Hormones are chemical messengers that travel throughout the
blood, acting on or stimulating various parts of the body. They are involved in a range of processes crucial to our health and wellbeing, including reproduction, thyroid health, blood sugar regulation and more. In this blog, we’re putting the spotlight on the major hormones associated with our digestive system and appetite!
How do hormones regulate our digestion?
Digestion begins as soon as we look at and smell food, causing saliva to increase in order to help break down the food that will soon be chewed in our mouths. Hormones come into play when our chewed food is swallowed and enters the stomach. Gastrin, intestinal gastrin, secretin, histamine, serotonin and somatostatin are the hormones secreted once food reaches the stomach. Each of these hormones plays a unique role in supporting the way we digest our food.
Gastrin is secreted when there are partially digested proteins in the stomach, and also by nerve signals. Its role is to increase the secretion of hydrochloric acid for proper food breakdown in the stomach, as well as stimulating the contraction of the intestinal muscles which helps move digested food through the rest of the digestive tract. Intestinal gastrin also targets the stomach where it too stimulates hydrochloric acid secretion, as well as protective mucous that lines and protects the stomach.
Secretin is a digestive hormone produced by the duodenum (the upper part of the small intestine). When food hits the stomach, secretin increases the output of bile from the liver and pancreatic juice from the pancreas which are both required for the breakdown of dietary proteins, carbohydrates and fats. Bile and pancreatic juice also play a role in intestinal motility.
We usually think of histamine as part of allergic reactions, however it also plays a role in digestive health. Histamine’s role is to activate stomach cells, called parietal cells, to release hydrochloric acid for food breakdown. In addition to its roles in mood balance and mental health, serotonin also gets secreted by the stomach cells and causes the contraction or churning action of the stomach muscle to further assist with food breakdown.
Somatostatin is the final hormone secreted once food enters the stomach however. Its role is protective and regulatory – it helps to regulate stomach acid levels by reducing gastric juice secretion and the output of bile and pancreatic juice. It works on the stomach, liver, gallbladder and pancreas to not only regulate stomach acid levels but also regulate the rate of intestinal motility.
What happens next?
After we’ve chewed our food, swallowed it and it mixes with stomach acid to break down even further, this compound is called chyme. When chyme enters the small intestine, four other digestive hormones get to work: gastric inhibitory peptide (GIP), cholecystokinin (CCK), motilin and vasoactive intestinal peptide (VIP).
GIP is produced in the top part of the small intestine, called the duodenum. Its release is stimulated by the presence of chyme, especially when it is rich in fat (i.e. after we’ve eaten a fatty meal). GIP targets the stomach and pancreas where it slows down the stomach’s churning rate and stimulates the release of insulin which is important for using the food we eat as fuel and for balancing blood sugar levels. CCK’s role is to increase the output of pancreatic juice and stimulate the gallbladder to expel bile, all important for intestinal motility to get the chyme moving through our intestines.
VIP plays a protective role on the stomach and small intestine – it decreases stomach acid production which it helps it from excessive acid exposure as stomach contents are moved into the small intestine. VIP also promotes the dilation of blood capillaries to promote the absorption of nutrients once chyme enters the small intestine.
Finally, motilin gets to work. Motilin’s secretion isn’t actually stimulated by the presence of chyme – it is naturally secreted every 1.5 to 2 hours by our nervous system and in times when we are fasting between meals. It targets the end part of the duodenum where it stimulates what is known as the migrating motor complex. This complex is thought to have a ‘housekeeping’ role throughout the lower digestive tract by sweeping any residual, undigested food through the rest of the intestines. Motilin controls the pattern of smooth muscle contractions that cause this sweeping action.
Hormones that regulate our appetite
As well as the above digestive hormones, the body has two additional hormones that are responsible for regulating appetite, called leptin and ghrelin.
Ghrelin is released by the stomach and targets the pituitary gland located in the brain, signalling to the body that it needs to eat! Ghrelin levels are typically higher during periods of fasting and/or when consuming a very low calorie diet. When we feel hungry, that’s ghrelin doing its job.
Leptin on the other hand is produced by adipose (fat) tissue and its main role is to suppress our appetite when we are full. Leptin sends signals to the brain, giving it information about the status of the body’s energy (fat) stores, resulting in a decreased appetite. What is interesting to note is that in individuals with a high fat mass and therefore high leptin levels, the body becomes desensitised to leptin and it no longer functions as an appetite suppressant.
REFERENCES
Marieb, E.N., Hoehn, K. (2013), Human Anatomy & Physiology (9 th Ed), Chapter: The Digestive System, pp. 844-905, Pearson Education, Inc.
Klok, M.D., Jakobsdottir, S., Drent, M.L. (2006), The role of leptin and ghrelin in the regulation of food intake and body weight in humans: a review, Obesity Reviews, 8(1): 21-34
