9.3 Absorption and assimilation of nutrients

The small intestine is a nutrient absorption powerhouse. Its unique structure, with circular folds, villi, and microvilli, maximizes surface area for efficient uptake of carbs, proteins, and fats. Specialized transporters and enzymes work together to break down and absorb these nutrients.

Absorbed nutrients then enter the bloodstream or lymphatic system. The liver plays a crucial role in processing and distributing these nutrients throughout the body. Various factors, including digestive disorders, medications, and genetics, can impact nutrient absorption and overall digestive health.

Nutrient Absorption in the Small Intestine

Anatomy and Surface Area Enhancement

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• The small intestine is the primary site of nutrient absorption, with its large surface area enhanced by circular folds, villi, and microvilli
• Circular folds (valves of Kerckring) increase the surface area by 3-fold and slow the passage of chyme
• Villi are finger-like projections that increase the surface area by 10-fold and contain capillaries and lacteals for nutrient absorption
• Microvilli are microscopic projections on the apical surface of enterocytes that increase the surface area by 20-fold, forming the brush border

Carbohydrate Absorption

• Carbohydrates are broken down into monosaccharides (glucose, fructose, galactose) by brush border enzymes (lactase, sucrase, maltase)
• Glucose and galactose are absorbed via sodium-dependent glucose transporter (SGLT1), which uses the sodium gradient established by the Na+/K+ ATPase
• Fructose is absorbed by facilitated diffusion through glucose transporter type 2 (GLUT2)
• GLUT2 also facilitates the basolateral exit of monosaccharides from enterocytes into the bloodstream

Protein Absorption

• Proteins are digested into amino acids, di-, and tripeptides by various peptidases (aminopeptidases, carboxypeptidases, endopeptidases)
• Amino acids are absorbed by specific amino acid transporters, which can be sodium-dependent or independent (e.g., neutral, basic, and acidic amino acid transporters)
• Di- and tripeptides are absorbed by the proton-coupled peptide transporter PepT1, which uses the H+ gradient established by the Na+/H+ exchanger
• Inside the enterocyte, di- and tripeptides are further hydrolyzed into amino acids by cytoplasmic peptidases

Lipid Absorption

• Lipids, primarily triglycerides, are emulsified by bile salts and hydrolyzed by pancreatic lipase into monoglycerides and free fatty acids
• These products form micelles with bile salts, which facilitate their absorption into enterocytes by passive diffusion
• Fat-soluble vitamins (A, D, E, and K) are incorporated into micelles and absorbed along with dietary lipids
• Inside the enterocyte, monoglycerides and free fatty acids are re-esterified into triglycerides and packaged into chylomicrons for lymphatic transport

Lymphatic System in Lipid Absorption

Chylomicron Formation and Exocytosis

• After absorption into enterocytes, monoglycerides and free fatty acids are re-esterified into triglycerides by the enzymes monoacylglycerol acyltransferase (MGAT) and diacylglycerol acyltransferase (DGAT)
• Triglycerides are packaged with cholesterol, phospholipids, and apolipoproteins (ApoB-48, ApoA-I, ApoA-IV) to form chylomicrons in the endoplasmic reticulum and Golgi apparatus
• Chylomicrons are exocytosed from enterocytes into the intercellular space and enter the lymphatic capillaries called lacteals

Lymphatic Transport and Circulation

• The lymphatic system, particularly the thoracic duct, transports chylomicrons and other lipoproteins from the intestines to the bloodstream, bypassing the liver
• Chylomicrons enter the bloodstream via the left subclavian vein, where they circulate and deliver triglycerides to peripheral tissues expressing lipoprotein lipase (LPL)
• LPL is anchored to the capillary endothelium by heparan sulfate proteoglycans and hydrolyzes triglycerides in chylomicrons, releasing free fatty acids for uptake by adjacent tissues (adipose, muscle)
• As triglycerides are hydrolyzed by LPL, chylomicrons shrink and become chylomicron remnants, which are enriched in cholesterol and apolipoprotein E (ApoE)
• Chylomicron remnants are taken up by the liver via the LDL receptor and LRP1 (LDL receptor-related protein 1) for further processing and recycling of components

Liver's Role in Nutrient Metabolism

Carbohydrate Metabolism

• The liver plays a central role in the metabolism of carbohydrates absorbed from the intestines
• Hepatocytes extract glucose from the blood and store it as glycogen (glycogenesis) when insulin levels are high, such as after a meal
• During fasting, the liver releases glucose through glycogenolysis (breakdown of glycogen) and gluconeogenesis (synthesis of glucose from non-carbohydrate precursors like amino acids and glycerol)
• The liver also converts excess glucose to fatty acids (de novo lipogenesis) when glycogen stores are full, contributing to the synthesis of triglycerides and VLDL particles

