Nutrition 331 Nutrition for Health

Study Guide

Unit 6: The Lipids: Fats, Oils, Phospholipids, and Sterols

Introduction

Dietary lipids have gained much attention because of growing evidence of their association with chronic lifestyle diseases such as heart disease, cancer, and obesity. Despite bad press, dietary lipids have an essential role. They provide the most concentrated form of energy (9 kcal per gram of fat) and are the source of essential fatty acids and omega‑3 fats, EPA, and DHA. As with all nutrients, overconsumption of lipids will result in adverse health effects. In our affluent society, most people tend to consume too much fat—especially the types that may contribute to chronic diseases.

In this unit, we examine the chemical and physical characteristics of various types of lipids and discuss their sources. While there are more than fifty types of fatty acids in the diet, this unit will focus on ten of them. We will study the processes of digestion, absorption, and transport of lipids, and review the functions of lipids and the effects of diet on blood cholesterol. Then we will look at trends in fat consumption by Canadians, the current recommendations for fat intake, and the effects of food processing on dietary lipids.

Objectives

After completing this unit, you should be able to

1. identify the chemical characteristics of triglycerides, phospholipids, and sterols.
2. describe seven functions of lipids in the body.
3. describe the processes of digestion, absorption, and transport of dietary lipids.
4. identify the two families of essential fatty acids.
5. describe nine roles of essential fatty acids.
6. describe the problem of lipid oxidation and explain how it can be prevented.
7. discuss the purpose of hydrogenating fats and how this affects the nutritional composition of fats.
8. discuss the health effects of trans fats.
9. identify the major sources of different types of dietary lipids and fatty acids discussed in this unit.
10. discuss current dietary recommendations on the dietary intake of fats.

Section 1: Chemistry and Classification of Lipids

Lipids are compounds composed primarily of carbon, hydrogen, and small amounts of oxygen. They can be defined in general terms as organic substances that can be used by living beings. Lipids are generally insoluble in water. Even though the terms lipids and fats are often used interchangeably (in this course, as elsewhere), in strict chemical terms, fats belong to a subgroup under the general nutrient class of lipids. This subgroup, called the triglycerides, includes fats that are usually solid and oils that are usually liquid at room temperature. Triglycerides make up 95 percent of dietary lipids. The other 5 percent is contributed by phospholipids, sterols, fat-soluble vitamins, waxes, and other minor complex lipid compounds. In the body, triglycerides make up 99 percent of stored fats.

Since terms like cholesterol and triglyceride may apply to both foods and to the body, make sure you clarify the topic under discussion.

You need not memorize chemical structures, but you should be able to identify the chemical components characteristic of each category of lipid, as outlined below.

Phospholipids (See the eText, p. 171 for a description of phospholipids.) Phospholipids are found in almost all cells as a major constituent of cell membranes. Unlike triglycerides and sterols, they have the ability to attract water-soluble as well as fat-soluble substances; they can therefore help to facilitate the passage of fats in and out of cells through cell membranes. Phospholipids in the blood and body fluids also act as emulsifiers, keeping fats in solution.

An example of a dietary phospholipid is lecithin. Lecithin and other phospholipids occur naturally in liver, egg yolk, and soybeans. As a food additive, lecithin serves as an emulsifier in such foods as processed cheese, margarine, candy, and chocolate bars. Phospholipids make up only a very small proportion of dietary lipids. When ingested, phospholipids are hydrolyzed by intestinal enzymes and can be used to produce energy. They are not an essential nutrient since the body can synthesize them as needed.

Sterol is a lipid with a characteristic hydrocarbon ring structure; all sterols are derived from cholesterol, and they resemble chicken wire. Sterols are not soluble in water. They do not contribute energy to the body.

The best known example of a sterol is cholesterol. About 80 percent of cholesterol is synthesized in the body (mainly by the liver); this portion is called endogenous cholesterol. Cholesterol from dietary sources, also called exogenous cholesterol, contributes the remainder. Adult dietary cholesterol intake generally ranges from 200 to 400 milligrams per day. All cholesterol is of animal origin; plants do not make cholesterol.

The body uses cholesterol as an essential component for cell membranes. Cholesterol is also the precursor for the synthesis of bile acids, hormones, and vitamin D. A high blood cholesterol level is a major risk factor for coronary heart disease, as we will see later in the unit.

