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PubMed This is a summary of 14 peer-reviewed journal articles Updated
Gastroenterology

Recognizing Symptoms and Avoiding Misdiagnosis

At a Glance

Congenital Sucrase-Isomaltase Deficiency (CSID) is a genetic inability to digest specific sugars and starches. In infants, symptoms like severe diarrhea and poor weight gain begin when solid foods are introduced. In adults, CSID is frequently misdiagnosed as Irritable Bowel Syndrome (IBS-D).

Identifying Congenital Sucrase-Isomaltase Deficiency (CSID) is often difficult because its symptoms mimic many other common digestive issues. For many, the diagnosis comes only after years of being told they have a “sensitive stomach” or a functional disorder [1][2]. Understanding the specific patterns of these symptoms and how they differ from common misdiagnoses is the first step toward effective management.

When Symptoms Emerge

The timing of when symptoms first appear is a major clue for doctors. In infants, CSID typically remains hidden while the baby is exclusively breastfed or on standard formula, as these do not usually contain the sugars that cause a reaction [3].

Symptoms usually erupt during “food diversification”—the transition to solid foods [3][4]. This includes the introduction of:

  • Baby cereals (which contain starches)
  • Fruit juices (which contain sucrose and fructose)
  • Pureed fruits and vegetables

In some cases, especially in adults or those with milder genetic variants, symptoms may be subtle for years and only become disruptive later in life as dietary habits change or the gut’s ability to compensate decreases [5][6].

Common Symptoms Across the Lifespan

While symptoms can vary in intensity, they center around the body’s inability to absorb nutrients properly.

  • In Infants and Children: The most concerning sign is often Failure to Thrive (FTT) [3]. This occurs because the child cannot absorb the calories and nutrients from starches and sugars, leading to poor weight gain or “falling off” their growth curve [1][7]. This is usually accompanied by chronic, watery diarrhea and significant irritability [3][4].
  • In Adults: Symptoms often mirror Irritable Bowel Syndrome (IBS) [1][5]. This includes chronic bloating, excessive gas, abdominal pain, and frequent loose stools (diarrhea) [6][8].

The Challenge of Misdiagnosis

CSID is frequently mistaken for other conditions because the symptoms are so similar.

“Toddler’s Diarrhea” vs. CSID

Many children are initially diagnosed with “toddler’s diarrhea” (chronic functional diarrhea). While toddler’s diarrhea is often related to high juice intake or rapid gut transit, CSID is a genetic inability to digest specific molecules [9][1]. If a child is also struggling to gain weight or has severe bloating, doctors should look closer at the possibility of CSID [3][4].

IBS-D vs. CSID

In adults, the most common misdiagnosis is IBS-D (diarrhea-predominant Irritable Bowel Syndrome) [5][10]. Because both conditions cause pain and diarrhea, many adults spend decades treating IBS without realizing that a specific enzyme deficiency is the root cause [11][12].

The Role of Heterozygous Mutations

Genetics aren’t always “all or nothing.” Traditionally, CSID was thought to only affect people who inherited two faulty genes (homozygous) [13]. However, we now know that heterozygous carriers—those with only one faulty gene—can also suffer from symptoms [5][11]. These individuals may have “milder” cases that are easily dismissed by doctors but still cause significant daily distress and mimic functional gut disorders [10][12]. Whether a person has one or two faulty genes, the treatment—dietary changes or enzyme replacement—can still provide life-changing relief [12][14].

Next steps: How CSID is Diagnosed

Common questions in this guide

When do CSID symptoms usually start in babies?
Symptoms typically emerge when babies transition to solid foods, such as baby cereals, fruit juices, and pureed fruits or vegetables. Infants who are exclusively breastfed or on standard formula usually do not show symptoms because these do not contain the trigger sugars.
Why is CSID often misdiagnosed as toddler's diarrhea?
In children, CSID is frequently mistaken for toddler's diarrhea because both conditions cause chronic loose stools. However, if a child also struggles to gain weight or has severe bloating, it may point to an underlying genetic deficiency like CSID.
Can adults have CSID?
Yes, adults can have CSID, and their symptoms often mirror diarrhea-predominant Irritable Bowel Syndrome (IBS-D). Many adults spend years treating IBS before discovering a specific enzyme deficiency is the actual cause of their chronic bloating, pain, and diarrhea.
Do I need two faulty genes to have CSID symptoms?
No. While it was once thought that you needed two faulty genes to have CSID, we now know that individuals with only one faulty gene can also experience significant digestive symptoms that mimic functional gut disorders.
What does 'failure to thrive' mean in the context of CSID?
Failure to thrive happens when an infant or child cannot absorb enough calories and nutrients from starches and sugars due to missing digestive enzymes. This leads to poor weight gain and causes the child to fall behind on their expected growth curve.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Given my (or my child's) history of 'toddler's diarrhea' or IBS-D, could CSID be the underlying cause of these persistent symptoms?
  2. 2.Are my symptoms consistent with a heterozygous mutation, and does that mean my treatment plan will be different than someone with homozygous CSID?
  3. 3.How can we differentiate between standard carbohydrate intolerance and a genetic sucrase-isomaltase deficiency using objective tests?
  4. 4.What specific changes should we look for in growth charts or weight patterns to determine if 'failure to thrive' is a concern?
  5. 5.Would a sucrose breath test or genetic testing be more appropriate for us at this stage of the diagnostic process?

