fatty acids, have over 100 different actions, such as regulating blood pressure, blood clotting, sleep/wake cycle, body temperature, inflammation or hypersensitivity reactions, stomach secretions, labor during childbirth, and immune and allergic responses. For example, some types of eicosanoids cause inflammation, whereas other types prevent the inflammation associated with inflammatory diseases and allergic reactions.

Eicosanoids also have other important roles in the body, many of which are only just being discovered. For example, they assist in :

• Maintaining normal kidney function and fluid balance
• Directing hormones to their target cells
• Regulating cell division rates, which may help prevent certain cancers or slow the growth of existing tumors and help prevent cancer from spreading to other parts of the body.
• Regulating ovulation, body temperature, immune system function and hormone synthesis.
• Regulating the flow of substances into and out of cells

Phospholipids

Many types of phospholipids are found in food and the body, especially the brain. The structure of these lipids is very similar to that of triglycerides __with 1 exception. One fatty acid is replaced with compound that contains the mineral phosphorus and often has nitrogen attached. Phosphate gives phospholipids an important quality __it lets these fats function in a watery environment without clumping together.

The phosphate end of the phospholipid is hydrophilic and will mix with water. The fatty acid end of the phospholipid is hydrophobic and is attracted to fats. When placed in water, phospholipids cluster together, with their hydrophilic heads facing outward in contact with the water and their hydrophobic tails extending into the cluster away from the water.

Functions:

In the body, phospholipids have 2 major roles; as a cell membrane component and an emulsifier. Phospholipids, along with fatty acids and cholesterol, are a primary component of cell membranes. Phospholipids provide structure to the cell membranes, which in turn keep organelles organized and divided to work efficiently, but this structure also aids in the membranes' flexibility and fluidity. Phospholipids also provide barriers in cell membranes to protect the cell, and they make barriers for the organelles with those cells. Phospholipids work to provide pathways for various substances across membranes. Membrane proteins stud the phospholipid bilayer, these respond to cell signals or act as enzymes or transporting mechanisms for the cell membranes.

Another function of phospholipids is that they work as emulsifiers in the body, such as when they are mixed with cholesterols an bile acid in the gallbladder to make micelles for fatty-substance absorption. Phospholipids also play the role of wetting of surfaces for such things as joints, alveoli and other parts of the body requiring smooth motion.

Phospholipids also involve in assembling circulating lipoproteins. These proteins play the essential role of transport for lipophilic triglycerides and cholesterol in the blood. 

The phospholipids in food often are used as an emulsifier in food preparation and manufacturing. Their ability to emulsify fats works the same in foods as it does in the body. Food manufactures add emulsifiers to keep the fat and watery compounds in them from separating. Emulsifying fats in foods such as cakes, muffins, and salad dressings gives them body and a smooth texture. Without emulsifiers, these foods would seem only and have a sandy or rough texture.

Food Sources of Phospholipids:

Phospholipids can be synthesized by the body or supplied by the diet. For example, lecithins are found in foods such as egg yolks, soybeans, wheat germ, and peanuts, Although lecithin supplements are available, they are not needed because the liver can produce enough amount of phospholipids. Lecithin supplements have been promoted as a way to lose weight, lower cholesterol, and reduce the risk of Alzheimer disease. Data are conflicting when it comes to lecithin's ability to lower cholesterol or Alzheimer disease risk. It is important to note that high doses of lecithin can cause gas, diarrhea, and weight gain.

Sterols

Sterols are the last type of lipid. Sterols are a type of steroid. The structure of sterols is very different from that of the long carbon chains seen in fatty acids and phospholipids. Instead, the carbons are mostly arranged in multi-ringed structures.

Sterol Functions:

From a nutrition perspective, cholesterol is the most well known sterol. This waxy substance is required to synthesize many compounds. For example, our bodies use cholesterol to make sex hormones, such as testosterone, and estrogens, the active form of vitamin D, and adrenal hormones such as cortisone. Cholesterol also is used to make bile, which is required to emulsify fat so that they can be digested normally.

In addition, cholesterol, along with phospholipids, forms cell membranes and allows fat-soluble substances to move into and out of the cells. Cholesterol, along with phospholipids and proteins, also forms the shell covering chylomicrons. This shell is what allows fat droplets to float through the water-based bloodstream.

Food Sources:

Cholesterol is found in foods or animal origin, such as meat, fish, poultry, eggs, and dairy products. Most people get about one-third of their cholesterol from the foods they eat, and the rest is manufactured by their bodies. Although plants do not contain or produce cholesterol, they do make other sterols, stanols, and sitostanols, such as ergosterol (a form of vitamin D) and beta sitosterols (added to some margarines, such as Take Control). Eating margarine that contains sitostanol can reduce the body's absorption of cholesterol and bile, which is made from cholesterol, thereby reducing blood cholesterol levels, which decreases the risk of heart disease.