Understanding Digestion – Hormones of the Gut

Overview

• By expanding the concept of the stomach to encompass the functions of the entire digestive tract, connections can be made with the idea of the Stomach in Chinese Medicine.
• The organs of the digestive system secrete a variety of hormones, many of which have effects on major endocrine glands or in the brain.
• Similar to how the Stomach functions in Chinese Medicine, these hormones control things like appetite, blood sugar, cardiovascular function, weight gain, muscle mass and bone density, and mood disorders.
• Understanding the digestive system from the more functional perspective of Chinese Medicine allows for greater flexibility in diagnosis and treatment.

From a biomedical standpoint, the stomach doesn't do that much: it accepts food and begins the digestive process, and it is seen mostly as a place to hold food after we eat so that the small intestine can complete the digestion and absorb nutrients. There is even a form of surgery that is done to help people lose weight, called gastric bypass surgery, that partially closes off the stomach so it can’t hold as much food. Western medicine obviously doesn’t think too much of the stomach. But if the concept of the stomach is expanded, as it is in Chinese Medicine, the importance of this essential organ becomes more and more apparent.

If you haven’t read part 1, on the Chinese Medicine perspective of the Stomach, I recommend taking a look before reading this section, as I will be referring to the concepts outlined there. By looking at the gastrointestinal tract from a functional perspective the rest of the structures of the GI tract begin to make more sense. The organs of the digestive tract secrete a variety of hormones, both into the hollow spaces of the stomach and intestines as well as into the blood and circulation. There are so many nerves that the enteric nervous system has been called a second brain, the gut-brain. All of the commonly known neurotransmitters – like serotonin, dopamine, acetylcholine, GABA – are used by the gut-brain as well. This more complete picture allows us to match the biomedical perspective of the GI tract to the Chinese Medicine understanding.

Gut Hormones

There are lots of hormones that come from the organs of the digestive system. Since there are signaling molecules that seem to act only on nearby organs, I’ll be focusing on the hormones that are known to have effects beyond the digestive system.

Ghrelin

(1, 2, 3, 4, 5, 6) – Secreted from the lining of the stomach, this hormone is best known for increasing appetite, but it does a lot of other things as well. Ghrelin inhibits the secretion of insulin (from the pancreas) and synergizes with glucagon (from the pancreas), catecholamines (from the adrenals), and growth hormone (from the pituitary) to increase blood sugar. Ghrelin stimulates dopamine receptors in the brain and increases the concentration of dopamine in the substantia nigra. It increases fat storage and weight gain. It exists in heart cells and calms down the heart, helping with arrhythmias and heart attacks. It also reduces inflammation in the heart. Ghrelin can help increase both muscle mass and bone density, but it also increases cell growth in cancers. Ghrelin seems to have an influence on memory and learning by acting in the hippocampus, as well as defending against anxiety and depression caused by stress. Interestingly, ghrelin plays a role in stimulating lung growth in the developing fetus.

You can see the relationship to the Stomach of Chinese Medicine in the effect on appetite, the ability to control blood sugar, the connection to dopamine, and the ability to increase weight and muscle mass. The downward movement of Stomach qi can help with certain types of heart problems, echoed here by the connection between ghrelin and the heart. The ability of ghrelin to defend against stress correlates with the Stomach’s action on “digesting” stimuli and experiences.

Leptin

(7, 8) – Despite leptin being known as a “fat” hormone, since it is primarily made in adipocytes (fat cells), leptin is also made in the fundus of the stomach. Leptin helps regulate energy homeostasis by producing a feeling of satiety, and low levels of leptin contribute to feeling hungry. Leptin regulates neuroendocrine function via the hypothalamus, stimulating GnRH (gonadotropin-releasing hormone), TRH (thyrotropin-releasing hormone), GHRH (growth hormone-releasing hormone), and CRH (corticotropin-releasing hormone). These hormones, respectively, stimulate the production of testosterone and progesterone, thyroid hormone, growth hormone, and cortisol. Leptin also increases insulin sensitivity and decreases the secretion of insulin from the pancreas. It helps to increase immune function and plays a role in the regulation of bone density. In people with low levels giving extra leptin leads to weight loss and better metabolic function.

