Digestion, Absorption, and Metabolism

• Lecture 2 focuses on digestion and absorption
• In class assignments are beginning soon, details are coming next week

Metabolism and Nutrition

• Metabolism is the sum of all chemical reactions in the body
• Digestion and absorption of food leads to a nutrient pool
• The body uses the nutrient pool for metabolism
• Anabolic activities build up growth, repair, storage, and reserves, leading to energy use
• Catabolic processes break down substances, producing energy/ATP, with 60% lost as heat and 40% as usable energy
• Anaerobic respiration (glycolysis) produces a small amount of energy quickly, without oxygen
• Aerobic respiration (citric acid/krebs cycle and electron transport chain) creates a large amount of energy, 90% of which the body uses

Fuel Sources

• Preferred fuels for the body include phosphocreatine, glucose, and fat
• Phosphocreatine provides energy for 5-10 seconds
• Glucose (glycolysis) provides energy for 10 sec - 2/3 minutes
• Fat (aerobic respiration) provides energy for 2/3 min – hours
• Protein is used during the synthesis process, but it can convert to glucose if needed

Importance of Nutrition

• Humans are continually renewing themselves
• 5% of body weight is replaced each day
• Optimal body function requires proper fuel, i.e. essential nutrients
• Food breaks down so smaller substances can be absorbed into the body and built back up as tissues, organs, and glands
• Macronutrients have energy stored as bonds, and thus, need to all be broken down and then absorbed
• Micronutrients also must be released from food and then absorbed

Digestion and Absorption

• Digestion is the mechanical and chemical process that converts food into smaller substances
• Smaller substances from digestion can be absorbed and used by the body
• Absorption is the process where nutrients and other substances are transferred from the digestive system into body fluids
• Body fluids transport nutrients
• Protein provides energy and amino acid building blocks
• Carbohydrates provide direct energy as glucose/glycogen
• Fat provides stored energy, cell membranes, and hormone synthesis
• Excess protein, carbs, or fats will become stored energy
• Vitamins are fat and water soluble and assist in regulating body processes/function
• Minerals come from the soil and are incorporated into plants, tissues of animals, and in drinking water
• Minerals assist in regulating body processes/function

Gum to Bum

• Digestion occurs in one continuous tunnel from mouth to anus; the body exists outside of this tunnel
• The digestive system is a 30-foot long muscular tube
• Major parts include the mouth, pharynx, esophagus, stomach, small and large intestine, rectum and anus
• Other supporting digestive organs include the liver, pancreas, and gallbladder, which secrete additional digestive juices

Mouth and Throat

• Includes teeth, tongue, and salivary glands
• The pharynx aids in the movement of both food and air
• Larynx protects the trachea from food aspiration
• Epiglottis seals the windpipe during eating
• Saliva contains enzymes, beginning carb and fat digestion

Esophagus

• A muscular tube, the esophagus transports food/drink to the stomach
• Smooth constrictor/sphincter muscles are present in the esophagus
• Peristalsis moves the food bolus along with relaxation and contraction

Food Bolus

• In the buccal phase, the food bolus is swallowed
• In the pharyngeal phase, the food bolus bypasses airway into the esophagus
• In the esophageal phase, peristaltic waves move the food bolus toward the stomach
• Sphincters that enter the stomach include the upper muscular sphincter and the lower (cardiac) muscular sphincter

Stomach

• The cardiac sphincter opens to the esophagus
• The pyloric sphincter opens to the small intestine
• Structures present are the greater and lesser curvatures
• Chyme is food mixed with gastric secretions (HCl, Pepsin)
• Protein digestion begins, and alcohol absorption occurs (30%)

Small Intestine

• The duodenum is the mixing bowl chyme in the small intestine that mixes with enzymes from pancreas and liver
• The jejunum is the bulk of chemical digestion, water and nutrient absorption in the small intestine
• The ileum has final nutrient absorption, fat soluble vitamins

Absorption

• Intestinal walls contain villi and circular folds to increase surface area for absorption
• The size of the intestinal wall is the size of a baseball infield
• Blood absorbs protein and carbs
• Lymph absorbs fat

