6.2.1: Functions of Lipids in the Body and in Food

Functions of Lipids in the Body: Storing Energy

The excess energy from the food we eat is digested and incorporated into adipose tissue, or fatty tissue. Carbohydrates and lipids provide most of the energy the human body requires. As discussed in the previous chapter, glucose is stored in the body as glycogen. While glycogen provides a ready energy source, lipids primarily function as an energy reserve. Glycogen is quite bulky and has a heavy water content; thus, the body cannot store too much for long. Alternatively, fats are packed together tightly without water and store far greater energy in a reduced space. A fat gram is densely concentrated with energy—it contains more than double the amount of energy than a gram of carbohydrate. Energy is needed to power the muscles for all our physical work and play. For instance, the stored energy in muscles propels an athlete down the track, spurs a dancer’s legs to showcase the latest fancy steps, and keeps all the moving parts of the body functioning smoothly.

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 result in undue stress on the body and can be detrimental to your health. A serious impact of excess fat is the accumulation of too much cholesterol in the arterial wall, which can thicken the walls of arteries and lead to cardiovascular disease. Thus, while some body fat is critical to our survival and good health, in large quantities, it can be a deterrent to maintaining good health.

Functions of Lipids in the Body: Regulating and Signaling

Fats control the body’s internal climate, maintaining a constant temperature. Those who don’t have enough fat tend to feel cold sooner, are often fatigued, and have pressure sores on their skin from fatty acid deficiency. Fats also help the body produce and regulate hormones. For example, adipose tissue secretes the hormone leptin, which regulates appetite. In the reproductive system, fatty acids are required for proper reproductive health; women who lack proper amounts may stop menstruating and become infertile. Omega-3 and omega-6 essential fatty acids help regulate cholesterol and blood clotting and control inflammation in the joints, tissues, and bloodstream. Fats also play important roles in sustaining nerve impulse transmission, memory storage, and tissue structure. More specifically in the brain, lipids are focal to brain activity in structure and function. They help form nerve cell membranes, insulate neurons, and facilitate the signaling of electrical impulses throughout the brain.

Figure \(\PageIndex{1}\): Lipids serve as signaling molecules; they are catalysts of electrical impulse activity within the brain. Nerve response in myelinated neurons (covered with a myelin coating on the right that is 70% lipids in dry mass) propagates faster than in unmyelinated neurons (left). ("Saltatory Conduction" by Dr. Jana is used under a Creative Commons license.).

Functions of Lipids in the Body: Insulating and Protecting

Did you know that up to 30% of body weight consists of fat tissue? Some of this is made up of visceral fat or adipose tissue surrounding delicate organs. Vital organs such as the heart, kidneys, and liver are protected by visceral fat. The composition of the brain is outstandingly 60% fat, demonstrating the major structural role that fat serves within the body. You may be most familiar with subcutaneous fat or fat underneath the skin. This blanket layer of tissue 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 or roller skating, horseback riding, or snowboarding.

Functions of Lipids in the Body: Aiding Digestion and Increasing Bioavailability

The dietary fats in the foods we eat break down in our digestive systems and begin the transport of precious micronutrients. By carrying fat-soluble nutrients through the digestive process, intestinal absorption is improved. This improved absorption is also known as increased bioavailability. Fat-soluble nutrients are especially important for good health and exhibit a variety of functions. Vitamins A, D, E, and K—the fat-soluble vitamins—are mainly found in foods containing fat. Some fat-soluble vitamins (such as vitamin A) are also found in naturally fat-free foods such as green leafy vegetables, carrots, and broccoli. These vitamins are best absorbed when combined with foods containing fat. Fats also increase the bioavailability of compounds known as phytochemicals, which are plant substances such as lycopene (found in tomatoes) and beta-carotene (found in carrots). Phytochemicals are believed to promote health and well-being. As a result, eating tomatoes with olive oil or salad dressing will facilitate lycopene absorption.

