Kinesiology 380, Fall 2007, Lecture 8-9 Digestion and Absorption

Kinesiology 380, Fall 2007, Lecture 8-9 Overview of Gastrointestinal System

Kinesiology 380, Fall 2007, Lecture 8-9 Energy Storage Overview of the gastrointestinal system: GI tube is OUTSIDE the body. Goal is to extract nutrients and get them into the body while keeping the filthy mess (bacteria, viruses) out.

Kinesiology 380, Fall 2007, Lecture 8-9 In the same way that you are not actually “in” the water as you drive through the Chunnel, food is not actually “in” your body as it passes through your GI system. Just as it would not be a good thing to allow your car to cross from inside the Chunnel to swimming with the fishes in the English Channel, the body prevents passage of dangerous materials from inside your GI system to the general circulation. How your insides are actually your “outsides” England France Chunnel

Kinesiology 380, Fall 2007, Lecture 8-9 General scheme for digestion of food H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O

Kinesiology 380, Fall 2007, Lecture 8-9 In the mouth: liquification and mechanical disruption of solid food. Not really necessary - can digest food added directly to the stomach but speeds up the process. Enzyme salivary amylase begins process of digesting starches to simple sugars. In the stomach, presence of HCl lowers pH and inactivates amylase. Instead, pepsin is activated and begins protein digestion

Kinesiology 380, Fall 2007, Lecture 8-9 How are carbohydrates absorbed? Once ingested carbohydrate is digested into monosaccharides, it is transported into absorptive cells in the lining of the small intestine. The monosaccharides then exit these cells and diffuse into capillaries and enter the general circulation. Absorptive Cell Absorptive Cells Goblet Cells Lacteal Arteriole Venule Vein Artery Lymph Duct Capillaries

Kinesiology 380, Fall 2007, Lecture 8-9 Upon arrival via the portal vein non-glucose sugars are converted to glucose. Glucose from portal vein is not stored but passes through. In general circulation, most glucose is taken up and stored by insulin-sensitive tissues, mainly in skeletal muscle as glycogen. glucose  fructose  galactose  GLUCOSE general circulation

Kinesiology 380, Fall 2007, Lecture 8-9

Some glucose returns to liver via hepatic artery. Unlike glucose entering via portal vein (through liver to general circulation) glucose entering via hepatic artery is stored as glycogen. glucose to general circulation glucose GLYCOGEN portal vein hepatic artery

Kinesiology 380, Fall 2007, Lecture 8-9

Protein Digestion and Absorption

Kinesiology 380, Fall 2007, Lecture 8-9 What exactly happens to that steak you ate for dinner? In the stomach: the digestion of proteins begins in the stomach. The low pH of the stomach activates the enzyme pepsin which begins breaking down protein into smaller peptide fragments. The pepsin becomes inactive once it enters the small intestine due to the more neutral pH environment. In the small intestine: the digestion continues by other proteases such as trypsin and chymotrypsin.

Kinesiology 380, Fall 2007, Lecture 8-9 How are proteins absorbed? The absorptive cells of the small intestine can only absorb individual amino acids, dipeptides or tripeptides. Once transported inside the cells all di/tripeptides are broken down into individual amino acids by proteases within the cells. Like monosaccharides, the amino acids are transported out of the absorptive cells and diffuse into the capillaries to move into general circulation.

Kinesiology 380, Fall 2007, Lecture 8-9 Fat Digestion and Absorption

Kinesiology 380, Fall 2007, Lecture 8-9 Fat-soluble compounds are handled very differently. Key processes are: 1. emulsification of lipid droplets to keep them suspended in solution (think detergent). 2. action of colipase to drill holes into bile acid coating to allow access by: 3. Lipases, which act to digest TG ---> two FFA + mono-glyceride. 4. After absorption (diffusion?), TG is reassembled, combined with cholesterol and fat sol. vitamins, and coated with protein to form a chylomicron

Kinesiology 380, Fall 2007, Lecture 8-9 In the stomach: the digestion of most lipids does not begin until they enter the small intestine (butterfat is one of the few lipids that is broken down by an enzyme – gastric lipase - secreted by the stomach). Lipids are hydrophobic and therefore do not mix with the rest of the food beginning its journey through your digestive system. Rather, they form large droplets on the surface of the chyme (mixture of food and digestive enzymes). As the chyme leaves the stomach most ingested lipids are on the surface massed together as globules.

