What role does melatonin play in the gut, are there any disorders which might involve decreased melatonin production in the gut, and would melatonin have any value as a supplement to support gut health?

It is certain that melatonin is required for healthy digestive function. Breast milk has been found to contain melatonin, which, in light of the fact that the infant GI tract cannot at first produce melatonin on its own, suggests that melatonin plays a critical role in digestion. Research has elucidated many of these critical functions:

• melatonin increases circulation to the gastric mucosa (an effect antagonized by its precursor serotonin)
• melatonin exerts a gastro-protective antioxidant effect
• melatonin diminishes visceral pain by reducing the sensitivity of nociceptive neurons in the GI tract
• melatonin enhances GI motility, helping to drive the digestive process
• melatonin decreases gastric acid production and increases gastrin and bicarbonate production
• through its effect on gastrin, melatonin increases the tone of the lower esophageal sphincter (LES)

Through a consideration of its functions above, melatonin would seem so intimately linked to disorders such as heartburn, gastroesophageal reflux disease (GERD), irritable bowel syndrome (IBS), and even peptic ulcer disease that one might wonder whether a melatonin deficiency is not in fact the cause of these disorders. This possibility has made melatonin the subject of considerable research, and so far the evidence supports the following conclusions:

• People suffering from heartburn and GERD have lowered GI melatonin levels
• Melatonin can be more effective than proton pump inhibitors in treating GERD
• Melatonin accelerates the healing of gastric ulcers in rats.
• Melatonin can improve symptoms of pain, bloating, and constipation in people with IBS

It is astounding that a natural molecule with so much potential has been ignored for so long. It is actually an embarrassingly common problem in science that the circumstances of something's discovery will heavily bias the understanding of its function. Consider the amino acid asparagine, so named because it was initially discovered as a component of asparagus. We now know that asparagine is an integral component of millions of proteins throughout the domains of life, but the name stuck because that is where it was discovered. Another example is the cytokine tumor growth factor beta, which is now known to have multiple, complex functions beyond that which became its namesake.

Melatonin was first discovered in the pineal gland, where its secretion is inhibited by light, and as such it was named "the sleep hormone" and wrongfully presumed for a long time to possess only this limited function. Promoting the onset of sleep in the brain is just one of melatonin's many functions, possibly even a minor one given its relatively staggering abundance elsewhere in the body.

In conclusion, the more we learn about the complexities of the gastrointestinal tract, including the gut-brain axis, the more we may come to appreciate how simple lifestyle interventions, such as sticking to a normal sleep schedule, can profoundly affect a diverse range of very common health problems, especially such as pertain to the digestive tract. This also bespeaks the power of relatively simple dietary supplements to promote healthy digestion and support the functions of the digestive tract, in ways which may at first not seem intuitive.