Digestion and Nutrient Absorption

• Digestion transforms food into basic nutrients for absorption and use by the body, fueling cellular growth, fighting infections, and providing energy.

Nervous System Involvement Before Food Intake

• Cognition, sound, odor, appearance, and taste all stimulate the hunger response and influence taste.

Breakdown of Food in the Mouth

• Begins in the mouth with chewing (mechanical).
• Involuntary muscle contractions, known as peristalsis, are used to move food mixtures along the digestive tract.
• Enzymes, acid, bile, and mucus chemically break down food.
• Enzymes are proteins that catalyze chemical reactions, breaking down food particles into smaller parts, preparing the nutrient for absorption.

Absorption

• Nutrient absorption happens across the wall of the gastrointestinal (GI) tract.
• The mucosa (intestinal wall) is the innermost layer, made of absorptive cells and glands.
• Circular and longitudinal muscles comprise the outer layers, mixing and moving food along the GI tract.
• Sphincters control the flow of food particles between different parts of the tract.

Nutrient Movement Processes

• Passive diffusion involves substances moving in and out of cells without energy, from high to low concentration.
• Facilitated diffusion needs a protein carrier but no energy, moving nutrients from high to low concentration.
• Active transport requires energy to move substances in or out of the cell, from low to high concentration.

Anatomy and Physiology of the GI Tract

• The GI tract includes the mouth, esophagus, stomach, small intestine, large intestine, and rectum.
• Accessory organs aiding digestion, but not part of the GI tract, are the salivary glands, liver, gallbladder, and pancreas.

Mouth

• The mouth alters food particles for swallowing through mechanical and chemical digestion.
• Mechanical digestion occurs as the teeth break food into smaller particles.
• Salivary glands secrete saliva containing mucus and enzymes, like salivary amylase (breaks down starches) and lingual lipase (breaks down fats).
• The tongue enables taste perception of sweet, salty, sour, bitter, and umami flavors.

Esophagus

• The esophagus transports food to the stomach.
• The epiglottis prevents food from entering the trachea during swallowing.
• Peristalsis propels food forward.
• The sphincter muscle prevents backflow from the stomach into the esophagus.

Stomach

• The stomach stores, mixes, dissolves, and continues the digestion of food.
• Food particles mix with gastric juice and enzymes.
• Acid in gastric juices prepares proteins for digestion and activates enzymes.
• Pepsin breaks down protein, and gastric lipase begins fat digestion.
• Chyme, the resulting substance, empties into the small intestine over 1-4 hours.
• Gastrin controls stomach acid concentration.
• Mucus protects the stomach wall from acidity.
• Intrinsic factor, a glycoprotein is produced in the stomach and is vital for vitamin B12 absorption.
• The pyloric sphincter controls the release of chyme into the small intestine.

Small Intestine

• The small intestine, about twenty feet long, connects the stomach to the large intestine, responsible for most digestion and absorption.
• The pyloric sphincter connects the stomach to the small intestine and is major for controlling chyme release.
• Composed of the duodenum, jejunum, and ileum.
• The mucosa is folded with villi that trap food for digestion and absorption, expanding the surface area.
• Fat-soluble nutrients absorb into the lymph system, while others absorb into the bloodstream.
• The liver secretes bile, stored in the gallbladder. The gallbladder releases bile into the duodenum to break down fats once it receives hormonal signals triggered by the presence of fat in the small intestine.
• The pancreas releases pancreatic juice (water, bicarbonate, enzymes) to break down carbohydrates, protein, and fats.
• Bicarbonate in pancreatic juice neutralizes the high level of acid to protect the small intestine.
• Glucagon and insulin, hormones also secreted by the pancreas, help with glucose regulation.
• The ileocecal sphincter connects the small intestine to the large intestine, preventing backflow.

Large Intestine

• The large intestine (colon) is the last section of the digestive tract, about five feet long.
• Divided into the ascending, transverse, descending, and sigmoid colon.
• It absorbs remaining water and certain minerals.
• Feces is produced and stored in the rectum until expulsion.
• Mucus-producing cells hold feces together and protect the intestine from bacteria.
• Bacteria break down remaining food products and some fiber.
• Bifidobacteria and lactobacilli are beneficial bacteria.
• Probiotics are foods containing these bacteria such as fermented milk and yogurt.
• Prebiotics stimulate bacterial growth in the large intestine.

