The Small Intestine: The Core of Digestion and Nutrient Absorption

The small intestine is the body’s central site for nutrient extraction. Stretching over six meters, its folded lining is covered with villi and microvilli, creating a massive surface area for absorbing nutrients into the bloodstream or lymph. This stage represents the climax of digestion—where macronutrients are broken down into absorbable units and vitamins and minerals are selectively taken up.

Normal Function of the Small Intestine

1. Duodenum
   • Receives chyme (acidic mixture from the stomach), bile (from the liver and gallbladder), and pancreatic enzymes.
   • Bile salts emulsify fats into tiny droplets, allowing enzymes to act more efficiently.
   • Pancreatic enzymes:
     • Amylase → breaks starch into maltose.
     • Lipase → breaks triglycerides into glycerol + fatty acids.
     • Proteases (trypsin, chymotrypsin, carboxypeptidase) → break proteins into amino acids.
   • Bicarbonate neutralizes stomach acid, creating the optimal pH for enzymes to function.
2. Jejunum & Ileum
   • Main site of nutrient absorption:
     • Carbohydrates, amino acids, water-soluble vitamins, and minerals → enter bloodstream via the portal vein to the liver.
     • Fats and fat-soluble vitamins (A, D, E, K) → absorbed into the lymphatic system as chylomicrons.

Abnormalities in Pancreatic Enzyme Production

If the pancreas fails to deliver enzymes or bicarbonate:

• Amylase deficiency → poor starch digestion → bloating, diarrhea, erratic blood glucose.
• Lipase deficiency → fat maldigestion → greasy, floating stools (steatorrhea) and fat-soluble vitamin deficiencies (A, D, E, K).
• Protease deficiency → incomplete protein breakdown → muscle wasting, poor healing, food sensitivities.

These conditions occur in chronic pancreatitis, cystic fibrosis, pancreatic cancer, or after surgical removal of pancreatic tissue.

Impact of Problems in the Previous Stage (Stomach)

If the stomach fails to prepare chyme correctly, the small intestine is immediately disadvantaged:

• Low stomach acid (hypochlorhydria) → weak signal for pancreatic enzyme and bile release.
• Poor pepsin activity → large protein fragments enter the duodenum, straining pancreatic proteases.
• Inadequately acidified chyme → disrupts bile and enzyme activation.

This leads to delayed digestion, incomplete breakdown of nutrients, and potential for malabsorption downstream.

Small Intestine Disorders Affecting Digestion

• Celiac disease: Autoimmune destruction of villi → severe malabsorption.
• Crohn’s disease: Chronic inflammation → narrowed intestine, nutrient loss.
• Infections (e.g., Giardia): Damage villi, causing diarrhea and nutrient depletion.
• Lactase deficiency: Inability to digest lactose → gas, bloating, diarrhea after dairy.

Consequences for Nutrient Digestion and Absorption

• Carbohydrates: Without amylase or brush-border enzymes, undigested carbs ferment → gas, bloating, diarrhea. Blood glucose may fluctuate unpredictably.
• Proteins: Without proteases, amino acid absorption declines → fatigue, weakened immunity, poor tissue repair. Larger peptides may provoke food sensitivities.
• Fats: Without bile or lipase, fats are excreted in stool → weight loss, energy deficits, deficiencies in fat-soluble vitamins (vision, bone health, clotting, nerve function).
• Vitamins & Minerals:
  • Fat-soluble vitamins (A, D, E, K) require fat digestion.
  • Minerals like calcium, iron, magnesium, zinc rely on stomach acid and healthy villi for absorption.

How It Affects the Next Stage

If nutrients are not properly digested or absorbed:

• The bloodstream and liver receive fewer building blocks (glucose, amino acids, fatty acids).
• The body compensates by breaking down glycogen, fat, and muscle tissue, leading to malnutrition and weakness.
• Hormone and immune systems suffer, as they depend on proteins, fats, and micronutrients.
• Undigested residues and excess water loss → diarrhea, dehydration, and electrolyte imbalance.

Summary

The small intestine is the engine of nutrient absorption, dependent on the stomach’s proper preparation of chyme, pancreatic enzymes, bile salts, and intact villi. If any of these are impaired, carbohydrates may ferment, proteins remain partially digested, fats pass into stool, and vitamins/minerals are lost. The result is malabsorption, nutrient deficiencies, and systemic consequences affecting every organ.

References

• Keller, J., & Layer, P. (2005). Human pancreatic exocrine response to nutrients in health and disease. Gut, 54(Suppl 6), vi1–vi28.
• DiMagno, E. P., Go, V. L. W., & Summerskill, W. H. J. (1973). Relations between pancreatic enzyme outputs and malabsorption in severe pancreatic insufficiency. New England Journal of Medicine, 288(16), 813–815.
• Fasano, A. (2005). Clinical presentation of celiac disease in the pediatric population. Gastroenterology, 128(4 Suppl 1), S68–S73.
• Camilleri, M. (2019). Functional gastrointestinal disorders and intestinal motility. New England Journal of Medicine, 381(11), 1068–1077.