The Use of Nutrients for Energy
As noted, glucose is the main source of energy in the body. Most of the carbohydrates in the diet are converted to glucose in the course of metabolism. Reserves of glucose are stored in liver and muscle cells as glycogen, a compound built from glucose molecules. When glucose is needed for energy, glycogen is broken down to yield glucose. Glycerol and fatty acids (from fat digestion) and amino acids (from protein digestion) can also be used for energy, but they enter the breakdown process at different points. Fat in the diet yields more than twice as much energy as do protein and carbohydrate (e.g., it is more “fattening”); fat yields 9 kcal of energy per gram, whereas protein and carbohydrate each yield 4 kcal per gram. Calories that are ingested in excess of need are converted to fat and stored in adipose tissue. Before they are oxidized for energy, amino acids must have their nitrogen (amine) groups removed. This removal, called deamination, occurs in the liver, where the nitrogen groups are then formed into urea by combination with carbon dioxide. The blood transports urea to the kidneys to be eliminated. There are no specialized storage forms of proteins, as there are for carbohydrates (glycogen) and fats (adipose tissue). Therefore, when one needs more proteins than are supplied in the diet, they must be obtained from body substance, such as muscle tissue or plasma proteins. Drawing on these resources becomes dangerous when needs are extreme. Fats and carbohydrates are described as “protein sparing,” because they are used for energy before proteins are and thus spare proteins for the synthesis of necessary body components.
Nutrient molecules are built into body materials by anabolic steps, all of which are catalyzed by enzymes.
Essential Amino Acids Eleven of the 20 amino acids needed to build proteins can be synthesized internally by metabolic reactions. These 11 amino acids are described as nonessential because they need not be taken in as food (Table 16-2). The remaining 9 amino acids cannot be made by the body and therefore must be taken in as part of the diet; these are the essential amino acids. Note that some nonessential amino acids may become essential under certain conditions, as during extreme physical stress, or in certain hereditary metabolic diseases.
Table 16•2 Amino acids
Essential Fatty Acids There are also two essential fatty acids (linoleic acid and linolenic acid) that must be taken in as food. These are easily obtained through a healthful, balanced diet.
Minerals and Vitamins
In addition to needing fats, proteins, and carbohydrates, the body requires minerals and vitamins. Minerals are chemical elements needed for body structure, fluid balance, and such activities as muscle contraction, nerve impulse conduction, and blood clotting. Some minerals are components of vitamins. A list of the main minerals needed in a proper diet is given in Table 16-3. Some additional minerals not listed are also required for good health. Minerals needed in extremely small amounts are referred to as trace elements. Vitamins are complex organic substances needed in very small quantities. Vitamins are parts of enzymes or other substances essential for metabolism, and vitamin deficiencies lead to a variety of nutritional diseases.
The water-soluble vitamins are the B vitamins and vitamin C. These are not stored and must be taken in regularly with food. The fat-soluble vitamins are A, D, E, and K. These vitamins are kept in reserve in fatty tissue. Excess intake of the fat-soluble vitamins can lead to toxicity. A list of vitamins is given in Table 16-4. Certain substances are valuable in the diet as antioxidants. They defend against the harmful effects of free radicals, highly reactive and unstable molecules produced from oxygen in the normal course of metabolism (and also from UV radiation, air pollution and tobacco smoke). Free radicals contribute to aging and disease. Antioxidants react with free radicals to stabilize them and minimize their harmful effects on cells. Vitamins C and E and beta carotene, an orange pigment found in plants that is converted to vitamin A, are antioxidants. There are also many compounds found in plants (e.g., soybeans and tomatoes) that are antioxidants.
Table 16•3 Minerals
Table 16•4 Vitamins