Digestion and Absorption

What are the GI hormones

• Gastrin

• Gastric Inhibitory Peptide (GIP)

• Vasoactive Intestinal Peptide (VIP)

Gastrin

• Primarily by stomach when full

• Increases secretion by parietal and chief cells of stomach

• Increased mixing of stomach

Gastric inhibitory peptide (GIP)

• Duodenum in presence of fats and glucose - carbohydrates

• Inhibit gastrin (likely not major effect)

• Increase secretion of insulin

Vasoactive intestinal peptide (VIP)

• Duodenum in presence of chyme

• Inhibits gastrin

• Vasodilates intestinal capillaries

  • Increases intestinal secretions

  • Increases motility

  • Overall effect: aids in digestion/absorption

Recall, secretin and CCK increase…

• Secretion of bile and pancreatic juice and inhibit gastric secretion and motility

Hormones involved in hunger

• Ghrelin

• Leptin

• Insulin

Ghrelin

• Released by stomach when empty

• Detected by hypothalamus

• Stimulates hunger

• Inhibits TSH which reduces ATP and ACTH which reduces glucose metabolism

Leptin

• Released by adipocytes

• Detected by hypothalamus

• Inhibits hunger

• Stimulates TSH and ACTH

Insulin

• Released by pancreas

• Same effect as leptin in the hypothalamus

Short term vs long term effects on hunger

• Ghrelin and insulin have short-term effects on hunger and eating

• Leptin is associated with more long-term regulation

Carbohydrate digestion

• Two step procedure:

  • Salivary and pancreatic amylase

  • Brush boarder enzymes

Salivary and pancreatic amylase of carbohydrate digestion

• Function at neutral pH

• Begins in the mouth, inactivates at stomach pH

• Begins again in duodenum

• Break down complex carbs to di and trisaccharides

Brush boarder enzymes of carbohydrate digestion

• Produce monosaccharides → hydrolysis reactions

  • Maltase - breaks down maltose

  • Sucrase - breaks down sucrose

  • Lactase - breaks down lactose

Carbohydrate absorption

• Requires carrier proteins

  • Facilitated diffusion

    • Fructose

    • 1 molecule through plasma membrane at a time

  • Co-transport

    • Glucose and galactose

    • Follows concentration gradient of Na+

• All three removed to capillaries of intestinal villi via facilitated diffusion

Lipid digestion

• Pancreatic lipase

• Bile salts

Pancreatic lipase of lipid digestion

• Unlimited activity of lingual lipase

• Functions at neutral pH

• Water-soluble enzyme

• Break down triglycerides into mono-glycerides

Bile salts of lipid digestion

• Emulsify large lipid drops into tiny droplets

• Creates surface area for lipase to work → break apart triglycerides - now there is more area to interact with the triglycerides

Lipids absorption

• Micelles: bile salts (amphipathic) - surrounding fatty acids (hydrophobic)

  • Attach to plasma membrane - fatty acids diffuse into epithelium

• ER of the epithelial cell:

  • Converts free fatty acids and monoglycerides back into triglycerides

  • Protein coat added

• Now known as chylomicrons

  • Secreted from epithelial cell

  • Move into lacteals

  • Travel in lymph to enter circulation

Protein digestion

• Mechanical digestion in mouth and acidity in stomach disrupt structure (folding) of proteins

• Stomach acid activates pepsinogen → pepsin

  • Active at pH of 1.5-2.0

  • Breaks peptide bonds in polypeptide chains

• Trypsin, chymotrypsin, elastase, carboxypepidase secreted from pancreas into duodenum (trypsin activates the others)

  • Cleave into short peptides and amino acids

• Dipeptidases in the brush border cleave short peptides into amino acids

Activation of proteolytic enzymes

• Stomach: low pH

• Small intestine: Enterokinase, an enzyme in the brush border of the small intestine catalyzes the conversion of trypsinogen to trypsin

  • Trypsin then catalyzes conversion of multiple proenzymes into active forms

Protein absorption

• Epithelium of small intestine will absorb amino acids, dipeptides, and tripeptides

• Facilitated diffusion:

  • Basic amino acids only

• Co-transport:

  • Acidic and neutral amino acid via Na+ co-transport

  • Di and tri peptides via H+ co-transport

    • Broken down into amino acids within the epithelial cell

• Amino acids removed to capillaries of intestinal villa via facilitated diffusion and co-transport (depends on the amino acid)

Water absorption

• Not actively absorbed or secreted

• Follows osmotic gradient

  • If chyme is dilute (watery) water will move across intestinal wall to interstitial fluid

  • If chyme is concentrated water will move into the lumen of the intestine

• As nutrients are absorbed in the small intestine - lowers solutes present in the lumen

• Water follows

• 90% reabsorbed in the small intestine

• 8-9% reabsorbed in the large intestine

Ions absorption

• Na+ (most abundant and the more we eat the more is absorbed)

  • Active transport, facilitated diffusion, co-transport

  • Aldosterone can increase Na+ absorption (insertion of Na+/H+ exchanger in small intestine)

• Calcium and phosphate

  • Active transport

  • Calcitriol (active vitamin D) increases Ca2+ and phosphate absorption (variety of calcium transporters and Na+/PO4- co-transporter)

• Magnesium, potassium, chloride, ion, bicarbonate

  • Facilitated diffusion/carrier proteins

Vitamin absorption

• Water soluble (B [8], and C)

  • Diffusion

  • Exception - B12, which must bind intrinsic factor and then be actively transported

• Fat soluble (A,D,E,K)

  • Sneak into micelles

  • Absorbed along with fatty acids