In the complex, elegant system called the human body, the nutrients that are ingested and absorbed are essential for the growth, maintenance, repair, and replacement of the trillions of cells of the organs and tissues that do the body’s work. These nutrients are the substances in the foods and beverages we ingest.

Nutrition is the sum total of the interaction between the foods we eat-our diets-and the ways in which our bodies process, use, and are nourished by the nutrients in those foods. Although human bodies are incredibly adaptable and may maintain themselves for a long time when nutrition is inadequate or improper, optimum health is dependent on optimum nutrition. When proper amounts of nutrients are lacking or out of balance, or when nutrients are not adequately processed, dysfunction, ill health, and disease are inevitable.

Nutrients for Life

Scientists and researchers are still learning to define optimal nutrition and understand how nutrients affect health and disease. So far, more than 40 nutrients have been identified as substances that are essential to life. They are grouped into six categories or classes. These classes are carbohydrates, proteins, fats, vitamins, minerals, and water.

Carbohydrates, fats, proteins, some minerals, and water are considered to be macronutrients, because the body needs them in large quantities. Vitamins and most minerals (often called trace minerals) are needed in relatively small amounts and are referred to as micronutrients. They are essential because they enable the body to use its macronutrients. No single food contains all essential nutrients, but most foods are a source of one or more in varying degrees. People must ingest a variety of foods in order to nourish their bodies.

Energy And Fuel

One of the most important functions of nutrients is to provide the fuel that powers the body’s functions. Glucose is the sugar from carbohydrates that provides the major source of fuel for the body, its organs, and its cells, but fats are another important source. When necessary, proteins can also be used as fuel. These macronutrients provide the body with 100% of its energy. Energy is measured in calories. Some foods, such as cheeseburgers, are calorie dense, while others, such as celery, provide few calories.

How much food is required to provide enough energy depends on the kinds of foods a person eats, as well as a person’s activity level, age, and size. In other words, how much energy is used up and how quickly it is used depends on individual factors and is never standard for all people at all times in their lives.

Because of the energy supplied by these macronutrients, muscles move, lungs breathe, and hearts beat. Bodies use the energy to maintain the correct temperature. Cells build the proteins that do the work in the body. With too few calories, cell activity and organ function falter. A complete lack of energy (meaning zero calories consumed) would lead to cell death and organ failure from starvation within about 8 to 12 weeks.

If too many calories are taken, the body stores the excess as fat, in case extra energy is needed in the future. Every healthy person needs to have some fat stores for emergencies, but an excess amount leads to unhealthy body weight and stress on organs, which also can lead to disease.