Digestive Disorders

• Heartburn or acid reflux occurs when stomach acid comes up the esophagus.
• GERD (gastroesophageal reflux disease) is frequent heartburn due to a weak esophageal sphincter.
• Dietary treatments include smaller, low-fat meals, chewing, avoiding overeating, limiting certain substances, and elevating the head during sleep.
• Risk factors: smoking, large or late-night meals, alcohol, coffee, soda, sleeping soon after meals, trigger foods, and medications.
• Constipation is infrequent, hard, dry stools due to slow movement through the large intestine.
• Alleviation could include dietary changes like eating high fiber foods and increasing liquids.
• Diarrhea is frequent, loose, watery stools, often from infection or antibiotics.
• Treatment involves increased fluid consumption and a low-fiber diet as tolerated.
• Diverticulosis: small pouches push out through weak spots in the colon.
• Diverticulitis caused by trapped food particles leading to inflammation or infection, due to low-fiber diet and constipation.
• Excess body weight or inactivity is also a risk factor
• Irritable bowel syndrome (IBS) includes bloating, abdominal pain, cramps, diarrhea and/or constipation.
• Aggravated by gas-producing foods, milk products, alcohol, caffeine, and fat.
• Treatment includes small, low-fat meals, high fiber intake, probiotics, and avoiding trigger foods.
• Ulcers (peptic ulcers) are sores in the stomach, lower esophagus, or small intestine.
• Caused by bacteria or medications (NSAIDs) eroding mucus lining as well as stress, smoking, and genetics.
• Symptoms include burning stomach pain, weight loss, nausea, vomiting, and loss of appetite.
• Treatment depends on lifestyle.

Macronutrients: Carbohydrates

• CHO is composed of carbon, hydrogen, and oxygen, providing 4 kcal/gram.
• Major energy source for cells (brain, nervous system, red blood cells).
• Available as blood glucose or glycogen (storage form) in muscle or liver.

Classification of Carbohydrates

• Monosaccharides (simple sugars): glucose, fructose, galactose.
  • Glucose (blood sugar): from breakdown of starches and sugars.
  • Fructose (fruit sugar): sweetest tasting.
  • Galactose: bound to glucose, primary sugar in milk.
• Disaccharides: two monosaccharide units joined together.
  • Sucrose (glucose + fructose): table sugar, honey, maple syrup.
  • Lactose (glucose + galactose): milk sugar.
  • Maltose (glucose + glucose): rarely found in food, used in the liquor industry.
• Complex carbohydrates (polysaccharides): many glucose units joined together.
  • Starch: plants store CHO as starch (vegetables, beans, pasta, rice).
  • Glycogen: storage form of glucose in liver or muscle.
  • Fiber: indigestible complex CHO
    • Insoluble (non-fermentable): increases stool mass;
    • Soluble (viscous): slows glucose absorption, lowers cholesterol, provides satiety;
    • Functional: added to products, promotes intestinal health.

Sources of Carbohydrates

• Fruits, vegetables, breads, pasta, and dairy products naturally contain carbohydrates and are sources of nutrients.
• Honey, jams, jelly, table sugar, soda, and desserts are sources of carbohydrates but low in nutritional value.
• High-starch foods are often good sources of fiber (whole grains, fruits, vegetables).

Carbohydrate Digestion

• Salivary amylase begins starch breakdown in the mouth.
• The acidic environment in the stomach deactivates amylase.
• Most digestion occurs in the small intestine.
• Pancreatic amylase breaks down polysaccharides into mono- and disaccharides.
• Small intestine enzymes break down disaccharides to monosaccharides:
  • Maltase digests maltose into two glucose units;
  • Sucrase digests sucrose into glucose and fructose;
  • Lactase digests lactose into glucose and galactose.
• Lactose intolerance is the inability to produce lactase.

