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A nutrient is either a chemical element or compound used in an organism's metabolism or physiology. A nutrient is essential to an organism if it cannot be produced by the organism and must be obtained from a food source.
Substances that provide energy
- Carbohydrates are compounds made of sugars. Carbohydrates are named by the number of sugar units they have.
- Proteins are organic compounds that are amino acids joined by peptide bonds. The body cannot make some of these; so they must come from food. In the body, proteins are broken down through digestion back into free amino acids.
- Fats are a glycerin molecule with three fatty acids attached. Fats are needed to keep cell membranes functioning properly, to insulate body organs against shock, to keep body temperature stable, and to maintain healthy skin and hair. The body does not manufacture certain fatty acids and the diet must supply these.
Substances that support metabolism
- Minerals are generally trace elements, salts, or ions such as copper and iron. These minerals are essential to human metabolism.
- Vitamins are organic compounds essential to the body. They usually act as coenzymes or cofactors for various proteins in the body.
- Water is an essential nutrient and is the solvent in which all the chemical reactions of life take place.
The following table gives an idea of what elements are essential for humans: 
Periodic table highlighting dietary elements
|The four organic basic elements||Quantity elements||Essential trace elements||Possible structural or functional role in mammals|
- Sizer F. & Whitney E. 2007. Nutrition: concepts and controversies. Belmont CA: Thompson Wadworth.
- CHNOPS: the six most abundant elements of life. Pearson BioCoach, 2010. "Most biological molecules are made from covalent combinations of six important elements, whose chemical symbols are CHNOPS".