Introduction

Nutrition is the process of consuming, absorbing, and using nutrients needed by the body for growth, development, and maintenance of life.

To receive adequate, appropriate nutrition, people need to consume a healthy diet, which consists of a variety of nutrients (the chemical substances in foods that nourish the body). A healthy diet enables people to maintain a desirable body weight and composition (the percentage of fat and muscle in the body) and to perform their daily physical and mental activities.

If a person does not consume sufficient amounts of nutrients, a nutritional deficiency disorder may result. To determine whether a person is consuming enough nutrients, a doctor asks about eating habits and diet and performs a physical examination to assess the composition and functioning of the body. Body composition, including the proportion of body fat, can be determined accurately by weighing the person under water (hydrostatic weighing) and can be approximated by measuring skinfold thickness or performing bioelectrical impedance analysis. Laboratory tests to measure the levels of nutrients in blood and tissues can be performed. For example, the level of albumin, the main protein in blood, can be measured. Nutrient levels decrease when nutrition is inadequate.

Generally, nutrients are divided into two classes: macronutrients and micronutrients. Macronutrients are required daily in large quantities. They include proteins, fats, carbohydrates, some minerals, and water. Water is required in amounts of 1 milliliter for each calorie of energy expended or about 2.6 quarts (2,500 milliliters) a day. Micronutrients are required daily in small quantities--in milligrams (one thousandth of a gram) to micrograms (one millionth of a gram). They include vitamins and trace minerals that enable the body to use macronutrients.

Foods consumed in the daily diet contain as many as 100,000 substances. But only 300 are classified as nutrients, and only 45 are classified as essential nutrients. However, food contains many other useful components, including some fibers, such as cellulose, pectins, and gums. Food contains additives (such as preservatives, emulsifiers, antioxidants, and stabilizers), which improve the production, processing, storage, and packaging of foods. Spices, flavors, substances that add odor or color, phytochemicals (substances in plants that have biologic activity in animals), and many other natural products improve the appearance, taste, and stability of food.

See the table Height-Weight Reference Chart for Adults.

See the sidebar Fat Versus Lean: Body Composition.