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Questions and Answers
What enzymes are responsible for hydrolyzing the disaccharides maltose, sucrose, and lactose?
What enzymes are responsible for hydrolyzing the disaccharides maltose, sucrose, and lactose?
In what part of the digestive system is most carbohydrate absorption completed?
In what part of the digestive system is most carbohydrate absorption completed?
Which type of fiber is not digested and can delay the absorption of other nutrients?
Which type of fiber is not digested and can delay the absorption of other nutrients?
What process allows glucose and galactose to traverse the cell lining of the small intestine?
What process allows glucose and galactose to traverse the cell lining of the small intestine?
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What compounds can short-chain fatty acids be converted to by liver cells?
What compounds can short-chain fatty acids be converted to by liver cells?
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What benefits does fiber provide in the digestive system?
What benefits does fiber provide in the digestive system?
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How does fructose primarily enter the bloodstream from the small intestine?
How does fructose primarily enter the bloodstream from the small intestine?
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What is one of the key functions of fiber in the large intestine?
What is one of the key functions of fiber in the large intestine?
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What are resistant starches primarily known for?
What are resistant starches primarily known for?
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Which of the following is NOT a disadvantage of increasing fiber in the diet?
Which of the following is NOT a disadvantage of increasing fiber in the diet?
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What is the primary rate at which carbohydrates provide energy?
What is the primary rate at which carbohydrates provide energy?
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Which function of carbohydrates is crucial for maintaining blood glucose levels?
Which function of carbohydrates is crucial for maintaining blood glucose levels?
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How does cellulose contribute to digestive health?
How does cellulose contribute to digestive health?
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What dietary fiber intake is considered high, exceeding 40 gm/day?
What dietary fiber intake is considered high, exceeding 40 gm/day?
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What is the consequence of the brain's lack of stored glucose?
What is the consequence of the brain's lack of stored glucose?
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Which of the following foods is likely to contain resistant starch?
Which of the following foods is likely to contain resistant starch?
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What primarily characterizes lactose intolerance?
What primarily characterizes lactose intolerance?
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Which statement correctly describes galactosemia?
Which statement correctly describes galactosemia?
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How does glucose function in carbohydrate metabolism?
How does glucose function in carbohydrate metabolism?
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What happens to excess glucose in the body?
What happens to excess glucose in the body?
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What is the primary enzyme required to break down lactose?
What is the primary enzyme required to break down lactose?
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What is the primary function of insulin in blood glucose regulation?
What is the primary function of insulin in blood glucose regulation?
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Which condition is characterized by symptoms like bloating and diarrhea?
Which condition is characterized by symptoms like bloating and diarrhea?
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What role does the liver play in glucose storage?
What role does the liver play in glucose storage?
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Which hormone is responsible for raising blood glucose levels after they fall?
Which hormone is responsible for raising blood glucose levels after they fall?
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What is the result of a condensation reaction involving glucose after a meal?
What is the result of a condensation reaction involving glucose after a meal?
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What is a common cause of Type 2 diabetes mellitus?
What is a common cause of Type 2 diabetes mellitus?
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What is the normal fasting blood glucose range?
What is the normal fasting blood glucose range?
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What condition results from an abnormally low blood glucose concentration?
What condition results from an abnormally low blood glucose concentration?
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Which hormone helps the liver convert glucose to fat?
Which hormone helps the liver convert glucose to fat?
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Which of the following is NOT a symptom of hypoglycemia?
Which of the following is NOT a symptom of hypoglycemia?
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How does epinephrine affect blood glucose levels?
How does epinephrine affect blood glucose levels?
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What is the primary function of noncellulose polysaccharides in the digestive system?
What is the primary function of noncellulose polysaccharides in the digestive system?
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Where does the final step of carbohydrate digestion occur?
Where does the final step of carbohydrate digestion occur?
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Which enzyme begins starch digestion in the mouth?
Which enzyme begins starch digestion in the mouth?
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How does chewing high-fiber food affect the digestive process?
How does chewing high-fiber food affect the digestive process?
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What occurs to starch digestion once food reaches the stomach?
What occurs to starch digestion once food reaches the stomach?
