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Irritable bowel syndrome (IBS) is a collection of symptoms, rather than a disease. If you’ve had constipation or diarrhoea for longer than three months, you’ve probably had a diagnosis of IBS. So, IBS isn’t what ‘happens’ when you get constipation or diarrhoea. It’s just the name modern medicine has given to those symptoms when there’s no known cause.

If you have IBS, take it as a sign from your body that you need to find out why. There are endless reasons why you could be getting diarrhoea or constipation. SIBO, dysbiosis in your large intestine or food intolerances are a few examples. We recommend working with an experienced practitioner who can guide you through the right gut health tests based on your symptoms and health history. 

Does IBS happen straight after eating?

IBS symptoms can happen straight after eating, but they can also happen any other time. Meals can trigger gut symptoms in people with IBS, but as IBS symptoms happen in the small and large intestine, it usually takes at least four hours after you eat something for you to feel them.

However, if you have irritable bowel syndrome, you might find that your gut symptoms seem to appear or get worse as soon as you start eating a meal. While that’s far too soon for the food in that meal to be having an effect on your intestines, there are other things going on in people with IBS that may help to explain why symptoms sometimes happen after eating.

Over-sensitive gastro-colic reflex

Your gastro-colic reflex is a movement in your gut that is triggered when you eat. It takes place to ‘make room’ for the food you have just eaten. It stimulates your large intestine to contract and push food and waste along. It’s the gastro-colic reflex that makes you feel the urge to have a bowel movement after a meal. 

We know from research that people with irritable bowel syndrome seem to have an exaggerated gastro-colic reflex, meaning that if you have IBS, you might feel stronger movements in response to eating a meal, like cramps, bloating, diarrhoea or pain, for instance. Big and fatty meals are especially likely to stimulate the reflex more powerfully. Large drinks can have the same effect, if you drink them quickly.

Disordered gut motility  

Motility is the movement of your gut. While the gastro-colic reflex happens after a meal, smaller movements are happening in your gut all the time. If you have IBS, studies have shown that you’re more likely to have disordered motility in one or more sections of your gut (that includes your stomach, small intestine and large intestine.

If you have diarrhoea, your gut contracts and moves things along too quickly. If you have constipation, you have the opposite problem: a slow transit time leads to fewer bowel movements. Strong contractions in your large intestine can contribute to abdominal pain.

If disordered movements happen with the gastro-colic reflex right after a meal, your IBS symptoms could be severe. Strong contractions in your large intestine can cause diarrhoea and contribute to abdominal pain.

Gut hypersensitivity 

After years of research into the causes of irritable bowel syndrome, we don’t have many conclusions, but one potential answer is an overly sensitive gut, or ‘visceral hypersensitivity’.

When foods ferment in your intestine, they produce gas, which makes your intestinal walls stretch. Because you have nerves around your intestines, that can hurt. When pain signals are sent to your brain, it can send signals back to your gut that hinder how it moves, causing more pain and IBS symptoms.

If your gut is hypersensitive, after a meal—when your gastrocolic reflex is moving food and gas along—could be the time when you feel your symptoms the most.

Does IBS cause diarrhoea immediately after eating?

If you have irritable bowel syndrome, you could get diarrhoea after eating. Research has revealed a few reasons why IBS makes it more likely.

While it’s not an official IBS symptom, a lot of IBS sufferers have acid reflux, which is usually associated with gastroesophageal reflux disease (GERD), which in turn has been linked to diarrhoea after eating. A small study found that people with IBS who took acid suppressants were less likely to have diarrhoea after eating. From this, we can assume that the excess acid may be contributing to the diarrhoea, but we need more research on this.

People with diarrhoea-predominant irritable bowel syndrome have also been found to have too much water in their large intestine, but not enough in their small intestine: it appears to pass quickly through their small intestine and builds up rapidly in their large intestine.

It’s also important to remember that diarrhoea after eating isn’t always IBS. There are other conditions that can cause it:

• bile acid malabsorption 
• pancreatic exocrine insufficiency
• glucosidase deficiency

If you have diarrhoea after eating, your doctor may have already tested you for these conditions. 

