Living with Congenital Sucrase-Isomaltase Deficiency (CSID): What Every Newly Diagnosed Person (and Their Loved Ones) Should Know

You just got a diagnosis – or maybe someone you love did and you have no idea what Congenital Sucrase-Isomaltase Deficiency, or CSID means. You may have spent months or even years being told it was IBS, anxiety, a “sensitive stomach,” or something you just had to live with. The diagnosis can bring both relief (finally, an answer) and a flood of new questions.

I would say that there is definitely a learning curve to this condition. Even as a GI dietitian myself, it took some time to understand the condition itself and figure out ways to best support my patients on this. Getting support from a gastroenterologist and GI specialized Registered Dietitian Nutritionist who is familiar with CSID will be very important to help ease that overwhelm, frustration, and confusion with navigating and managing this long-term condition. 

What Is Congenital Sucrase-Isomaltase Deficiency (CSID)?

CSID is a rare, inherited autosomal recessive disorder in which the small intestine doesn’t produce enough, or any, of two critical enzymes: sucrase and isomaltase. There are more than 40 mutations identified recently (Irlayıcı et al 2024), and sucrase-isomaltase enzyme deficiency is the most common disaccharidase deficiency. Since CSID is difficult to diagnose and symptoms can change with age, the true prevalence of this disease is likely to be underestimated and may be more common than expected.

These enzymes are responsible for breaking down specific sugars:

-> Sucrase (enzyme) breaks down sucrose. Sucrose is commonly found in table sugar, fruits, juices, sweets, some starchy vegetables and grains, and many processed foods.

-> Isomaltase (enzyme) breaks down isomaltose. Isomaltose is a sugar derived from the digestion of starches like bread, pasta, rice, and potatoes.

When these enzymes are absent or significantly reduced, undigested sugars travel into the large intestine, where gut bacteria ferment them. The result? Gas, bloating, cramping, watery diarrhea, and nausea. This can often occur within minutes to hours of eating.

CSID is genetic, caused by mutations in the SI gene, and is typically present from birth. It’s estimated to affect roughly 1 in 500 people of European descent, and may be significantly more common in certain Indigenous populations, such as Greenlandic Inuit, where prevalence may approach 1 in 10.

CSID is rare and uncommon in the Asian population. In 2014, the first case series of CSID in Chinese children was published (Geng et al 2014). It was indicated that CSID is probably an underdiagnosed or misdiagnosed disease and should be considered in children with chronic watery diarrhea if other gastrointestinal workup is unremarkable.  

What CSID Is NOT

There is a lot of confusion, and (potentially) a lot of misdiagnosis surrounding CSID. Let’s address some of the myths below.

CSID is not IBS. Irritable Bowel Syndrome is a disorder of the gut-brain interaction with no identifiable structural or biochemical cause. CSID has a clear, measurable cause: missing or significantly reduced enzyme activity. Many people with CSID can be misdiagnosed with IBS for years before the real answer is found.

CSID is not a food allergy. Food allergies involve an immune system response. CSID is purely an enzyme deficiency. Eating sucrose with CSID will not trigger anaphylaxis, but it will trigger significant GI distress. 

CSID is not lactose intolerance (though they can coexist). Lactose intolerance involves a different enzyme (lactase) and different sugars (lactose in dairy). They are separate conditions, though some people have both.

CSID is not caused by diet choices or lifestyle. This is not something a person did to themselves. It is a genetic condition. It cannot be “fixed” by willpower, stress reduction, or eating more fiber.

CSID is not always obvious in infancy. Some infants are diagnosed early when they react to sucrose-containing formula or fruit. Others don’t show significant symptoms until their diet expands, or even into adulthood – making diagnosis notoriously delayed.

CSID is not the same for everyone. Enzyme activity exists on a spectrum. Some individuals retain partial enzyme function and tolerate small amounts of sucrose or starch. Others have near-zero function and react to trace amounts. This variability is why CSID can look so different from person to person. 

How Is CSID Treated?

The good news: CSID is manageable. It is not a progressive disease, it doesn’t damage the intestine over time (the way celiac disease can), and with the right approach, most people can live full, active, well-nourished lives.

Treatment typically involves a combination of the following:

1. Enzyme Replacement Therapy

The most targeted treatment is sacrosidase (Sucraid®), an FDA-approved liquid enzyme supplement derived from baker’s yeast. Taken with meals and snacks, it provides the sucrase enzyme the body is missing. For many patients, this is a game-changer – allowing a significantly more flexible diet.

Note that sacrosidase (the “S” in CSID) does not replace isomaltase (the “I” in CSID), so starch tolerance must still be managed separately, often through dietary limits with the help of a Registered Dietitian who specializes in GI conditions.