Note that removing the fat from food may lessen the food’s fat-soluble vitamin content. When products such as grain and dairy are processed, these essential nutrients are lost.

Label Reading Activity

Remember, fat-soluble nutrients require fat for effective absorption. The next time you pick up a food with a nutrient facts panel, check the Percent Daily Value for Vitamin A and Vitamin D (a high source is 20% or higher). Next check if the food contains any fat that increases bioavailability (absorption) of the vitamins. If not, think of ways to add a bit of healthy fat to the snack or meal to aid in fat-soluble vitamin absorption.

Role of Lipids in Food: High Energy Source

Fat-rich foods naturally have a high caloric density. Foods that are high in fat contain more Calories than foods high in protein or carbohydrates. As a result, high-fat foods are a convenient source of energy. For example, 1 gram of fat or oil provides 9 calories of energy, compared with 4 calories in 1 gram of carbohydrate or protein. Depending on the level of physical activity and nutritional needs, fat requirements vary greatly from person to person. When energy needs are high, the body welcomes the high-caloric density of fats. For instance, infants and growing children require appropriate amounts of fat to support normal growth and development. If an infant or child is given a low-fat diet for an extended period, growth and development will not progress normally. Other individuals with high-energy needs are athletes, people who have physically demanding jobs, and those recuperating from illness. Higher-fat foods may also help someone with a poor appetite reach their Calorie goals to maintain or gain weight.

Figure \(\PageIndex{2}\): Athletes have high-energy requirements. "Caitlin Sinclair at IGSSA Swimming Carnival 2010" by MLC School is in the Public Domain.)

Depending on the intensity, duration, and training level of a person engaging in exercise, the proportion of carbohydrate and fat used for energy changes. Fat is an important fuel in lower to moderate aerobic exercise, but it also comes in handy as a long-term fuel supply when carbohydrate levels drop during long-duration activities. A professional swimmer must consume large amounts of food energy to meet the demands of swimming long distances, so eating fat-rich foods makes sense. That same swimmer, however, may want to choose lower-fat foods before training as this may be more comfortable for digestion. In contrast, if a person who leads a sedentary lifestyle eats the same high-density fat foods, they will take in more fat Calories than their body requires within just a few bites. Use caution—consumption of Calories over and beyond energy requirements is a contributing factor to obesity.

Role of Lipids in Food: Smell and Taste

Fat contains dissolved compounds that contribute to mouth-watering aromas and flavors. Fat also adds texture to food. Baked foods are supple and moist. Frying foods locks in flavor and lessens cooking time. How long does it take you to recall the smell of your favorite food cooking? What would a meal be without that savory aroma to delight your senses and heighten your preparedness for eating? It is possible to have great-tasting food with little or no fat for those with health conditions that require such. Adding aromatic herbs and spices in the cooking process is a great way to flavor food!

Fat plays another valuable role in nutrition. It contributes to satiety, or the sensation of fullness. When fatty foods are swallowed, the body responds by enabling the processes controlling digestion to slow the movement of food along the digestive tract, thus promoting an overall sense of fullness. Often, before the feeling of fullness arrives, people overindulge in fat-rich foods, finding the delectable taste irresistible. Indeed, the very things that make fat-rich foods attractive also make them a hindrance to maintaining a healthful diet.

Tools for Change

Although fats provide delicious smells, tastes, and textures to our foods, they also provide numerous Calories. To allow your body to experience the satiety effect of the fat before you overindulge, try savoring rich foods. Eating slowly will allow you to both fully enjoy the experience and be satiated (feeling of fullness) with a smaller portion. Remember to take your time. Drink water between bites or eat a lower-fat food before and after a higher-fat food. The lower-fat foods will provide bulk but fewer Calories.

Attributions

Zimmerman, "An Introduction to Nutrition (Zimmerman)", CC BY-NC-SA 3.0. Text was updated.