Understanding Congenital Sucrase-Isomaltase Deficiency (CSID)

Understanding Congenital Sucrase-Isomaltase Deficiency (CSID): Causes, Symptoms, and Treatment

Congenital Sucrase-Isomaltase Deficiency (CSID) is a genetic disorder which is linked as the culprit of many cases of sucrose intolerance – a type of food intolerance which masquerades as diarrhea-predominant irritable bowel syndrome (IBS-D).

This very rare inherited condition isn’t yet well-known or understood, even among gastrointestinal (GI) doctors and gut health dietitians.

As a result, many folks with a congenital sucrase-isomaltase deficiency are left feeling at a loss when interventions such as the low FODMAP diet and/or specific carbohydrate diets (like GAPS and Paleo) offer little to no relief for what appears to be IBS!

In this article we’ll delve into the root causes, symptoms, and available treatment options for navigating CSID, from an evidence-based and holistic nutrition standpoint.

Disclaimer:  This article was written for educational purposes, not to replace medical or nutritional advice and 1:1 consultation from your healthcare team.

What is Congenital Sucrase-Isomaltase Deficiency (CSID)?

Congenital Sucrase-Isomaltase Deficiency (CSID) is an inherited disorder characterized by the body’s inability to break down certain sugars, particularly sucrose (table sugar) and starches.

People with CSID have a deficiency in the sucrase-isomaltase enzyme, which is a digestive enzyme responsible for breaking down sucrose (a disaccharide, aka a two-molecule carbohydrate) into glucose + fructose (two monosaccharides, which are single molecules and the smallest unit of carbohydrates).

This disorder is caused by mutations in the gene responsible for producing the sucrase-isomaltase digestive enzyme, which is normally secreted by the brush border of your small intestine.

Without this key enzyme, your body can’t digest sugar properly, leading to what may look and feel a whole lot like lactose intolerance and IBS-D.


According to the National Organization for Rare Disorders and CSID Cares, this rare metabolic disorder impacts ~0.2% of people in the U.S., while it seems to impact ~5-10% of people living in Canada and Greenland.

What causes it?

CSID is a genetic disorder that follows an autosomal recessive pattern of inheritance. This means that an individual must inherit two copies of the mutated gene (one from each parent) to develop it.


If both parents are carriers of the mutated gene, there’s a 25% chance with each pregnancy for the child to have CSID.

CSID Symptoms

Eating “normal” food when you have CSID is pretty comparable consuming lactose when you have a lactose intolerance! (After all, sucrose is found in almost everything – especially highly processed foods which are typically made with flour and sugar as primary staple ingredients.)

The symptoms  typically manifest during infancy when a baby starts consuming solid foods containing sucrose and starches.

Unfortunately it’s difficult to identify most of this early on, since infants can’t express how they are feeling.

Due to bio-individuality, everyone’s CSID symptoms vary case-by-case and exist on a spectrum.  Still, there’s a lot of overlap and common ground in terms of CSID symptoms.

Common symptoms include, but aren’t limited to:

  • Diarrhea
  • Abdominal pain
  • Bloating
  • Excessive gas
  • Failure to thrive
  • Unintentional weight loss
  • Greasy stools
  • Nausea & vomiting (in some cases)
  • Chronic fatigue


Frequent loose, watery stools are a hallmark symptom of CSID due to undigested sugars reaching the colon and causing osmotic diarrhea.

Abdominal pain and bloating

Incomplete digestion of sugars can lead to gas production and abdominal discomfort.

Failure to thrive

Poor weight gain and growth may occur due to malabsorption of nutrients from undigested sugars.

Excessive gas

Fermentation of undigested sugars by gut microbes can result in excessive gas and foul-smelling flatulence.

Unintentional weight loss

When you have CSID, undigested sucrose is getting fermented by microbes in the colon, which interferes with your body’s ability to digest and absorb nutrients from the food you eat.

If nutrients don’t get properly absorbed into the body, it will lead to malabsorption, malnutrition, and unintentional weight loss.

Greasy stools

Greasy, floating stools are a sign of fat malabsorption, caused by the fermentation of undigested sucrose in the gut.  This can happen often in cases of CSID.  Fat malabsorption is also called “steatorrhea”.

Nausea and vomiting

Some but not all people with CSID may experience symptoms of nausea and vomiting.

Chronic fatigue

Poor nutrient absorption is common among CSID patients, so chronic fatigue is inevitable until symptoms are under control.

