Digestion and Absorption

Function of Trypsin in Macronutrient Digestion

1. Location:

• Produced and released by the pancreas into the small intestine, specifically in the duodenum.

2. Substrate:

• Acts on proteins, breaking down large protein molecules into smaller peptides.

3. Specificity:

• Trypsin is a protease enzyme that specifically cleaves peptide bonds on the carboxyl side of basic amino acids, such as lysine and arginine.

4. Activation:

• Trypsinogen, the inactive form of trypsin, is secreted by the pancreas. It is activated into trypsin by an enzyme called enterokinase, which is produced by the small intestine.

5. Synergy with Other Enzymes:

• Works in conjunction with other proteases like chymotrypsin and peptidases to further break down peptides into individual amino acids.

6. Role in Protein Digestion:

• Hydrolyzes (breaks down) proteins into smaller peptides and amino acids, facilitating their absorption in the small intestine.

7. Importance in Nutrient Absorption:

• The breakdown of proteins into amino acids by trypsin is crucial for the absorption of these amino acids across the intestinal wall and into the bloodstream.

Gallbladder

• The gallbladder is a small, pear-shaped organ located beneath the liver.

• Its primary function is to store and concentrate bile, a digestive fluid produced by the liver.

Bile Storage:

• The liver continuously produces bile, which consists of water, electrolytes, bile salts, bilirubin, and cholesterol. Instead of releasing bile directly into the small intestine, the excess bile is stored in the gallbladder.

Concentration of Bile:

• While bile is stored in the gallbladder, it becomes more concentrated. Water and electrolytes are absorbed, making the bile more potent and efficient for the digestion of fats.

Bile Release:

• When the body requires bile for the digestion of fats, especially in response to the presence of fatty foods in the small intestine, the gallbladder contracts and releases the concentrated bile into the common bile duct.

Aid in Fat Digestion:

• Bile plays a crucial role in the digestion and absorption of fats.

• Bile salts emulsify large fat globules into smaller droplets, increasing the surface area for enzymes (such as lipase) to efficiently break down fats into fatty acids and glycerol.

Assist Nutrient Absorption:

• By aiding in the digestion of fats, bile indirectly supports the absorption of fat-soluble vitamins (A, D, E, K) and other lipophilic nutrients in the small intestine.

Absorption of Amino Acids

Digestion in the Small Intestine:

• Proteins from the diet are broken down into peptides and amino acids by various enzymes, including pepsin in the stomach and proteases like trypsin and chymotrypsin from the pancreas, in the small intestine.

Final Breakdown by Peptidases:

• Peptidases, enzymes present on the brush border of the small intestine's epithelial cells (enterocytes), further break down peptides into individual amino acids.

Transporter Proteins:

• Amino acids are absorbed across the apical membrane of the enterocytes (the side facing the intestinal lumen) through various transporter proteins. These transporter proteins are specific to different types of amino acids.

Intracellular Processing:

• Inside the enterocytes, amino acids are processed and may undergo various modifications before being transported to the basolateral membrane (the side facing the bloodstream).

Transport Across the Basolateral Membrane:

• Amino acids are transported across the basolateral membrane of the enterocytes into the interstitial fluid, facilitated by specific transporter proteins.

Capillary Network:

• Once in the interstitial fluid, amino acids are readily absorbed into the capillary network (blood vessels) that surrounds the small intestine.

Portal Circulation:

• The absorbed amino acids enter the portal circulation, a system of veins that carries blood from the digestive organs directly to the liver.

Liver Processing:

• Before reaching the systemic circulation, blood from the portal vein passes through the liver. The liver plays a role in regulating amino acid levels, as well as in amino acid metabolism.

Systemic Circulation:

• Finally, amino acids enter the systemic circulation, where they are transported to various tissues and organs throughout the body.

Typical PH range found in the mouth:

• The typical pH range found in the mouth is slightly acidic to neutral.

• Saliva, which is produced by the salivary glands and present in the mouth, helps maintain a pH within the range of approximately 6.2 to 7.6.

  • The pH of saliva can vary among individuals and throughout the day, influenced by factors such as diet, hydration, and overall health.

