The Chemistry of Lipids

The diverse and vital functions that lipids perform in the body reveal why eating too little fat can be harmful. As mentioned earlier, though, too much fat in the diet seems to be the greater problem for most people. To understand both the beneficial and harmful effects that fats exert on the body, a closer look at the structure and function of members of the lipid family is in order.

Triglycerides

When people talk about fat—for example, “I’m too fat” or “That meat is fatty”—they are usually referring to triglycerides. Among lipids, triglycerides predominate—both in the diet and in the body. The name triglyceride almost explains itself: three (tri) fatty acids attached to a glycerol “backbone.”

Fatty Acids

When energy from any energy-yielding nutrient is to be stored as fat, the nutrient is first broken into small fragments. Then the fragments are linked together into chains known as fatty acids. The fatty acids, three at a time, are then packaged with glycerol to make triglycerides.

Chain Length and Saturation

Fatty acids may differ from one another in two ways—in chain length and in degree of saturation. The chain length refers to the number of carbons in a fatty acid. Saturation also refers to its chemical structure—specifically, to the number of hydrogen atoms the carbons in the fatty acid are holding. If every available carbon is filled to capacity with hydrogen atoms, the chain is called a saturated fatty acid. A saturated fatty acid is fully loaded with hydrogen atoms and has only single bonds between the carbons.

Unsaturated Fatty Acids

In some fatty acids, including most of those in plants and fish, hydrogen atoms are missing from the fatty acid chains. The places where the hydrogen atoms are missing are called points of unsaturation, and a chain containing such points is called an unsaturated fatty acid. An unsaturated fatty acid has at least one double bond between its carbons. If there is one point of unsaturation, the chain is a monounsaturated fatty acid.

Hard and Soft Fat

A triglyceride can contain any combination of fatty acids—long chain or short chain and saturated, monounsaturated, or polyunsaturated. The degree of saturation of the fatty acids in a fat influences the health of the body (discussed in a later section) and the characteristics of foods. Fats that contain the shorter chain or the more unsaturated fatty acids are softer at room temperature and melt more readily.

A comparison of three fats—lard (which comes from pork), chicken fat, and safflower oil—illustrates these differences: lard is the most saturated and the hardest; chicken fat is less saturated and somewhat soft; and safflower oil, which is the most unsaturated, is a liquid at room temperature.

Stability

Saturation also influences stability. Fats can become rancid when exposed to oxygen. Polyunsaturated fatty acids spoil most readily because their double bonds are unstable. The oxidation of unsaturated fats produces a variety of compounds that smell and taste rancid; saturated fats are more resistant to oxidation and thus less likely to become rancid. Other types of spoilage can occur due to microbial growth, however. Manufacturers can protect fat-containing products against rancidity in three ways—none of them perfect.

First, products may be sealed airtight and refrigerated— an expensive and inconvenient storage system.

Second, manufacturers may add antioxidants to compete for the oxygen and thus protect the oil (examples are the additives BHA and BHT and vitamins C and E).

Third, manufacturers may saturate some or all of the points of unsaturation by adding hydrogen atoms—a process known as hydrogenation.

Hydrogenation

Hydrogenation offers two advantages: it protects against oxidation (thereby prolonging shelf life) and also alters the texture of foods by increasing the solidity of fats. When partially hydrogenated, vegetable oils become spreadable margarine.

Hydrogenated fats make piecrusts flaky and puddings creamy. A disadvantage is that hydrogenation makes polyunsaturated fats more saturated. Consequently, any health advantages of using polyunsaturated fats instead of saturated fats are lost with hydrogenation.

Trans-Fatty Acids

Another disadvantage of hydrogenation is that some of the molecules that remain unsaturated after processing change shape from cis to trans. In nature, most unsaturated fatty acids are cis-fatty acids—meaning that the hydrogen atoms next to the double bonds are on the same side of the carbon chain.

Only a few fatty acids in nature (notably a small percentage of those found in milk and meat products) are trans-fatty acids—meaning that the hydrogen atoms next to the double bonds are on opposite sides of the carbon chain. These arrangements result in different configurations for the fatty acids, and this difference affects function: in the body, trans-fatty acids behave more like saturated fats, increasing blood cholesterol and the risk of heart disease.

Researchers are trying to determine whether the health effects of naturally occurring trans fats differ from those of commercially created trans fats. In any case, the important distinction is that intake of naturally occurring trans-fatty acids is typically low. At current levels of consumption, naturally occurring trans fats are unlikely to have adverse effects on blood lipids. The naturally occurring trans-fatty acid conjugated linoleic acid may even have health benefits.

Essential Fatty Acids

Using carbohydrate, fat, or protein, the human body can synthesize all the fatty acids it needs except for two—linoleic acid and linolenic acid. Both linoleic acid and linolenic acid are polyunsaturated fatty acids. Because they cannot be made from other substances in the body, they must be obtained from food and are therefore called essential fatty acids.

Linoleic acid and linolenic acid are found in small amounts in plant oils, and the body readily stores them, making deficiencies unlikely. From both of these essential fatty acids, the body makes important substances that help regulate a wide range of body functions: blood pressure, clot formation, blood lipid concentration, the immune response, the inflammatory response to injury, and many others. These two essential nutrients also serve as structural components of cell membranes.

Linoleic Acid: An Omega-6 Fatty Acid

Linoleic acid is an omega-6 fatty acid, found in the seeds of plants and in the oils produced from the seeds. Any diet that contains vegetable oils, seeds, nuts, and whole-grain foods provides enough linoleic acid to meet the body’s needs. Researchers have long known and appreciated the importance of the omega-6 fatty acid family.