Kinesiology 380, Fall 2007, Lecture 8-9 A conundrum arises because the lipases are water soluble and thus can only act on the surface of the globules – therefore the lipases can not actually act on most of the lipid molecules in the globule. An easy way to visualize this is a bottle of oil and vinegar. Imagine that the enzymes that could digest the oil were dissolved in the vinegar – the only oil that could be digested would be that on the border between the oil and vinegar. Lipid “core” that is inaccessible to lipases. Lipase Globule

Kinesiology 380, Fall 2007, Lecture 8-9 In the small intestine: lipid digestion is able to occur because of the action of bile. Bile is a substance produced by the liver and stored and released from the gall bladder. Bile does not actually digest fat, rather after the churning action of the stomach breaks apart the fat globules into fat droplets, bile acts to keep the droplets from reforming into globules. This process is called emulsification. Globule Bile salt Droplets Churning

Kinesiology 380, Fall 2007, Lecture 8-9 Bile salts are responsible for emulsification of fats. They are amphipathic, meaning that one side of the molecule is hydrophilic (water-loving) and the other side is hydrophobic (water-hating) When the bile salts come into contact with a fat droplet their hydrophobic side faces inwards towards the fat and their hydrophilic side faces outwards into the water. This provides a coating around the droplets which keeps them in solution and prevents them from reaggregating into larger droplets. Hydrophilic group Hydrophobic Bile Salt

Kinesiology 380, Fall 2007, Lecture 8-9 Pancreatic lipase, an enzyme for fat digestion, is active in the duodenum of the small intestine. Lipases act on triglycerides (TG) to break the bonds that link the fatty acid chains to the glycerol backbone of the TG molecule. The result are two fatty acid chains and a monoglyceride (fatty acid + glycerol). glycerol As the globules continue to move through the small intestine they are all eventually broken down by the action of lipases before reaching the colon. The bile salts are reabsorbed in the ileum (terminal portion of small intestine) and recycled in the liver to be reused. FA

Kinesiology 380, Fall 2007, Lecture 8-9 The first step of fat absorption occurs in the small intestine. Monoglycerides and fatty acids are absorbed into intestinal cells via simple diffusion. (Lets briefly revisit HS biology – cell membranes are composed of a lipid bilayer which is hydrophobic. CHO and protein are hydrophilic and thus do not pass through the cell membrane unaided. However, lipids are hydrophobic and can pass through the cell membrane.

Kinesiology 380, Fall 2007, Lecture 8-9 Chylomicrons are too big to pass through capillary beds in the small intestine. Instead, they are secreted into the LYMPH system and go to the thoracic duct where they enter the sub-clavian vein. In arterioles, LPL binds chylomicrons and hydrolyzes TG  FFA + glycerol.FFA taken up by fat cells (muscle as well) and re-esterified to TG for storage. TG FFA action of lipoprotein lipase

Kinesiology 380, Fall 2007, Lecture 8-9 Energy Density: Person weighs 167 pounds (lb.) with 15% body fat, of which 3% is “structural” and unavailable for energy. 167 lbs. * 12% = 20 lb. fat At 3500 kcal/ lb. how much energy stored in body fat? How many pounds of glycogen would be required to store the same amount of energy? (the energy density of stored glycogen is about 600 kcal/lb). How much would the person weigh? Total energy stores: CHO:about 1500 kcal (1200 muscle liver) FAT: 50,000 to > 400,000 kcal PRO:no true storage but >10,000 kcals are “accessible”.