Unit 3 AOS 2: How Does the Body Produce Energy?

6.1 Food Fuel for Energy Production

• Introduction

  • Consuming food provides the body with the energy required for muscle contraction.
  • The energy in food cannot be directly used. It is used to form ATP (adenosine triphosphate), which is stored in muscle fibers.

• ATP: The Energy Currency

  • ATP is a chemical compound responsible for producing energy for movement.

  • ATP consists of an adenosine molecule with three phosphate molecules attached.

  • When ATP is broken down, energy is released.

  • The breaking of the phosphate bond releases energy for movement.

    ATP
    ightharpoonup ADP + Pi + Energy

• ATP Resynthesis

  • Only a small amount of ATP is stored in muscles, enough for a few maximal contractions lasting 2-3 seconds.

  • ATP must be continually resynthesized to provide energy for longer periods.

  • ATP can be rebuilt from:

    • Breakdown of phosphocreatine (PC).
    • Breakdown of macronutrients (carbohydrates, proteins, and fats) from our diet.

• Food as Energy - The Energy Path

  • From ingestion to ATP production for muscular contraction, food is broken down and either:

    • Used immediately.
    • Excreted.
    • Stored as chemical energy (converted to mechanical energy for muscle contractions).

  • Energy from food supports:

    • Body functions.
    • ATP production.
    • Mechanical, concentration, and chemical work. Breakdown of fuel sources

• Energy Source vs. Fuel Source

  • Energy source: The energy system used to produce ATP (ATP/PC, anaerobic glycolysis, aerobic).
  • Fuel source: What fuels the exercise (carbohydrates, fats, protein, creatine phosphate).

• Food Fuels vs. Chemical Fuels

  • Food fuels: Carbohydrates, fats, and proteins.
  • Chemical fuels: ATP, phosphocreatine.
  • Energy from the breakdown of PC, carbohydrates, fats, and protein are used to re-join ADP & inorganic Phosphate (Pi) to resynthesise ATP.

• Proteins: Roles and Sources

  • Found in meat, fish, poultry, legumes, eggs, and grains.

  • Make a small contribution to energy production during exercise (5-10% in ultra-endurance events).

  • Essential for:

    • Building connective tissue and muscle cells.
    • Acting as enzymes to speed up chemical reactions.
    • Producing red blood cells, hormones, and antibodies.

  • Stored in muscles and around the body, primarily for growth and repair.

  • Only used as a fuel in extreme circumstances

  • Divided into:

    • Animal foods (meat, etc.): Rich in protein and contain all essential amino acids.
    • Plant foods (cereals, lentils, etc.): Rich in protein but may not contain all essential amino acids.
    • They need to be obtained through dietary intake.
    • Non-essential amino acids, which can be synthesized by the body from other amino acids

• Fats: Types and Functions

  • Found in butter, margarine, cheese, oil, nuts, and fatty meats.

  • Broken down into fatty acids (FFA) and triglycerides.

  • Most plentiful source of potential energy and main fuel source at rest and during prolonged submaximal exercise.

  • Trained athletes can utilize fats at higher intensities, leading to glycogen sparing.

  • Types of fat:

    • Saturated fats:

      • Found in animal foods (dairy, meat).
      • Contain cholesterol, potentially leading to cardiovascular disease.
      • Provide energy, assist with nutrient absorption, and play a role in hormone production.

    • Unsaturated fats:

      • Polyunsaturated fats: Found in vegetable oils and oily fish; help lower cholesterol and contain essential fatty acids.
      • Mono-unsaturated fats: Found in olive oil, avocados, and nuts.

• Carbohydrates: Types and Glycemic Index

  • Found in sugars and starches in fruits, cereals, bread, pasta, and vegetables.

  • Body’s preferred fuel source, especially during exercise.

  • Broken down into glucose for blood transportation.

  • Excess glucose is converted to glycogen and stored in muscles and the liver.

  • When glycogen stores are full, remaining glycogen is stored in adipose tissue.

  • Divided into:

    • Simple carbohydrates (sugars): Metabolized quickly, providing immediate energy (e.g., sugars, watermelon).
    • Complex carbohydrates (starches): Metabolized slowly, providing gradual energy (e.g., sweet potatoes, broccoli).

• Glycemic Index (GI)

  • Ranking of carbohydrates based on their effect on blood glucose levels (scale of 1-100).

    • High GI foods (e.g., white bread, honey): Cause rapid increase in blood sugar; best consumed during and immediately after exercise to replenish glycogen stores.
    • Moderate GI foods (e.g., corn, sweet potatoes):
    • Low GI foods (e.g., apples, lentils): Result in a slow release of glucose; best for pre-event meals and post-exercise replenishment.