Triglyceride is a lipid composed of a glycerol molecule, to which are attached three fatty acids (organic acids that consist of hydrocarbon chains). Each chain has a carboxylic acid at one end and a methyl group at the other. Figure 5-3: Triglyceride Formation (p. 168) describes how three fatty acids are attached to a glycerol to form a triglyceride. If only one fatty acid is attached to a glycerol molecule, a monoglyceride is formed; if two fatty acids are attached, a diglyceride is formed. Monoglycerides and diglycerides are used by food processors. They are used as emulsifiers to give foods a smooth texture and to keep fats evenly dispersed. Foods such as ice cream, baked goods, margarine, lard, and shortening may contain monoglycerides and diglycerides. Monoglycerides can also be found in the human gastrointestinal tract after a fatty meal, as a product of fat digestion.

Dietary lipids are typically composed of mixed triglycerides, which means that three different fatty acids are attached to each glycerol. These fatty acids can vary in length (even-numbered, from 4–30 carbon atoms) and in degree of unsaturation (number of double carbon bonds along the chains). The nutritional significance of triglycerides is governed by the composition of these fatty acids, which we will discuss later. Triglycerides predominate in dietary fats. Visible dietary fats are found in foods such as butter, margarine, lard, vegetable oil, salad dressing, the fatty portion of meat, and chicken skin. Foods that contribute invisible fats include cream, milk, cheese, egg yolk, meat, fish, poultry, nuts, seeds, olives, avocados, baked foods, and fried foods.

Fatty acids are the building blocks of lipids. They can be classified according to chain length:

• Short-chain fatty acids have 4–8 carbons.
• Medium-chain fatty acids have 10–14 carbons.
• Long-chain fatty acids have 16–22 carbons.

Another way of classifying fatty acids is by the degree of unsaturation; that is, the number of double carbon bonds formed along the chain. The following are characteristics of fatty acids:

• Saturated fatty acids have no double bond.
• Monounsaturated fatty acids have one double bond.
• Polyunsaturated fatty acids have two or more double bonds.

The length of the carbon chains and the degree of unsaturation of fatty acids determine the hardness of a fat or oil: the longer and more unsaturated the fatty acids, the more liquid the fat or oil. Dietary lipids that contain saturated short- and medium-chain fatty acids are butterfat and coconut oil. The majority of fats and oils found in nature are made up of long-chain fatty acids.

* Omega‑6 and omega‑3 fatty acids are often referred to as n‑6 and n‑3 fats. These terms are interchangeable.

Natural fats and oils contain a great variety of different types of fatty acids and triglycerides. In general, fats of animal origin tend to contain large amounts of saturated fats, which are solid at room temperature. Oils found in plants and seeds, however, usually have large amounts of oleic and linoleic acids, which are unsaturated and liquid at room temperature (and hence referred to as oils). Exceptions are coconut and palm oils (tropical oils) that are of plant origin but are highly saturated. Among the animal fats, the degree of unsaturation also varies depending on the diet consumed by the animal. For instance, beef tallow and mutton tallow are harder and more saturated than pork fat, and pork fat is harder and more saturated than poultry fat.

Table 6.1: Fatty acids

Figure 5-5: Fatty Acid Composition of Common Food Fats (p. 170) shows a comparison of dietary fats. Note the major sources for the different types of fats. Also note that oils and fats contain varying levels of saturated fats, MUFA, and PUFA. Fats are usually classified according to their predominant category, based on saturation.

Section 2: Digestion, Absorption, and Transport

Dietary lipids, which are primarily triglycerides, are water-insoluble. Lipid molecules tend to float in watery fluids. This tendency presents the body with two challenges. The first is to mix lipids with the watery digestive juices so that lipids are exposed to digestive enzymes. Mixing is made possible through the emulsifying action of bile, which produces fat droplets, called micelles, in the digestive juices.

The second challenge is transporting lipids around the body via body fluids. After digestion and absorption, lipids (triglycerides, phospholipids, and cholesterol) cluster together with protein, forming lipoprotein complexes. The lipoprotein packages serve practical roles in the body. They are also used as indicators of the health of the arteries. Arterial health will be explored further in Unit 12. These lipoproteins can be transported by the aqueous media of lymph and blood without separating. All lipoproteins contain triglycerides, phospholipids, cholesterol, and proteins in varying proportions.

In the body, there are four main classes of lipoproteins: chylomicrons, very low-density lipoproteins, low-density lipoproteins, and high-density lipoproteins.

