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 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 do their daily physical and mental activities.

If people consume too much food, obesity may result. If they consume large amounts of certain nutrients, usually vitamins or minerals, harmful effects (toxicity) may occur. If people do not consume enough nutrients, a nutritional deficiency disorder may result.

To determine whether people are consuming a proper amount of nutrients, doctors ask them about their eating habits and diet and do a physical examination to assess the composition and functioning of the body. Height and weight are measured, and body mass index (BMI) is calculated. BMI is calculated by dividing weight (in kilograms) by the square of the height (in meters). A BMI between 19 and 24 is usually considered normal for men and women.

Body composition, including the percentage of body fat, is sometimes estimated by measuring skinfold thickness or doing bioelectrical impedance analysis. More accurate ways to determine this percentage include weighing people under water (hydrostatic weighing) and doing a dual-energy x-ray absorptiometry (DEXA) scan, but these methods are seldom used.

Levels of many nutrients can be measured in blood and sometimes in tissues. For example, measuring the level of albumin, the main protein in blood, may help determine whether people are deficient in protein. Nutrient levels decrease when nutrition is inadequate.

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 (BMI) is more reliable. 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: Hydrostatic weighing: People are weighed underwater in a small pool. Bone and muscle are denser than water, so people with a high percentage of lean tissue weigh more in water and people with a high percentage of fat weigh less. Although this method is considered the most accurate, it requires special equipment, considerable time, and expertise to do. Skinfold thickness: Body composition can be estimated by measuring the amount of fat under the skin (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: This test measures the resistance of body tissues to the flow of an undetectable low-voltage electrical current. Typically, people stand 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 muscle tissue does. 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): This imaging procedure accurately determines the amount and distribution of body fat. DEXA uses a very low dose of radiation and is safe. However, it is too expensive to use routinely.

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Who Is Overweight? *Normal Overweight Obese Extremely Obese 19–24 25–29 30–34 35–39 40–47 48–54 Height† (inches) Body Weight† (pounds) 60–61 97–127 128–153 153–180 179–206 204–248 245–285 62–63 104–135 136–163 164–191 191–220 218–265 262–304 64–65 110–144 145–174 174–204 204–234 232–282 279–324 66–67 118–153 155–185 186–217 216–249 247–299 297–344 68–69 125–162 164–196 197–230 230–263 262–318 315–365 70–71 132–172 174–208 209–243 243–279 278–338 334–386 72–73 140–182 184–219 221–257 258–295 294–355 353–408 74–75 148–192 194–232 233–272 272–311 311–375 373–431 76 156–197 205–238 246–279 287–320 328–385 394–443 *BMIs lower than those listed as normal are considered underweight. †Calculations are done using height in meters and weight in kilograms. Height is without shoes. Weight is without clothes. BMI = body mass index. Who Is Overweight? *Normal Overweight Obese Extremely Obese 19–24 25–29 30–34 35–39 40–47 48–54 Height† (inches) Body Weight† (pounds) 60–61 97–127 128–153 153–180 179–206 204–248 245–285 62–63 104–135 136–163 164–191 191–220 218–265 262–304 64–65 110–144 145–174 174–204 204–234 232–282 279–324 66–67 118–153 155–185 186–217 216–249 247–299 297–344 68–69 125–162 164–196 197–230 230–263 262–318 315–365 70–71 132–172 174–208 209–243 243–279 278–338 334–386 72–73 140–182 184–219 221–257 258–295 294–355 353–408 74–75 148–192 194–232 233–272 272–311 311–375 373–431 76 156–197 205–238 246–279 287–320 328–385 394–443 *BMIs lower than those listed as normal are considered underweight. †Calculations are done using height in meters and weight in kilograms. Height is without shoes. Weight is without clothes. BMI = body mass index.

Components of the Diet: Generally, nutrients are divided into two classes:

• Macronutrients: Macronutrients are required daily in large quantities. They include proteins, fats, carbohydrates, some minerals, and water.
• Micronutrients: 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 certain minerals that enable the body to use macronutrients. These minerals are called trace minerals because the body needs only very small amounts.

Water is required in amounts of 1 milliliter for each calorie of energy expended or about 2.6 quarts (2,500 milliliters) a day. The requirement for water can be met by the water naturally contained in many foods, and by drinking fruit or vegetable juices and caffeine-free coffee or tea as well as water. Alcoholic beverages and caffeinated coffee, tea, and sodas may make people urinate more, so they are less useful.

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: vitamins, minerals, some amino acids (components of protein), and some fatty acids (components of fats). Essential nutrients cannot be synthesized by the body and must be consumed in the diet.

Foods contain many other useful components, including fibers (such as cellulose, pectins, and gums). Foods also contain additives (such as preservatives, emulsifiers, antioxidants, and stabilizers), which improve the production, processing, storage, and packaging of foods (see Overview of Nutrition: Food Additives and Contaminants).

Last full review/revision July 2008 by Margaret-Mary G. Wilson, MD