What causes bloating and dark stools in an adult with a possible history of gastrointestinal issues?

Bloating and Dark Stools: Causes and Evaluation

Dark stools combined with bloating should immediately raise concern for gastrointestinal bleeding, which requires urgent evaluation to exclude serious pathology including peptic ulcer disease, gastritis, malignancy, or vascular lesions.

Critical Red Flag Assessment

Dark stools (melena) indicate upper gastrointestinal bleeding until proven otherwise. This occurs when blood is digested by gastric acid and intestinal bacteria, producing the characteristic black, tarry appearance. When combined with bloating, this presentation demands immediate investigation for:

• Upper GI bleeding sources: peptic ulcers, gastritis, esophageal varices, Mallory-Weiss tears, or gastric/esophageal malignancy
• Iron supplementation: the most common benign cause of dark stools, which can also cause bloating and constipation as a side effect 1
• Medications: bismuth subsalicylate (Pepto-Bismol), iron supplements, or certain antibiotics can darken stools without bleeding

Immediate Diagnostic Priorities

If true melena is suspected (not just darkened stools from iron or bismuth), obtain:

• Complete blood count to assess for anemia and acute blood loss
• Comprehensive metabolic panel to evaluate for organ dysfunction 2
• Upper endoscopy is the gold standard for identifying bleeding sources in the esophagus, stomach, and duodenum

Bloating Evaluation Framework

Once bleeding is excluded or attributed to benign causes (iron supplements), bloating requires systematic evaluation through six primary mechanisms 3:

1. Dietary Carbohydrate Malabsorption

• Lactose intolerance affects 51% of patients with bloating 3
• Fructose intolerance is present in 60% of bloating patients 3
• Fructans in gluten-containing foods may be the actual culprit rather than gluten itself in self-reported gluten sensitivity 2, 3

2. Constipation-Related Bloating

• Evaluate for constipation using Rome IV criteria for IBS-C or chronic constipation 2
• Consider anorectal manometry if pelvic floor dyssynergia is suspected 2
• Abdominal x-ray may reveal increased stool burden suggesting slow transit constipation 2

3. Small Intestinal Bacterial Overgrowth (SIBO)

High-risk patients warrant testing:

• Chronic watery diarrhea with malnutrition and weight loss >10% 3
• Systemic diseases causing small bowel dysmotility (cystic fibrosis, Parkinson disease) 2
• Diagnosis via hydrogen-based breath testing with glucose or lactulose, or small bowel aspirates 2, 3

4. Celiac Disease Screening

Mandatory testing if any of the following are present:

• Iron-deficiency anemia (present in 2-6% of asymptomatic celiac patients) 4
• Weight loss >10% 2, 3
• Diarrhea-predominant symptoms 2
• Test with tissue transglutaminase IgA and total IgA levels to exclude IgA deficiency causing false-negatives 2, 4

5. Functional Gastrointestinal Disorders

• Irritable bowel syndrome presents with bloating in >50% of patients due to visceral hypersensitivity 3, 5
• Functional dyspepsia frequently includes bloating 3
• Functional bloating affects 3.5% of the population (4.6% in women, 2.4% in men) when Rome IV criteria are met 3

6. Age and Gender-Specific Considerations

For women ≥50 years old:

• Ovarian cancer commonly presents with bloating and abdominal fullness 2, 6
• Transvaginal and transabdominal ultrasound with Doppler is the initial imaging modality 6

When to Pursue Advanced Testing

Order imaging and endoscopy ONLY when alarm features are present 2, 3:

• Weight loss >10% 2, 3
• Gastrointestinal bleeding or iron-deficiency anemia 2, 4
• Recent worsening nausea or pain 2
• Family history of inflammatory bowel disease 2
• Age >40 years with new-onset dyspeptic symptoms in high Helicobacter pylori prevalence regions 2

Common Pitfall to Avoid

Do not attribute dark stools to diet alone without excluding bleeding. While iron supplements and bismuth commonly darken stools, true melena has a distinctive tarry consistency and foul odor that differs from dietary causes. When in doubt, obtain a complete blood count and consider fecal occult blood testing, though this may be falsely positive with upper GI bleeding 1.

Iron supplementation itself causes gastrointestinal discomfort including nausea, constipation, or diarrhea 1, which can contribute to bloating. If iron is the culprit for dark stools, taking it with meals may minimize gastrointestinal side effects 1.