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Pronunciations x-ray absorptiometry

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.

Height-Weight Reference Chart for Adults* Weight (Pounds) Height Women Men 4' 10" 92—121 — 4' 11" 95—124 — 5' 0" 98—127 — 5' 1" 101—130 105—134 5' 2" 104—134 108—137 5' 3" 107—138 111—141 5' 4" 110—142 114—145 5' 5" 114—146 117—149 5' 6" 119—150 121—154 5' 7" 122—154 125—159 5' 8" 126—159 129—163 5' 9" 130—164 133—167 5' 10" 134—169 137—172 5' 11" — 141—177 6' 0" — 145—182 6' 1" — 149—187 6' 2" — 153—192 6' 3" — 157—197 *Height is without shoes; weight is without clothes.

Fat Versus Lean: Body Composition Maintaining an appropriate weight is important for physical and psychologic health. A standardized height-weight table can be used as a guide. But body mass index is more reliable. It is calculated by dividing the weight (in kilograms) by the height (in meters squared).(see Determining Body Mass Index ) A body mass index between 18 and 25 is usually considered normal for men and women. A less obvious but important consideration is how much of the body is fat and how much is muscle (body composition). There are several ways to determine body composition. Underwater (hydrostatic) weighing in an immersion pool can determine body composition. Bone and muscle are denser than water, so a person with a high percentage of lean tissue weighs more in water and a person with a high percentage of fat weighs less. Although this method is considered the most accurate, it requires special equipment, considerable time, and expertise to perform. Body composition can be estimated by measuring the amount of fat under the skin (skinfold thickness) or by bioelectric impedance analysis. For skinfold thickness, a fold of skin on the back of the left upper arm (triceps skinfold) is pulled away from the arm and measured with a caliper. A skinfold measurement of about ½ inch in men and about 1 inch in women is considered normal. This measurement plus the circumference of the left upper arm can be used to estimate the amount of skeletal muscle in the body (lean body mass). Bioelectric impedance analysis measures the resistance of body tissues to the flow of an undetectable low-voltage electrical current. Typically, a person stands barefoot on metal footplates, and the electrical current is sent up one foot and down the other. Body fat and bone resist the flow much more than does muscle tissue. By measuring the resistance to the current, doctors can estimate the percentage of body fat. This test takes only about 1 minute. Dual-energy x-ray absorptiometry (DEXA) accurately determines the amount and distribution of body fat. This imaging procedure uses a very low dose of radiation and is safe. However, it is too expensive to use routinely.

Last full review/revision February 2003