Maldigestion and Malabsorption

• Maldigestion and malabsorption can have subtle clinical presentations, including extraintestinal manifestations such as anemia, bone loss, and menstrual disturbance.
• Subtle malabsorption of single nutrients like calcium or vitamin B12 might lead to irreversible complications if not recognized and treated on time.
• The 72-hour fecal fat determination test, used to diagnose malabsorption, has limited availability.
• Maldigestion is defined as defective intraluminal hydrolysis of nutrients, whereas malabsorption is defined as defective mucosal absorption.
• The clinical presentation and complications of maldigestion and malabsorption are similar.
• The terms digestion and absorption, or maldigestion and malabsorption, are used separately when discussing pathophysiology, otherwise, the terms absorption and malabsorption are used.
• Malabsorption may be caused by diseases of the small intestine, pancreas, liver, biliary tract, or stomach.

Etiology and Pathophysiology

• Mechanisms causing malabsorption are divided into pre-mucosal (luminal), mucosal, and postmucosal (vascular and lymphatic) factors.
• Normal nutrient uptake requires several steps, including solubilization and digestion.
• Micelles help solubilize fat and fat-soluble vitamins.
• Acidification in the GI lumen helps solubilize Calcium.
• Polysaccharides, triglycerides, and proteins must be digested into monosaccharides, fatty acids, and amino acids, respectively, by digestive enzymes.

Fats

• Defective mixing of dietary fat with digestive secretions results in fat malabsorption.
• Gastric resections or GI motility disorders with rapid gastric emptying can cause fat malabsorption.
• Micelle formation decreases if conjugated bile acids fall below critical concentrations, leading to fat malabsorption.
• Impairment of pancreatic lipase and co-lipase secretion decreases dietary fat hydrolysis, leading to decreased lipolysis.

Decreased Mucosal Absorption and Chylomicron Formation

• Generalized mucosal diseases often correlate with fat malabsorption as defective uptake of free fatty acids and monoglycerides lowers mucosal surface area, enterocyte function, and mucosal inflammation.
• Disturbance of chylomicron intracellular formation and lipid accumulation within enterocytes impairs intestinal fat absorption.

Defective Lymphatic Transport of Chylomicrons

• Postmucosal dietary fat malabsorption is due to the impairment of lymphatic transport of chylomicrons
• Decreased lymphatic transport results if lymphatic vessels are obstructed by metastatic solid tumors, lymphoma, Whipple disease, retroperitoneal fibrosis, or trauma, or a congenital disease.

Proteins and Amino Acids

• Protein-losing enteropathy must be differentiated from defective digestion or absorption of dietary proteins
• Protein digestion suffers in patients who has undergone gastric resection, owing to poor mixing with digestive secretions
• Exocrine pancreatic insufficiency causes deficient proteolysis
• Congenital diseases impair proteolytic enzyme synthesis or proenzyme activation

Defective Mucosal Hydrolysis of Peptides and Decreased Absorption of Oligopeptides and Amino Acids

• Lack of hydrolysis of oligopeptides and defective mucosal absorption leads to global malabsorption including malabsorption of oligopeptides and AAs
• Short bowel syndrome or jejunoileal bypass, can also result in protein and AA malabsorption.
• Selective malabsorption can be caused by congenital defects of AA transporters on the enterocytes.

Carbohydrates

• Pancreatic a-amylase is normally secreted in excess and carbohydrate digestion is usually preserved partially, but severe pancreatic insufficiency can result in clinically apparent carbohydrate malabsorption and diarrhea.
• Late-onset lactose malabsorption is the most common cause of carbohydrate malabsorption due to decreased levels of the intestinal brush border enzyme lactase
• Acquired malabsorption of carbohydrates typically transpires after extensive intestinal resections, in diffuse mucosal diseases, or postinfection, temporarily.

Vitamins

• Fat-soluble vitamins become commonly malabsorbed through diseases that cause malabsorption of dietary fat as fat-soluble vitamins and dietary fat have comparable absorptive mechanisms
• Impaired micelle formation because of bile salt deficiency also leads to malabsorption of fat-soluble vitamins.
• Vitamin B12 malabsorption with clinical consequences, such as pernicious anemia, happens stemming from the deficiency of gastric IF secretion

Minerals

• Severe calcium malabsorption transpires among the ailments impacting the intestinal mucosa leading to a reduction of the intestinal surface area
• Diseases with malabsorption and deficiency in vitamin D may contribute to an intestinal calcium malabsorption as with fat malabsorption

Mechanisms that Compensate for Malabsorption

• The colon has the capacity to absorb substances and nutrients, yet its nutritive role in patients with malabsorption is clinically relevant as deficiencies and other adverse effects rise
• The colon's ability to salvage malabsorbed nutrients results in complications like the formation of renal stones with the hyperabsorption of oxalate

Role of the Colon

• Carbohydrates that arrive in the colon cannot be absorbed by it, but colonic bacteria can metabolize them breaking down oligosaccharides and polysaccharides to mono- and disaccharides
• In short bowel syndrome, colonic salvage of malabsorbed carbohydrates can save up to 700-950 kcal/day, given continuity with the small intestine

Clinical Features and Evaluation

• Suspect malabsorption generally on the basis of the patient's medical record, symptoms or signs shown such as abnormal lab results
• Find malabsorption source through measuring amplified stool concentration or its urinary excretion.

Signs and Symptoms

• Classic symptoms of malabsorption including malabsorption can have other causes and shouldn't be assumed as confirmation
• Anemia and more extraintestinal problems are now being discovered from the current obesity picture and the increase of patients being overweight
• The current blood tests for abnormalities are not considered highly specific

Diagnostic Approach

• Reviewing the patient's exposure and laboratory results provides possible diseases
• The location and source determines the approach tests and procedures such as colonoscopy and x-ray

Anatomic Investigations

• Endoscopic inspection of the duodenal mucosa provides clues to a few causes of malabsorption such as the number of duodenal folds
• Radiologic imaging and CT scans are useful as imaging small intestine

Maldigestion involves impaired nutrient hydrolysis, while malabsorption is defective mucosal absorption. Both conditions share similar clinical presentations and complications. Malabsorption can stem from diseases affecting the small intestine, pancreas, liver, biliary tract, or stomach, impacting nutrient uptake.