Ever since the discovery of the first hormone secretin, in the year 1902, a wealth of hormones and growth factors have been described that influence and regulate all three steps of human nutrition, viz. digestive process, intestinal nutrient absorption and storage and utilization of nutritional substrate.
When the food enters stomach, several hormones help in its digestion. Gastrin causes the stomach to produce an acid for dissolving and digesting some foods, and for destroying harmful pathogenst present in the food. Secretin causes the pancreas to send out a digestive juice that is rich in bicarbonate. The bicarbonate helps neutralize the acidic stomach contents as they enter the small intestine. Secretin also stimulates the stomach to produce pepsin, an enzyme that digests protein, and stimulates the liver to produce bile. Cholecystokinin, or CCK causes the pancreas to produce the enzymes of pancreatic juice, necessary for fat digestion, and causes the gallbladder to empty.
In terms of regulating appetite, two more hormones play crucial roles: Ghrelin is produced in the stomach and upper intestine in the absence of food in the digestive system and stimulates appetite. Peptide YY is produced in the digestive tract in response to a meal in the system and inhibits appetite. Again, there is another hormone called leptin, which tells the body it is full and thus prevents overfeeding.
As the food enters small intestine, two more hormones start their regulatory influence upon its absorption:
Gastric inhibitory Peptide (GIP) is secreted in the duodenum and decreases the stomach churning in turn slowing the emptying in the stomach. Its another function is to induce insulin secretion which regulates blood glucose levels.
Motilin is secreted in the duodenum and controls contraction of stomach and intestine. It also stimulates the production of pepsin.
Then there are a host of hormones that regulate the storage of additional glucose and utilization of stored nutritional substrates under exigent conditions.