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 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: dark green; red and orange; beans, peas, and lentils; starchy; and other vegetables
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
If people consume too much food, obesity Obesity Obesity is excess body weight. Obesity is influenced by a combination of factors, which usually results in consuming more calories than the body needs. These factors may include physical inactivity... read more 
may result. If they consume large amounts of certain nutrients, usually vitamins Overview of Vitamins Vitamins are a vital part of a healthy diet. The recommended dietary allowance (RDA)—the amount most healthy people need each day to remain healthy—has been determined for most vitamins. A safe... read more or minerals Overview of Minerals Minerals are necessary for the normal functioning of the body’s cells. The body needs relatively large quantities of Calcium Chloride Magnesium read more , harmful effects (toxicity) may occur. If people do not consume enough nutrients, undernutrition Undernutrition Undernutrition is a deficiency of calories or of one or more essential nutrients. Undernutrition may develop because people cannot obtain or prepare food, have a disorder that makes eating or... read more 
may develop, resulting in a nutritional deficiency disorder.
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 countries with low rates of food insecurity, 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).
Components of the Diet
Generally, nutrients are divided into two classes:
Macronutrients: Macronutrients are required daily in large quantities. They include proteins Proteins Carbohydrates, proteins, and fats are the main types of macronutrients in food (nutrients that are required daily in large quantities). They supply 90% of the dry weight of the diet and 100%... read more , fats Fats Carbohydrates, proteins, and fats are the main types of macronutrients in food (nutrients that are required daily in large quantities). They supply 90% of the dry weight of the diet and 100%... read more , carbohydrates Carbohydrates, Proteins, and Fats Carbohydrates, proteins, and fats are the main types of macronutrients in food (nutrients that are required daily in large quantities). They supply 90% of the dry weight of the diet and 100%... read more , 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 2.7 liters (2.8 quarts) for women and 3.7 liters (3.9 quarts) for men 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:
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.
Nutrients that the body can make from other compounds are called nonessential. However, under certain conditions, such as illness or stress, the body may not make enough of these nonessential nutrients. Then, they need to be consumed in the diet, making them conditionally essential nutrients.
Foods contain many other useful components, including fiber Fiber Some foods contain fiber, which is a tough complex carbohydrate. Fiber may be Partly soluble: It dissolves in water, and the body may be able to digest some of it. Insoluble: It does not dissolve... read more (such as cellulose, pectins, and gums).
Foods also contain additives Food Additives and Contaminants Additives and contaminants are often components of commercially grown, prepared, or packaged foods. Noncommercially grown food can also become contaminated due to pollution or pesticides. Substances... read more (such as preservatives, emulsifiers, antioxidants, and stabilizers), which improve the production, processing, storage, and packaging of foods.
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: 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