2. Dietary Modification

Even with enzyme therapy, some dietary awareness is necessary. A registered dietitian with GI experience (like myself!) can be invaluable here. The core goals are:

-> Reducing sucrose. Limiting table sugar, high-sugar fruits, fruit juices, sweetened beverages, candy, and many packaged foods.

-> Managing starch intake. Finding an individual’s tolerance level for bread, pasta, rice, and root vegetables.

-> Reading labels carefully. Sucrose hides under many names: cane sugar, evaporated cane juice, fruit concentrate, and more.

Low-sucrose, low-starch diets don’t have to be joyless. Many whole proteins, fats, non-starchy vegetables, and certain fruits (like berries in moderation) are well-tolerated.

3. Identifying Individual Tolerances

Because CSID varies in severity, treatment is highly individualized. I utilize a food diary or a structured elimination-and-reintroduction process with my patients to help map out food tolerances and symptoms. Some people with partial enzyme function tolerate cooked starches better than raw, or find that spreading intake across the day helps.

4. Managing Secondary Nutritional Gaps

I commonly see patients not being able to hit their nutrition goals due to long-term dietary restriction way before their CSID diagnosis. This can sometimes lead to gaps in fiber, certain vitamins, or caloric intake in both children and adults. Regular check-ins with a GI dietitian help ensure nutritional needs are met. 

How Family and Friends Can Help

A new diagnosis can feel isolating. Food is social, emotional, and cultural. Suddenly having to navigate every meal differently can be exhausting and lonely. The people around someone with CSID have more power to help than they may realize.

1. Learn What It Actually Means

Understanding the basics – that sucrose and starch aren’t processed correctly, and that it causes real, physical pain, is not something they can just “push through”. Having this level of understanding can go a long way. Ask questions. Read along with them. Your willingness to understand communicates that this matters to you.

2. Don’t Minimize It

“Just have a little bit” or “one bite won’t hurt” are phrases that can feel dismissive. For someone with severe enzyme deficiency, even small amounts can cause significant symptoms. Trust that the person living in this body knows their body. 

3. Make Gatherings Feel Safe, Not Stressful

Before hosting a meal, ask what they can eat. It can be helpful to include food options that are deemed safe by the patient to start. As a GI dietitian, I typically navigate and support this process both short and long term with CSID patients. It may not require making full separate selections. Sometimes small tweaks, side dishes or an entree that they can eat freely makes them feel seen. If you’re not sure, ask rather than guess.

4. Be a Co-investigator at Restaurants

Eating out is one of the biggest challenges. Sucrose hides in sauces, marinades, and dressings. Starch is in most side dishes. Help research menus ahead of time, ask the server questions alongside them, or simply be patient when they need a few extra minutes to figure it out.

5. Don’t Make Food the Centerpiece of Your Bond

Especially in the early adjustment phase, it helps when friends and family find ways to connect that don’t revolve entirely around food. A walk, a movie, a game night can give the person space to breathe without every interaction feeling like a dietary obstacle course.

6. Ask How They’re Really Doing (Or, Give Them Space)

Beyond the logistics, a new chronic diagnosis comes with grief. Grief for a simpler relationship with food, for spontaneity, for the version of life that felt easier. Check in (or give them space) on the emotional side as needed.

7. Celebrate the Wins

Tried a new food and tolerated it well? Found a restaurant with safe options? Figured out a great low-sucrose recipe? Made it through a holiday meal without symptoms? These are real victories. Acknowledge them.

A Final Word

Living with CSID is a lifelong process of learning your own body. The early months after diagnosis are usually the hardest. There’s a steep learning curve, a lot of trial and error, and an emotional adjustment that doesn’t happen overnight.

But with the right medical team, a good dietitian, enzyme therapy where appropriate, and people in your corner who actually get it, this is absolutely manageable. Many people with CSID describe their diagnosis not as a loss, but eventually as a kind of clarity: finally knowing what was wrong, and finally having the tools to feel better.

If you’re newly diagnosed, you’re not alone. Feel free to reach out to me (or other GI dietitians near you) for support. And if you’re supporting someone who is – thank you for showing up for them.

Sources: 

Irlayıcı Fİ, Özbaş H, Salman H et al. Frequency of Congenital Sucrase-Isomaltase Deficiency by Whole Exome Sequencing: Is It Really Rare?. J Pediatr Res. 2024;11(4):225-231. doi:10.4274/jpr.galenos.2024.65625

Senftleber NK, Ramne S, Moltke I, et al. Genetic Loss of Sucrase-Isomaltase Function: Mechanisms, Implications, and Future Perspectives. Appl Clin Genet. 2023;16:31-39. Published 2023 Mar 23. doi:10.2147/TACG.S401712

Geng L, Li DY, Ou W, et al. Congenital sucrase-isomaltase deficiency: an under-diagnosed disease in Chinese children. BMC Pediatr. 2014;14:11. Published 2014 Jan 16. doi:10.1186/1471-2431-14-11