Could sugar intolerance be causing your IBS symptoms?

If you’ve tried the low FODMAP diet but still have symptoms like bloating, diarrhoea, or cramps, it’s worth looking at sugar digestion.

The low FODMAP diet doesn’t restrict sucrose (table sugar), but for a small number of people, that might be part of the problem.

How carbohydrates are digested

Carbohydrates make up a big part of our diet — around 50% of calories in a typical Western diet. These carbs come from foods like bread, pasta, potatoes, rice, and of course, sugar.

To absorb and use these carbs for energy, your body breaks them down into simple sugars like:

• Glucose

• Fructose

• Galactose

For example:

• Sucrose (table sugar) → breaks into glucose + fructose

• Lactose (from dairy) → becomes glucose + galactose

• Maltose (from cereals or sweet potatoes) → becomes two glucose molecules

Digestion starts in the mouth (chewing and saliva help), but most sugar digestion happens in the small intestine using specialised enzymes.

You might notice chewing bread or rice for a long time can make it taste sweet – this is the sugars breaking down. This is one reason I really focus on chewing your food very well – it helps to give yourself more time to break down your starchy foods – read more here.

Small intestine digestion of sugar

As food moves into the small intestinal digestive enzymes help to break apart the larger molecules of carbs into smaller parts that can be absorbed. 

For example, the lactase turns lactose (the sugar from dairy) into glucose and galactose. 

Another key carbohydrate digestive enzyme we need for sugar digestion is sucrase-isomaltase (SI). 

This works on multiple types of carbohydrate, and it’s responsible for all sucrase activity and about 60%–80% of maltase activity in the intestine. 

If this enzyme doesn’t work well, you might get symptoms like:

• Bloating

• Cramping

• Diarrhoea

• Excess gas

Causes of maldigestion of sugar

You can either be born with an issue digesting carbs, or it can develop throughout life.

1 Genetic enzyme issues

Primary carbohydrate maldigestion comes from genetic issues producing enzymes. This might be more likely if your close family members also have a lot of digestive issues.  For example, congenital sucrase-isomaltase deficiency (CSID) affects how people digest sucrose and starch. It’s caused by modifications to the  sucrase-isomaltase (SI) enzyme. 

This enzyme breaks down most of the sucrose in our diet and around 60-80% of the maltose. 

• Sucrose is table sugar and is found in many processed foods like cereals, sweets, cakes, yoghurts etc. 
• Maltose is two molecules of glucose stuck together and occurs in foods like honey, malted cereals, and cooked sweet potatoes. 

Other enzymes that work on sugars include maltase-glucoamylase (MGAM) and trehalase. 

Those people who have issues with breaking down these sugars, can normally manage some sugar digestion. It’s thought people with IBS are more likely to have mutations to the genes which code for this enzyme. 

These inherited conditions are rare but could overlap with people who have IBS. One study found that 9.2% of adults with unexplained IBS symptoms had a deficiency in all four common disaccharides (e.g. lactase, sucrase). They looked at 120 adults with at least a year of IBS and found the enzyme wasn’t working as it should. 

Other studies found sucrase deficiency more common at around 34.4% of 258 adults with IBS.

So how do you get this in later life? 

2. Secondary causes of sugar intolerance: damage to digestive enzyme production

Secondary problems with breaking down sucrose is much more common in IBS and other gut conditions. This happens when infections, inflammation, or damage to the small intestine affect your production of enzymes.

This could be down to

• physical injuries to the intestine from accidents or surgery,
• uncontrolled coeliac disease
• Inflammatory bowel disease
• Medication such as Codeine (pain medication) and ranitidine with antihistamine effect can also inhibit sucrase activity in the intestine 
• infections such as Giardiasis or autoimmune disorders.
• nutrient deficiencies – Low iron or vitamin A

The good news is that in many cases, enzyme activity can improve once the underlying cause is treated.

Why do unabsorbed sugars cause symptoms in IBS?

When carbs aren’t fully digested or absorbed, they reach the colon and become food for gut bacteria.
This leads to gas (hydrogen, methane, etc.) and short-chain fatty acids being produced — which can cause:

• bloating and distension
• pain or cramping
• flatulence
• diarrhoea due to increased water in the gut

These symptoms are especially noticeable in people with visceral hypersensitivity (a common feature of IBS).

Signs sugar intolerance might be affecting you

It’s not always easy to spot a pattern with sugar intolerance. Your response may depend on:

• The amount and type of sugar

• How well your enzymes work

• The speed of digestion

• Your gut bacteria

• Your individual sensitivity

Keeping a symptom diary can help you work this out.

Foods and sweeteners to be mindful of

Keep an eye on:

• Processed foods with sucrose, maltose, glucose syrup, honey, or malted cereals

• Sweeteners like:

  • Acesulfame K

  • Maltitol or maltitol syrup

  • Brown rice syrup

  • Neotame or NutraSweet

  • Stevia

Read more about artificial sweeteners and how it affects IBS – Which sweeteners are low FODMAP

Sometimes it’s not about having an intolerance — just eating too much sugar at once can overwhelm your digestion, even in people without IBS. 

The role of testing: when it’s helpful, and when it might not be

There are two main ways to check for sucrose intolerance. The gold standard test for sucrase deficiency is the sucrase enzyme activity assay from duodenal biopsies. Alternatively, the sucrose breath test is a noninvasive way to diagnose sucrose malabsorption 

The breath test measures how much hydrogen you produce when you consume sucrose. 

You may find it hard to access this test through the NHS, and as a starting point keeping food-symptom diary or elimination diet might sometimes be more useful than testing.

Practical strategies for managing sucrose intolerance

You don’t need to cut out all sugars completely. Different foods contain varying levels of sucrose and other sugars, so it’s about finding your personal tolerance.

• Try the low FODMAP diet first
  Most people benefit from removing fermentable fibres first before exploring sugar-specific issues. if you need help with following the diet check out my Ultimate Low FODMAP Diet Guide for recipes, process steps and checklists. 

• Consider enzyme supplements
  Saccharomyces cerevisiae (e.g. Brewer’s yeast-based) may help with sucrase activity.

• Don’t avoid carbs completely
  Some carbohydrate intake can support enzyme production. In fact, the enzyme maltase increases when your diet includes more carbs.

• Gradual reintroduction
  Start with small amounts of low-sucrose foods and monitor symptoms.

If you suspect sugar digestion is a problem for you, start by tracking your symptoms, and get in touch to talk about working with me. Remember: it’s about finding what works for your gut.