Lipid metabolism is associated with carbohydrate metabolism, as products of glucose such as acetyl CoA can be converted into lipids. Lipid metabolism begins in the intestine where ingested triglycerides are broken down into free fatty acids and a monoglyceride molecule see Figure When food reaches the small intestine in the form of chyme, a digestive hormone called cholecystokinin CCK is released by intestinal cells in the intestinal mucosa.
CCK stimulates the release of pancreatic lipase from the pancreas and stimulates the contraction of the gallbladder to release stored bile salts into the intestine. CCK also travels to the brain, where it can act as a hunger suppressant. Once the bile salts have emulsified the triglycerides, the pancreatic lipases down triglycerides into free fatty acids. These fatty acids can be transported across the intestinal membrane.
However, once they cross the membrane, they are recombined to again form triglyceride molecules. Within the intestinal cells, these triglycerides are packaged along with cholesterol molecules in phospholipid vesicles called chylomicrons Figure The chylomicrons enable fats and cholesterol to move within the aqueous environment of your lymphatic and circulatory systems.
Chylomicrons leave the enterocytes by exocytosis and enter the lymphatic system via lacteals in the villi of the intestine. From the lymphatic system, the chylomicrons are transported to the circulatory system. Once in the circulation, they can either go to the liver or be stored in fat cells adipocytes that comprise adipose fat tissue found throughout the body.
To obtain energy from fat, triglycerides must first be broken down by hydrolysis into their two principal components, fatty acids and glycerol. This process, called lipolysis , takes place in the cytoplasm.
The glycerol that is released from triglycerides after lipolysis directly enters the glycolysis pathway as DHAP. Because one triglyceride molecule yields three fatty acid molecules with as much as 16 or more carbons in each one, fat molecules yield more energy than carbohydrates and are an important source of energy for the human body. Triglycerides yield more than twice the energy per unit mass when compared to carbohydrates and proteins. Therefore, when glucose levels are low, triglycerides can be converted into acetyl CoA molecules and used to generate ATP through aerobic respiration.
This fatty acyl CoA combines with carnitine to create a fatty acyl carnitine molecule, which helps to transport the fatty acid across the mitochondrial membrane. As a result, eating tomatoes with olive oil or salad dressing will facilitate lycopene absorption. Other essential nutrients, such as essential fatty acids, are constituents of the fats themselves and serve as building blocks of a cell.
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 compared with 4 calories found in 1 gram of carbohydrate or protein.
Depending on the level of physical activity and on 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 higher 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.
When the body has used all of its calories from carbohydrates which can occur after just twenty minutes of exercise , it initiates fat usage.
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. In contrast, if a person who leads a sedentary lifestyle eats the same fat-rich foods, they will likely get more fat calories than their body requires. Fat contains dissolved compounds that contribute to mouth-watering aromas and flavors.
Fat also adds texture to food and helps keep baked foods moist. Vital organs such as the heart, kidneys, and liver are protected by visceral fat. The composition of the brain is outstandingly 60 percent 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- or roller skating, horseback riding, or snowboarding.
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. Finally, lipid reserves containing , kcal of energy can maintain human body functions without food for days with sufficient water. Lipids or fats represent about 24 pounds of the body weight in a pound male. Lipids provide the sole source of energy in hibernating animals and migrating birds.
Fortunately, lipids are more compact and contain more energy per gram than glycogen, otherwise body weight would increase approximately pounds if glycogen were to replace fat as the energy reserve.
Lipids or fats are stored in cells throughout the body principle in special kinds of connective tissue called adipose tissue or depot fat. In addition to energy storage, depot fat provides a number of other functions. Fat serves as a protective cushion and provides structural support to help prevent injury to vital organs such as the heart, liver, kidneys, and spleen.
Fat insulates the body from heat loss and extreme temperature changes. At the same time, fat deposits under the skin may be metabolized to generate heat in response to lower skin temperatures. Click for larger image Lipids in the Blood:. Lipids ingested as food are digested in the small intestine where bile salts are used to emulsify them and pancreatic lipase hydrolyzes lipids into fatty acids, glycerol, soaps, or mono- and diglycerides.
There is still some dispute about the lipid form that passes through the intestinal wall -- whether as fatty acids or as glycerides. In either case, triglycerides are found in the lymph system and the blood.
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