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Overview of Nutrition

by Adrienne Youdim, MD

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.

Evaluation of Nutritional Status

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. In the United States and other developed countries, many people have a BMI that is higher than 24.

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 not easy to use, can be expensive, and are not always readily available. They are used mainly in research.

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.

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)

  • 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 Food Additives and Contaminants).

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