What can you do for diarrhoea after eating?

If you have diarrhoea after eating, there are a few things you can try to ease your symptoms when they happen, or even prevent them from happening.

Don’t get dehydrated

Because your food is passing quickly through your system, you won’t absorb all the fluids and minerals that you need when you have diarrhoea. It’s important to not just drink extra water to replace the lost fluids, but electrolytes like magnesium and potassium too. 

You can buy electrolyte water from a pharmacy, but at IBS Clinics we usually recommend coconut water, which contains natural electrolytes. You can also try drinking ginger or peppermint tea, which soothes your gut and can help nausea. 

Take probiotics

If you have diarrhoea because your microbiome (the population of bacteria and other organisms in your gut) is unbalanced, taking probiotics can help. Probiotics are ‘friendly’ bacteria that come in supplements, like pills, or probiotic foods like yogurt, kefir, kombucha, kimchi, and sourdough bread. 

However, probiotics don’t help everyone. If you have SIBO (an overgrowth of bacteria in your small intestine) they could make your symptoms worse. Saccharomyces Boulardii is a ‘friendly’ yeast (as opposed to bacteria) that has been proven to help diarrhoea, and it doesn’t usually worsen SIBO symptoms. At IBS Clinics, we recommend it to a lot of clients with diarrhoea. 

Try to relax

It’s easier said than done, but anything that calms your mind will also calm your body. Your brain and your gut are intricately connected: Most of your serotonin (your ‘happy’ hormone) is in your gut. If you think of your gut as kind of a second brain, it’s no surprise that stress can be a trigger for diarrhoea. 

Whatever makes you relax can also ease your symptoms. Some people find deep-breathing exercises or meditation helpful. You may like taking a hot bath, or lying down under heavy blankets and listening to calming music. You could also try the strategic tensing and relaxing of muscles, known as ‘progressive muscle relaxation’: Focusing on parts of your body outside your gut can take the tension away from it, slowing down diarrhoea.

How quickly can IBS be triggered?

It depends if by ‘triggered’, you want to know how quickly IBS symptoms can manifest after a specific trigger like eating garlic or onions, for example, or you want to know how quickly the syndrome can be triggered. In other words, how long does it take to develop irritable bowel syndrome as a condition that persists, rather than a one-off attack.

How long does it take for IBS symptoms to start after a trigger food?

How quickly you get IBS symptoms after a trigger food depends on what’s going on to trigger your symptoms.

If what you’ve just eaten is responsible for your symptoms like gas, diarrhoea or bloating, fermentation—and the release of gases—is the culprit, and that can begin at any time from 2-8 hours after eating the food. Two hours is a very fast ‘transit’ time: a typical period is closer to 4-5 hours.

The reason for these timings is because food takes time to travel down to your small and large intestines, where IBS symptoms manifest. Of course, you might feel symptoms much sooner after eating than that, but the food you’ve just eaten isn’t responsible.

If you have to go to the toilet immediately after eating, it’s not down to the fermentation of that food: there simply hasn’t been enough time for that to happen. It takes about 12-48 hours for food to move from your mouth to your anus. This means something other than fermentation is at play if you need to empty your bowels right after a meal.

A common explanation, which we explained above, is an over-sensitive gastro-colic reflex: your gut’s natural way of moving food and waste forwards after you eat. Normally, when you eat food, your gut releases hormones and generates nerve signals as your intestinal walls are stretched by the incoming food. The gastro-colic reflex explains why so many people need to visit the bathroom after a meal.

If you have irritable bowel syndrome, you might have an extra-powerful gastro-colic reflex, so that when you eat a small amount, your system pushes a large amount of food into your large intestine, which then causes your symptoms.

So, it’s not the meal you just finished eating that sends you rushing to the bathroom. It could be the one before, one you ate yesterday, or even the day before, if you have a slower transit time.

How long does it take to develop IBS?

Though it may take a long time to develop in the background, with no noticeable symptoms, like any other condition, IBS usually has a rough start date.