  • The mildly acidic to neutral pH in the mouth is conducive to the activity of enzymes like salivary amylase, which initiates the digestion of carbohydrates by breaking down starches into simpler sugars.

Role of Enzymes in Digestion

• Enzymes play a crucial role in digestion by facilitating the breakdown of complex food molecules into smaller, absorbable units.

• Digestion involves the conversion of large macromolecules, such as carbohydrates, proteins, and fats, into smaller components like sugars, amino acids, and fatty acids.

• Enzymes act as biological catalysts, accelerating these chemical reactions.

Key roles of enzymes in digestion

Carbohydrate Digestion

• Enzyme: Amylase (salivary amylase in the mouth, pancreatic amylase in the small intestine)

• Role: Breaks down complex carbohydrates (starches) into simpler sugars (such as maltose and glucose).

Protein Digestion

• Enzymes: Pepsin (in the stomach), trypsin, chymotrypsin, and peptidases (in the small intestine)

• Role: Breaks down proteins into peptides and amino acids. Each enzyme has specificity for certain peptide bonds.

Fat Digestion

• Enzymes: Lipase (produced by the pancreas)

• Role: Hydrolyzes triglycerides (fats) into fatty acids and glycerol, allowing for their absorption.

Nucleic Acid Digestion

• Enzymes: Nucleases (e.g., DNAase and RNAase)

• Role: Breaks down nucleic acids (DNA and RNA) into nucleotides.

Lipid Emulsification

• Enzymes: Bile salts (produced by the liver, stored in the gallbladder)

• Role: Bile salts emulsify large fat globules into smaller droplets, increasing the surface area for lipase action and aiding in fat digestion.

Absorption Facilitation

• Enzymes: Brush border enzymes (e.g., lactase, sucrase, maltase)

• Role: Located on the surface of intestinal epithelial cells, these enzymes further break down disaccharides into monosaccharides for absorption.

Activation of Enzymes

• Enzymes: Enterokinase

• Role: Activates trypsinogen into trypsin in the small intestine.

Typical PH range found in small intestine

Duodenum (proximal part of the small intestine)

• The pH in the duodenum is typically around 6 to 7.

• This slightly acidic to neutral pH is due to the presence of bicarbonate ions, which are released from the pancreas to neutralize the acidic chyme entering from the stomach.

Jejunum (middle part of the small intestine)

• As chyme progresses through the jejunum, the pH gradually increases.

• The pH in the jejunum is generally around 7 to 8, becoming more alkaline.

Ileum (distal part of the small intestine)

• The ileum continues the trend of increasing pH, and the pH in this section is also around 7 to 8.

Enzymes that are primarily responsible for the digestion of proteins

Pepsin:

• Location: Stomach

• Role: Pepsin is an enzyme produced by the chief cells in the stomach lining.

• It works in the acidic environment of the stomach and is responsible for breaking down large protein molecules into smaller peptides.

• Pepsinogen, an inactive form of pepsin, is secreted by the stomach, and it is activated to pepsin by the acidic conditions in the stomach.

Trypsin:

• Location: Small intestine (duodenum)

• Role: Trypsin is produced by the pancreas and released into the duodenum of the small intestine.

• It further breaks down peptides into smaller fragments, including individual amino acids.

• Trypsin works in conjunction with other proteases like chymotrypsin and peptidases to complete the digestion of proteins.

Enzyme responsible for the digestion of protein from the mouth to the small intestine is Pepsin

Mouth:

• Enzyme: No specific protein-digesting enzyme is secreted in the mouth.

• However, mechanical breakdown of food occurs through chewing, and salivary amylase initiates the digestion of carbohydrates.

Stomach

• Enzyme: Pepsin

• Role: Pepsin is secreted by the chief cells in the stomach lining as an inactive precursor called pepsinogen.

• When pepsinogen comes into contact with the acidic environment of the stomach (around pH 1.5-3.5), it is activated to pepsin.

• Pepsin then plays a crucial role in breaking down large protein molecules into smaller peptides.

Small Intestine (Duodenum)

• Enzymes: Trypsin, Chymotrypsin, and Peptidases

• Role: In the small intestine, pancreatic enzymes are released, including trypsin and chymotrypsin.

  • These enzymes continue the digestion of peptides into smaller fragments.

