Essential nutrients are nutrients required for normal body functioning that either cannot be synthesized by the body, or cannot be synthesized in amounts adequate for good health (niacin, choline), and must be obtained from a dietary source. Essential nutrients are also defined by the collective physiological evidence for their importance in the diet, as represented in US government approved tables for Dietary Reference Intake. Some categories of essential nutrients include vitamins, dietary minerals, essential fatty acids, and essential amino acids. Water and oxygen are also essential for human health and life, as oxygen cannot be synthesized by the body, and water, while a biochemical reaction product of metabolism, is not created in sufficient amounts. Both are necessary as biochemical reactants in some processes, and water is used in various ways such as a solvent, carrier, coolant, and integral polar structural member, but both are often not included as nutrients.