IBS diets don’t work for everyone. New research shows why – and it’s not just about the food

If you’ve ever tried a diet to fix gut symptoms, you’ll know it can be hit or miss. One person swears it changed their life. Another follows it carefully and feels no better.

This is especially true for irritable bowel syndrome, or IBS. It’s a common condition that causes stomach pain, bloating and changes in bowel habits.

Many people with IBS are told to try the low-FODMAP diet. This reduces certain carbohydrates (known as FODMAPs) that the gut absorbs poorly. These are fermented by gut bacteria, producing gas and drawing water into the bowel, which can trigger symptoms.

Reducing FODMAPs – found in foods such as onions, garlic, apples, wheat and some dairy products – can help ease symptoms. The diet usually involves restricting these foods for a short period, then slowly reintroducing them to identify which ones trigger symptoms in each person.

For many people, it works. But for many others, it doesn’t. Our new research helps explain why.

We found the effectiveness of a low-FODMAP diet for IBS doesn’t come down to food alone, but also how the gut and brain work together.

Different levels of gut sensitivity

IBS affects how the brain and gut communicate. Signals travel between them, shaping how sensitive the gut is and how strongly symptoms are felt.

A simple way to think about it is as a volume dial. For some people, the gut is turned up, so even normal digestion can feel uncomfortable or painful. For others, the dial is lower.

Food matters, but it is only part of the picture. The brain can also turn symptoms up or down, influenced by stress, anxiety about gut symptoms, and expectations about how the body will respond.

To understand this, we studied 112 adults with IBS over six months as they completed the three phases of the low-FODMAP diet. Participants worked with a dietitian through restriction, reintroduction and personalisation, allowing us to track how symptoms changed as foods were removed and then reintroduced.

We measured symptoms, quality of life and psychological factors such as anxiety and expectations. We used statistical modelling to identify response patterns and what predicted improvement.

What we found

Some people improved quickly and stayed better. Others improved only slightly, or not at all, even after completing all phases of the diet. We found psychological factors played a major role in whether the diet worked.

Importantly, the difference was not just what people ate, but how they thought and felt about their symptoms and treatment.

People who believed the diet would help were more likely to improve. This is called “treatment expectancy” and is seen across health care.

People with high gut-focused anxiety were less likely to improve. This means they were very worried about their gut and more sensitive to normal sensations, like gas or movement in the bowel.

People who felt more in control of their symptoms also tended to do better.

These factors often changed before symptoms improved. This suggests the brain may help drive changes in symptoms.

This doesn’t mean IBS is “all in your head”. The symptoms are real and can have a big impact on daily life.

The gut and brain are closely linked. Stress and anxiety can change how sensitive the gut feels and how strongly symptoms are experienced – for example, many people notice “butterflies” in their stomach during stress.

What does this mean?

Right now, IBS treatment is often trial and error, with diet changes commonly tried first, followed by psychological therapies if needed.

Our findings suggest we may need to rethink this approach.

Some people may benefit more from psychological approaches, such as stress-reduction or cognitive behavioural therapy (CBT). These can help people reframe unhelpful thoughts about their gut, reduce anxiety, and gradually face foods or situations they fear may trigger symptoms.

Others may respond well to diet alone. And many may need both.

If we can identify these differences earlier, for example by assessing anxiety or expectations, we could better match people to the right treatment.

This research marks a shift in how we understand IBS. It’s not just a food problem. It’s shaped by the interaction between diet, the gut and the brain.

For people living with IBS, this could mean fewer restrictive diets, less frustration and faster access to treatments that work.

For clinicians, it opens the door to more personalised care, where treatment is tailored to how a person’s gut-brain system is working.

In the end, improving IBS care may not be about finding the perfect diet. It may be more about understanding how the gut and brain work together, and using that to guide the right treatment.