Introduction

This chapter will learn about the three main types of lipids and their functions in our bodies. In the body, fat functions as an important depot for energy storage offers insulation and protection and plays important roles in regulating and signaling. Large amounts of dietary fat are not required to meet these functions because they can synthesize most fat molecules from other organic molecules like carbohydrates and protein (except for two essential fatty acids). However, fat also plays unique roles in the diet, including increasing the absorption of fat-soluble vitamins and contributing to food flavor and satisfaction. Let’s take a closer look at each of these fats’ functions in the body and the diet. Let’s begin by watching the video below for a brief introduction to lipids.

What is Lipid?

Lipids are important fats that serve different roles in the human body. The three main types of lipids are triacylglycerols (also known as triglycerides), phospholipids, and sterols.

1) Triglycerides make up more than 95 percent of lipids in the diet and are commonly found in fried foods, butter, milk, cheese, and some meats. Naturally occurring triacylglycerols are found in many foods, including avocados, olives, corn, and nuts. We commonly call the triglycerides in our food “fats” and “oils.” Fats are lipids that are solid at room temperature, whereas oils are liquid.

2) Phospholipids make up only about 2 percent of dietary lipids. They are water-soluble and are found in both plants and animals. Phospholipids are crucial for building the protective barrier, or membrane, around your body’s cells. In fact, phospholipids are synthesized in the body to form cell and organelle membranes. In blood and body fluids, phospholipids form structures in which fat is enclosed and transported throughout the bloodstream.

3) Sterols are the least common type of lipid. Cholesterol is perhaps the best well-known sterol. Though cholesterol has a notorious reputation, the body gets only a small amount of its cholesterol through food—the body produces most of it. Cholesterol is an important component of the cell membrane and is required to synthesize sex hormones, vitamin D, and bile salts.

What Lipids Do In the Body

Fats satisfy appetite (the desire to eat) because they add flavor to foods. Fat contains dissolved compounds that contribute to mouth-watering aromas and flavors. Fat also adds texture, making baked foods moist and flakey, fried foods crispy, and adding creaminess to foods like ice cream and cream cheese. Consider fat-free cream cheese; when fat is removed from the cream, much of the flavor is also lost. As a result, it is grainy and flavorless—nothing like its full-fat counterpart—and many additives are used to replace the lost flavor.

Fats satisfy hunger (the need to eat) because they’re slower to be digested and absorbed than other macronutrients. Dietary fat thus contributes to satiety—the feeling of being satisfied or full. When fatty foods are swallowed, the body responds by enabling the processes controlling digestion to slow the movement of food along the digestive tract, giving fats more time to be digested and absorbed and promoting an overall sense of fullness. Sometimes, before the feeling of fullness arrives, people overindulge in fat-rich foods, finding the delectable taste irresistible. Slowing down to appreciate the taste and texture of foods can give your body time to send satiety signals to your brain, so you can eat enough to be satisfied without feeling overly full.

Lipids perform many functions within the body:

1) Store Energy – When we take in more energy than we need, the body stores it as adipose tissue (fatty tissue, which we call fat). Carbohydrates and lipids provide most of the energy required by the human body. As discussed in the Carbohydrates unit, glucose is stored in the body as glycogen. While glycogen provides a ready source of energy, it is quite bulky with heavy water content, so the body cannot store much of it for long. Fats, on the other hand, can serve as a larger and more long-term energy reserve. Fats pack together tightly without water and store far greater amounts of energy in a reduced space. A fat gram is densely concentrated with energy, containing more than double the amount of energy as a gram of carbohydrate.

We draw on the energy stored in fat to help meet our basic energy needs when we’re at rest and fuel our muscles for movement throughout the day, from walking to class, playing with our kids, dancing through dinner prep, or powering through a shift at work. Historically, when humans relied on hunting and gathering wild foods or on crops’ success, storing energy as fat was vital to survival through lean times. Hunger remains a problem for people worldwide, and being able to store energy when times are good can help them endure a period of food insecurity. In other cases, the energy stored in adipose tissue might allow a person to weather a long illness.

Unlike other body cells that can store fat in limited supplies, fat cells are specialized for fat storage and can expand almost indefinitely in size. An overabundance of adipose tissue can be detrimental to your health, from mechanical stress on the body due to excess weight and hormonal and metabolic changes. Obesity can increase the risk for many diseases, including type 2 diabetes, heart disease, stroke, kidney disease, and certain types of cancer. It can also interfere with reproduction, cognitive function, and mood. Thus, while some body fat is critical to our survival and good health, it can be a deterrent to maintaining good health in large quantities.

2) Regulate and Signal – Lipids regulate the temperature of your body, keeping it steady, not too hot, and not too cold.  Lipids also help the body produce and regulate hormones for everything from appetite to the reproductive system to blood clotting.  Lipids are key to brain structure and function; the lipids form nerve cell membranes, insulate neurons (the cables that send messages throughout the body), and help send signals within the brain.

3) Insulate and Protect – Our bodies are padded with fat, protecting us from everyday friction. The average body fat for a man is 18 to 24 percent and for a woman is 25 to 31 percent^{1. Still, adipose} tissue can comprise a much larger percentage of bodyweight depending on the degree of obesity of the individual. Some of this fat is stored within the abdominal cavity, called visceral fat, and some are stored just underneath the skin, called subcutaneous fat. Visceral fat protects vital organs—such as the heart, kidneys, and liver. The blanket layer of subcutaneous fat insulates the body from extreme temperatures and helps keep the internal climate under control. It pads our hands and buttocks and prevents friction, as these areas frequently come in contact with hard surfaces. It also gives the body the extra padding required when engaging in physically demanding activities such as ice skating, horseback riding, or snowboarding. There are two types of fat stored as adipose tissue: subcutaneous fat and visceral fat.