  • Peptidases, which are enzymes on the brush border of the small intestine, further break down peptides into individual amino acids.

Function of Enzymes in Digestion of Micronutrients

Vitamin Digestion

• Some vitamins are precursor compounds that need to be converted into their active forms before the body can use them.

• Enzymes facilitate these conversion processes.

• For example, provitamins (inactive forms of vitamins) are often converted to active forms by enzymes. An enzyme called 25-hydroxycholecalciferol is involved in the activation of vitamin D in the liver.

Mineral Digestion

• Enzymes are not directly involved in the breakdown of minerals in the same way they are for macronutrients.

• Instead, minerals are typically in their elemental forms or simple chemical compounds in food.

• Absorption of minerals often involves transport proteins, carriers, or channels rather than enzymatic breakdown.

• For example, calcium absorption in the small intestine involves a calcium transport protein.

Enzymes in Coenzyme Functions

• Some micronutrients, such as certain B vitamins, function as coenzymes.

• Coenzymes are essential for the activity of various enzymes involved in metabolic pathways.

• For instance, thiamine (vitamin B1) is a coenzyme that participates in reactions related to energy metabolism.

Antioxidant Enzymes

• Certain micronutrients, such as vitamins C and E, function as antioxidants. Antioxidant enzymes, like superoxide dismutase, catalase, and glutathione peroxidase, play a role in neutralizing harmful free radicals.

• These enzymes help protect cells and tissues from oxidative damage caused by reactive oxygen species.

Enzymes in Digestive Processes

• Enzymes involved in the digestion of macronutrients indirectly support the absorption of micronutrients.

• For example, pancreatic enzymes released during the digestion of fats can aid in the absorption of fat-soluble vitamins (A, D, E, K).

Two forms of Digestion

Mechanical Digestion

• Process: Chewing or mastication

• Description: Mechanical digestion involves the physical breakdown of food into smaller particles.

• In the mouth, teeth play a crucial role in mechanically breaking down food into smaller fragments, increasing the surface area for chemical digestion. Chewing also mixes food with saliva, forming a semiliquid mixture known as chyme.

Chemical Digestion

• Process: Salivary digestion by enzymes

• Description: Chemical digestion involves the breakdown of complex molecules into simpler ones through the action of enzymes.

• In the mouth, salivary glands release saliva that contains the enzyme amylase. Amylase initiates the digestion of carbohydrates by breaking down starches into simpler sugars like maltose and glucose.

• While the process begins in the mouth, salivary amylase's activity continues in the stomach until it is eventually inactivated by the acidic environment.

Needs of Enzymes in Digestion

Large Molecule Breakdown

• Most dietary nutrients come in the form of large macromolecules, such as carbohydrates, proteins, fats, and nucleic acids.

• Enzymes are necessary to break down these large molecules into smaller components that the body can absorb and utilize.

Specificity

• Enzymes are highly specific to particular substrates (molecules they act upon).

• Each type of enzyme is designed to catalyze a specific chemical reaction.

• This specificity ensures that the right enzyme acts on the right substrate, preventing random or wasteful reactions.

Acceleration of Reactions

• Enzymes significantly speed up the rate of chemical reactions involved in digestion.

• Without enzymes, these reactions would occur too slowly to meet the body's metabolic needs.

Optimal Conditions

• Enzymes work under specific conditions, such as a particular pH and temperature range.

• Their activity is optimized for the physiological environment in which they operate.

• This ensures efficient digestion within the body's constraints.

Energy Efficiency

• Enzymes allow the body to extract energy from nutrients in a controlled and efficient manner.

• Breaking down large molecules step by step allows for the gradual release of energy, which can be harnessed by the body for various physiological processes.

Specific Enzymes for Each Nutrient Type

• Different types of nutrients (carbohydrates, proteins, fats) require specific enzymes for their digestion.

• For example, amylase digests carbohydrates, proteases break down proteins, and lipases act on fats.

  • This specificity ensures the effective digestion of diverse nutrients in the diet.

Prevention of Nutrient Waste

• Enzymes help prevent nutrient waste by ensuring that the breakdown of complex molecules is directed toward the production of useful components (such as amino acids, fatty acids, and sugars) that can be absorbed and utilized by the body.