Consider running a comprehensive micronutrient panel with a functional dietitian nutritionist to rule out nutritional deficiencies, if your fatigue doesn’t improve after a few months of following a nutritionally balanced low sucrose diet.

Diagnosing CSID

If CSID is suspected, a healthcare professional may perform the following diagnostic tests:

Disaccharide biopsy

This is the “gold standard” test for identifying the presence of sucrase-isomaltase deficiency.  This test measures the amount of various types of digestive enzymes (sucrase-isomaltase, lactase, and others) in your intestinal brush border.

Unfortunately, this test is also the most invasive in that it requires an endoscopy or colonoscopy with a gastrointestinal (GI) doctor.

This is not a routine test; most GI doctors don’t yet know about it, so make sure to advocate for yourself and leave no stone unturned until you feel realigned!

Sucrose intolerance breath test

This test involves measuring the presence of hydrogen gas levels before and after consuming a specific amount of sucrose.

However, even if this test confirms a sucrose intolerance, this won’t necessarily tell us whether or not it’s genetic (versus acquired later in life, secondary to a condition that damages the intestinal brush border – such as small intestinal bacterial overgrowth / “SIBO”, celiac disease, Crohn’s disease, or ulcerative colitis).

Generally speaking, if you’ve had these severe digestive issues your entire life, it’s safe to assume your sucrose intolerance was genetic versus acquired.

On the other hand, if you didn’t have issues until a certain timeframe in your life, it’s less likely that your sucrose intolerance is congenital (inherited).

Either way the best way to know for sure is to run a genetic test.

Genetic testing: Identifying mutations in the sucrase-isomaltase gene can confirm a CSID diagnosis.

Food logging

While I’m a big advocate of testing over guessing, going on a sucrose elimination diet (and tracking via a food-symptom journal, like this one) may be enough to provide you with some data and clarity on whether or not you feel better after removing sucrose from your diet.

This diet is extremely restrctive, however, so don’t do this without the supervision and guidance of a gut health dietitian who is CSID-informed.

Managing CSID

While there’s currently no cure for CSID, managing the condition primarily involves dietary modifications:

  1. Dietary modification via a low-sucrose diet
  2. Enzyme replacement therapy

Sucrose and starch restriction

Limiting or eliminating the intake of foods high in sucrose and starches can alleviate symptoms. A registered dietitian can help create an appropriate meal plan.

Enzyme replacement therapy

Taking sucrase and/or isomaltase enzyme supplements (specifically Sucraid) with meals can aid in the digestion of sugars. (Read more about how to get started with Sucraid here.)

You may also benefit from a supplement called Starchway if your body doesn’t break down the maltose in starch easily.

On the other hand, if you have a sucrose intolerance but NOT CSID, you may be able to recover if you properly rule out and address SIBO and/or inflammatory bowel disease. which are the most common underlying root-cause culprits of a sucrose intolerance that isn’t inherited.  (Read more about SIBO here!)

CSID vs acquired sucrase-isomaltase deficiency

If your genetic test was negative for CSID but you’re testing positive for (or suspect) a sucrose intolerance, chances are you’ve got acquired sucrase-isomaltase deficiency.

Acquired sucrase-isomaltase deficiency usually develops secondary to SIBO.  This happens when microbes invading your small intestine are left unchecked – so they eventually start to damage your brush border.  (The “brush border” is the site in your intestines where sucrase-isomaltase enzyme is supposed to be made and secreted.)

Tips for living with CSID

Living with CSID can be challenging, but with proper management and support, you can still lead a healthy life.

It’s essential to maintain open, ongoing communication with your healthcare professionals, adhere to the recommended dietary modifications, and seek emotional support from community support groups.

  • If you’re in need of 1:1 support and guidance from a CSID dietitian, feel free apply HERE to hop on a complimentary meet & greet phone call with yours truly! (Space is limited in my 1:1 practice, but if it sounds like a great mutual fit, I’d LOVE to help you!)

More resources

Sucrose Intolerance and CSID Diet Plan PDF with Meal Planning Guide and 21 CSID Recipes


Congenital Sucrase-Isomaltase Deficiency (CSID) is a rare genetic disorder that affects your body’s ability to digest sucrose and starches. It is crucial to raise awareness about CSID to facilitate early diagnosis and proper management.

If you suspect CSID in yourself or your child, consult a healthcare professional for an accurate diagnosis and guidance on managing the condition effectively. With the right strategies in place, individuals with CSID can thrive and lead fulfilling lives.

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