IBD, Malabsorption & Irritable Bowel Syndrome

Inflammatory bowel disease (IBD) is chronic immune-system inflammation of the digestive tract, mainly Crohn's disease and ulcerative colitis. Malabsorption means the small intestine fails to absorb nutrients properly, whatever the cause. Irritable bowel syndrome (IBS) is a functional disorder — recurrent abdominal pain with altered bowel habits, but with no inflammation or damage visible on tests.

These matter because they are easily confused yet treated completely differently. IBD needs lifelong immune therapy and raises colon-cancer risk; malabsorption silently starves children of growth and adults of vitamins; IBS is extremely common and, though not dangerous, ruins quality of life until recognized and managed.

Key Concepts

• Ulcerative colitis (UC): inflammation limited to the colon, starting at the rectum and spreading continuously upward. Causes bloody diarrhea and urgency; biopsy shows crypt abscesses. Severe flares can dilate the colon dangerously (toxic megacolon).
• Crohn's disease: can strike anywhere from mouth to anus, in "skip lesions" with healthy bowel between. Inflammation goes through the full wall (transmural), causing strictures, fistulas and a cobblestone appearance on endoscopy.
• IBD treatment: 5-ASA drugs (mesalamine) for mild UC, steroids for flares, and biologics such as anti-TNF agents for moderate-to-severe disease; surgery removes damaged segments but Crohn's can recur.
• Malabsorption causes: celiac disease (gluten-triggered immune damage, diagnosed by anti-tTG antibodies and duodenal biopsy), pancreatic insufficiency, lactose intolerance, bile-acid problems and extensive bowel surgery. Clues: chronic diarrhea, weight loss, anemia, low albumin and vitamin deficiencies.
• IBS (Rome IV criteria): recurrent abdominal pain at least one day per week, related to defecation or change in stool frequency or form. Subtypes: IBS-D (diarrhea), IBS-C (constipation), IBS-M (mixed). Managed with fiber, antispasmodics, gut-directed therapy and low-FODMAP diet — not with steroids or biologics.

Formulas

• Body mass index: BMI = weight (kg) ÷ [height (m)]². It is the standard bedside measure of nutritional status in suspected malabsorption.
• BMI bands: under 18.5 is underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and above obese. A falling BMI in chronic diarrhea points to malabsorption.

Worked Example

1. A 24-year-old woman with newly diagnosed celiac disease weighs 48 kg and is 1.62 m tall, with months of diarrhea.
2. BMI = 48 ÷ (1.62 × 1.62) = 48 ÷ 2.6244 = 18.29, about 18.3.
3. Since 18.3 is below 18.5, she is underweight — objective evidence of malabsorption from untreated celiac disease. She starts a strict gluten-free diet plus iron and vitamin D replacement, with weight and BMI rechecked in three months.

Common Misconceptions

• Myth: IBS and IBD are the same thing. Fact: IBS has no inflammation or tissue damage; IBD destroys bowel wall and needs immune-suppressing drugs.
• Myth: Avoiding milk fixes all gut problems. Fact: lactose intolerance is a specific enzyme deficiency diagnosed by testing; most IBD and IBS patients tolerate dairy fine.
• Myth: Stress causes IBD. Fact: IBD is an immune disease; stress only worsens symptoms, and blaming stress delays real treatment.

Real-World Applications

• Gastroenterology: colonoscopy with biopsy separates UC from Crohn's and IBS, guiding completely different treatments.
• Dietetics: gluten-free plans for celiac disease and low-FODMAP plans for IBS.
• Pediatric growth monitoring: falling growth curves are often the first sign of malabsorption in children.

IBD (Crohn's and ulcerative colitis) is immune inflammation needing powerful drugs; malabsorption starves the body of nutrients and shows in a falling BMI; IBS is diagnosed by the Rome IV symptom pattern and managed without anti-inflammatory drugs. Same neighborhood, three very different diseases.