What the tongue tells the brain about taste.

The chemical stimuli of special significance to taste are sugars (sweet), amino acids (umami), sodium chloride and other salts (salty), alkaloids (bitter) and acids (sour). Sugars and amino acids tend to be preferred, while alkaloids and acids tend to be avoided. Intake of salts depends on electrolyte balance. The gustatory system codes taste qualities and their associated hedonic attributes. The ability to distinguish foods from poisons is so important that it is hard-wired in the receptor cells themselves. Yet despite teleological appeal, neither nutritional value nor toxicity is a stimulus dimension that is coded per se. Sweet and bitter may be considered metaphors for nutritious and toxic, respectively. Toxicity and nutrition are not chemical properties of a stimulus nor are they properties of the senses. They are defined in terms of critical metabolic events, most of which are beyond sensation. For example, sucrose and saccharin are both sweet, but saccharin has no caloric value. Salts such as NaCl and LiCl have similar tastes, yet lithium salts are toxic but sodium salts are not. There are many examples of neurotoxins that are tasteless yet extremely toxic. Strychnine and sucrose octaacetate (SOA) are both bitter, though strychnine is toxic while SOA is not.