1. The process by which food is broken down mechanically and chemically in the gastrointestinal tract and converted into absorbable forms. Salts (minerals), water, and monosaccharides can be absorbed unchanged, but starches, fats, and proteins must be broken down into smaller molecules. This is brought about by enzymes, each of which acts on a specific type of food and requires a specific pH to be effective.

SEE TABLE: Action of Digestive Enzymes on Food
Hormones released by the gastrointestinal mucosa stimulate the secretion of digestive enzymes and bile and influence the motility (peristalsis) of the stomach and intestines. Starches and disaccharides are digested to monosaccharides; fats are digested to fatty acids and glycerol; proteins are digested to amino acids. During digestion vitamins and minerals are liberated from these large organic molecules.
SEE: intestinal hormone

2. In biochemistry, the consumption of a substrate by an enzymatic process.
Action of Digestive Enzymes on Food

Food ComponentEnzymeSecretionSite of Action
ProteinsPepsinGastric juice, acidStomach
TrypsinPancreatic juice, alkalineSmall intestine
PeptidasesIntestinal juiceSmall intestine
FatsLipaseGastric juiceStomach
Pancreatic juiceSmall intestine
CarbohydratesSalivary amylaseSaliva, alkalineMouth
Pancreatic amylasePancreatic juiceSmall intestine
Sucrase, maltase, lactaseIntestinal juiceSmall intestine

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