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Overview of Nutrition

By

Adrienne Youdim

, MD, David Geffen School of Medicine at UCLA

Last full review/revision Dec 2021| Content last modified Dec 2021
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Topic Resources

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), to do their daily physical and mental activities, and to minimize risk of disease and disability.

The United States Department of Agriculture (USDA) Dietary Guidelines for Americans 2020-2025, 9th edition state that: “A healthy dietary pattern consists of nutrient-dense forms of foods and beverages across all food groups, in recommended amounts, and within calorie limits.” According to these guidelines, the core elements that make up a healthy dietary pattern include:

  • Vegetables of all types and colors (including beans and legumes, such as peas and lentils)

  • Fruits, especially whole fruit

  • Grains, at least half of which are whole grain

  • Dairy, including fat-free or low-fat milk, yogurt, and cheese, and/or lactose-free versions and fortified soy beverages and yogurt as alternatives

  • Protein foods, including lean meats, poultry, and eggs; seafood; beans, peas, and lentils; and nuts, seeds, and soy products

  • Oils, including vegetable oils and oils in food, such as seafood and nuts

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 18.5 and 24.9 is usually considered normal or healthy for men and women. In the United States and other developed countries, many people have a BMI that is higher than 24. Maintaining an appropriate weight is important for physical and psychologic health. A standardized height-weight table can be used as a guide, but BMI is more reliable.

BMI, however, does not account for differences in body composition Body composition 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... read more . Waist circumference can be measured instead; the fat in the midsection is sometimes a more accurate measure of excess weight or harmful fat that is deposited in the internal organs and that tends to predict risk of heart disease and metabolic disorders.

Levels of many nutrients can be measured in blood, inside some cells, 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. However, whether these measures reliably indicate nutritional status may depend on what the measurement reflects (for example, whether inside cells or in the blood) as cellular levels of nutrients may be more reflective of usable or available nutrient as opposed to the amount that is carried in the blood.

Body composition

Body composition usually refers to how much of the body is fat and how much is muscle, typically expressed as the percentage of body fat. Body composition is sometimes estimated by

  • Measuring skinfold thickness

  • 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 (DXA) scan, computerized tomography (CT) scan, and magnetic resonance imaging (MRI). However, these more accurate methods are not easy to use, can be expensive, expose people to unnecessary radiation (CT scans), and are not always readily available. They are used mainly in research.

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 1/2 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, which people cannot feel, 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.

Hydrostatic weighing: People are weighed underwater in a small pool and that weight is compared to their weight on dry land. 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 that is not readily available, as well as considerable time and expertise to do.

Dual-energy x-ray absorptiometry (DXA): This imaging procedure accurately determines the amount and distribution of body fat. DXA uses a very low dose of radiation and is safe. However, it is too expensive to use routinely.

CT scan and MRI, although not routinely available for health enhancement alone, provide the most detailed and accurate body composition analysis because they can determine more precisely how much fat is in the tissues, including inside muscles and organs, and can differentiate the more harmful abdominal and internal organ (visceral) fat from the less harmful fat under the skin (subcutaneous fat).

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Components of the Diet

Generally, nutrients are divided into two classes:

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:

Essential nutrients cannot be synthesized by the body and must be consumed in the diet.

Spotlight on Aging: Nutrition

A diet is whatever a person eats, regardless of the goal. The best diet for older people has not been determined. However, people may benefit from changing some aspects of their diet as they age, based on the way the body changes as it ages. No changes are required for some nutrients such as carbohydrates and fats.

Older people are more likely to have disorders or take drugs that can change the body’s nutritional needs or the body’s ability to meet those needs. Disorders and drugs can decrease appetite or interfere with the absorption of nutrients. When older people see their doctor, they should ask their doctor whether the disorders they have or the drugs they take affect nutrition in any way.

More Information

The following English-language resources may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

  • Dietary Guidelines for Americans 2020-2025, 9th edition: U.S. Department of Agriculture (USDA) and U.S. Department of Health and Human Services nutrition recommendations by life stage, from birth through older adulthood

  • MyPlate Plan: USDA's food guidance system promoting a healthy eating routine with a variety of fruits, vegetables, grains, protein foods, and dairy and fortified soy alternatives

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