Melatonin, a hormone produced by the brain in response to darkness, is used by millions of people around the world as a natural sleep aid. And for good reason. Research has shown that the popular supplement “decreases sleep onset latency, increases total sleep time, and improves overall sleep quality,” according to a 2013 meta-analysis published in PLOS One.
Other research has shown that melatonin demonstrates significant antioxidant and immunomodulatory activity, and may show promise in the prevention and treatment of cancer. With all that going for it, it’s easy to understand why melatonin is such a popular supplement, but a new study in the journal Microorganisms finds that those benefits may come with a catch.
“It’s generally thought to be harmless. After all, it’s a hormone and can help regulate sleep,” says Cristina Ribeiro de Barros Cardoso, a professor of immunology at the University of São Paulo’ in Brazil. “However, our study shows that people should be careful about taking hormone supplements and that the ingestion of melatonin supplement can have adverse effects on health.”
Cardoso studies inflammatory bowel diseases such as Crohn’s disease and ulcerative colitis, specifically targeting the role that the immune system can play in managing these conditions. “Our lab works to achieve a better understanding of these diseases and propose novel treatments that are more affordable…in recent years we have pursued novel therapeutic options, primarily based on immune response modulation or regulation,” she says.
To that end, Cardoso and her team set out to determine whether melatonin’s antioxidant and immunomodulatory properties could make it a potential treatment for colitis. Their study involved inducing colitis in a group of rats, and then administering either 10mg/kg of melatonin per day, or a placebo solution. Mice were then evaluated daily for weight change and clinical signs of colitis.
What the researchers found shocked them—bowel inflammation in the mice treated with melatonin actually got worse. This unexpected result led Cardoso’s team to experiment further. “We then began trying to understand why,” says Cardoso. “We found that melatonin had a positive effect on the disease if the effect on gut microbiota was ignored and the mice were treated with wide-spectrum antibiotics to eliminate all the bacteria.”
So the negative impact that melatonin showed on colitis inflammation depended entirely upon bacteria that live in the intestine and are also associated with inflammatory bowel diseases. Cardoso’s team discovered that certain features of gut microbiota increase inflammation and dysregulate the immune system in response to treatment with melatonin, damaging the digestive system.
“We started out in this study with the assumption that we might be able to develop a novel treatment for Crohn’s disease and ulcerative colitis, but to our surprise, we found exactly the opposite, and patients should be made aware of this danger,” Cardoso says. “I’m not saying by any means that melatonin doesn’t have beneficial effects. On the contrary, in fact…[but] It’s a hormone, and regulation of the interaction between all hormones and the immune system is very delicate…We should take great care with medications, hormone supplements, or hormones offered as food supplements.”