Nutrition is the process of consuming, absorbing, and using nutrients needed by the body for growth, development, and the maintenance of life; nutrients are chemical substances in foods that nourish the body.
Many nutrients can be synthesized in the body’. Those that cannot be synthesized in the body called essential nutrients-must be consumed in the diet. They include amino acids (in proteins), certain fatty acids (in fats and oils), minerals, and vitamins. Nine of the 20 amino acids in proteins are essential nutrients. If essential nutrients are not supplied in the quantities required, nutritional deficiency disorders may result. To determine whether a person is getting enough nutrients, a doctor asks about eating habits and diet, performs a physical examination to assess the composition (the amount of fat and muscle) and functioning of the body, and orders laboratory tests to measure the nutrient content of blood and tissues.
Generally, nutrients are divided into two classes: macronutrients and micronutrients. Macronutrients, which include proteins, fats, carbohydrates, and some minerals, are required daily in large quantities. They constitute the bulk of the diet and supply the energy and building blocks needed for growth, maintenance, and activity. Micronutrients are required in small quantities milligrams (one thousandth of a gram) to micrograms (one millionth of a gram). They include vitamins and trace minerals that catalyze the utilization of macronutrients. Other useful components of food aren’t digested or metabolized to any appreciable extent. These components include some fibers, such as cellulose, pectin, and gums. Authorities recommend that 20 grams of fiber be consumed daily to improve movement in the gastrointestinal tract, moderate the changes in blood sugar and cholesterol that occur after meals, and increase the elimination of cancer-causing substances produced by the bacteria in the large intestine. Food additives such as preservatives, emulsifiers, antioxidants, and stabilizers improve the production, processing, storage, and packaging of foods. Substances such as spices, flavors, odors, colors, phytochemicals (nonnutrients in plants, which have biologic activity in animals), and many other natural products improve the appearance, taste, and stability of food. Food in the daily diet contains as many as 100,000 substances, of which only 300 are nutrients and 45 are essential nutrients.
Macronutrients
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The organic macronutrients are carbohydrates, fats, and proteins, which supply 90 percent of the dry weight of the diet and 100 percent of its energy. They are digested in the intestine and broken down into their basic units: sugars from carbohydrates, fatty acids and glycerol from fats, and amino acids from proteins. The energy content is 4 calories in a gram of protein or carbohydrate and 9 calories in a gram of fat (1 gram equals 1 /28 of an ounce). As sources of energy, carbohydrates, fats, and proteins are interchangeable in proportion to their energy content.
Energy intake varies markedly from about 1,000 to more than 4,000 calories a day depending on age, sex, and physical activity. Typically, sedentary women, young children, and older adults need about 1,600 calories a day; older children, active adult women, and sedentary men need about 2,000 calories; and active adolescent boys and young men need about 2,400 calories. About 55 percent of the calories commonly come from carbohydrates, about 30 percent from fats, and about 15 percent from proteins. If the energy intake is insufficient for the body’s needs, weight loss occurs, and fat stored in the body and to a lesser degree protein is used to supply the energy needed. Total starvation causes death in 8 to 12 weeks.
The essential fatty acids make up about 7 percent of the fat consumed in a normal diet (which is 3 percent of the total calories or about 8 grams) and thus are considered macronutrients. They include linoleic acid, linolenic acid, arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid. Linoleic acid and linolenic acid are found in vegetable oils; eicosapentaenoic acid and docosahexaenoic acid, which are essential for brain development, are found in fish oils. In the body, arachidonic acid can be formed from linoleic acid, and eicosapentaenoic and docosahexaenoic acid can be formed from linotenic acid, although fish oil is a more efficient source.
The macrominerals are calcium, phosphorus, sodium, chloride, potassium, and magnesium. They are considered macronutrients because they are required in fairly large quantities (about 1 or 2 grams a day). Water, also a macronutrieni, is required in amounts of 1 milliliter for each calorie of energy expended or about 2,500 milliliters (2.6 quarts) a day.
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Micronutrients
Vitamins and trace minerals are micronutrients. Vitamins are classified as water soluble (vitamin C and eight members of the vitamin B complex) or fat soluble (vitamins A, D, E, and K).
Essential trace minerals include iron, zinc, copper, manganese, molybdenum, selenium, iodide, and fluoride. Except for fluoride, all of these minerals activate enzymes required in metabolism. Fluoride forms a stable compound with calcium, helping stabilize the mineral content of bones and teeth and helping prevent tooth decay. Trace minerals such as arsenic, chromium, cobalt, nickel, silicon, and vanadium, which may be essential in animal nutrition, have not been established as requirements in human nutrition. All trace minerals are toxic at high levels, and some (arsenic, nickel, and chromium) have been identified as causes of cancer.