Absorption of Carbohydrates

• Most monosaccharides are actively absorbed with a carrier protein and energy.
• Fructose uses facilitated absorption (carrier protein but no energy).
• Monosaccharides are transported to the liver, released as glucose into the bloodstream, stored as glycogen, or used for fat production.
• Fat production occurs with excess calories and carbohydrates.

Quiz Answers

• Monosaccharides commonly found in foods: glucose, fructose, galactose.
• Primary sugar in milk: Lactose is the primary sugar in milk, composed of galactose and glucose.
• Difference between polysaccharides and monosaccharides: A polysaccharide has many glucose units joined together, a disaccharide has two monosaccharide units joined together.
• Difference between fiber and starch: Both are complex carbohydrates but fiber cannot be digested.
• Enzymes that breakdown disaccharides into monosaccharides: maltase, sucrase, lactase. If an inadequate amount of carbohydrates are consumed the body will instead begin to convert protein derived from muscle into glucose for fuel which is known as gluconeogenesis

Carbohydrate Functions

• Main function: forms glucose to provide calories for fuel.
• Muscles and red blood cells rely on glucose.
• The brain and CNS can use ketones if glucose is not available.
• Secondary function: preserves protein (protein sparing) and prevents ketosis.
• Proteins used for building and maintaining muscles or organs, -If carbohydrates are lacking, the body converts protein from muscle into glucose.
• Prevent gluconeogenesis by consuming 45-65% of calorie intake from carbohydrates (vegetables, fruits, whole grains, seeds, nuts, beans).

Carbohydrates and Your Health

• Glucose regulation is important.
• Blood glucose levels should be between 70-100 mg/dl.
• Regulated by insulin and glucagon.
• As levels rise, the pancreas produces and releases insulin causing the liver cells to take up glucose from the blood and store it as glycogen muscles, fat and various cells absorb the glucose into their cells.
• High blood sugar levels are referred to as hyperglycemia and blood sugar levels return to normal range.
• As levels decline glucagon is released from the pancreas, happens when someone doesn’t eat for several hours and blood sugar level are referred to as hypoglycemia
• Glucagon converts glycogen stored in the liver as glucose to raise blood sugar levels.
• An imbalance of hormones (insulin and glucagon) in regulation of blood sugar levels would be loss.

Imbalance of Hormones

• Causing diseases such as Diabetes, types 1 and 2 where blood sugar levels are high, is a result of a lack of insulin leading to excessive thirst/urination/hunger, weight loss, fatigue, ketosis.
• Type 1: pancreas does not make insulin, genetic link, diagnosed early in life: Insulin therapy, diet changes, and exercise required for treatment.
• Type 2: linked with being overweight : The Pancreas makes some insulin but the body is resistant or does not produce enough; Diet changes, exercise, and oral medications can treat type 2 diabetes; A diet high in soluble fiber can also help with blood sugar control as well as better weight control, lower cholesterol levels and improved bowel health.

High levels of refined Carbohydrates and sugars

• Provide calories but no nutritive value, can cause Obesity, type 2 diabetes and dental caries.
• Diagnosis can be syndrome X if exhibiting at least three of :Obesity, Poor glucose control, Hypertension, Increased blood triglycerides, Low HDL cholesterol levels.

Macronutrients: Fats

• Lipids (fats and oils): composed of fatty acids, providing 9 kcal/gram.
• Lipids do not easily dissolve in water.
• Three primary lipids: triglycerides, phospholipids, sterols.

