Nutrition and Metabolism Chapter 21 Nutrition and Metabolism

Energy Expenditure As shown here, some body systems demand a greater supply of energy than others.

Energy Expenditure After the digestive system breaks food into usable food components (called nutrients), they are absorbed into the bloodstream. The nutrients then enter cells and undergo chemical reactions, called metabolism. It’s through the process of metabolism that nutrients are transformed into energy or materials the body can either use or store.

Units of Energy Energy expenditure: Measured by the output of heat from the body Calorie: The amount of heat needed to raise the temperature of 1 gram of water by 1 degree Celsius Basal metabolic rate: The amount of energy the body needs at rest Factors that increase metabolic rate include anxiety, fever, eating (the metabolic rate increases as the body processes the ingested food), and thyroid hormones. Physical activity also affects caloric need. To maintain weight, calorie intake must equal output.

Factors Affecting Metabolic Rate Body size and composition People who are larger or have more muscle mass burn more calories. Gender Males tend to have less body fat and more muscle than women of the same age and weight; consequently, they burn more calories. Age Muscle mass declines with age, slowing the metabolic rate.

Hunger and Satiety Leptin: Suppresses appetite Peptide YY (PYY): Signals satiety Ghrelin: Produces hunger Insulin: Suppresses appetite Cholecystokinin (CCK): Suppresses appetite

Question Which hormone stimulates the appetite? A. Ghrelin B. Leptin C. Peptide YY D. Cholecystokinin

Nutrients Macronutrients: Carbohydrates, lipids, protein, and water Micronutrients: Vitamins and minerals Essential nutrients: Obtained through diet Nonessential nutrients: Synthesized by the body

Carbohydrates Monosaccharides: Simple sugars; absorbed without being broken down Disaccharides: Broken down into monosaccharides Polysaccharides: Complex carbohydrates

Carbohydrates Carbohydrates are the body’s primary energy source. Dietary carbohydrates occur in three forms: monosaccharides, disaccharides, and polysaccharides. Examples of monosaccharides: Glucose, fructose, galactose Examples of disaccharides: Sucrose (table sugar), lactose, maltose Examples of polysaccharides: Starches found in vegetables, grains, potatoes, rice, legumes Adults should obtain 40% to 50% of their calories from carbohydrates.

Lipids Act as reservoir of excess energy Enable absorption of certain vitamins Contribute to cellular structure Insulate and protect the body

Lipids Fat is an ideal substance for storing energy: not only is it more compact than carbohydrates, it’s also more energy dense. Each gram of fat contains 9 calories per gram, compared to 4 calories for each gram of carbohydrate. Fats can be saturated or unsaturated. Ideally, fat should comprise no more than 30% of an adult’s daily intake, with no more than 20% being saturated fat.

Proteins Complete proteins: Supply all essential amino acids Incomplete proteins: Lack one or more essential amino acids

Proteins The body contains over 100,000 different proteins, with each consisting of various combinations of just 20 different amino acids. During the process of digestion, proteins are broken down into their individual amino acids. Once absorbed, the body recombines the amino acids to create a new protein for a specific purpose.

Proteins Foods that supply all the essential amino acids—making them available at the same time—are called complete proteins. Foods that lack one or more essential amino acids are called incomplete proteins. Complete proteins mainly come from animal sources; incomplete proteins come from plant sources.

Vitamins Water-soluble vitamins Fat-soluble vitamins Absorbed with water in the small intestine Excreted by the kidneys Fat-soluble vitamins Absorbed with dietary fat Stored in liver and fat tissues Almost all vitamins (except for vitamin D, which can be formed by exposure of the skin to the sun) must be obtained through food. Vitamins A, E, E, and K are fat-soluble vitamins. Because fat-soluble vitamins are stored, megadosing may lead to toxicity.

Minerals Needed for numerous vital functions. The most abundant minerals are calcium and phosphorous.

Question Nonessential amino acids are: A. not necessary for metabolic processes. B. synthesized by the body. C. not necessary for the synthesis of new proteins. D. unavailable in most foods.

Metabolism Catabolism: Breaks down complex substances into simpler ones or into energy Anabolism: Forms complex substances out of simpler ones Inside the cell, nutrients are transformed through metabolism into energy that the body can use immediately or store for later use.

Carbohydrate Metabolism Involves three phases Glycolysis Anaerobic fermentation Aerobic respiration

Carbohydrate Metabolism All ingested carbohydrates are converted to glucose, most of which is immediately burned as energy. If the body doesn’t need the glucose for immediate energy, it stores it as glycogen or converts it to lipids. The primary goal of glucose catabolism is to generate ATP. This occurs in three phases: glycolysis, anaerobic fermentation, and aerobic respiration. Processes that occur without oxygen are called anaerobic processes; those that require oxygen are called aerobic.

Glycolysis Occurs without oxygen Splits one glucose molecule into two molecules of pyruvic acid Releases only a fraction of available energy When oxygen is not available—such as during a period of intense exercise when the demand for energy exceeds the supply of oxygen—anaerobic fermentation occurs. When oxygen is available, aerobic respiration is the pathway of choice.

Anaerobic Fermentation Pyruvic acid → lactic acid Lactic acid → liver for storage Lactic acid → pyruvic acid → aerobic respiration When oxygen is in short supply, pyruvic acid is converted into lactic acid. The lactic acid travels through the bloodstream to the liver, where it is stored. Later, during a period of rest when oxygen becomes available, the lactic acid is converted back into pyruvic acid. At that point, the pyruvic acid can enter the series of reactions in aerobic respiration.

Aerobic Respiration Pyruvic acid → acetyl CoA → citric acid cycle Electron transport chain → CO2 + H2O + energy When oxygen is available, pyruvic acid enters the mitochondria and is converted into acetyl coenzyme A. This beings a series of reactions called the citric acid cycle. This is followed another series of reactions called the electron-transport chain. The end result is the complete breakdown of glucose into carbon dioxide, water, and large amounts of ATP.

Question All ingested carbohydrates are converted to: lipids. proteins. glucose. glycogen.

Lipid Metabolism Stored fat → glycerol + 3 fatty acid molecules Glycerol → small amount of energy Fatty acids → acetyl CoA → large amounts of energy

Protein Metabolism Used to build tissue During digestion, proteins are broken down and amino acids recombined to form new proteins.

Thermoregulation Body temperature normally fluctuates Three main methods of heat loss Radiation Conduction Evaporation Most of the body’s heat results as a byproduct of the chemical reactions occurring in cells.

Regulation of Body Temperature

Regulation of Body Temperature The hypothalamus acts as the body's thermostat: it monitors the temperature of the blood and sends signals to blood vessels and sweat glands. When the body temperature rises too high, the hypothalamus signals cutaneous blood vessels to dilate. More warm blood flows close to the body's surface and heat is lost through the skin. If the temperature remains high, sweat glands produce sweat and the evaporation of sweat produces cooling.

Regulation of Body Temperature When the body temperature falls below normal, the hypothalamus signals cutaneous blood vessels to constrict. Instead of flowing to the skin, warm blood remains confined deep in the body. As a result, less heat is lost through the skin. If the temperature remains below normal, the body begins to shiver. Muscle contractions associated with shivering release heat and raise the body's temperature.

Question Proteins are primarily used to: build tissue. store energy. produce energy. generate heat.