Carbohydrates Classification and Metabolism

Carbohydrates Classification

• Classify carbohydrates by chemical composition: monosaccharides, disaccharides, and polysaccharides.
• Can also be categorized as:
  • Sugars and starches
  • Simple and complex
  • Minimally processed vs. highly processed
  • “Good” vs. “bad”
  • Whole vs. refined

Carbohydrate Digestion and Absorption

• Carbohydrates are broken down into monosaccharides in the small intestine
• Monosaccharides are absorbed into the bloodstream
• Glucose is used for energy, stored as glycogen, or converted into fat
• The glycemic response is the rate at which blood glucose levels rise after eating

Metabolism of Carbohydrates

• Glucose is metabolized differently depending on cell type, enzymatic ability of the cell, energy state, hormonal status, training history, and exercise intensity.
• Glucose can be used immediately or stored for later use

The Importance of Carbohydrates for Exercise

• Carbohydrates are the primary fuel source for exercise
• Muscle glycogen is the primary source of carbohydrate during exercise
• Blood glucose is used to replenish muscle glycogen during exercise

Carbohydrate Recommendations for Athletes

• Athletes need to consume more carbohydrates than sedentary individuals.
• Carbohydrate recommendations vary depending on the intensity, duration, and frequency of exercise.
• Carb needs differ between pre-, during, and post-exercise.

Carbohydrate Periodisation

• Optimise performance using Carbohydrate Periodisation
• Vary carb intake in training to adapt to exercise requirements, like low vs high carb days.

Low CHO Availability Strategies

• Adapting to low CHO availability is vital for fitness performance.
• Employ strategies like training fasted, no CHO during training, delayed rescue, twice-daily training (second session no CHO), and sleep low.

Carbohydrates and Gut Training

• Training with high carbohydrate availability ensures athletes are prepared for competition, especially if significant carbohydrate intake is required during their event.

Benefits of training with low carbohydrate availability

• Improved mitochondrial function, increased capillarisation, enhanced lactate oxidation, and enhanced ability to metabolize fat as a substrate.

Glycogen threshold hypothesis

• Post-exercise muscle glycogen concentration influences adaptive response, suggesting a “threshold" value (around 300mmol/dry wt.muscle) for maximising training adaptions.

Carbohydrates in the body

• Primarily found in the form of glucose and glycogen.
• Glucose circulates in the bloodstream.
• Glycogen is stored in muscle and liver tissue.
• Carbohydrates are consumed through foods in three categories: monosaccharides, disaccharides, and polysaccharides.
• The body primarily uses monosaccharides (single sugar units) in the form of glucose.
• The body needs to break down polysaccharides (complex sugars) for energy.

Carbohydrate recommendations for athletes

• Athletes need adequate carbohydrate intake to fuel their performance.
• A diet with 70% of total calories from carbohydrates is likely too high for most athletes.
• Athletes often underconsume carbohydrates.

Glucose metabolism

• Refers to the body's process of using glucose for energy.
• Glucose is either used immediately for energy or stored for later use.
• Whether glucose is used or stored depends on the specific needs of the cell.
• Exercise intensity and body mass influence how much carbohydrate is used during activity.
• Carbohydrate availability, glycogen stores, and habitual carbohydrate intake affect how much energy is derived from carbohydrates.
• The respiratory exchange ratio can be used to determine how much carbohydrate is being burned during activity.
• A respiratory exchange ratio of 100% indicates that almost all energy being used is coming from carbohydrates.
• Glycogen is a more efficient source of energy than starch because it can be broken down into glucose units more quickly.

Carbohydrate types

• Sugars are simple carbohydrates (monosaccharides).
• Starches are complex carbohydrates (polysaccharides).
• Simple carbohydrates include minimally processed whole foods containing single sugar units.
• Complex carbohydrates are longer chains of sugar units that are more branched than simple carbohydrates.
• Refined grains are processed grains that have been stripped of the bran and germ, leaving primarily starch.
• Whole grains include the bran, germ, and endosperm, providing more nutrients than refined grains.
• Carbohydrates in sports gels are often high-glycemic, which means they can be quickly absorbed by the body.