Supporting Organs

• The liver is responsible for detoxification, glycogen storage and production of bile for fat digestion
• The pancreas secretes enzymes for carb, protein, fat breakdown and produces insulin for blood sugar regulation
• The gallbladder stores and secretes bile for fat digestion

Liver Diseases

• Cirrhosis of the Liver is the formation of fibrous scarring, which impacts liver function
• A Fatty Liver is a warning sign for Cirrhosis
• Jaundice (yellowish skin and eyes) results from buildup of bilirubin/red blood cell waste in skin that liver normally processes
• Alcohol abuse and genetic factors may cause Cirrhosis
• Gallstones are hardened deposits of digestive fluid
• Gallstones can be the size of a grain of sand to a golf ball
• Passing a gallstone through the ducts is painful
• Lasers can break up gallstones

Major Structures

• Large intestine contains the ascending colon, transverse colon, and descending colon
• Water and electrolyte absorption continues, concentrating feces
• Intestinal bacteria aid in digestion, otherwise little digestion occurs
• The appendix stores healthy gut bacteria
• The rectum stores waste products for elimination
• Sphincters include internal (involuntary) and external (voluntary)
• Elimination occurs by holding the rectum closed

The Digestive Process

• Salivary glands produce enzymes that break down starch and fats
• The tongue undergoes mastication and mixing of food with saliva to form bolus
• Esophagus transfers the food bolus to the stomach
• The stomach secretes enzymes that break down proteins and mixes and liquefies food (Chyme = gastric secretions + food)
• The liver produces bile that aids fat digestion
• The gallbladder stores and secretes bile that aids in fat digestion
• The pancreas secretes enzymes that break down carbs, proteins, and fats
• The small intestine is the primary site where enzymes break down carbs, proteins, and fats
• Water/electrolyte reabsorption occur in the large intestine
• The rectum stores waste products for elimination
• The anus holds rectum closed and then opens to allow elimination
• The entire digestive process takes 24 to 72 hours
• Mouth watering, chewing, and swallowing help the digestive process
• Bolus contracts and relaxes as it moves through the esophagus, entering the stomach
• Liquified contents are ejected into the pulse, roughly in the form of 1 to 2 teaspoons
• Then the bolus liquefies its contents at a time until emptied into small intestine (Chyme)
• Intestinal smooth muscle continues to contract/relax to move food along to the small intestine
• The small intestine contains intestinal walls that are lined with finger-like villi
• Majority of nutrient absorption occurs in the small intestine, the exception being 30% alcohol in the stomach
• Lymphatic/circulatory in the blood systems take up nutrients in order for absorption to continue
• Large intestines absorb water, sodium and chloride
• Large intestine compacts substances that cannot be digested/absorbed, e.g. bacteria and cellular waste, as stool

Macronutrient Absorption Rates

• 99% of absorbed carbs turn into glucose, providing direct or stored energy (glycogen or adipose)
• 95% of absorbed fats turn into fatty acids and glycerol
• 92% of absorbed proteins turn into amino acids, to provide materials for building tissue or to provide energy

Lipogenesis

• Excess energy is converted into fat (adipose) stores regardless of the source (carb, fat, or protein)
• It's regulated by insulin and stimulated by simple carbs
• Excess Calories will always be stored

Gut MicroBiome

• Gut microbiome contains beneficial bacteria/gut flora that reside in the large intestine
• This serves as a signal immune system that fights harmful bacteria
• Gut digests parts of fiber to produce energy (and gas)
• Gut produces vit K and biotin (B7)
• Antibiotics use reduces healthy bacteria

Gut as a Second Brain

• Gut is the second brain as it can regulate taste centers (salty, sweet, bitter, sour and rich/savory)
• Gut has either benefits or can be toxic
• Stress slows down digestion
• Digestion is parasympathetic

Gut Diseases

• Constipation occurs with no more than 3 bowel movements per week
• Diarrhea occurs with more than 3 bowel movements per day
• Crohn's and colitis are chronic inflammations of the digestive tract, potentially leading to ulcers and bleeding
• A symptom of irritable bowel syndrome (IBS) is an irritated large intestine