You might have had normal bowel movements all your life, then start to notice constipation or diarrhoea quite suddenly. If, after three months, you still have symptoms, your doctor could diagnose you with IBS, but even doctors can have trouble working out if you have IBS or something else with similar symptoms.

There are a few reasons for this:

• IBS symptoms often change over time. Perhaps yours change every day, or they may get better for weeks or months at a time and then get worse. 
• Symptoms are similar to other gut disorders, like Crohn’s disease or coeliac disease, for example.
• Symptoms are different for each person with IBS. They can be severe and life-limiting, or mild.
• There are no diagnostic tests for IBS. It can’t be diagnosed with a lab test, x-ray or biopsy.

Most doctors diagnose IBS based on something called the Rome IV criteria. If you’ve had recurring abdominal pain at least once a week in the last three months, and the pain is associated with at least two of these criteria, you have IBS according to the Rome IV criteria.

1. The pain is related to bowel movements
2. The bowel movements occur more or less often than usual
3. The stool is different than usual (harder, softer)

However, your doctor may not use these criteria and diagnose you with IBS based on your own symptom pattern and history.

How long after eating does bloating occur?

Bloating can happen at any time, but it usually happens after you’ve eaten a meal because that’s when your gastro-colic reflex pushes food from a previous meal along your gut, causing your gut bacteria to release gases.

However, we can also swallow air when we eat, trapping it in our digestive system. The gas in fizzy drinks can also make you feel bloated, but it normally escapes out of the top end in burps rather than the other end in a less socially-acceptable way!

That kind of gas comes from the gut bacteria in your microbiome. As they break down your food, they release gases that can fill up your intestines—a bit like a balloon being blown up—and distend your gut.

How bloated you become depends on what you’ve eaten and the types and numbers of different microbes in your gut. The longer your ‘transit time’ (the length of time food takes to travel through your gut) the more likely you are to bloat.

What causes sudden onset of IBS?

IBS is a syndrome, rather than a disease, which means it’s a collection of symptoms: diarrhoea, constipation and abdominal pain. To be diagnosed with IBS, your symptoms must have been going on for three months or more. They may have started suddenly, after a bout of food poisoning, for example, or built up slowly over time.

A lot of people who develop IBS symptoms out of the blue have been through an illness, operation or mental or physical trauma. For instance, ten percent of people develop persistent symptoms of diarrhoea after an outbreak of gastroenteritis. A similar proportion of women develop persistent abdominal pain after they have had a hysterectomy. 

Interestingly, in both of these examples, researchers found that if someone is suffering from anxiety or depression, or they’re going through divorce, bereavement or any other deeply stressful event at the time of the original infection or surgery, their symptoms are more likely to persist. We also know that IBS symptoms are more likely to endure if a traumatic event instigated the symptoms: for instance, immediate diarrhoea that happens as a result of a spouse dying is more likely to endure than immediate diarrhoea that happens as a result of food poisoning or an operation.

Post-infectious IBS

If your IBS symptoms started after a stomach bug, you could be in a subgroup of IBS sufferers with ‘post-infectious IBS’.

Post-infectious IBS happens suddenly, after what appears to be an acute infection in your stomach and intestines (gastroenteritis). While most people recover completely after the initial illness, some don’t. Interestingly, post-infectious IBS seems to happen more often after a GI infection caused by bacteria, rather than a virus.

British doctors first recognised post-infectious IBS during World War II in soldiers returning home with persistent diarrhoea after suffering bacterial dysentery. Today, research has shown that it can happen after a range of infections from a range of bacteria including C. difficile, Campylobacter, Salmonella, and Shigella. 

Around 6-17% of people with IBS think their symptoms began suddenly after a stomach bug. And about 10% of people who suffer bacterial gastroenteritis develop IBS symptoms.

Post-infectious IBS is almost always the diarrhoea-predominant type, rather than the constipation-predominant type.

