contribute 30 percent of the calories. Saturated fat intake should be no more than 10 percent of the total calories. Cholesterol may be limited to 300 milligrams (mg) per day.
Many fats act as carriers for the fat-soluble vitamins A, D, E, and K; they also act both as a padding for vital organs, particularly the kidneys, and as subcutaneous tissue to help conserve body heat. Fat is stored as adipose (fatty) tissue to form a reserve supply in time of need. Fats delay gastric emptying and promote satiety. Excess calories from fat may produce obesity, the forerunner of arteriosclerosis, hypertension, gallbladder disease, and diabetes. Too little fat in the diet may lead to being underweight, having insufficient padding for the vital organs, and lowered energy. Dietary sources of fats are butter, margarine, cream cheese, fatty meats, whole milk, olives, avocados, egg yolks, nuts, and vegetable oils.
Carbohydrates (sugar and starches) are the most efficient sources of energy and are known as the fuel of life. They are abundant in food. The Senate Select Committee on Nutrition and Human Needs, 1977 Dietary Goals for the United States, Second Edition, recommends that complex carbohydrates and naturally occurring sugars make up 48 percent of total caloric intake, and refined and processed sugars 10 percent.
Each g of carbohydrate yields 4 calories in the process of its metabolism. Carbohydrates must be reduced to glucose before they can be used by the body. Carbohydrates are stored in the liver as glycogen. The glycogen is broken down and released as glucose at the exact rate needed by the body. This mechanism is controlled largely by insulin from the pancreas. During fasting glycogen is rapidly depleted, which leads the body to use its fat for energy.
The main functions of carbohydrates are to (1) furnish the main source of energy for muscular work and nutritive processes, (2) help maintain body temperature, (3) form reserve fuel, (4) assist in oxidation of fats, and (5) spare protein for growth and repair. Excess carbohydrates are converted into adipose tissue. Dietary sources of carbohydrates are fruits, honey, sweets, legumes, potatoes, grains, sugars, and grain products.
Although the mineral elements constitute only a small portion of the total body weight, they enter into the activities of the body to a much greater degree than their mere weight would indicate. Table 3-1 lists the essential elements, the foods that contain them, and their functions. Certain mineral elements are essential for specific body functions. It is not known exactly how many of the mineral elements are indispensable to the body functions, but changes of concentration that may seem small can be fatal. These essential inorganic elements contribute overwhelmingly to the skeletal framework of the body and the teeth and are an essential part of many organic compounds. They form an integral part of all cell structure and circulate in body fluids. In addition, they exercise specific physiological influences on the function of body tissues. For the mineral needs to be met satisfactorily, the consumption of each element must be sufficient to cover body tissue requirements and to meet changing physiological needs. It was once believed that any diet that was adequate in other respects would also provide an adequate intake of the essential minerals. This is not true. Foods vary greatly in their mineral content. Depending on growing conditions and storing and preparation procedures, they may vary considerably in nutritional content.
Vitamins are essential substances present in food in minute quantities. Although they do not furnish energy or act as tissue building materials, they do act as catalysts in many body chemical reactions and are necessary for normal metabolic functions, growth, and health of the human body. Their absence results in malnutrition and specific deficiency diseases. Their chemistry is complex and nutritional experimentation is difficult, so our knowledge of them is being continually supplemented and revised. It is quite possible that additional vitamins will be discovered or that some of those already recognized may prove to contain more than one factor.
Vitamins are so widely distributed in food that a properly prepared normal diet usually provides an adequate amount. Some are destroyed in preparing or preserving certain foods. Some manufacturers add vitamins to their products to replace those destroyed or removed in processing. Fat-soluble vitamins A, D, E, and K can be stored in the body. It is possible to consume excessive