BIOCHEM 3.7 - DIGESTION & ABSORPTION

• Digestion, digestive enzymes and absorption are the key topics
• These notes are section 3, lecture 6

Learning Objectives

• Define key terms: ingestion, digestion, absorption, maldigestion, malabsorption, and micronutrients
• Summarize the mechanisms involved in digestion and nutrient absorption
• Identify and explain the activation of enzymes involved in digestion and absorption
• Compare absorptive, fasting, and starved states
• Identify changes in energy sources and metabolism in various organs (liver, muscle, brain, adipose, RBC, kidney) in fed, fasting, and starved states
• Assess hormone expression patterns and metabolic behavior in different states
• Compare carbohydrate, protein, and fat digestion and absorption
• Assess the causes and consequences of altered digestion or absorption, focusing on enzymes
• Link altered enzymes to observed changes or identify consequences

Case Study 11

• A 6-week-old male with chronic diarrhea and failure to thrive
• Upper gastrointestinal endoscopy showed a straight duodenum
• Small bowel biopsies were histologically normal
• Laparotomy indicated a bowel length of 50 cm

Terminology

• Digestion is the breakdown of food into absorbable products
• Absorption is the uptake of absorbable products
• Maldigestion is impaired breakdown of food
• Malabsorption is defective uptake and transport of digested nutrients
• Micronutrients are vitamins and minerals
• Ingestion of food leads to digestion and absorption
• Some food is oxidized for energy; excess food is stored

Digestion and Absorption

• Food breakdown occurs in the stomach
• Absorption occurs throughout the intestines
• Salivary glands, stomach, and pancreas secrete digestive enzymes (inactive zymogens)
• Hydrogen ions are secreted in the stomach
• Bicarbonate ion secreted throughout the GI tract, concentrated in pancreatic juice
• Aldosterone regulates potassium secretion in the colon

Digestive Enzymes

• Most zymogen activation occurs in the gut lumen
• Digestive enzymes are hydrolases

Basal Metabolism

• Declining glucose levels characterizes this state
• Insulin decreases while glucagon increases
• Liver maintains blood glucose
• Adipose triacylglycerols are the major energy source, involving lipolysis

Absorptive State

• Absorption is underway until complete
• Macronutrients are oxidized for energy
• Regulation is determined with oxidation of fuel or storage for further use with endocrine hormones (insulin increases, glucagon is inhibited)

Short Term Fasting

• This state is similar to the Basal State
• Starts about 2-4 hours after a meal
• Blood glucose declines to basal levels
• State ends as blood glucose rises
• Reductions in insulin and increases in glucagon levels occur
• Blood glucose is sustained by the liver
• Adipose triacylglycerols are a major energy source during fasting and lipolysis occur

Prolonged Fasting

• Occurs after 3-5 days, considered the starved state
• Adipose tissue breaking down provides the fatty acid fuel
• Overall glucose needs reduce
• Brain utilizes ketone bodies
• Glucose remains needed for neurotransmitters
• RBCs only uses glucose if there are no mitochondria
• Muscle decreases the body's utilization of ketone bodies, and rely mostly on fatty acids
• Liver glycogen storage depletes and the liver produces amino acids for glucose

Digestion and Absorption of Lipids

Digestion

• is complex due to their insolubility in water, requiring emulsification
• Pancreatic lipase in the intestinal lumen converts triacylglycerols into fatty acids and 2-monoacylglycerols
• Bile salts are synthesized by the liver, stored in the gall bladder and released into the intestine
• Bile interacts with fatty acids and 2-monoacylglycerols to form micelles

Absorption

• Intestinal epithelial cells absorb micelles and then create triacylglycerols
• These compounds mix with phospholipids, cholesterol, and proteins to create chylomicrons, which are transported to the blood via lymph

Digestion and Absorption of Carbohydrates

Digestion

• Conversion into monomers/Monosaccharides
• Starch is major diet component
• Start with salivary amylase
• Pancreatic amylase further digests
• Intestinal epithelial cells produce enzymes to break down oligosaccharides, disaccharides and trisaccharides
  • Sucrase converts sucrose to glucose and fructose
  • Lactase converts lactose to glucose and galactose

Absorption

• Absorbed by intestinal epithelial cells and released into hepatic portal vein for transport to liver

Digestion and Absorption of Proteins

Digestion

• Breakdown of peptide bonds via proteinases
  • Stomach: Pepsin
  • Small Intestine (pancreatic enzymes): trypsin, chymotrypsin, elastase, & carboxypeptidase
• Intestinal epithelial cells generate aminopeptidases, carboxypeptidases, dipeptidases, and tri-peptidases

Absorption

• Absorbed by intestinal epithelial and released into hepatic portal vein

Case Study 11

• A 6-week-old male with chronic diarrhea and failure to thrive is indicative of Congenital Short Bowel Syndrome
• Historically high mortality rate for this condition
• Parenteral (intravenous) specialized foods provide nutrients
• Enteral (feeding tube) feeding brings nutrition directly into the stomach or small bowel using amino acid based formula that contains long-chain fatty acids

Case Study 12

• A 33 year old male demonstrates Re-feeding Syndrome from previous bowel surgery
• First identified in post-World War II
• Characterized with fluid/electrolyte uptake and disturbance, increased uptake of nutrients, and nutrient depleting
• High mortality rate requires proper patient assessment

Case Study 13

• A 15 year old with acquired lactose deficiency after complaining about bloating, abdominal pain and diarrhea
• Deficiencies develop from time and repeated exposures
• Evaluation occurs similarly to glucose tolerance tests with lactose intake and plasma analysis