• High GI Foods: During and After Exercise

  • Break down quickly, increasing blood sugar levels immediately.

  • During exercise:

    • Rapid absorption provides energy to top up glycogen stores, delaying depletion.

  • Immediately after exercise (within 30 minutes):

    • Muscles are most responsive to topping up fuel supplies.

• Low GI Foods: Pre- and Post-Exercise

  • Break down slowly, releasing glucose slowly into the bloodstream.

  • Pre-event meal (1-4 hours prior):

    • Helps keep blood glucose levels topped up.

  • After exercise (1-24 hours post-exercise):

    • Assists with repletion of muscle and liver glycogen stores.

• Blood Glucose Concentration and GI

  • High GI: rapid glucose release over a short time.
  • Low GI: slow glucose release over an extended period.

• Rebound Hypoglycemia

  • Athletes should avoid high GI foods 30-120 minutes before an event to avoid rebound hypoglycaemia.
  • Eating carbs causes a rise in blood sugar, prompting insulin release to lower blood sugar.
  • Consuming high GI foods before activity causes a rapid blood sugar spike, leading to an insulin overshoot.
  • This reduces blood sugar levels, impairing CNS functioning and negatively affecting performance.

• Energy for Rest and Activity

  • The body creates energy (ATP) under two main conditions:

    1. Rest conditions:

       • Sufficient oxygen (O2) is available for the body to function at a resting level.

    2. Active conditions:

       • Insufficient O2 is available for the body to function without a significant increase in O2 intake during or after exercise.

• ATP Production During Rest Conditions

  • Abundant O2 supply:

    • Approximately 2/3 of ATP is produced from fat stores.
    • Fat is a richer energy source but requires more O2 to release energy than glucose.
    • Other 1/3 of ATP is produced from CHO stores in the blood glucose and glycogen stores in muscle and liver.

• Fuel Sources at Rest and During Submaximal Exercise

  • At rest: fats and carbohydrates.
  • Submaximal exercise: fats and carbohydrates.

• ATP Production During Physical Activity

  • Any physical activity increases the demand for O2 in working muscles.

  • Submaximal exercise can be met by the aerobic system, as O2 supply meets the muscles' demand.

  • High-intensity exercise causes the respiratory and circulatory systems to be unable to supply the muscle cells with enough O2.

  • Causes them to burn glucose anaerobically.

  • Anaerobic systems produce ATP quickly but:

    • Produce a limited amount
    • Produce fatiguing by-products.

• Crossover Concept

  • Theoretical model explaining the balance of CHO and fat usage during sustained exercise.
  • The crossover point is the intensity at which energy from CHO predominates over energy from fats.
  • Increased intensity results in greater CHO use and decreased fat oxidation.
  • Trained individuals delay the switch to CHO at low to moderate intensities, better utilizing fats and sparing glycogen.

• Fuel Source Based on Duration and Intensity

  1. Low intensity / rest - stored fats are the main fuel source.

  2. As intensity of exercise increase, the contribution of muscle glycogen increases to meet the more immediate demands for fuel.

     • There is enough glycogen stored in muscles to fuel up to 60 minutes, depending on intensity, training, and CHO loading. Athletes “hit the wall” when muscle glycogen runs out.

  3. When muscle glycogen stores run out, the stored liver glycogen becomes the primary fuel source allowing exercise to continue but performance starts to diminish

     • Depletion of liver glycogen affects the brain, and decision-making abilities diminishes.

  4. Fats now become the primary fuel source and intensity of exercise is reduced, as fats require more complex chemical reactions and greater amounts of oxygen.

  5. Depletion of fats results in protein being used as a fuel source, which is only likely to occur in ultra-endurance events.

• Food Sources, Transport Forms, and Storage Forms

  • Carbohydrates:

    • Gradual release: apples, carrots, quinoa, whole grain bread, oats, chickpeas, low-fat cheese.
    • Fast release: candy, soda, white potatoes, instant oatmeal, white rice, fruit juice.
    • Transport form: Glucose.
    • Storage form: Glycogen in liver and muscle, converted into fats (adipose tissue) if excess.

  • Fats (lipids):

    • Saturated fats: butter, coconut oil, cream, pork, beef.
    • Unsaturated fats: flaxseed oil, salmon, almonds, sunflower seeds, olives.
    • Transport form: Free fatty acids and glycerol.
    • Storage form: Triglycerides in adipose tissue.

  • Protein:

    • Animal foods: lean beef, chicken breast, turkey, fish, eggs.
    • Plant foods: quinoa, tofu, black beans, chia seeds, edamame.
    • Transport form: Amino acids.
    • Storage form: Amino acids in adipose tissue.