Glucose metabolism in the cell

• Glucose can be used immediately for energy or stored for later use by the cell.
• Which path is selected depends on the requirements of the cell, influencing how glucose is metabolized.

Quiz Information

• The quiz is due on Monday of week four
• The quiz will be open for 40 minutes, from 12:35 p.m. to 1:15 p.m.
• The quiz has 30 multiple choice questions
• The quiz can be completed at home, in the library, or during lecture time
• The quiz must be completed individually to prevent collaboration

Carbohydrate Basics

• Carbohydrates are found in the body primarily as:
  • Glucose in the bloodstream
  • Glycogen stored in muscle and liver tissue
• Carbohydrates in food can be categorized into:
  • Monosaccharides: single sugars
  • Disaccharides: two sugars joined together
  • Polysaccharides: many sugars joined together
• Monosaccharides include:
  • Glucose: glycemic index (GI) of 100, found in blood and stored as glycogen, known as dextrose when added to food
  • Fructose: found in fruit, sweeter than glucose, doesn’t raise blood sugar as quickly
  • Galactose: found in milk, not common in the diet

Disaccharides

• Disaccharides always contain one molecule of glucose, and a second monosaccharide that determines its name.
• Disaccharides include:
  • Sucrose: glucose + fructose, known as table sugar (white or brown)
  • Lactose: glucose + galactose, known as milk sugar
  • Maltose: glucose + glucose, not commonly found in the human diet

Polysaccharides

• Polysaccharides are formed by connecting monosaccharides through reducing ends
• Polysaccharides include:
  • Glycogen: the storage form of carbohydrates in humans
  • Starch: the storage form of carbohydrates in plants, two main types are amylopectin and amylose
  • Fiber: polysaccharides that humans cannot digest well due to their structure (e.g., cellulose)

Whole Grain vs. Refined Grains

• Whole grains include the bran and germ, containing fiber, vitamins, minerals and phytochemicals
• Refined grains only contain the endosperm, mainly starchy carbohydrates
• Refined grains are often recommended for athletes due to their rapid energy delivery but may lack the nutritional benefits of whole grains

Carbohydrate Recommendations for Athletes

• Carbohydrate intake for athletes can be higher than for the general population
• High sugar, low fiber foods and beverages might be necessary for short-term energy bursts during competition
• Some examples include lollies, sugar water, sports drinks (like Gatorade)

Digestion and Absorption

• Digestion is the breakdown of food into smaller molecules that the body can absorb
• Absorption is the process of transferring nutrients from the digestive tract into the bloodstream
• Most digestion takes place in the stomach
• Most absorption takes place in the small intestine

Carbohydrate Digestion

• Digestion of carbohydrates begins in the mouth with salivary amylase.
• The stomach inactivates salivary amylase, halting starch digestion.
• The majority of carbohydrate digestion occurs in the small intestine with pancreatic amylase.
• Disaccharides are hydrolyzed into monosaccharides on the surface of intestinal cells.
• Dietary fiber is not digested and delays gastric emptying.
• Some dietary fiber is digested by bacterial enzymes in the large intestine, creating short chain fatty acids and gas.

Carbohydrate Absorption

• The small intestine has villi and microvilli, increasing surface area to optimize absorption.
• The small intestine is only one cell thick, allowing for efficient nutrient absorption.
• Glucose and galactose are absorbed through the sodium-glucose transporter, a secondary active transport mechanism.
• Fructose is absorbed through facilitated diffusion, using a glucose transporter.
• Training can increase the expression of the sodium-glucose transporter, leading to more sugar absorption.

Glucose Metabolism

• The body prioritizes regulating blood glucose concentration within a homeostatic range.
• Insulin, secreted from the beta cells of the pancreas, promotes glucose uptake by the liver and muscle.
• Glucagon, secreted from the alpha cells of the pancreas, stimulates the liver to release stored glycogen into the bloodstream.
• Glycemic response varies depending on the type and structure of carbohydrates in food.

Glycemic Index (GI)

• Orange juice has a higher GI than apple sauce, leading to quicker blood sugar spikes.
• Foods with high GI provide immediate energy.
• Foods with low GI provide a slower release of carbohydrates.
• The GI is influenced by the type of carbohydrates present (e.g., monosaccharides, polysaccharides), fiber content, and the food's overall composition.
• GI values can be confusing and inconsistent across food sources.
• Sports gels, often containing fructose and glucose, have a high GI, facilitating rapid absorption.

