ElyDave said:
but your stomach acid rather rapidly destroys the structure of most enzymes, after all that is what it is meant to do. For enzyme read protein, stomachs digest proteins, hence no oral insulin delivery mechanism.
No you've got role of enzymes completely wrong I'm afraid. They are selective catalysts, starting or stopping biological reactions. Gastric acids don't destroy enzymes they activate them and enzymes are not the same as proteins at all. Rather they break down proteins.
"Gastric acid is a digestive fluid, formed in the stomach. It has a pH of 1.5 to 3.5 and is composed of hydrochloric acid (HCl) (around 0.5%, or 5000 parts per million) as high as 0.1 M,[1] and large quantities of potassium chloride (KCl) and sodium chloride (NaCl). The acid plays a key role in digestion of proteins, by activating digestive enzymes, and making ingested proteins unravel so that digestive enzymes break down the long chains of amino acids."
http://en.wikipedia.org/wiki/Gastric_acid
Digestive enzymes are secreted by different exocrine glands including:
Salivary glands
Secretory cells in the stomach
Secretory cells in the pancreas
Secretory glands in the small intestine
Of the many stomach or gastric enzymes, Pepsinogen is the main one. It is produced by the stomach cells called "chief cells" in its inactive form pepsinogen, which is a zymogen. Pepsinogen is then activated by the stomach acid into its active form, pepsin. Pepsin breaks down the protein in the food into smaller particles, such as peptide fragments and amino acids. Protein digestion, therefore, first starts in the stomach, unlike carbohydrate and lipids, which start their digestion in the mouth.
http://en.wikipedia.org/wiki/Pepsin