Questions For You

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References

References (14)
  1. 1

    Sucrase-Isomaltase Deficiency Causing Persistent Bloating and Diarrhea in an Adult Female.

    Chiruvella V, Cheema A, Arshad HMS, et al.

    Cureus 2021; (13(4)):e14349 doi:10.7759/cureus.14349.

    PMID: 33972906
  2. 2

    Prevalence of Disaccharidase Deficiency in Adults With Unexplained Gastrointestinal Symptoms.

    Viswanathan L, Rao SSC, Kennedy K, et al.

    Journal of neurogastroenterology and motility 2020; (26(3)):384-390 doi:10.5056/jnm19167.

    PMID: 32380581
  3. 3

    Two Novel Mutations in the SI Gene Associated With Congenital Sucrase-Isomaltase Deficiency: A Case Report in China.

    Zhou J, Zhao Y, Qian X, et al.

    Frontiers in pediatrics 2021; (9()):731716 doi:10.3389/fped.2021.731716.

    PMID: 34926337
  4. 4

    Congenital sucrase-isomaltase deficiency in Türkiye; a single center experience.

    Barut D, Kıran Taşcı E, Kunay B, et al.

    Scandinavian journal of gastroenterology 2024; (59(6)):647-651 doi:10.1080/00365521.2024.2324961.

    PMID: 38459691
  5. 5

    Sucrase-isomaltase Gene Variants in Patients With Abnormal Sucrase Activity and Functional Gastrointestinal Disorders.

    Deb C, Campion S, Derrick V, et al.

    Journal of pediatric gastroenterology and nutrition 2021; (72(1)):29-35 doi:10.1097/MPG.0000000000002852.

    PMID: 32732636
  6. 6

    Differential Effects of Sucrase-Isomaltase Mutants on Its Trafficking and Function in Irritable Bowel Syndrome: Similarities to Congenital Sucrase-Isomaltase Deficiency.

    Husein DM, Rizk S, Naim HY

    Nutrients 2020; (13(1)) doi:10.3390/nu13010009.

    PMID: 33375084
  7. 7

    Improved Starch Digestion of Sucrase-deficient Shrews Treated With Oral Glucoamylase Enzyme Supplements.

    Nichols BL, Avery SE, Quezada-Calvillo R, et al.

    Journal of pediatric gastroenterology and nutrition 2017; (65(2)):e35-e42 doi:10.1097/MPG.0000000000001561.

    PMID: 28267073
  8. 8

    Intestinal Disaccharidase Deficiency in Adults: Evaluation and Treatment.

    Viswanathan L, Rao SS

    Current gastroenterology reports 2023; (25(6)):134-139 doi:10.1007/s11894-023-00870-z.

    PMID: 37199899
  9. 9

    Molecular pathogenicity of novel sucrase-isomaltase mutations found in congenital sucrase-isomaltase deficiency patients.

    Gericke B, Amiri M, Scott CR, Naim HY

    Biochimica et biophysica acta. Molecular basis of disease 2017; (1863(3)):817-826 doi:10.1016/j.bbadis.2016.12.017.

    PMID: 28062276
  10. 10

    Exploring congenital sucrase-isomaltase deficiency in autism spectrum disorder patients with irritable bowel syndrome symptoms: A prospective SI gene sequencing study.

    Zubarioglu T, Ulgen D, Akca-Yesil S, et al.

    Autism research : official journal of the International Society for Autism Research 2025; (18(1)):44-55 doi:10.1002/aur.3293.

    PMID: 39676735
  11. 11

    Heterozygotes Are a Potential New Entity among Homozygotes and Compound Heterozygotes in Congenital Sucrase-Isomaltase Deficiency.

    Husein DM, Wanes D, Marten LM, et al.

    Nutrients 2019; (11(10)) doi:10.3390/nu11102290.

    PMID: 31557950
  12. 12

    Prevalence of congenital sucrase-isomaltase deficiency in Turkey may be much higher than the estimates.

    Taskin DG, Civan HA, Sari EE, et al.

    Journal of genetics 2023; (102()).

    PMID: 37349966
  13. 13

    Congenital Sucrase-Isomaltase Deficiency: Same Mutation with Different Clinical Presentations.

    İryancı Fİ, Güven B, Çakır M

    The Turkish journal of gastroenterology : the official journal of Turkish Society of Gastroenterology 2024; (35(4)):343-349 doi:10.5152/tjg.2024.23250.

    PMID: 39128102
  14. 14

    Hypomorphic SI genetic variants are associated with childhood chronic loose stools.

    Chumpitazi BP, Lewis J, Cooper D, et al.

    PloS one 2020; (15(5)):e0231891 doi:10.1371/journal.pone.0231891.

    PMID: 32433684

This page explains the symptoms and common misdiagnoses of CSID for educational purposes only. Always consult a pediatric gastroenterologist or your primary care provider for a proper diagnosis of chronic digestive issues.

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