Glucose Metabolism

• Glucose metabolism can occur anaerobically or aerobically, depending on the needs of the cell.
• Different cell types utilize glucose differently.
• Red blood cells primarily utilize anaerobic metabolism, while heart cells and slow-twitch muscle fibers rely on aerobic metabolism.
• Enzyme activity in cells influences glucose metabolism.
• Exercise training can lead to upregulation of enzymes associated with aerobic metabolism.
• Muscle glycogen levels, hormonal status, training history, and exercise intensity influence glucose metabolism.

Muscle Glycogen Utilization During Exercise

• Muscle glycogen depletion occurs during moderate-to-high-intensity exercise.
• Muscle glycogen depletion increases with exercise intensity.
• Fatigue at high-intensity exercise (100% and 130% VO2max) may be more related to hydrogen ion accumulation than low muscle glycogen.
• Fatigue at 70% VO2max is associated with low muscle glycogen levels.
• Higher starting muscle glycogen stores extend time to exhaustion at 70% VO2max.

Muscle Glycogen Storage and Training Status

• Muscle glycogen storage capacity increases with fitness levels (VO2max).
• Dietary carbohydrate intake influences muscle glycogen storage.
• Individuals with higher VO2max can store more muscle glycogen and may require higher carbohydrate intake to maintain optimal levels.

Carbohydrate Oxidation Rates

• Carbohydrate oxidation rates are affected by total energy expenditure (exercise intensity and body mass).
• Factors influencing the proportion of energy derived from carbohydrates include carbohydrate availability, habitual intake, training status, and exercise intensity.
• Indirect calorimetry can be used to measure carbohydrate oxidation rates during exercise.
• A respiratory exchange ratio (RER) of 1.0 indicates primarily carbohydrate utilization.

Carbohydrate Intake During Exercise

• Consuming carbohydrates during exercise can extend time to fatigue compared to a placebo.
• A meta-analysis of studies comparing carbohydrate intake to a placebo shows a significant increase in time to exhaustion for those consuming carbohydrates.

Carbohydrates and Exercise Performance

• Carbohydrate consumption during exercise is generally beneficial, especially for endurance activities exceeding two hours.
• Single-source carbohydrates (glucose only) offer performance benefits up to two hours of exercise, while multi-source carbohydrates (glucose and fructose) provide substantial benefits for longer duration activities.
• Studies show that consuming carbohydrates during exercise can increase performance time by up to 10% for events lasting over two hours.
• The study highlights the potential of carbohydrate depletion followed by high intensity training to enhance fat oxidation and increase the intensity at which fat is utilized as fuel.
• A crossover design trial demonstrated the advantages of high carbohydrate consumption during a multi-day high-intensity training block compared to a control group consuming lower carbohydrates.
• The study suggests that the high carb group could sustain higher speed and performance during the intense training block.
• The long-term effects of a low carbohydrate, high fat diet for athletes were examined in a study involving 10 participants.
• Participants on the low carb diet demonstrated enhanced fat oxidation during exercise, suggesting a potential shift in fuel utilization.
• However, the study also showed that athletes on the low carbohydrate, high-fat diet exhibited reduced speed compared to those on the high carbohydrate diet, potentially due to limitations in carbohydrate availability and glycogen stores.
• Metabolic adaptations that enhance performance at submaximal work rates include increased capillary density, mitochondrial density, and enzyme activity associated with glucose and lactate oxidation.
• The glycogen threshold hypothesis suggests that training with low carbohydrate availability enhances metabolic adaptations. This occurs when post-exercise glycogen concentrations remain below 300 millimoles of kilogram dry weight, leading to increased fat oxidation and changes in carrier proteins and enzymes.
• Alternative nutritional strategies to achieve the effects of the glycogen threshold hypothesis include fasting before exercise, excluding carbohydrates during training, employing a delayed rescue strategy (carbohydrates during training, protein post-training), training twice daily with the second session without carbohydrates, and maintaining a low carbohydrate intake before sleep.