Human nutrition is the provision to obtain the materials necessary to support life. In general, people can survive for two to eight weeks without food, depending on stored body fat and muscle mass. Survival without water is usually limited to three or four days.
Nutritional science investigates the metabolic and physiological responses of the body to diet. With advances in the fields of molecular biology, biochemistry, genetics, the study of nutrition is increasingly concerned with metabolism and metabolic pathways: the sequences of biochemical steps through which substances in living things change from one form to another.
The human body contains chemical compounds, such as water, carbohydrates (sugar, starch, and fiber), amino acids (in proteins), fatty acids (in lipids), and nucleic acids (DNA and RNA). These compounds in turn consist of elements such as carbon, hydrogen, oxygen, nitrogen, phosphorus, calcium, iron, zinc, magnesium, manganese, and so on. All of these chemical compounds and elements occur in various forms and combinations (e.g. hormones, vitamins, phospholipids, hydroxyapatite), both in the human body in the plant and animal organisms that humans eat.
The human body consists of elements and compounds ingested, digested, absorbed, and circulated through the bloodstream to feed the cells of the body. Except in the unborn fetus, the digestive system is the first system involved in obtaining nutrition. In a typical adult, about seven liters of digestive juices enter the digestive tract. These break chemical bonds in ingested molecules, and modulate their conformations and energy states.
There are seven major classes of nutrients:
- Dietary fiber,
- Vitamins, and
These nutrient classes can be categorized as either:
- macro-nutrients (needed in relatively large amounts) or
- micro-nutrients (needed in smaller quantities)
The macro-nutrients are carbohydrates, fats, fiber, proteins, and water. The micro-nutrients are minerals and vitamins.
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