Carbohydrates range from simple monosaccharides (glucose, fructose, galactose) to complex polysaccharides (starch).
Fats are triglycerides, made of assorted fatty acid monomers bound to glycerol backbone.
By breaking down existing protein, some glucose can be produced internally; the remaining amino acids are discarded, primarily as urea in urine.
Other dietary substances found in plant foods (phytochemicals, polyphenols) are not identified as essential nutrients but appear to impact health in both positive and negative ways.
For example, salt provides sodium and chloride, both essential nutrients, but will cause illness or even death in too large amounts.
A diet that contains adequate amounts of amino acids (especially those that are essential) is particularly important in some situations: during early development and maturation, pregnancy, lactation, or injury (a burn, for instance).
A few amino acids from protein can be converted into glucose and used for fuel through a process called gluconeogenesis; this is done in quantity only during starvation.
[7] Animal intestines contain a large population of gut flora which are essential to digestion, and are also affected by the food eaten.