A Whole9 guest post by Emily Deans M.D., a board certified psychiatrist with a practice in Massachusetts and a clinical instructor in psychiatry at Harvard Medical School.

Irritable bowel syndrome (IBS) is a functional disorder of the gut that can lead to cramping, abdominal pain, constipation, and diarrhea. It’s symptoms often come hand in hand with chronic anxiety and depression, so as a psychiatrist, I see lots of patients with IBS symptoms. While some people take the idea that “people with IBS have lots of anxiety” to mean that the mysterious gut issue is “all in the head.” I think it is more likely that in many cases, the same problem is causing both the symptoms of irritable bowel and anxiety/depression.

What is that problem? In general terms, it’s likely an issue with the microbiota, those hundred trillion little beasties who live in our guts. In more specific terms it involves an intolerance to dietary FODMAPs (that’s fermentable oligosaccharides, disaccharides, monosaccharides, and polyols so yes, just say FODMAPs.) Some people might have a genetic issue as well, fructose malabsoroption, that allows too much fructose into the large intestine, leading to a feeding frenzy by the microbiota there perhaps causing IBS symptoms. If your microbiota have too much of a taste for FODMAPs, eating these fermentable oilgosaccharides and polyols can lead to inflammation (which can likely cause several systemic problems, including depression and anxiety). In addition, it’s possible that fructose and fructans (fructose-like molecules found in some some foodstuffs like wheat), if not absorbed in the upper gut, degrade and diminish the amount of the amino acid tryptophan that gets absorbed. Why does that matter? Well, the neurotransmitter serotonin is made from tryptophan, and we need plenty of tryptophan to make serotonin to maintain a happy, non-anxious mood state.

Common FODMAPs in the diet include high fructose foods like soda, fruit juice, watermelon, apples, and peaches, foods made from wheat like bread (particularly commercial breads made with high fructose corn syrup), onions, garlic, cabbage, lactose-containing foods such as cow’s milk, and anything made with sugar alcohols (like many sugar-free gums). Many people who find intestinal or even mood relief with a gluten-free, dairy-free diet may be responding to switching to a relatively low FODMAP diet in comparison to the standard diet they were eating before.

If you have IBS symptoms, whether it is associated with anxiety and depression or not, it may be worthwhile to try a low FODMAP diet for six weeks to see if there is any improvement in your symptoms. Stanford has a very good online instruction manual here. After six weeks, however, it’s important to try to add back your onions, avocados, watermelon, and the rest (at least of the whole, nutritious foods…the processed garbage and cola you can go without if you like), to see which ones you can tolerate again. Dietary changes are followed rapidly by changes in the microbiota… starving them for a short period can resolve gastrointestinal symptoms, allow other microbiota to grow in and replace them, and you may not have to restrict the diet for all that long.

Ultimately, the oligosachharides and fermented foods are prebiotics, meaning they feed the gut, and going entirely low FODMAP for the long term might not foster the healthiest type of microbiota. The health of the gut is something of a moving target, and Dr. Grace Liu of the colorful blog Animal Pharm uses the term “seed and weed” when it comes to fixing gut issues. In terms of FODMAP-sensitive IBS, that means weed out the offending microbiota (those beasties that go nuts and cause bloating and discomfort when eating watermelon) through a low FODMAP diet, and then using certain probiotics (or very careful, small, slowly increasing amounts of fermented foods such as water kefir) to seed the gut with different populations of microbiota. Eventually, the new refurbished gut microbiota should be able to tolerate a bit more FODMAP without you having symptoms.

Evidence suggests many folks with depression have a dietary intolerance to fructose, fructans, and lactose, and in those that do, going on a low FODMAP diet did relieve the depressive symptoms by 65%. Ultimately the gut that supports human health including mental health is fed by a healthy, natural foods diet.

Emily Deans M.D. is a board certified psychiatrist with a practice in Massachusetts and a clinical instructor in psychiatry at Harvard Medical School. She writes articles about nutrition, lifestyle, and mental health at Psychology Today. You can also find her on Twitter at @evolutionarypsy. (Please note: she can’t give medical advice over the internet, so please don’t request it.)