4) Aid Digestion and Increase Bioavailability – When food breaks down in the gut, fat is released and combines with fat-soluble nutrients.  The combination of the fat and the nutrients allows the nutrients to be digested more easily and absorbed into the body.  This improved absorption is called increased bioavailability. Dietary fats can also increase the bioavailability of compounds known as phytochemicals—non-essential plant compounds considered beneficial to human health. Many phytochemicals are fat-soluble, such as lycopene found in tomatoes and beta-carotene found in carrots, so dietary fat improves the absorption of these molecules in the digestive tract.

In addition to improving the bioavailability of fat-soluble vitamins, some of the best dietary sources of these vitamins are also foods that are high in fat. For example, good vitamin E sources are nuts (including peanut butter and other nut butter), seeds, and plant oils such as those found in salad dressings. It isn’t easy to consume enough vitamin E if you’re eating a very low-fat diet. (Although fried foods are usually cooked in vegetable oils, vitamin E is destroyed by high heat so that you won’t find much vitamin E in french fries or onion rings. Your best bets are minimally-processed, whole foods.) Vegetable oils also provide some vitamin K, and fatty fish and eggs are good sources of vitamins A and D.

Below are images of foods that contain the four fat-soluble vitamins.

Vitamin A	Vitamin D	Vitamin K	Vitamin E

Vitamin A, D, K, and E are the four fat-soluble vitamins and can be found in an array of foods like butternut squash, broccoli, and salmon. Eating dietary fat in a balanced diet helps you absorb these fat-soluble vitamins, such as a kale salad with olive oil dressing and walnuts.

Food Sources of Lipids

Dietary lipids are primarily oils (liquid) and fats (solid).  Commonly consumed oils are canola, corn, olive, peanut, safflower, soy, and sunflower oil.  Foods rich in oils include salad dressing, olives, avocados, peanut butter, nuts, seeds, and some fish.  Fats are found in animal meat, dairy products, and cocoa butter.

Saturated and Unsaturated Fats

Most oils are high in monounsaturated or polyunsaturated fat and low in saturated fat.  Monounsaturated fats help regulate blood cholesterol levels, thereby reducing the risk of heart disease and stroke. Both monounsaturated fats and polyunsaturated fats provide nutrition that is essential for normal cell development and healthy skin.

• Monounsaturated fat – This type of fat is found in plant oils. Common sources are nuts (almonds, cashews, pecans, peanuts, and walnuts) and nut products, avocados, extra virgin olive oil, sesame oil, high oleic safflower oil, sunflower oil, and canola oil.
• Polyunsaturated fat – This type of fat is found mainly in plant-based foods, oils, and fish. Common sources are nuts (walnuts, hazelnuts, pecans, almonds, and peanuts), soybean oil, corn oil, safflower oil, flaxseed oil, canola oil, and fish (trout, herring, and salmon).
• Saturated fat – This fat is found in animal products, dairy products, palm and coconut oils, and cocoa butter. Limit these products to less than 10 percent of your overall dietary fat consumption. Saturated fat, which is found in meat, dairy products, and some plant oils, is associated with increased bloodstream cholesterol.  High cholesterol levels indicate that a person is at a major risk for disease, such as heart attack.  Avoid saturated fat, or at least consume in moderation.

Lipids & Disease

An increased intake of lipids is associated with heart disease, obesity, cardiovascular disease, and other problems.  Making dietary choices that limit the intake of saturated (and trans, another type of fat) fats to the recommended levels, replacing saturated and trans fats with unsaturated fats, increasing physical activity, and quitting smoking can reduce the risk of developing heart disease and other ailments.  Note that a diet too low in fat is also problematic; fat is essential for many body functions, making food taste great and satiating the appetite.  Like with so many things in life, the best results come from balance:  eat enough lipids but not too much.

Omega-3 fatty acids

Omega-3 fatty acids, also known as alpha-linolenic acid, are two major classes of polyunsaturated fats. Omega-3 fatty acids play an important role in the diet for overall cell health and growth and include additional heart health benefits. Two commonly known omega-3 fatty acids are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) and are commonly found in fatty fish like salmon, sardines, and herring. Other sources of omega-3 fatty acids include flaxseeds, walnuts, soybean oil, and chia seeds.

The recommended daily intake of omega-3 fatty acids for an individual’s diet should make up about 0.6 – 1.2% of overall total calories. Studies have shown that intake of omega-3 fatty acids, especially at least two EPA and DHA servings a week, can reduce cardiovascular disease risk.

Cholesterol – Made by you for you!

Cholesterol is primarily produced by the body and is important for synthesizing Vitamin D, bile salts, and reproductive hormones. Cholesterol is also an essential component of the cell membrane. Minimal intake of cholesterol is consumed through food like animal sources, cheese, or egg yolks. An increased build-up of cholesterol can contribute to the build-up of plaque and lead to many coronary artery disease complications. Factors that can contribute to high cholesterol include unhealthy eating habits with increased intake of processed and fried foods, lack of physical activity, and smoking.

The liver produces lipoproteins that help carry fat and cholesterol through your bloodstream. Two commonly discussed lipoproteins include low-density lipoprotein (LDL) and high-density lipoprotein (HDL). LDL is known as the “bad” cholesterol and helps contribute to the build-up of plaque in your arteries’ walls. HDL cholesterol is known as the “good” cholesterol and helps remove cholesterol and transport it back to the liver.

The Facts About Fat