(CSID) What is Congenital Sucrase-Isomaltase Deficiency (CSID)? 

Congenital Sucrase-Isomaltase Deficiency (CSID) is a rare genetic disorder that affects the body's ability to digest certain sugars and often results in chronic diarrhea. This is due to a missing key enzyme (sucrase-isomaltase) that normally lives in our intestines. Without this enzyme, the body cannot digest disaccharides (learn more about disaccharides here) or oligosaccharides (learn more about oligosaccharides here). The Canadian Medical Association Journal published a study identifying the exact genetic mutation that results in CSID and found that this mutation may be more prevalent than we thought. Interestingly, this mutation is also more commonly found in people of American or European ancestry specifically living in northern regions. While CSID and Irritable Bowel Syndrome (IBS) share certain characteristics, true CSID can be misdiagnosed as IBS instead. 

The gold standard for accurate CSIB diagnosis is a biopsy of the small bowel tissue, and can be confirmed with a complete or near-complete absence of the necessary enzymes (sucrase and isomaltase). CSID can also occur as a result of severe trauma to the integrity of the small bowel tissue, such as significant infections, ulcers, or uncontrolled chronic inflammatory bowel disease. 

Symptoms of CSID 

The sucrase-isomaltase enzyme plays an important role in digestion and absorption of certain foods. The breakdown of monosaccharides, disaccharides, and polysaccharides is completely dependent on this enzyme. According to the Journal of Biological Chemistry, without this enzyme people can experience chronic diarrhea, abdominal distention, bloating, and/or cramps, and occasionally vomiting.

Since these enzymes play a large role in the digestion and absorption of sugars and starches, nutritional deficiencies are a possible outcome of CSID. Without adequate amounts of complex carbohydrates (i.e. oatmeal, whole wheat bread or pasta) or lactose (dairy products), several vitamin levels may need to be checked and routinely monitored. Without adequate carbohydrate products, people are at a greater risk for developing vitamin B (B1, B2, B5, B6, etc.) and calcium deficiencies. According to CSID Cares, the sucrase-isomaltase enzyme is responsible for the digestion of 60 - 80% of starches consumed in the diet, leaving a lot of wiggle room for potential deficiencies. 

Diagnosis and Testing 

As mentioned above, the gold standard for diagnosing CSID is a biopsy of the small bowel tissue to better identify the presence of (or lack of) the sucrase-isomaltase enzyme. This biopsy-focused test is called the disaccharidase assay. There are several simple breath tests that can be performed, the first of which is called the carbon-13 sucrose breath test and the second is called the sucrose hydration breath test. Both of these tests involve consuming sucrose in the form of a sugary drink and measuring certain gas levels. A physician may also proceed with a therapeutic trial of enzyme replacements. See the Sucraid website for more details on testing options. 

Managing CSID: Dietary Modifications and Treatment 

If your physician places you on a dietary restriction during the testing period and throughout treatment, it is very important to adhere closely to the restrictions. Even trace amounts of sugary or starchy foods can cause an adverse and uncomfortable reaction. There are lists of ‘safe foods’ that upon first glance seem very limited, but working with a Registered Dietitian (RD) can help expand this list and tailor it to better fit your needs. Generally speaking, you may be asked to avoid carbohydrates and sugar, some examples of this include: bread products, pastas, dairy products (dairy free nut milks may be fine but need to be evaluated for amount of added sugars), sweets and candies, and any other products with sucrose listed as an added ingredient. 

Enzyme replacement therapy does offer the potential to consume a more regular diet (including starches and sugars). Sucraid is a FDA-approved oral solution that once in the gut starts to break down sucrose into glucose and fructose, simple sugars that can be easily absorbed by the small intestine. Learn more about this enzyme replacement therapy here. 

Living with CSID: Coping Strategies and Support 

Building a support group with friends or family is important when dealing with any chronic illness. Since this is a rare disease, it is also crucial that you become an ‘expert’ of CSID so that you can appropriately advocate for yourself. Building a team of trusted medical professionals will provide long-term support and ongoing treatment options. Consider asking your provider for a referral to a RD to also assist in the restrictive nature of this diet. It may also be a good idea to keep lists of ‘safe foods’ and ‘no-no foods’ on your person at all times to aid in ordering out at restaurants or grocery shopping. 

In general, coping with this diagnosis can be a long-term challenge. Consider seeking local or virtual support groups - if there is a lack of CSID specific group options in your area, consider joining a support group for IBS or IBD as well. 

1. _{Diagnosing congenital sucrase-isomaltase deficiency (CSID). Sucraid. (2024, August 2). https://www.sucraid.com/about-csid/diagnosis/} 

2. _{Gericke, B., Schecker, N., Amiri, M., & Naim, H. Y. (2017). Structure-function analysis of human sucrase-isomaltase identifies key residues required for catalytic activity. The Journal of biological chemistry, 292(26), 11070–11078. https://doi.org/10.1074/jbc.M117.791939}

3. _{Marcadier, J. L., Boland, M., Scott, C. R., Issa, K., Wu, Z., McIntyre, A. D., Hegele, R. A., Geraghty, M. T., & Lines, M. A. (2015). Congenital sucrase-isomaltase deficiency: identification of a common Inuit founder mutation. CMAJ : Canadian Medical Association journal = journal de l'Association medicale canadienne, 187(2), 102–107. https://doi.org/10.1503/cmaj.140657}