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What role does the pancreas play in carbohydrate digestion?
What role does the pancreas play in carbohydrate digestion?
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What is the effect of fiber on gastric emptying?
What is the effect of fiber on gastric emptying?
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What is the primary goal of carbohydrate digestion?
What is the primary goal of carbohydrate digestion?
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What happens to liver glycogen when blood glucose levels fall?
What happens to liver glycogen when blood glucose levels fall?
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Which part of the body relies almost exclusively on glucose for energy?
Which part of the body relies almost exclusively on glucose for energy?
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What is the primary role of carbohydrates (CHO) in relation to protein during energy metabolism?
What is the primary role of carbohydrates (CHO) in relation to protein during energy metabolism?
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How long can liver glycogen stores last without additional carbohydrate intake?
How long can liver glycogen stores last without additional carbohydrate intake?
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What is gluconeogenesis?
What is gluconeogenesis?
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What may occur if blood glucose levels fall below normal?
What may occur if blood glucose levels fall below normal?
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Which of the following correctly describes the concept of glucose homeostasis?
Which of the following correctly describes the concept of glucose homeostasis?
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Which is a consequence of treating fluctuations in blood glucose levels inadequately?
Which is a consequence of treating fluctuations in blood glucose levels inadequately?
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Study Notes
Introduction to Human Nutrition
- Human nutrition is the study of nutrients in food and how the body uses them.
- Food provides the energy and essential nutrients needed for growth, development, and overall health.
Total Fibers
- Total fibers can be the sum of dietary fibers and functional fibers.
- Dietary fibers are fibers that occur naturally in plants.
- Functional fibers are fibers that have been extracted from plants or manufactured and then added to foods or used in supplements.
Resistant Starches
- Some starches are classified as dietary fibers, known as resistant starches.
- These starches escape digestion and absorption in the small intestine.
- Starch resistance can be due to individual digestive efficiency or food properties.
- Resistant starch is found in whole legumes, raw potatoes, and unripe bananas.
High Fiber Diet
- A high fiber diet contains more than 40 grams of dietary fiber per day.
- Common food sources of fiber include fruits, vegetables, bread, and cereals.
- Disadvantages of increasing fiber intake include abdominal fullness, increased flatulence, nausea, vomiting, gas, interference with mineral absorption, and obstruction of the gastrointestinal tract.
Functions of Carbohydrates: Basic Energy Supply
- Carbohydrates are the primary energy source for the body.
- Carbohydrates release energy at a rate of 4 kcal/gram.
- Carbohydrates provide energy for physical activity and bodily functions.
Functions of Carbohydrates: Reserve Energy Supply
- The body stores carbohydrates as glycogen in the liver and muscles.
- Glycogen stores maintain a consistent blood glucose level preventing fat and protein breakdown.
- Regular consumption of carbohydrates is necessary to meet energy demands.
Functions of Carbohydrates: Special Tissue Function (Liver)
- Glycogen reserves in the liver and muscles provide constant energy balance for the body.
- Liver glycogen stores protect cells from metabolic depression and injury.
Functions of Carbohydrates: Central Nervous System
- The brain depends on a continuous supply of glucose from the blood.
- Sustained or profound blood sugar shock can result in brain damage.
Fibers: Cellulose
- Cellulose remains undigested in the gastrointestinal tract.
- Cellulose provides bulk to the diet.
- Fiber aids in moving food along the digestive system.
- Fiber stimulates normal muscle actions in the intestines and in the formation of feces to eliminate waste products.
Fibers: Noncellulose Polysaccharides
- Noncellulose polysaccharides absorb water and swell into a large mass.
- Noncellulose polysaccharides slow the emptying of food from the stomach.
- Noncellulose polysaccharides bind bile acids and cholesterol in the intestines.
- Noncellulose polysaccharides act as fermentation material for colon bacteria.
Digestion, Absorption, and Metabolism of Carbohydrates
- The goal of carbohydrate digestion is to break down sugars and starches into small molecules like glucose, for absorption and usage.
- Starch is broken down extensively.
- Disaccharides need to be broken down once.
- Monosaccharides don't need to be broken down.
