GESH220: Digestive System

• The digestive system is covered in this study note
• Course code is GESH220
• The instructor for this module is Joelle Dib

What is Digestion?

• It involves a process where food must be digested and absorbed for it to become part of the body
• The phrases of digestion are:
  • Ingestion
  • Movement (peristalsis)
  • Mechanical and Chemical Digestion
  • Absorption
  • Elimination/excretion

Digestion vs Absorption

• Digestion is the body's way of breaking down foods into nutrients in preparation for absorption
• Digestion can either be mechanical or chemical
• Absorption involves passage of nutrients from the GI (Gastrointestinal) tract into either the blood or lymph

Mechanical vs Chemical Digestion

• Mechanical (physical) digestion occurs with these steps:
  • Chew
  • Tear
  • Grind
  • Mash
  • Mix
• Chemical (Enzymatic) digestion involves enzymatic reactions to improve digestion of:
  • Carbohydrates
  • Proteins
  • Lipids

Digestive System Organization

• The gastrointestinal (GI) tract is the pathway food takes from the mouth to anus
• Propulsion involves swallowing and peristalsis in the esophagus, stomach, small intestine, and large intestine
• Mechanical digestion involves chewing in the mouth, churning in the stomach, and segmentation in the small intestine
• Chemical digestion reduces food

Mouth

• Teeth mechanically break down food into small pieces
• Saliva is released from salivary glands
• Saliva contains salivary amylase, which converts starch into simple sugars like glucose & maltose
• Saliva softens food to ease swallowing
• The tongue mixes food with saliva to form a bolus
• The tongue contains taste buds for tasting sweet, sour, salty, and bitter flavors

Esophagus

• It's approximately 20 cm long and connects the mouth to the stomach
• The functions of the esophagus include secreting mucus and moving food from the throat to the stomach using muscle movement (peristalsis)
• Heartburn occurs if acid from the stomach goes back to the esophagus

Stomach

• It is a J-shaped muscular bag that receives food from the esophagus
• Acid (HCl) in the stomach kills microorganisms such as bacteria
• The stomach releases mucus to protect its surface from acid
• It mixes food with digestive/gastric juices that contain enzymes to break down only proteins & lipids
• Food mixed with juices creates a semiliquid mass called chyme
• It takes around 4 hours for the stomach to digest food, depending on the type of food, and then empties the chyme into the small intestine
• The stomach does NOT absorb nutrients, it only absorbs some water & alcohol

Small Intestine

• It's the primary site for digestion & absorption
• It is approximately 6-7 meters in length
• Peristalsis moves the chyme
• Accessory organs (pancreas, liver, gallbladder) secretions complete the chemical breakdown of food
• Nutrients must be in their simplest form to be absorbed
• Carbohydrates break down to become simple sugars
• Proteins break down to become amino acids
• Fats break down to become fatty acids & glycerol
• Villi are fingerlike protrusions that cover the surface of the small intestine and absorb food
• Absorption surface area increases due to the villi
• Microvilli further increase surface area for absorption
• Once food particles enter the capillaries in the villi, they are carried to the liver through the bloodstream or lymphatic system
• Water, vitamins, and minerals are absorbed
• Water soluble components go to the blood, fat soluble to the lymph
• Carbohydrates and proteins are absorbed in simplest form
• Lipids are absorbed in the simplest form and transferred to the lymph

Absorption

• Most absorption occurs in the small intestine; some occurs in the large intestine
• Water is absorbed in the stomach, small intestine, and mainly in the large intestine

Accessory Organs

• These are not part of the path of food, but play a critical role
• These include the liver, gallbladder and pancreas

Liver

• It filters out toxins and waste, including drugs, alcohol, and poisons
• The production of bile helps digest and emulsifies fat for enzyme to break it down

Gallbladder

• Stores bile from the liver and releases it into the small intestine
• Rapid weight loss and fatty diets can cause gallstones

Pancreas

• It produces digestive enzymes to digest fats, carbohydrates, and proteins
• Bicarbonate is released into the intestine to neutralize the acid pH of the chyme secreted by the stomach
• Blood sugar is regulated via hormone insulin production

Large Intestine

• It receives food from the small intestine
• Stores and concentrates undigested food
• Little absorption and no digestive enzymes exist
• There are no villi
• About 1.5 meters long
• Short-term storage via rectum which holds feces/stools before they are expelled
• Absorbs more water and concentrates waste to form feces
• Large intestine bacteria ferment undigested carbohydrates (soluble fibers)
• Some B vitamins and vitamin K are synthesized by bacteria

Elimination

• Harmless bacteria in the large intestine change the consistency of undigested food into a semisolid waste, called feces
• The expulsion of undigested food and body wastes
• Feces passes from the body through the anus
• Termed "bowel movement"

Conclusion

• It's important to break down food through mechanical and chemical digestion into nutrients that can be absorbed
• Enzymes act to break down nutrients into their simplest & smallest absorbable form
• Absorption predominantly occurs in the small intestine

Myth vs Reality: Food Combining

• Myth: Certain food combinations cause poor digestion, health problems
• Reality: The body has a complex system for digesting and absorbing a variety of foods
• Reality: Eating fruit and meat together aids absorption of components found in each (Vit C and iron)
• Reality: There is no physiological effect of food combining

Prebiotics and Probiotics

• Probiotics are healthy "good" bacteria that naturally live in our colon
• Living bacteria naturally found in certain foods (i.e., yogurt, laban) or added to some processed foods to increase or replenish the colonic microflora
• Linked to specific health benefits such as improved digestion, decreased bloating, and better immunity
• Prebiotics are non-digestible food ingredients that favor the growth of beneficial bacteria
• Prebiotics are mainly dietary fibers
• Prebiotics act as "food" for probiotics
• Probiotics may alleviate lactose intolerance, enhance immune function, protect against GI cancer, lower blood cholesterol and treat diarrhea

Food Sources of Probiotics

• Fermented dairy foods: yogurt, kefir products, and aged cheeses (Cheddar, Gruyere, Manchego, Gouda and Parmesan) are good sources
• Non-dairy foods: fermented cabbage (sauerkraut), miso (Japanese seasoning produced by fermenting soybeans), tempeh (fermented soybean), and cultured non-dairy yogurts are other sources

Types of Prebiotics

• Inulin is the most commonly used prebiotic
• It belongs to a class of dietary fibers known as fructo-oligosaccharides (FOS)
• Inulin is naturally found in chicory (hindbeh), artichoke, garlic, onions, bananas & whole grains (i.e. barley, rye)
• It's increasingly added to processed foods for its renowned health benefits

Prebiotics and Probiotics

• When probiotics ("good" bacteria) and prebiotics (good bacteria promoters) are combined, they form a symbiotic.
• These substances work even better synergistically than separately.