You’re more likely to get post-infectious IBS if:

• your initial illness was long and severe.
• you have depression or anxiety: Stressful life events in the months leading up to the  infection doubles your risk about two-fold.
• you’re female
• you were infected with a bacteria that produces high levels of toxins. The bacteria Campylobacter jejuni produces a toxin which has been shown to alter how your bowel moves and functions.
• you’re aged 19-29: People in this age group are three times more likely than people over the age of 60 to develop post-infectious irritable bowel syndrome. 
• You didn’t vomit during the initial illness: Interestingly, if you vomited during the initial illness, you’re half as likely to develop post-infectious IBS as someone who didn’t. This could be because vomiting rids your body of a good amount of the bacteria before they have a chance to cause havoc in your gut.

What to eat after IBS attack

You’ve probably heard of the BRAT diet, which stands for bananas, rice, apple sauce, and toast, which health practitioners often recommend to relieve diarrhoea, as they’re low in fibre (which can irritate an inflamed gut) and easy to digest. However, apples are high in FODMAPs (specific types of carbohydrates that can cause IBS symptoms in sensitive people), so they may not be the best choice for people with IBS. Gluten is another common IBS trigger food, so bread could be problematic.

Bananas are usually a safe bet, and if you have time, plain broth made from meat bones is very healing for your gut. Starchy foods like oats help to bind loose stools. Avoid foods that can worsen diarrhoea, like dairy products, fatty foods, and foods that give you gas.

IBS trigger foods

Any food can be an IBS trigger food, because no gut, body or brain is alike, and everyone has their own individual list of triggers. While some people with irritable bowel syndrome have to avoid fatty foods completely, others do very well on a keto diet, which is usually around 50-95% fat.

However, there are some common offenders. The low-FODMAP diet has become very popular over the last decade or so, after researchers at Monash University in Australia found that certain types of carbohydrate are much more likely to cause IBS symptoms than others.

FODMAP is an acronym that describes the carbohydrates that some people have trouble digesting. The ‘F’ stands for fermentable.

The sugars are:

Oligosaccharides 

Disaccharides 

Monosaccharides

Polyols 

High FODMAP foods

• Artichoke
• Apple
• Garlic
• Leek
• Onion
• Mushrooms
• Cauliflower
• Wholemeal bread
• Rye bread
• Wheat pasta
• Kidney beans
• Split peas
• Falafels
• Baked beans
• Soft cheeses
• Milk
• Yoghurt

The low-FODMAP diet isn’t the only diet that tackles IBS symptoms. The low-FODMAP diet works by starving the bacteria in your gut of some of their favourite food sources: fermentable carbohydrates.

However there are other diets that take away various types of carbohydrates that can also help to reduce IBS symptoms.

For example:

• The ‘elemental’ diet: a liquid diet where you only consume the nutrients and calories you need, without feeding the bacteria in your gut
• The specific carbohydrate diet (SCD): a lower-carb diet that restricts complex carbohydrates
• The GAPS diet: a gut healing diet invented by a doctor to address her daughter’s autism. 

Conclusion

While you can get an attack of your IBS symptoms right after eating, or even during a meal, it’s not the food you just ate that’s causing your problem. Because it takes at least two hours for food to reach the start of your small intestine, your symptoms are much more likely to be happening as a result of your gastro-colic reflex moving previous meals along your gut, ‘making room’ for what you’ve just eaten.

If you have diarrhoea-predominant IBS, you could have an extra-strong gastro-colic reflex that moves large amounts of food into your large intestine when you eat, forcing it out the other end too powerfully and quickly. 

At IBS Clinics, we see IBS symptoms after eating as a message from your gut that it needs some support. While it’s natural and normal to have a bowel movement after a meal, if it’s loose and comes with pain or urgency, work with a health professional to start digging in to the health of your gut, body and mind to find out why.

Author

Alexandra Falconer MA (Dist) DipCNM mBANT is a Nutritionist and Registered Nutritional Therapist. After graduating from Brighton’s College of Naturopathic Medicine in 2018, she set up her nutrition practice specialising in gut health, IBS, IBD and related conditions.

Before becoming a nutritionist, Alex was a journalist and copywriter. Now, she combines her two great loves—words and natural medicine—to help bring vibrant health to everyone who needs it.

Reviewed by: Tracey Randell