- Carbohydrate splitting begins in the mouth and final splitting/absorption occurs in the small intestine.
- Glucose formation occurs in the liver.
Carbohydrate Digestion (In the Mouth)
- Mastication is the process of food crushing and grinding by teeth.
- High fiber foods are ground and chewed slowly to stimulate the flow of saliva.
- Saliva moistens food.
- Salivary amylase begins carbohydrate digestion.
- Amylase breaks down starch into smaller polysaccharides (e.g. maltose).
- Fiber is mechanically crushed and mixed with saliva for swallowing.
Carbohydrate Digestion (In the Stomach)
- Peristalsis are wave-like muscular contractions that mix food with stomach acid and protein-digesting enzymes.
- Saliva enzymes are deactivated by stomach acid stopping starch digestion.
- Stomach acids partially break down starch.
- Fiber remains indigested in the stomach and delays gastric emptying causing a feeling of fullness and satiety.
Carbohydrate Digestion (In the Small Intestine)
- Pancreatic amylase is released from the pancreas into the small intestine.
- Pancreatic amylase continues breaking down polysaccharides.
- In the duodenum, amylase breaks down smaller polysaccharides into disaccharides.
Carbohydrate Absorption
- The final step of digestion occurs on the outer membrane of intestinal cells.
- Disaccharide enzymes break down disaccharides into monosaccharides.
- Maltose is broken down by maltase into glucose + glucose
- Sucrose is broken down by sucrase into fructose + glucose
- Lactose is broken down by lactase into galactose + glucose
- Intestinal cells absorb these monosaccharides.
- Fiber is not digested, and can delay the absorption of other nutrients.
Large Intestine
- Within one to four hours after a meal, sugars and starches are primarily digested.
- Only fibers remain undigested in the digestive tract.
- Fibers in the large intestine draw in water softening the stool for easier passage.
- Bacteria in the large intestine ferment some fibers.
- Bacteria enzymes digest some fibers producing short-chain fatty acids, gas and water.
- Colon cells use these substances for energy and some energy is absorbed by the body.
- Fiber holds water, regulates intestinal activity, and binds substances like bile, cholesterol, and minerals, removing them from the body.
Carbohydrate Absorption
- Peristaltic movements move monosaccharides to the jejunum, where digestion is completed and absorption begins.
- Absorption increases due to intestinal villi.
- Monosaccharides pass through blood capillaries via diffusion or active transport.
- Glucose can be absorbed through the mouth lining.
- Glucose and galactose are absorbed through active transport.
Fructose
- Fructose is absorbed by facilitated diffusion and absorbed slower producing a smaller glucose rise in blood.
- Unbranched starch chains are digested more slowly than branched chains which have many more places for enzymes to act on and release glucose rapidly.
- Fructose and galactose are processed by liver cells and converted into other substances, often glucose.
- All disaccharides in food provide at least one molecule of glucose, and potentially another via fructose and galactose conversion.
Lactose Intolerance
- Lactose intolerance is the inability to digest lactose, a milk sugar.
- It is characterized by bloating, gas, abdominal discomfort, and diarrhea.
- It differs from milk allergy (immune response to milk protein).
- It occurs due to insufficient lactase (enzyme that breaks down lactose).
- Treatment involves limiting milk intake and supplementing with other sources of riboflavin, vitamin D, and calcium.
Galactosemia
- Galactosemia is a rare genetic disorder impacting the ability to metabolize galactose properly.
- It is not related to lactose intolerance.
- Lactose is broken down by lactase into glucose and galactose.
- Individuals with galactosemia have reduced or absent enzymes for galactose metabolism.
- Galactosemia leads to high levels of galactose 1-phosphate, causing various issues like organ enlargement (hepatomegaly), cirrhosis, kidney failure, cataracts, brain damage, and ovarian failure.
Carbohydrate Metabolism
- Glucose is central to carbohydrate metabolism.
- Metabolism is the sum of all chemical processes in a living organism.
- Metabolism includes building and breaking down tissues.
- Metabolites are the products of metabolic processes.
- The cell is the functional unit of human life.
- Energy metabolism happens in all cells to sustain life.
