Editing Digestion (section)

==Breakdown into nutrients==
{{Expand section|digestion of other substances|date=August 2011}}

===Protein digestion===
Protein digestion occurs in the stomach and [[duodenum]] in which 3 main enzymes, pepsin secreted by the stomach and [[trypsin]] and [[chymotrypsin]] secreted by the pancreas, break down food proteins into [[polypeptides]] that are then broken down by various [[exopeptidases]] and [[dipeptidases]] into [[amino acids]].  The digestive enzymes however are mostly secreted as their inactive precursors, the [[zymogens]].  For example, trypsin is secreted by pancreas in the form of [[trypsinogen]], which is activated in the duodenum by [[enterokinase]] to form trypsin.  Trypsin then cleaves [[protein]]s to smaller polypeptides.

===Fat digestion=== <!--Fat digestion redirects here-->
{{Main|Fatty acid metabolism#Dietary sources of fatty acids, their digestion, absorption, transport in the blood and storage}}
Digestion of some fats can begin in the mouth where [[lingual lipase]] breaks down some short chain lipids into [[diglyceride]]s. However fats are mainly digested in the small intestine.<ref name="mehta">[http://pharmaxchange.info/press/2013/10/digestion-of-fats-triacylglycerols/ Digestion of fats (triacylglycerols)]</ref>  The presence of fat in the small intestine produces hormones that stimulate the release of [[pancreatic lipase]] from the pancreas and [[Bile acid|bile]] from the liver which helps in the emulsification of fats for absorption of [[fatty acids]].<ref name="mehta" />  Complete digestion of one molecule of fat (a [[triglyceride]]) results a mixture of fatty acids, mono- and di-glycerides, but no [[glycerol]].<ref name="mehta" />

===Carbohydrate digestion===
{{further|Carbohydrate metabolism|Carbohydrate catabolism}}
In humans, dietary starches are composed of [[glucose]] units arranged in long chains called amylose, a [[polysaccharide]]. During digestion, bonds between glucose molecules are broken by salivary and pancreatic [[amylase]], resulting in progressively smaller chains of glucose.  This results in simple sugars glucose and [[maltose]] (2 glucose molecules) that can be absorbed by the small intestine.

[[Lactase]] is an enzyme that breaks down the [[disaccharide]] [[lactose]] to its component parts, glucose and [[galactose]].  Glucose and galactose can be absorbed by the small intestine.  Approximately 65 percent of the adult population produce only small amounts of lactase and are unable to eat [[Fermentation|unfermented]] milk-based foods.  This is commonly known as [[lactose intolerance]].  Lactose intolerance varies widely by genetic heritage;  more than 90 percent of peoples of east Asian descent are lactose intolerant, in contrast to about 5 percent of people of northern European descent.<ref>{{cite web|title=Genetics Home Reference|url=http://ghr.nlm.nih.gov/condition/lactose-intolerance|website=US National Library of Medicine|publisher=US National Institutes of Health|access-date=27 June 2015}}</ref>

[[Sucrase]] is an enzyme that breaks down the disaccharide [[sucrose]], commonly known as table sugar, cane sugar, or beet sugar. Sucrose digestion yields the sugars [[fructose]] and glucose which are readily absorbed by the small intestine.

===DNA and RNA digestion===
{{main|nucleic acid metabolism}}
DNA and RNA are broken down into [[Nucleotide|mononucleotides]] by the [[nuclease]]s [[deoxyribonuclease]] and [[ribonuclease]] (DNase and RNase) from the pancreas.