Let’s Not Sugarcoat the Truth: Your IBS May Actually Be CSID (Sucrase-Isomaltase Deficiency)

Sugar is more than just a sweetener. For some people, it’s the source of significant digestive symptoms due to an underlying enzyme deficiency.

Since my teens, I’ve struggled with digestive issues, including constant bloating, reflux, diarrhoea, abdominal pain, and discomfort that no one could quite explain. I learned to never be far from a toilet or a hot water bottle.

My first visit to a gastroenterologist was at 15 years old, and I was told it was just stress. Since then, I’ve had multiple upper GI investigations, tried cutting out gluten and dairy, followed a strict low FODMAP diet, and taken countless supplements like peppermint oil, Iberogast, and digestive enzymes. I even tried acupuncture, naturopathy, osteopathy, and Chinese herbal medicine (including those unforgettable bitter brews). Each time, the outcome was the same:

“We don’t know what’s wrong.”

After aligning with a very savvy GP who knew which test to order, we finally got to the bottom of it: I had low levels of a little-known digestive enzyme called sucrase-isomaltase.

What is sucrase-isomaltase? First we need to understand how sugar digestion works.

Sugars come in two main types:

• Single sugars (monosaccharides) like glucose and fructose

• Double sugars (disaccharides) like sucrose (table sugar) and lactose (milk sugar) which are made up of two single sugars bonded together.

  
  

Before absorption, double sugars must be split into single sugars by digestive enzymes. For sucrose, that enzyme is sucrase-isomaltase (SI), which is found in the lining of the small intestine. It breaks the bond between glucose and fructose so each can be absorbed into the bloodstream. Sucrase-isomaltase also helps break down maltose, a sugar produced during starch digestion from foods such as bread, pasta, rice and potatoes.

What happens if you don’t produce enough sucrase-isomaltase?

If your body does not produce enough of this enzyme, sucrose and starch are not broken down properly. Instead, they move to the colon where gut bacteria ferment them, producing excess gas, bloating and abdominal discomfort. This process can also draw water into the bowel, which may lead to diarrhoea. For some people, it can cause constipation or reflux instead.

It’s similar to lactose intolerance, where a lack of the enzyme lactase prevents proper digestion of milk sugar, leading to similar symptoms.

Genetic and acquired forms

For years, sucrase-isomaltase deficiency was thought to occur only in the rare genetic condition Congenital Sucrase-Isomaltase Deficiency (CSID), sometimes referred to as classical CSID. In this form, the enzyme is nearly or completely absent, making it impossible to digest sucrose and maltose. Symptoms often appear in infancy, with babies struggling to gain weight and thrive.

However, new research shows that many people may have a partial deficiency of the SI enzyme, leading to symptoms that mimic IBS. One common genetic variant (known as Val15Phe) has been shown to reduce enzyme activity by around 40%, potentially contributing to digestive issues in otherwise healthy adults.

Research has also revealed there is another form of deficiency which can develop later in life, known as Acquired Sucrase-Isomaltase Deficiency (ASID). ASID may be temporary and often occurs after gut inflammation or irritation, such as after a viral infection or food poisoning. It can also occur secondary to coeliac disease or other conditions that damage the intestinal lining.

The sugar-starch connection

In today’s food environment, sugar is everywhere. It’s added to everything from tomato sauce to peanut butter, which may explain why so many people experience digestive distress. I recently wrote about this in News.com.au, where I discussed how added sugars sneak into everyday foods and the effect this can have on our health. For those with sucrase-isomaltase deficiency, this hidden sugar load can make symptoms much worse, often without realising why.

But I thought I should follow a low FODMAP diet for IBS?

You’ve probably heard of the low FODMAP diet, which is widely used for managing IBS. FODMAPs are short-chain carbohydrates that are not well absorbed in the small intestine and can ferment in the gut, causing bloating and gas. Yet even this approach only helps about 60% of people. So, what if for some, the problem is not FODMAPs but sucrose and starch?

Research spotlight: low-sucrose, low-starch diets

Several clinical trials from Lund University and Skåne University Hospital in Sweden have tested a low-sucrose, low-starch diet in people diagnosed with IBS. The results were impressive. Participants reported significantly less abdominal pain, bloating and diarrhoea, as well as improvements in fatigue, muscle aches and joint pain.

A follow-up study compared the low-sucrose, low-starch diet with the traditional low FODMAP diet. Both approaches improved symptoms, but the low-sucrose, low-starch diet led to greater reductions in weight and sugar cravings.

What does a low-sucrose, low-starch diet look like?

In these studies, participants:

• Completely removed table sugar (sucrose)

• Limited starch to two serves per day, such as:

  • ½ cup oats or one slice of whole grain bread

  • ½ cup brown rice or whole grain pasta

• Avoided high sucrose containing fruits and vegetables (e.g., mangoes, apples, watermelon, pumpkin, carrots, beetroot)

• Chose lower sucrose options (e.g., berries, papaya, pears, leafy greens, green beans, broccoli)

Continued to enjoy dairy, eggs, chicken, meats, and low-sucrose sweeteners such as honey and agave

You can read the full study here: Starch and Sucrose Reduced Diet Has Similar Efficiency as Low FODMAP in IBS

How do you know if you have CSID or ASID?

To diagnose sucrose intolerance, your GP may order a breath test, a genetic test, or refer you for an intestinal biopsy, which remains the gold standard for measuring enzyme activity.

You may also come across the “4-4-4 test”, a simple at-home challenge used in the US:

• Mix 4 tablespoons of table sugar (around 60 g) in 4 ounces (120 mL) of water, drink it while fasted, and monitor symptoms for 4 hours. If you develop bloating, gas or diarrhoea, it may suggest poor sucrose absorption.

  
  

However, neither the 13C-sucrose breath test nor the 4-4-4 test are validated diagnostic tools at this stage, so results should be interpreted with caution and ideally discussed with a clinician who understands SID.

Alternatively, you can try reducing your intake of sucrose and starch to see if symptoms improve. Even if you don’t have an official diagnosis, many people find that lowering their sugar intake helps digestion and overall well-being.

The bottom line

If you’ve been managing “IBS” for years and nothing seems to work, it might be time to consider a different root cause. Sucrase-isomaltase deficiency is more common than once thought and once identified, it can be life-changing.

Understanding how your body processes sugar and starch can be the missing piece in feeling better again.