Fat Composition

• Triglycerides, the major form of fats, are composed of fatty acids.
• Fatty acids: chains of carbons linked together surrounded by hydrogen, with an acid group on one end and a methyl group on the other.
• Fats can be divided as saturated (no double bonds) or unsaturated ( have at least one double bond) - dependent on the bonds that link the carbon units together .
• Saturated fatty acids: all single carbon-carbon bonds, carbons saturated with hydrogens.
• Unsaturated fatty acids: have at least one double bond with classification of: (1)mono -unsaturated (3)poly -unsaturated (4)Trans-unsaturated fatty acids - Monounsaturated fatty acids contain a single double bond between carbon atoms. - Polyunsaturated fatty acids: two or more carbon-carbon double bonds like corn, soybean and sunflower oil. - Omega 3 (alpha-linolenic acid): the first carbon-carbon double bond in the fatty acids is located on the third carbon from the left side or the methyl end of the chain. - Omega 6 essential fatty acids: first double bond is on the sixth C from the methyl end Classified as linoleic acid - Essential fatty acids cannot be made by the body and must be supplied by food such as Soybean, canola oil, walnuts, flaxseed oil, salmon, tuna, and mackerel for omega 3 and vegetable oils for omega 6
• Trans unsaturated fatty acids: found in stick margarine, shortening and deep-fried foods, fat that is partially hydrogenated unsaturated fat that is largely a byproduct of the food industry
• Triglycerides: three fatty acids bonded with glycerol - a main role is source of energy, insulation and protection, carrier of fat-soluble vitamins, improve food taste
• Phospholipids: - important in the formation of cell membranes, used to transport lipoproteins and act as emulsifiers. They consist of two major components: 1. A phosphate group or the head group 2. Two fatty acid chains or the tail group are linked together by a molecule of glycerol and can be found in egg yolk, peanuts, liver and soybeans
• Sterols: - include hormones and(steroids) and cholesterol, has different structure than triglycerides or phospholipids, no glycerol backbones - Cholesterol found in cell membranes, needed for hormones and bile acids.

Digestion and Absorption of Lipids.

• Lipase produced by salivary glands and stomach begin initial digestion.
• The pancreas secretes further lipase.
• Triglycerides are broken down into monoglycerides and fatty acids.
• Lipoproteins are required in order for fats to travel through blood and lymph to get to various cells within the body,

Fat Functions

• The are four Lipoproteins = lipids surrounded by a shell of protein, phospholipids and cholesterol. This coating allows the fats to be transported in watery substances, such as blood to tissue and cells, such as:
• Chylomicrons: for transport dietary fat from the intestine to the cells and dietary cholesterol to the liver. They are a combination of: - Triglyceride, come cholesterol and fat soluble vitamins made in the small intestine - Deliver dietary fat from the small intestine and Bring dietary cholesterol to the liver
• Very low denisty lipoproteins: delivers lipids to the cells - Triglyceride and cholesterol made in the liver - Transports lipid from the liver to the cells
• Low-density lipoprotein (LDL): delivers cholesterol to cells - cholesterol, and triglyceride (VLDL loses it triglyceride content) made in the blood - Transports cholesterol to cells
• High-density lipoproteins (HDL): pick up cholesterol from cells for removal via the liver - proteins made in the liver - Transports cholesterol from the liver to be eventually excreted

Fats emulsifiers

• Food manufactures also use a variety of fats as emulsifiers and to prevent certain fats from becoming rancid
• Lecithin: a phospholipid used as an emulsifier to keep fats and water from separating

Rancidity

• Occurs when double bonds in unsaturated fats break down due to light and oxygen.

Fats and your Health.

• HDL and LDL are good and bad cholesterol, because high levels of LDL can lead to plaque formation and inhibit to proper blood flow through the arteries because of atherosclerosis
• In addition, to heart disease there can also be metabolic syndrome, a cluster of symptoms that include excess abdominal fat, high blood glucose levels, high triglycerides, low HDL cholesterol, syndrome X

Lipid Consumption

• The american Heart association and food and nutrition board recommends americans consume no more than 30-35% of their calories from Fat

Review

Typical sources of Omega 6 fatty acids: vegetable oils

• converted into glucose for fuel: gluconeogenesis
• A severe deficiency is both calories and protein in children: marasmus
• Preferred energy for the brain and central nervous system: glucose
• Individual who only consumes plant foods: vegan
• Build up of plaques in the arteries: atherosclerosis
• Delivers cholesterol to cells: LDL
• When Fat is being used for energy: ketosis
• Souce of omega 3 fatty acids : salmon
• Plays a role in acid/base balance : protein

Explore the primary functions of digestion, the role of enzymes, and peristalsis. Understand nutrient absorption mechanisms like active transport and diffusion. Learn about the impact of mucus inhibition and dietary modifications for digestive health.