Nutrients are chemicals in foods that provide energy for powering life processes; chemicals aiding or enabling life processes; or materials to construct molecules for the normal development, growth, and maintenance of the body. There are six groups of nutrients: carbohydrates, lipids, proteins, vitamins, minerals, and water. All six groups provide raw materials for constructing new molecules, but only carbohydrates, lipids, and proteins provide energy to sustain life processes.
Essential nutrients are those nutrients that the body cannot synthesize and must obtain in food in order to construct other molecules necessary for life. The essential nutrients include certain amino acids, certain fatty acids, most vitamins, minerals, and water. Because the essential nutrients are not all present in any one food, a balanced diet is required. The conversion of raw materials into molecules for life processes is a major role of the liver.
The Institute of Medicine of the U.S. National Academy of Sciences has developed nutritional recommendations, called the Dietary Reference Intake (DRI), to assist in the planning and assessment of nutrient intake. DRI includes the Recommended Daily Allowance (RDA) for each nutrient. The RDA for a nutrient is the average daily intake level that is sufficient to meet the nutritional needs of a healthy person. Most nutritional labels provide RDA but not DRI recommendations. DRI and RDA recommendations can be found through the U.S. Department of Agriculture (USDA) website (fnic.nal.usda.gov).
Carbohydrates, fats, and proteins are called “energy foods” because they are used in cellular respiration to release the energy in their chemical bonds for ATP production. Cellular respiration includes both anaerobic and aerobic components. Anaerobic respiration, or glycolysis, occurs within the cytosol of a cell, while aerobic respiration occurs within mitochondria, where the enzymes catalyzing the reactions are located. There are two sequential, linked aerobic processes: the citric acid cycle and the electron transport chain. When oxygen is available, a nutrient, such as glucose, is completely degraded to carbon dioxide and water, in order to release energy. Approximately, 40% of the energy is captured in ATP, while the remainder is lost as heat. ATP does not store energy, but it carries it to where it is needed to power life processes. The released heat energy is important in maintaining a normal body temperature.
Follow the cellular respiration of glucose in figure 15.20 . A molecule of glucose (containing 6 carbon atoms) is split during glycolysis to form 2 molecules of pyruvic acid (each containing 3 carbon atoms) and 2 ATP. Each pyruvic acid molecule is converted to acetyl- CoA (each molecule containing 2 carbon atoms), which releases CO2 . Each acetyl-CoA molecule enters the citric acid cycle. With each turn of the cycle, one acetyl-CoA molecule is broken down to release CO2, H+, and high- energy electrons. Substrate reactions associated with the citric acid cycle produce 2 ATP.
The higher-energy electrons pass along the molecular carriers of the electron transport chain from a higher- energy level to a lower-energy level-much like water flowing down a staircase. At each transfer (step) within the chain, energy is released to form ATP and the electrons move to the next lower energy level (next lower step in the staircase). Ultimately, all of the available energy is extracted, and the electrons, H+, and O2 combine to form water (H2O). Electron transfer yields a total of 32 to 34 ATP, depending upon the cell in which cellular respiration occurs. When added to the 2 ATP from glycolysis and the 2 ATP from the citric acid cycle, one molecule of glucose can yield a total of 36 to 38 ATP.
The end products of digestion for lipids (fats) and proteins also may be used in cellular respiration, either directly or after conversion into compatible molecules. Figure 15.20 shows the major points of entry into cellular respiration for these molecules. The number of ATP produced varies with the type of molecule broken down.
Nearly all carbohydrates in the diet come from plant foods. Glycogen is the only carbohydrate in animal foods. While there is very little of this polysaccharide in meat, animal liver is an abundant source. Monosaccharides are in honey and fruits; disaccharides are found in table sugar and dairy products; and starch, a polysaccharride, occurs in cereals, vegetables, and legumes (e.g., beans, peas, peanuts).
Cellulose is a polysaccharide that is abundant in plant foods, but it cannot be digested by humans because they lack the necessary digestive enzymes. However, it is an important dietary component, because it provides fiber (roughage) that increases the bulk of the intestinal contents, which aids the function of the large intestine. Evidence suggests that high-fiber diets reduce the risk of certain colon disorders, such as diverticulitis and colon cancer.
Carbohydrates are used mostly as an energy source, with glucose as the primary carbohydrate molecule used in cellular respiration. The hormone insulin plays a crucial role in moving glucose into cells. Most cells can live by obtaining energy from fatty acids or amino acids via cellular respiration, but some cells, notably neurons, are dependent upon a steady supply of glucose. For this reason, the functions of the nervous system decline if the concentration of blood glucose decreases.
The liver, along with the hormones insulin and glucagon, is involved in the regulation of glucose concentration in the blood. In response to insulin, excess glucose is converted into glycogen for storage primarily in the liver but also in skeletal muscles. If excess glucose still remains, it is converted into triglycerides and is stored in adipose tissues. When blood glucose levels decline, glucagon signals the liver to convert glycogen into glucose. If still more glucose is needed, triglycerides are converted into glycerol and fatty acids. Then glycerol may be converted into glucose.
Lipids include triglycerides, phospholipids, steroids, and lipid-soluble vitamins, (A, D, E, and K), but triglycerides are the most common lipids in the diet. Triglycerides may be either saturated or unsaturated. Fats and oils contain a mixture of saturated, monounsaturated (one double bond), and polyunsaturated (more than one double bond) fatty acids. Coconut and palm oils, dairy products, and beef fat contain mostly saturated fatty acids. Peanut, olive, and canola oils consist mostly of monounsaturated fatty acids. Safflower, sunflower, and corn oils contain mostly polyunsaturated fatty acids. Cholesterol is present in dairy products, red meats, and egg yolks.
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