Let's look at the facts first: the link between traumatic experiences, such as emotional abuse or sexual assault, and digestive problems is a strong one. One of the biggest disorders in question? Irritable bowel syndrome, or IBS.

While IBS's causes are debated and aren't the same for everybody, researchers think a combination of physical factors, over-sensitivity in the gut area, and various triggers like infections or stress all play a part. And we've increasingly discovered that an experience of trauma may increase the likelihood of developing IBS. Back in 1997, a review of the science found "a significant association between a history of sexual, physical or emotional abuse and IBS symptoms," adding that "Abused patients with symptoms of IBS are more likely than non-abused patients to seek health care for gastrointestinal symptoms and to report somatic stimuli as painful."

More recent studies have cemented this particular link, and broader ones. One in 2014, for example, looked at over 21,000 African Americans in the U.S. and found that people with IBS were "significantly more likely" to suffer from PTSD than those with healthy guts. And it goes further; a 2011 review found that people who's suffered from severe abuse tended to show up with a higher likelihood of gastrointestinal disorders in general. So the link is definitely there. The question is: why?

If we want to explain the trauma-gut link, we've actually got a multitude of possibilities on our hands. The relationship between the traumatized brain and our digestion is pretty complex; the scientists who did the 2011 review call it "stress-mediated brain-gut dysfunction" (in case you want a fancy condition to explain your stomach aches to a date), and pinpoint a range of stuff that might cause it, "from altered stress-induced mucosal immune function to impaired ability of the central nervous system to downregulate incoming visceral or somatic afferent signals." If that makes zero sense to you, don't worry — we're going to unpack it.

One of the biggest causes of digestive malfunctions as a result of trauma is actually your nervous system. The brain and the gut are connected by various nervous connections; Harvard Medical School explains that we have both a sympathetic nervous system (the one that triggers our "fight or flight" response when we're very stressed or panicked) and a few others, including one that lines the gut. They interact with one another to determine how to react to things. A traumatized nervous system, though, is likely to send the wrong signals, or to send far too many and panic everything in your stomach lining. That's the "impaired ability" the scientists were talking about.

It's an explanation that seems to be particularly useful when it comes to IBS. A major study that came out in 2011 by the Mayo Clinic, which interviewed 2,600 people, found a distinct link between multiple types of trauma and IBS, and hypothesized that it was partially down to nervous system communication issues. (That's not all of it, as IBS can be caused by a variety of stuff, but it's a pretty interesting addition to the party.)

Here's where things get really specific: a few studies have found that there's a particular neurotransmitter that seems to link PTSD, the most extreme form of traumatic condition, and IBS. Neuropeptide Y, as it's charmingly called, has a big role in PTSD in the brain, because it's involved with our stress responses and how they're regulated; it might even be at the root of what goes wrong in the brain during PTSD flashbacks. And new research coming out next month shows up something pretty cool: there seems to be a link between neuropeptide Y levels in PTSD sufferers and the occurrence of IBS.

Basically, the researchers found that neuropeptide Y levels in people with PTSD may actually be partially causing irritable bowels and digestion issues, because it seems to signal the colon to misbehave. It remains to be seen whether this is more widely applicable (whether neuropeptide Y levels are weird in people with general traumatic experiences, for example, and if the problem might explain other gastrointestinal disorders).

The gut-brain axis, as it's called, is a weird one. It's not just all about the nervous system; there's a relationship between our gut microbes and our mental health, too. (It's one of the most promising areas of research for figuring out and helping people with depression, for example.) And that microbial content might also play a role in how we respond to trauma. It's a complicated thing.

We now know that the way in which our gut microbes develop can actually affect how susceptible we are to PTSD and bad reactions to trauma in later life, which is weird enough. But 2016 experiments with mice at the Office of Naval Research, of all places, found that trauma seriously disrupts gut bacteria, killing off various varieties and making it less diverse. "The gut and bowels are a very complex ecology," the researchers said. "The less diversity, the greater disruption to the body." A lack of diversity of gut flora's been seen elsewhere, like in humans who've spent a lot of time in hospital, and it is very much not good news. However, the researchers also found something interesting: when they transplanted more diverse gut flora into the traumatized mice, the mice became much calmer and showed a lot less misery. Healing PTSD via the gut? Hey, it might be the future.

This is the interesting thing: if we've experienced trauma and are having gut troubles as part of our body's response to it, it may not actually be a one-way street. It was suggested, back in the 1997 review, that it might be "a vicious cycle of mutual aggravation," in which trauma causes altered sensitivity in the gut via nervous system issues, and then that discomfort "acts to reinforce central nervous system abnormalities." We know this actually happens when the trauma's caused by the gut problems themselves; a 2010 study of Crohn's disease found that people suffered severe psychological stress as a result of the illness, which then made their symptoms worse. It's no stretch to wonder if trauma-induced gut issues of all kinds might stress us out and reinforce bad gut-brain behavior.

Co-morbidity, or the existence of other illnesses alongside our gut issues, may make things worse, too. It's pretty common for gut-related problems in traumatized people to coincide with other medical issues, like depression and cardiovascular disorders, which are related to PTSD in their own way. Those can interact in the body to make either stress or gut disorders worse, and it all becomes a bit of a nasty knot. One that we'll hopefully untie in the future. Yay science!