Are You Eating Enough? Under-Fuelling and IBS

When you have irritable bowel syndrome (IBS), it can feel like food is constantly causing problems.

You eat something, your gut reacts, so you remove that food. Then another meal causes bloating, abdominal pain, diarrhoea or constipation, so you start eating smaller portions or skipping meals altogether.

Before long, you might find yourself eating a surprisingly small amount of food.

While reducing or changing certain foods can be a useful part of managing IBS, not eating enough can affect digestive function and become another factor that complicates IBS management.

Let’s explore what under-fuelling is, how it can affect your digestive system, and why making sure you’re eating enough is an important part of managing IBS.

 

What is under-fuelling?

Under-fuelling means you aren’t consistently eating enough food to meet your body’s energy needs.

This doesn’t necessarily mean you’re deliberately dieting or trying to lose weight.

Under-fuelling can happen when:

• Nausea, bloating or abdominal pain reduce your appetite.
• You regularly skip meals to avoid symptoms
• A busy lifestyle means meals and snacks regularly get missed.
• IBS symptoms make you afraid to eat.
• You eat very small portions because larger meals feel uncomfortable.
• You’ve removed lots of foods from your diet without replacing the energy and nutrients they provided.
• You’re exercising regularly but haven’t increased your food intake to match your body’s additional energy needs.

For people with IBS, under-fuelling can creep up gradually. You might start by avoiding a few foods that seem to trigger symptoms and gradually find that your diet becomes increasingly restricted.

Research has found that food avoidance and restriction frequently occur and are associated with lower overall energy, carbohydrate and protein intake. For a subgroup of people with IBS this restriction becomes severe enough to create additional anxiety and gut symptoms which makes getting back to ‘regular eating’ feel like an up-hill battle.

 

How can under-fuelling affect your digestive system?

Your digestive system needs adequate nutrition too.

When you consistently don’t eat enough, several things can happen that may affect how your gut functions.

 

1. Your gut motility can change

Your digestive tract uses coordinated muscular contractions to move food and waste through your system.

Research outside of IBS has linked low energy availability with changes in digestive function, including slower gastric emptying and constipation. While this doesn’t mean under-fuelling is causing your IBS symptoms, consistently eating too little could be another factor affecting your digestive function.

If you’re already prone to IBS-C, it’s therefore worth considering whether you’re eating enough when looking at the bigger picture of your bowel habits.

 

2. You might not be getting enough fibre

When someone starts reducing their food intake, they often aren’t just reducing calories. Their intake of fibre and other important nutrients can drop too.

Research suggests fibre intake is already commonly suboptimal in people with IBS.

Fibre plays several roles in digestive health, including helping regulate bowel movements and stool consistency. The type and amount of fibre matter with IBS, and suddenly adding large amounts of fibre isn’t necessarily helpful. However, consistently eating too little fibre can contribute to constipation. If you’re prone to diarrhoea or loose bowel movements, getting enough of the right types of viscous/gel-forming (soluble) fibre may also help support more well-formed stools.

 

3. Your eating pattern can become irregular

Skipping breakfast, delaying lunch and then trying to eat most of your food later in the day can mean your digestive system isn’t receiving food regularly throughout the day.

Eating stimulates activity within the digestive tract. This includes the gastrocolic response, where eating stimulates movement in the colon.

Regular meals can therefore be an important part of supporting regular bowel habits.

 

4. Your overall nutrition can suffer

Food isn’t just about calories. If you’re consistently eating too little, it can become harder to consume enough protein, carbohydrates, fats, vitamins and minerals. These are all essential for supporting your body, including one of your most important organs: your brain.

When your brain isn’t receiving enough balanced fuel, it can shift into a more protective mode and create rigid rules, often around avoiding foods or situations you associate with gut symptoms, as a way to try and protect you from experiencing symptoms. This can make it harder to see other options, while anxiety and fear around food and symptoms can increase. You might also notice that getting through your day feels harder and that you have less energy.

Losing variety, spontaneity and abundance in your diet can also leave you at risk of not getting enough fibre, calcium, iron and vitamin D to support your body’s needs.

This is why continuing to remove foods or reduce your food intake without making appropriate substitutions isn’t a great long-term strategy.

 

The IBS under-fuelling cycle

This is where things can become tricky. Let’s look at this scenario: You experience gut symptoms after eating. So you eat less. You start skipping meals or removing more foods because you’re worried they’ll trigger symptoms.

Your overall food and fibre intake can drop and your eating pattern may become less regular. These changes can affect digestive function and nutritional adequacy, adding another layer of complexity when you’re trying to manage your IBS.

Those symptoms then reinforce the feeling that food is the problem, so you restrict even further.

This can create a frustrating cycle:

Gut symptoms → food avoidance or restriction → lower food and nutrient intake → potential changes in digestive function, nutritional adequacy and brain function → IBS management becomes more complicated → further food restriction.

Importantly, this doesn’t mean your original IBS symptoms weren’t real or that simply eating more will make IBS disappear.

IBS is a complex disorder of gut-brain interaction, and many different factors can influence symptoms. The point here is that under-fuelling can be another piece of the symptom puzzle.

 

What about the low FODMAP diet?

The low FODMAP diet is one of the most evidence-based dietary therapies available for IBS, but it isn’t designed to be a low-calorie diet. It also isn’t intended to involve removing high FODMAP foods indefinitely.