Linolenic Acid and Other Omega-3 Fatty Acids

Linolenic acid belongs to a family of polyunsaturated fatty acids known as omega-3 fatty acids, a family that also includes EPA and DHA. EPA and DHA are found primarily in fish oils. As mentioned, the human body cannot make linolenic acid, but given dietary linolenic acid, it can make EPA and DHA, although the process is slow.

The importance of omega-3 fatty acids has been recognized since the 1980s, and research continues to unveil impressive roles for EPA and DHA in metabolism and disease prevention. The brain has a high content of DHA, and both EPA and DHA are needed for normal brain development.6 DHA is also especially active in the rods and cones of the retina of the eye.

Today, researchers know that these omega-3 fatty acids are essential for normal growth and development and that they may play an im- portant role in the prevention and treatment of heart disease.

Phospholipids

Up to now, this discussion has focused on one class of lipids, the triglycerides (fats and oils), and their component parts, the fatty acids. Two other classes of lipids, the phospholipids and sterols, make up only 5 percent of the lipids in the diet, but they are nevertheless worthy of attention. Among the phospholipids, lecithins are of particular interest.

Structure of Phospholipids

Like the triglycerides, the lecithins and other phospholipids have a backbone of glycerol; they differ from the triglycerides in having only two fatty acids attached to the glycerol. In place of the third fatty acid, they have a phosphate group (a phosphorus-containing acid) and a molecule of choline or a similar compound. The fatty acids make the phospholipids soluble in fat; the phosphate group enables them to dissolve in water. Such versatility benefits the food industry, which uses phospholipids as emulsifiers to mix fats with water in such products as mayonnaise and candy bars.

Phospholipids in Foods

In addition to the phospholipids used by the food industry as emulsifiers, phospholipids are also found naturally in foods. The richest food sources of lecithin are eggs, liver, soybeans, wheat germ, and peanuts.

Roles of Phospholipids

Lecithins and other phospholipids are important constituents of cell membranes. They also act as emulsifiers in the body, helping to keep other fats in solution in the watery blood and body fluids. In addition, some phospholipids generate signals inside the cells in response to hormones, such as insulin, to help alter body conditions.

Sterols

Sterols are large, complex molecules consisting of interconnected rings of carbon. Cholesterol is the most familiar sterol, but others, such as vitamin D and the sex hormones (for example, testosterone), are important, too.

Sterols in Foods

Foods derived from both plants and animals contain sterols, but only those from animals—meats, eggs, fish, poultry, and dairy products—contain significant amounts of cholesterol. Organ meats, such as liver and kidneys, and eggs are richest in cholesterol; cheeses and meats have less. Shellfish contain many sterols but much less cholesterol than was previously thought. Sterols other than cholesterol are naturally found in plants. Being structurally similar to cholesterol, plant sterols interfere with cholesterol absorption. Food manufacturers have fortified foods such as margarine with plant sterols, creating a functional food that helps to reduce blood cholesterol.

Cholesterol Synthesis

Like the lecithins, cholesterol can be made by the body, so it is not an essential nutrient. Right now, as you read, your liver is manufacturing cholesterol from fragments of carbohydrate, protein, and fat. Most of the body’s cholesterol ends up in the membranes of cells, where it performs vital structural and metabolic functions.

Cholesterol’s Two Routes in the Body

After it is made, cholesterol leaves the liver by two routes:

  1. It may be incorporated into bile, stored in the gallbladder, and delivered to the intestine.
  2. It may travel, via the bloodstream, to all the body’s cells.

The bile that is made from cholesterol in the liver is released into the intestine to aid in the digestion and absorption of fat. After bile does its job, most of it is absorbed and reused by the body; the rest is excreted in the feces.

Cholesterol Excreted

While bile is in the intestine, some of it may be trapped by soluble fibers or by some medications, which carry it out of the body in feces. The excretion of bile reduces the total amount of cholesterol remaining in the body.

Cholesterol Transport

Some cholesterol, packaged with other lipids and protein, leaves the liver via the arteries and is transported to the body tissues by the blood. These packages of lipids and proteins are called lipoproteins. As the lipoproteins travel through the body, tissues can extract lipids from them. Cholesterol can be harmful to the body when it forms deposits in the artery walls. These de- posits contribute to atherosclerosis, a disease that can cause heart attacks and strokes.

Summary

The predominant lipids both in foods and in the body are triglycerides, which have glycerol backbones with three fatty acids attached.

Fatty acids vary in the length of their carbon chains and their degree of saturation. Those that are fully loaded with hydrogen atoms are saturated; those that are missing hydrogen atoms and therefore have double bonds are unsaturated (monounsaturated or polyunsaturated).

Most triglycerides contain more than one type of fatty acid.

Fatty acid saturation affects the physical characteristics and storage properties of fats.

Hydrogenation, which makes polyunsaturated fats more saturated, gives rise to trans-fatty acids, altered fatty acids that may have health effects similar to those of saturated fatty acids.

Linoleic acid and linolenic acid are essential nutrients. In addition to serving as structural parts of cell membranes, they make powerful substances that help regulate blood pressure, blood clot formation, and the immune response.

Phospholipids, including the lecithins, have a unique chemical structure that allows them to be soluble in both water and fat.

In the body, phospholipids are major constituents of cell membranes; the food industry uses phospholipids as emulsifiers.

Sterols include cholesterol, bile, vitamin D, and the sex hormones.

Only animal-derived foods contain significant amounts of cholesterol.

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