Chylomicrons are the lipoproteins formed in the mucosal cells after absorption of dietary lipids. Since chylomicrons are composed mainly of triglycerides, they have a very low density (i.e., they float easily). After a meal, chylomicrons are first released into the lymphatic system and then into the bloodstream at a point near the heart to circulate to different parts of the body. Cells all over the body remove the lipids from the chylomicrons as they pass by. Liver and adipose cells are particularly active in taking up lipids from the chylomicrons.

Very low-density lipoproteins (VLDL) are the lipoproteins formed in the liver. The liver picks up various compounds from the blood as well as excess carbohydrates, proteins, and alcohol and converts them into triglycerides, cholesterol, fatty acids, and other compounds. Like chylomicrons, VLDL are composed of a high proportion of triglycerides. VLDL are carried to other parts of the body where cells remove the triglycerides and use them for energy, storage, or synthesis of other body compounds. Removal of the triglycerides results in the formation of low-density lipoproteins (LDL).

LDL contain little triglyceride but are high in cholesterol. As LDL circulate throughout the body, peripheral tissues (outside the liver) such as muscle, arterial walls, adipose tissue, and mammary glands pick up cholesterol and phospholipids for synthesis of membranes, hormones, and so on. Special LDL receptors in the liver normally remove LDL from circulation thereby controlling the cholesterol concentration in the blood.

High-density lipoproteins (HDL) contain much protein. HDL differ from LDL in their transportation role in the body. Instead of transporting cholesterol from the liver to peripheral tissues, HDL carry cholesterol and phospholipids from peripheral tissues back to the liver for recycling or disposal. HDL have an important role in helping to remove excess cholesterol.

Section 3: Functions of Fats and Essential Fatty Acids

The study of lipids and their roles in health and disease has been the focus of research for many years, particularly since the demonstration of a relationship between the blood cholesterol level and heart disease. A phenomenal number of research studies have been reported, many of which are controversial. Nevertheless, strong scientific evidence confirms both beneficial and harmful health effects of dietary fats.

While too much dietary fat, particularly saturated and trans fats, may cause health problems, fats do play important roles in our body. These functions are outlined on pages 165–166 of the eText and are summarized in Table 5-1: The Usefulness of Fats. There are also the essential fatty acids, discussed next.

When the term essential appears with a nutrient, it means the body is unable to produce that nutrient in amounts sufficient for normal functioning; the nutrient must be supplied by the diet.

Two families of polyunsaturated fatty acids are considered essential: the omega‑6 and the omega‑3 fatty acid families. These fatty acid series are not interchangeable. The position of the double bonds is a small but critical difference in the chemical structure, creating very different effects on the body. This will be discussed later in the unit.

Linolenic acid is the “parent” fatty acid of the omega‑3 series. Linolenic acid has distinct nutritional functions and serves as the precursor for two other fatty acids in this series: EPA and DHA.

The functions of essential fatty acids in the body are described in Table 5-3: Functions of the Essential Fatty Acids (p. 181). Essential fatty acids are also parts of the lipoproteins used to transport fat. The roles of EPA and DHA in supporting infant growth and development and lowering risks for heart disease and cancer are listed in Table 5-4: Potential Health Benefits of Fish Oils (p. 182). The role of fish in reducing the risk of heart disease will be discussed further in Unit 12.

The omega‑6 and omega‑3 fatty acids have different abilities to stimulate or slow down processes in the body. The body needs about ten times as much linoleic acid as linolenic acid to balance eicosanoid effects. (See Dietary Reference Intakes for young adults (19–30 years old) for the recommendations for certain fatty acids in the eText, p. 180.)

Food sources of omega‑6 and omega‑3 fats are presented in Table 5-5 (p. 183). A few important comments on this list follow:

• Leafy vegetables (e.g., spinach, kale, collard greens) contain low levels of linoleic acid because they are virtually fat free.
• Soybean oil contains both omega‑3 and omega‑6 fats.

Deficiency of essential fatty acids is relatively rare in populations that consume a varied diet. However, cases of deficiency have occurred in infants fed with a low-fat formula or skim milk; patients suffering from serious burns, with loss of body fluid; people with bowel resections, who may suffer from severe fat malabsorption.

Classic signs of essential fatty acid deficiency in humans are growth retardation, skin lesions with characteristic dermatitis (eczema) and dry scaly skin, increased susceptibility to infections, and possible peripheral neuropathy and blurred vision.