Feeling full is a definite function of the Stomach in Chinese Medicine, and when the Stomach is healthy fullness and hunger are appropriate. Interestingly, people who are overweight typically have leptin resistance; this could be seen as there being plenty of nourishment in the body but somehow it’s not recognized, and hunger persists – clearly a Stomach issue. Since the Stomach provides qi for all the other organs it’s no surprise that there is at least one hormone in the gut that stimulates the hypothalamus-pituitary axis; these two organs control much of the endocrine system. Any connection to insulin is also going to point towards Spleen/Stomach function.

Cholecystokinin

(9) – Cholecystokinin (CCK) is made primarily in the duodenum (the first part of the small intestine) and has a number of functions, both in the GI tract as well as neurologically. It inhibits the stomach from emptying, as well as stimulating the pancreas and gallbladder to secrete digestive enzymes and bile. It also increases the feeling of satiety. Its metabolites in the central nervous system can induce anxiety, panic, or hallucinations.

The effects in the digestive system and on appetite place CCK in the realm of Stomach function in Chinese Medicine. Interestingly, Stomach qi counterflow can lead to the same states of anxiety, panic, and hallucinations described for CCK.

Secretin

(10) – This peptide hormone is made primarily in the duodenum but also in the jejenum and ileum, the rest of the small intestine. Secretin regulates digestive secretions, reducing stomach acid and increasing bicarbonate and bile in the duodenum. It also has diuretic effects, helping to regulate water metabolism. Secretin has actions in the central nervous system and appears to increase neural healing and growth. It has cardiovascular effects, increasing heart rate and cardiac output while simultaneously decreasing arteriolar resistance.

The connection between the digestion and the cardiovascular system is present again in secretin, as with ghrelin. Beyond the regulation of digestive secretion, the Stomach as the provider of qi nourishes all cells, so it’s interesting to note that secretin has neuro-restorative properties. Traditionally the Spleen is involved in water metabolism, and secretin has actions in that realm as well.

Somatostatin

(11, 12) – Somatostatin is a broad-acting inhibitory hormone, inhibiting both digestive hormones as well as pituitary ones. It inhibits gastrin, cholecystokinin, secretin, insulin, glucagon, and other digestive hormones; it also inhibits growth hormone, thyroid-stimulating hormone, and prolactin.

This is a balancing hormone to the activating hormones associated with the Stomach function so far. It’s interesting to note that again there is a connection between the GI and the hypothalamus-pituitary axis. This shows the role of the Spleen and Stomach in regulating the endocrine system via these central glands.

Insulin

(13) – Insulin is one of the more well-known gut hormones. It is secreted from the pancreas directly in the blood and is a major regulator of blood sugar. Insulin drives glucose out of the blood and into cells, especially the liver and muscle tissue. It is known as the diabetes hormone; in diabetes type 1 the beta cells of the pancreas get destroyed and cannot produce enough insulin, whereas in diabetes type 2 insulin resistance develops and the body becomes less responsive to the influence of insulin.

The connection with nourishment, and therefore the Spleen and Stomach, is clear. Diabetes type 1 is very similar to a disease known in Chinese Medicine as wasting and thirsting disease; this disease is considered to be a Spleen issue. In diabetes type 2 there is often Spleen and Stomach deficiencies, compounded by the presence of dampness. Dampness can manifest as high blood sugar or excess weight, two very common issues for diabetes patients.

Glucagon

(14) – Glucagon is the antagonist to insulin, directly countering insulin’s effects and helping to increase blood sugar. This hormone is also secreted by the pancreas and interestingly tends to be elevated in diabetes just as insulin often is. Glucagon goes up when the person hasn’t eaten recently, during fasting, exercise, and stress.

Just as with insulin the actions of glucagon on blood sugar, and the fact that it is secreted from the pancreas, place it within the realm of the Spleen and Stomach. The ability to nourish the body in the absence of food is a trait of the digestion too.

There are more than 50 gut hormones and bioactive peptides; hopefully, the ones above as the most well-known are enough evidence to begin to connect the GI system with the Stomach of Chinese Medicine. See Part 3 next part of this series to examine the role of the enteric nervous system and its neurotransmitters, as they are also integral to understanding how the digestive system influences the health of the body.