The first phase of the low FODMAP diet is a short-term diagnostic phase, generally followed for 2–6 weeks before moving into FODMAP reintroduction and then a personalised long-term diet.

When starting the low FODMAP diet, the goal should be to swap higher FODMAP foods for suitable low FODMAP alternatives, rather than simply removing foods.

For example, if you remove a higher FODMAP breakfast cereal, you need another satisfying breakfast option. If you reduce certain breads, fruits, vegetables, or legumes, you should replace them with appropriate low FODMAP alternatives.

Otherwise, it’s surprisingly easy for your overall food intake to shrink. We can help you avoid this in the FODMAP Made Easy Program, where you’ll find low FODMAP food swap sheets and 1,200+ low FODMAP recipes to help you build satisfying meals while navigating the diet.

 

Signs you might not be eating enough

Under-fuelling can look different for everyone, but it may be worth reviewing your food intake if you:

• Regularly skip meals.
• Go long periods without eating during the day.
• Find yourself eating most of your daily food in one large meal.
• Frequently feel hungry or find yourself reaching for lots of snacks in the evening (this can be a sign you aren’t eating enough earlier in the day).
• Feel hungry but avoid eating because you’re worried about gut symptoms.
• Have progressively removed more and more foods from your diet.
• Struggle to find enough foods you feel comfortable eating.
• Have unintentionally lost weight.
• Feel that your portion sizes have become very small.
• Avoid eating before work, travel or social activities because you’re worried about symptoms.
• Are exercising regularly without adjusting your food intake.
• Regularly feel low in energy or struggle to concentrate.
• Feel like managing your IBS has become centred around eating as little as possible.

These signs don’t automatically mean you’re under-fuelling, but they can be useful prompts to take a closer look at whether you’re meeting your nutritional needs.

 

How can you start fuelling your body more consistently?

If you think you might be under-fuelling, the solution doesn’t have to be suddenly eating enormous meals. This is because large meals can feel uncomfortable when you have a sensitive gut. Instead, focus on establishing a regular eating pattern that meets your nutritional needs.

You might:

• Aim for regular meals throughout the day.
• Add snacks if there are long gaps between meals.
• Gradually increase portions if they’ve become very small.
• Include carbohydrate, protein and fat across your meals.
• Make sure you’re replacing foods you’ve removed with suitable alternatives.
• Review whether all of your current food restrictions are still necessary.
• Work through FODMAP reintroduction rather than staying strictly low FODMAP long term. If you’re not sure where to start, our FODMAP Made Easy Program can guide you through the reintroduction process step by step.

If increasing your food intake feels difficult because eating reliably triggers significant symptoms, then that’s also useful information to share with your healthcare team.

 

When should you get extra support?

If you’ve lost weight without intending to, are struggling to meet your nutritional needs, have a very limited range of foods you feel able to eat, or feel anxious about increasing your food intake, talk to your doctor and a dietitian experienced in gastrointestinal conditions.

It’s also important not to assume that ongoing or changing digestive symptoms are automatically caused by IBS. New symptoms, unexplained weight loss, rectal bleeding or other concerning symptoms should be assessed by your healthcare provider.

 

Final Thoughts

When you have IBS, it’s understandable to focus heavily on what you need to remove from your diet. But sometimes we need to ask a different question: Are you actually eating enough?

Under-fuelling won’t explain everyone’s IBS symptoms, and eating more isn’t a cure for IBS. However, inadequate food intake, irregular eating patterns, low fibre intake and increasingly restrictive diets can all make managing your gut symptoms more complicated.

Successful IBS management isn’t about eating as little as possible or creating an ever-growing list of foods to avoid. The goal is to help you build a varied, nourishing diet that gives your body enough fuel while keeping your gut symptoms manageable.

 

References:

Melchior C, Algera J, Colomier E, et al. (2022). Food Avoidance and Restriction in Irritable Bowel Syndrome: Relevance for Symptoms, Quality of Life and Nutrient Intake. Clinical Gastroenterology and Hepatology, 20(6), 1290–1298.e4. DOI: 10.1016/j.cgh.2021.07.004.

Veraza GC, et al. (2024). A systematic review and meta-analysis of diet and nutrient intake in adults with irritable bowel syndrome. Neurogastroenterology & Motility, 36, e14698. DOI: 10.1111/nmo.14698.

Bek S, et al. (2022). Association between irritable bowel syndrome and micronutrients: A systematic review. Journal of Gastroenterology and Hepatology. DOI: 10.1111/jgh.15891.

Bijkerk CJ, de Wit NJ, Muris JWM, et al. (2009). Soluble or insoluble fibre in irritable bowel syndrome in primary care? Randomised placebo controlled trial. BMJ, 339, b3154. DOI: 10.1136/bmj.b3154.

Whelan K, Ford AC, Burton-Murray H, Staudacher HM. (2024). Dietary management of irritable bowel syndrome: considerations, challenges, and solutions. The Lancet Gastroenterology & Hepatology, 9(12), 1147–1161. DOI: 10.1016/S2468-1253(24)00238-3.

Moayyedi P, Quigley EMM, Lacy BE, et al. (2014). The effect of fiber supplementation on irritable bowel syndrome: a systematic review and meta-analysis. American Journal of Gastroenterology, 109(9), 1367–1374. DOI: 10.1038/ajg.2014.195.