While essential fatty acid deficiencies may be rare, a person’s actual intake may be less than ideal. When people are overzealous in consuming a low-fat diet, they may eliminate essential fatty acids, and a low intake of omega‑3 fatty acids may occur.

Section 4: Hydrogenation and Oxidation

Oxidation of lipids occurs when oxygen combines with an unsaturated fatty acid at the double bond. Oxidation gives oil the unpleasant odour and flavour characteristic of rancid fats. Oils, which contain a high percentage of polyunsaturated fatty acids, are the most susceptible to oxidation. Conditions that accelerate oxidation include the presence of oxygen, light, and heat and exposure to metals such as iron and copper.

Oils high in polyunsaturated fatty acids can be protected from oxidation in a number of ways. They should be stored in air-tight, dark containers at cool temperatures. Some highly unsaturated oils need refrigeration (e.g., flaxseed oil). Antioxidants such as BHA, BHT, and vitamin E may be added to oils to help prevent oxidation. These substances combine with oxygen, serving as oxygen scavengers. Vitamin E is not often used as a food additive because of cost. A final way to prevent oxidation is hydrogenation of oil, which reduces the number of double bonds and increases saturation.

Hydrogenation reduces oxidation of fats and prolongs the shelf life of foods. The eText explains how trans fatty acids are formed (Fig. 5-12, p. 185). The quantity of PUFA and essential fatty acids decrease as double bonds are eliminated.

There is now compelling evidence that trans fat is the worst type of fat with respect to heart health. For this reason, intake should be kept as low as possible. Until a few years ago, foods that listed partially hydrogenated oils among the first three ingredients usually contained substantial amounts of trans fatty acids as well as some saturated fat. Hard margarine had an especially high content. Other important sources were shortenings, most peanut butters, cookies, doughnuts, and french fries. In recent years, foods such as margarine and baked goods have been reformulated to reduce levels of trans fats. Foods sold in Canada now contain virtually none of these fats.

Section 5: Recommendations for Dietary Fat

During the 1980s and 1990s, dietary fat was widely seen as a major cause of many health problems. As a result, nutrition recommendations strongly emphasized reducing the amount of fat in the diet. A common recommendation was that dietary fat should not exceed 30 percent of energy intake. However, it is now generally accepted that the type of fat is far more important than the amount of fat. In Unit 3, Section 2, we discussed the acceptable macronutrient distribution ranges. These state that for all people older than 18 years, fats should provide 20 to 35 percent of energy.

For the purposes of this course, use the guideline of less than 10 percent of calories from saturated fat while consuming a diet that is nutritionally adequate. This recommendation is challenging to interpret as some foods that contain saturated fats provide significant amounts of protein, vitamins, and minerals. A few such nutritious foods are cheese and beef. The recommendation for saturated fat is based heavily on the belief that saturated fat causes a rise in the blood cholesterol and thence increases the risk of heart disease. However, this relationship has been questioned in recent years, as will be discussed in Unit 12.

Linoleic and linolenic acid are the only two fatty acids with specific adequate intakes.

Section 6: Fats in the Diet

Of all the nutrients, lipids are most often associated with conditions such as obesity and heart disease. The eText points out that the Canadian diet today provides about 31 percent of energy as fat (p. 187). As mentioned in the previous section, fat should provide between 20 percent and 35 percent of energy. At first glance, therefore, dietary fat is not a problem. However, the average Canadian consumes too much dietary energy overall, and fat is an integral part of this problem. Excess dietary fat and carbohydrate is the fundamental cause of obesity and all its associated problems.

As you review sources of fat in the diet, be cautious when choosing lower fat foods. Some foods, such as low-fat yogurt, may have a similar calorie level as a higher fat product. This is because sugar or starch is often added.

You are expected to know which foods are relatively high or low in fat, saturated fat, MUFA, PUFA, omega‑3, omega‑6, and dietary cholesterol. You should also know the specific fatty acids presented in Table 6.1: Fatty acids (Unit 6 of this Study Guide). Continue to examine food labels in your refrigerator and cupboards for their levels of total fats and types of fats.

Review Questions

See eText Chapter 5, Self-Check:

• questions 1, 2, 4, 5, 6, and 10 (p. 203)

Examination Request

You are required to write a midterm examination after you have completed Unit 8. If you haven’t already done so, make arrangements to write your examination and ensure you are requesting the midterm exam.