Food is one of the basic necessities of life. Food contains nutrients—substances essential for the growth, repair, and maintenance of body tissues and for the regulation of vital processes. Nutrients provide the energy our bodies need to function.
The energy in food is measured in units called calories. Age, sex, weight, height, and level of activity determine the number of calories a person needs each day. Depending on age, sex, and activity level, the recommended daily caloric intake for a child aged 11 to 14 can range anywhere from 1,600 to 2,600 calories per day, with sedentary girls needing the fewest calories and active boys needing the most. For adults, this can range from 1,800 to about 3,000.
Kinds of Nutrients
Scientists divide nutrients into six major groups: carbohydrates, fats, proteins, minerals, vitamins, and water. Most foods contain all or most nutrient groups, in different amounts.
Carbohydrates provide energy for the body. Nearly all the carbohydrates we eat come from plants. They include starches found in cereal grains and plants like potatoes and yams. Sugars, found in fruits, vegetables, and milk, are also carbohydrates. Sugarcane and sugar beets are grown specifically for their high sugar content.
Many of the starches and sugars we eat have been processed into products, such as flour and corn syrup. These processed carbohydrates are used in cookies, cakes, breads, pastas, and pies.
Fats provide more than twice as much energy as carbohydrates. They also help protect and insulate the body and its internal organs. Common fats include vegetable oils, such as soybean, cottonseed, and corn oil. They are used in cooking and in the processing of many foods. Fats that come from animal products include butter and lard. Eggs, milk, cheese, meats, poultry, and fish also contain high levels of fats.
Proteins are the body’s chief tissue-builders. They help keep skin, bones, muscles, and blood healthy. Proteins also help regulate bodily processes, including transporting oxygen and nutrients into and out of cells; the clotting of blood; and the formation of antibodies, which help fight disease. Animal products, such as beef, fish, poultry, eggs, and dairy products, are high in protein. Grains, nuts, and some beans are also protein-rich foods.
Minerals and vitamins are called micronutrients because they are needed in very small quantities compared with carbohydrates, fats, and proteins (known as macronutrients). Minerals provide building materials for the body and help regulate its activities, much as proteins do. Calcium and phosphorus build strong bones and teeth, iron contributes to healthy blood, and iodine helps keep the thyroid gland working.
Vitamins help the body make full use of other nutrients by assisting the chemical reactions that make those nutrients work. For example, vitamin B1, or thiamine, helps regulate the release of energy from carbohydrates, promotes a healthy appetite, and aids the functioning of the nervous system. Vitamin D helps in the growth and maintenance of healthy bones.
Other essentials for the body’s health include water, oxygen, and fiber. Some scientists include water in the list of basic nutrients. Water makes up more than half of a human body’s weight. It is involved in most body processes, such as the regulation of temperature, the transporting of nutrients into cells, and the elimination of waste products from cells.
Oxygen is not a nutrient, since it is breathed in and not eaten, but it is essential to life. It permits the release of energy from food inside the body.
Fiber is indigestible material found in most plant foods. It adds bulk to the diet, helping to keep the intestines healthy. Fiber-rich foods include whole grains, dried beans, and fresh fruits and vegetables.