Enzymes responsible for the digestion of carbohydrates in the mouth and small intestine are:

In the Mouth:

• Enzyme: Salivary Amylase

• Role: Salivary amylase is produced by the salivary glands and released into the mouth.

  • It initiates the digestion of carbohydrates by breaking down starches into smaller molecules like maltose and glucose.

  • However, salivary amylase's activity is limited in the stomach due to the acidic environment.

In the Small Intestine:

• Enzyme: Pancreatic Amylase

• Role: Pancreatic amylase is produced by the pancreas and released into the duodenum of the small intestine.

  • It continues the digestion of carbohydrates by breaking down complex polysaccharides into maltose and other smaller sugars.

  • The activity of pancreatic amylase is crucial for further carbohydrate digestion in the small intestine.

Digestion of Fats

In the Mouth:

• No specific fat-digesting enzyme is present in the mouth.

In the Stomach:

• Enzyme: Gastric Lipase

• Role: Gastric lipase is produced in the stomach.

  • While its contribution to fat digestion is limited compared to other lipases, it does play a role in breaking down triglycerides into fatty acids and glycerol.

In the Small Intestine:

• Enzyme: Pancreatic Lipase

• Role: Pancreatic lipase is produced by the pancreas and released into the small intestine.

  • It is a key enzyme for the digestion of fats, breaking down triglycerides into fatty acids and monoglycerides.

  • Bile salts, which are not enzymes but aid in emulsification, also play a crucial role in fat digestion by breaking down large fat globules into smaller droplets, increasing the surface area for pancreatic lipase action.

Digestion of Proteins

In the Stomach:

• Enzyme: Pepsin

• Role: Pepsin is produced in the stomach and is activated from its inactive precursor, pepsinogen, in the acidic environment of the stomach.

  • Pepsin plays a crucial role in breaking down proteins into smaller peptides.

In the Small Intestine:

• Enzymes: Trypsin, Chymotrypsin, and Peptidases

• Role: These enzymes are produced by the pancreas and released into the small intestine.

  • Trypsin and chymotrypsin further break down peptides into smaller fragments, and peptidases on the brush border of the small intestine complete the process by breaking down peptides into individual amino acids.

Water and Electrolyte Balance

Reasons why humans cannot live without water for a prolonged period:

• Metabolic Reactions: Water is a crucial component for various metabolic reactions, including those involved in the production of ATP, the body's primary energy source.

• Thermoregulation: Water plays a vital role in regulating body temperature through processes like sweating.

  • Without proper temperature regulation, heat-related illnesses can occur.

• Transport Medium: Water serves as a solvent for nutrients and gasses, facilitating their transport within the body.

  • It is essential for the functioning of the circulatory system.

• Cell Structure: Water is a major component of cells, providing structure and support.

  • Without water, cellular integrity is compromised.

Location of extracellular fluid throughout the body:

• Extracellular fluid (ECF): This fluid is found outside cells and includes interstitial fluid (between cells) and plasma (within blood vessels).

Comparison of water distribution in trained and untrained individuals:

• Trained Individuals: Trained individuals may have a higher percentage of lean muscle mass, which contains more water.

• This can affect overall body water distribution compared to untrained individuals.

Explanation of homeostasis and negative feedback mechanisms:

• Homeostasis: Homeostasis is the maintenance of stable internal conditions.

• Negative feedback mechanisms work to counteract changes and return the system to a set point.

Roles of the loop of Henle, medulla, collecting duct, and ADH in maintaining water balance:

• Loop of Henle: This structure in the kidney creates an osmotic gradient, allowing for the concentration of urine.

• Medulla: The medulla assists in concentrating urine by reabsorbing water.

• Collecting Duct: The collecting duct further concentrates urine and regulates its final concentration.

• ADH (Antidiuretic Hormone): ADH is released by the pituitary gland and increases water reabsorption in the kidneys, reducing urine volume and helping maintain water balance.

Description of how the hydration status of athletes can be monitored:

• Monitoring Urine Color: Dark urine may indicate dehydration.

• Weighing Before and After Exercise: Changes in body weight can indicate fluid loss.

• Thirst Perception: Athletes can monitor their thirst perception as an indicator of the need for fluid intake.