Protein Metabolism

• Amino acids are taken up by the liver and used for protein synthesis, including the production of albumin, clotting factors, and acute-phase proteins
• Excess amino acids are catabolized by the liver, with their carbon skeletons used for glucose production (gluconeogenesis) or oxidized for energy
• The liver converts ammonia, a byproduct of amino acid catabolism, into urea for excretion by the kidneys, preventing neurotoxicity

Lipid Metabolism

• The liver is essential for lipid metabolism, as it synthesizes and secretes lipoproteins (VLDL, HDL), cholesterol, and bile acids
• Hepatocytes package triglycerides, cholesterol, and apolipoproteins (ApoB-100, ApoE) into VLDL particles for export to peripheral tissues
• The liver also synthesizes HDL particles, which participate in reverse cholesterol transport, removing excess cholesterol from peripheral tissues and returning it to the liver for excretion or recycling
• Hepatocytes oxidize fatty acids for energy production, especially during fasting or prolonged exercise, through the process of beta-oxidation in mitochondria and peroxisomes

Vitamin and Mineral Storage

• Hepatocytes store fat-soluble vitamins (A, D, E, and K) and regulate their release into the circulation as needed
• Vitamin A (retinol) is stored in hepatic stellate cells and released bound to retinol-binding protein (RBP) for transport to target tissues
• The liver hydroxylates vitamin D to its active form, 25-hydroxyvitamin D, which is further activated in the kidneys to 1,25-dihydroxyvitamin D
• The liver stores and releases minerals such as iron (ferritin and hemosiderin) and copper (ceruloplasmin) to maintain homeostasis
• Hepatocytes synthesize transferrin, the primary iron transport protein in the blood, and hepcidin, a key regulator of iron homeostasis

Factors Affecting Nutrient Absorption

Digestive Disorders

• Digestive disorders, such as celiac disease, Crohn's disease, and inflammatory bowel disease, can impair nutrient absorption by damaging the intestinal lining or causing inflammation
• Celiac disease is an autoimmune disorder triggered by gluten, leading to villous atrophy and malabsorption of nutrients (iron, calcium, fat-soluble vitamins)
• Crohn's disease and ulcerative colitis are inflammatory bowel diseases that cause intestinal inflammation, ulceration, and scarring, disrupting nutrient absorption

Surgical Interventions

• Gastrointestinal surgery, such as bariatric surgery or resection of the small intestine, can reduce the surface area available for absorption and lead to nutrient deficiencies
• Roux-en-Y gastric bypass (RYGB) and biliopancreatic diversion (BPD) are bariatric procedures that bypass segments of the small intestine, potentially causing malabsorption of iron, calcium, and fat-soluble vitamins
• Small bowel resection, performed for conditions like Crohn's disease or intestinal tumors, can result in short bowel syndrome and malabsorption of nutrients

Microbial Imbalances

• Bacterial overgrowth in the small intestine (SIBO) can interfere with nutrient absorption and cause malabsorption
• SIBO occurs when excessive bacteria colonize the small intestine, competing for nutrients and producing metabolites that damage the intestinal lining
• Probiotics, beneficial microorganisms like Lactobacillus and Bifidobacterium, can promote nutrient absorption by maintaining a healthy gut microbiome and supporting intestinal barrier function

Medications and Substance Use

• Certain medications, such as proton pump inhibitors, antibiotics, and cholestyramine, can alter digestive processes or bind to nutrients, reducing their absorption
• Proton pump inhibitors (PPIs) reduce stomach acid production, which can impair the absorption of calcium, magnesium, and vitamin B12
• Antibiotics can disrupt the gut microbiome, leading to diarrhea and malabsorption of nutrients
• Cholestyramine, a bile acid sequestrant used to treat hypercholesterolemia, can bind to fat-soluble vitamins and reduce their absorption
• Alcohol consumption can impair the absorption and metabolism of nutrients, particularly water-soluble vitamins (B-complex and C), by damaging the intestinal lining and interfering with nutrient transport systems

Age-Related Changes

• Aging can lead to decreased production of digestive enzymes, reduced intestinal motility, and changes in the gut microbiome, all of which can affect nutrient absorption
• Atrophic gastritis, a condition common in older adults, results in decreased stomach acid production and impaired absorption of vitamin B12, calcium, and iron
• Reduced intestinal motility can lead to constipation and bacterial overgrowth, further compromising nutrient absorption

Genetic Factors

• Genetic factors, such as mutations in nutrient transporter genes or enzymes involved in nutrient metabolism, can impact an individual's ability to absorb and utilize specific nutrients
• Lactose intolerance is caused by a deficiency in the brush border enzyme lactase, leading to malabsorption of lactose and gastrointestinal symptoms
• Familial hypercholesterolemia is a genetic disorder characterized by mutations in the LDL receptor gene, impairing the liver's ability to clear LDL cholesterol from the bloodstream
• Hemochromatosis is an inherited disorder of iron metabolism, causing excessive iron absorption and accumulation in tissues, leading to organ damage