Overview of Nutrition
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, undernutrition may develop, resulting in a nutritional deficiency disorder.
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 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.
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 (DXA) 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.
Another important consideration is how much of the body is fat and how much is muscle (body composition). Hydrostatic weighing, skinfold thickness, bioelectric impedance analysis, and dual-energy x-ray absorptiometry can be used 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 that is not readily available, as well as 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 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 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 (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.
Body Mass Index (BMI)
Generally, nutrients are divided into two classes:
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:
Essential nutrients cannot be synthesized by the body and must be consumed in the diet.
Foods contain many other useful components, including fiber (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.