Explanation of why endurance athletes require a greater water intake:

• Increased Sweat Loss: Endurance athletes experience higher sweat rates, leading to increased fluid loss.

• Electrolyte Loss: Prolonged exercise can lead to significant electrolyte loss through sweat.

• Prevention of Dehydration: Adequate water intake is crucial to prevent dehydration, which can impair performance and pose health risks.

Discussion of the regulation of electrolyte balance during acute and chronic exercise:

• Acute Exercise: Electrolyte balance is influenced by factors such as sweating, leading to the loss of sodium and other electrolytes. Rehydration strategies during and after exercise are crucial.

• Chronic Exercise: Training adaptations may affect electrolyte balance, emphasizing the need for proper diet and hydration practices to maintain electrolyte levels.

Energy Balance and Body Composition

Basal metabolic rate (BMR):

• Definition: Basal Metabolic Rate (BMR) is the amount of energy expended by the body at rest to maintain basic physiological functions such as breathing, circulation, and cell production.

  • It is measured under standardized conditions, including resting and fasting, and is often expressed in calories per unit of time (e.g., calories per day).

State the components of daily energy expenditure:

• Basal Metabolic Rate (BMR): Energy expended at rest to maintain basic physiological functions.

• Physical Activity: Energy expended during physical activities, including exercise, sports, and daily activities.

• Thermic Effect of Food (TEF): Energy expended during the digestion, absorption, and metabolism of food.

The relationship between energy expenditure and intake:

• Energy Balance: The relationship between energy intake and expenditure determines whether an individual is in a state of energy balance, surplus, or deficit.

• Energy Balance Equation: Energy Intake - Energy Expenditure = Energy Balance.

• Implications: A positive balance leads to weight gain, a negative balance leads to weight loss, and a balanced equation results in weight maintenance.

The association between body composition and athletic performance:

• Lean Body Mass (LBM): Athletes often strive to optimize their lean body mass, as it contributes to strength, power, and athletic performance.

• Body Fat Percentage: Maintaining an optimal body fat percentage is crucial for endurance athletes, as excess body fat may hinder performance.

Dietary practices employed by athletes to manipulate body composition:

• Caloric Manipulation: Athletes may adjust caloric intake to create a surplus for muscle gain or a deficit for fat loss.

• Macronutrient Composition: Manipulating the ratio of carbohydrates, proteins, and fats can impact body composition and performance.

• Nutrient Timing: Timing nutrient intake around workouts can influence muscle building and recovery.

Nutritional Strategies

The approximate glycogen content of specific skeletal muscle fiber types:

• Glycogen Content:

  • Type I (Slow-Twitch) Fibers: About 10-15 grams of glycogen per 100 grams of muscle tissue.

  • Type II (Fast-Twitch) Fibers: About 3-5 grams of glycogen per 100 grams of muscle tissue.

Reference to exercise intensity, typical athletic activities requiring high rates of muscle glycogen utilization:

• High-Intensity Activities:

  • Sprinting, weightlifting, and activities requiring rapid bursts of power heavily rely on muscle glycogen.

The pattern of muscle glycogen use in skeletal muscle fiber types during exercise of various intensities:

• Low-Intensity Exercise: Type I fibers predominantly use glycogen for energy.

• High-Intensity Exercise: Both Type I and Type II fibers utilize glycogen, with Type II fibers relying more heavily on anaerobic glycolysis.

Glycemic index (GI):

• Definition: The glycemic index (GI) is a measure of how quickly a carbohydrate-containing food raises blood glucose levels.

  • It ranks foods on a scale from 0 to 100 based on their effect on blood sugar.

List foods with low and high glycemic indexes:

• Low GI Foods: Oatmeal, legumes, whole grains, most fruits.

• High GI Foods: White bread, sugary cereals, white rice, potatoes.

The relevance of GI with regard to carbohydrate consumption by athletes pre- and post-competition:

• Pre-Competition: Athletes may consume high-GI foods to quickly raise blood glucose levels for immediate energy.

• Post-Competition: Consuming a mix of high and low-GI foods helps replenish glycogen stores gradually.