- Enzymes break down glucose for energy.
- Excess glucose can be stored as fat adipose tissue.
Storing Glucose as Glycogen
- The liver stores about 1/3 of the total body glycogen.
- The liver releases glucose into the bloodstream as needed.
- When blood sugar rises after a meal, liver cells join excess glucose molecules through condensation reaction forming glycogen (long branching chains).
- When blood sugar levels decrease, liver cells break down glycogen through hydrolysis into individual glucose molecules and release them into the bloodstream.
Using Glucose for Energy
- Glucose fuels many body cells.
- Liver glycogen stores last 4 hours.
- Carbohydrates must be eaten frequently to maintain glucose levels.
Gluconeogenesis
- Gluconeogenesis is the conversion of protein to glucose.
- Sufficient carbohydrates prevent protein use for energy.
- This effect is known as protein-sparing action of carbohydrates.
Metabolism of Carbohydrates
- Catabolism (breakdown) of carbohydrates: Glycogenolysis (breakdown of glycogen to glucose).
- Oxidation of glucose for ATP synthesis.
- Anabolism (synthesis) of carbohydrates: Glycogenesis (synthesis of glycogen from glucose).
Glucose in the Body
- Every body cell utilizes glucose for energy.
- The cells of the nervous system depend almost exclusively on glucose.
- Cells have limited storage for glucose.
- Glucose supplies are maintained in consistent streams to all cells from either food or liver storage.
Maintaining Glucose Homeostasis
- Optimal body function needs maintenance of blood glucose within limits.
- Low blood glucose causes dizziness/weakness.
- High blood glucose causes fatigue.
- Fluctuation of blood glucose to extremes can be fatal (not treated).
Regulating Hormones
- Blood glucose homeostasis is mainly regulated by two hormones:
- Insulin: moves glucose from blood into cells
- Glucagon: releases stored glucose from liver.
After Meal/Elevated Glucose
- High blood glucose triggers special pancreas cells to release insulin into the bloodstream.
- Insulin levels correspond with glucose.
- Body cells use glucose for energy.
- Some glucose is stored as glycogen in the liver and muscle cells.
- Extra glucose is converted to fat and stored.
When Blood Glucose Falls (Between Meals)
- Low blood glucose triggers special pancreas cells to release glucagon.
- Glucagon triggers liver to release stored glucose into the bloodstream.
- Another hormone that triggers liver release is epinephrine.
Normal blood glucose ranges
- Normal: 70-120 mg/dL
- Prediabetes: 100—125 mg/dL
- Diabetes: >126 mg/dL
Falling Outside of Normal Ranges
- Blood glucose regulation can fail, causing diabetes or hypoglycemia in some individuals.
- Diabetes is a chronic disorder of carbohydrate metabolism often due to insufficient or ineffective insulin.
- Type 1 diabetes is caused by insufficient insulin production by the pancreas.
- Type 2 diabetes is caused by inadequate response to insulin in cells (often due to obesity).
Hypoglycemia
- Hypoglycemia is an abnormally low blood glucose concentration.
- Symptoms include weakness, rapid heartbeat, sweating, anxiety, hunger, and trembling.
- Hypoglycemia can be a consequence of poorly managed diabetes.
- Possible causes include too much insulin, strenuous exercise, or inadequate food intake.
Glycemic Response
- The glycemic response is the extent to which a food raises blood glucose and elicits insulin response.
- It describes how quickly glucose is absorbed after eating, how high blood glucose rises, and how quickly it returns to normal.
- Low glycemic response foods are desirable.
Glycemic Index
- Glycemic index is a method for classifying foods by their potential to raise blood glucose.
Source of Blood Glucose
- Dietary carbohydrates are a primary source supplying high percentage of blood glucose.
- Glycogen is another glucose source.
- Byproducts of carbohydrate metabolism (lactic acid and pyruvic acid) can contribute to blood glucose levels
- Glucose can be created from non-carbohydrate sources such as protein and fat.
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Test your knowledge on the functions and digestion of carbohydrates with this quiz. Explore enzyme roles, fiber benefits, and nutrient absorption in the digestive system. Ideal for students of nutrition or health sciences.