Interaction of carbohydrate loading and training program modification prior to competition:

• Carbohydrate Loading: Involves consuming a high-carbohydrate diet to maximize glycogen stores.

• Training Program Modification: Tapering exercise intensity while maintaining volume helps conserve glycogen and optimize performance.

Reasons for adding sodium and carbohydrate to water for the endurance athlete:

• Sodium: Helps maintain electrolyte balance, preventing hyponatremia during prolonged exercise.

• Carbohydrate: Provides a source of energy to delay fatigue during endurance events.

The use of nutritional ergogenic aids in sports:

• Supplements: Athletes may use supplements like caffeine, creatine, and beta-alanine to enhance performance.

• Caution: Athletes should be cautious and consult professionals due to potential risks and limited evidence for some supplements.

The daily recommended intake of protein for adult male and female non-athletes:

• Adult Male and Female Non-Athletes: Approximately 0.8 grams of protein per kilogram of body weight.

List sources of protein for vegetarian and non-vegetarian athletes:

• Non-Vegetarian Sources: Meat, poultry, fish, eggs.

• Vegetarian Sources: Beans, lentils, tofu, dairy, nuts, seeds.

The significance of strength and endurance training on the recommended protein intake for male and female athletes:

• Strength Training: Athletes engaged in strength training may require higher protein intake (1.2–2.0 g/kg) to support muscle repair and growth.

• Endurance Training: Athletes engaged in endurance training may also benefit from slightly higher protein intake (1.2–1.6 g/kg) to support recovery.

Possible harmful effects of excessive protein intake:

• Kidney Strain: Excessive protein intake may strain kidneys, especially in individuals with pre-existing kidney conditions.

• Nutrient Imbalance: High protein intake without a balanced diet may lead to nutrient imbalances.

The Effects of Alcohol on Performance and Health

Acute effects of excess alcohol on the body:

• Central Nervous System (CNS) Depression: Alcohol acts as a depressant, impairing cognitive function, coordination, and reaction time.

• Dehydration: Alcohol is a diuretic, leading to increased urine production and potential dehydration.

• Metabolic Effects: Alcohol can interfere with nutrient metabolism, affecting the utilization of carbohydrates, proteins, and fats.

The possible effects of excessive chronic alcohol intake on body systems:

• Liver Damage: Chronic alcohol abuse can lead to liver diseases such as fatty liver, alcoholic hepatitis, and cirrhosis.

• Cardiovascular System: Increased risk of high blood pressure, cardiomyopathy, and arrhythmias.

• Immune System: Impaired immune function, increasing susceptibility to infections.

• Central Nervous System: Cognitive deficits, memory loss, and an increased risk of neurological disorders.

The effects of alcohol on athletic performance:

• Dehydration: Alcohol contributes to fluid loss, potentially impacting endurance and performance.

• Impaired Recovery: Alcohol can hinder post-exercise recovery by affecting muscle protein synthesis and glycogen replenishment.

• Coordination and Reaction Time: Impaired cognitive function can lead to decreased coordination and delayed reaction times, affecting skill-based sports.

Antioxidants

The role of antioxidants in the body:

• Definition: Antioxidants are substances that neutralize free radicals, protecting cells from oxidative damage.

• Cellular Protection: Antioxidants help maintain cellular health and reduce the risk of chronic diseases.

The harmful effects of free radicals at the cellular level:

• Oxidative Stress: Free radicals, produced during normal cellular processes and external factors, can cause oxidative stress, leading to cellular damage.

• Cellular Dysfunction: Oxidative stress can damage lipids, proteins, and DNA, contributing to cellular dysfunction and aging.

Free radical production during exercise:

• Mitochondrial Respiration: Exercise increases oxygen consumption, leading to an elevated production of free radicals during mitochondrial respiration.

• Inflammation: Intense exercise, especially in untrained individuals, may lead to increased inflammation and free radical production.

The role of antioxidants for combating the effects of free radicals:

• Neutralization of Free Radicals: Antioxidants neutralize free radicals, preventing oxidative damage to cellular structures.

• Reduced Inflammation: Antioxidants may help modulate inflammation, promoting recovery after exercise.

• Performance Enhancement: Some studies suggest that antioxidants can contribute to improved exercise performance and reduced muscle soreness.