Module 1 (3): Bioenergetics and Energy Substrates (Pg. 53-58)

“Hitting the Wall” in Marathon Running

What does “Hitting the Wall” mean?

• A sudden drop in energy, usually around miles 20-22

• Symptoms: heavy legs, tingling/numbness, mental fog, extreme fatigue

Why does it happen and its impact on performance

• The body burns CHO and fats for energy. primarily relying on CHO (glycogen) as it is a means of provide energy quickly, but stores are limited (about 2000-2200 calories stored)

• Once glycogen is depleted, body switches to → fat, but it is slower in breaking down for energy

• Leads to a slower pace due to a slower fat metabolism

Methods of preventing “Hitting the Wall”

• (1) Carb loading before a race to maximize glycogen stores

• (2) Consuming carbohydrates (gels, sports drinks) during the run

• (3) Training the body to use fat more efficiently through long runs and proper nutrition

Energy Conversion in Nature

• Photosynthesis: plants convert sunlight → stored energy

  • Humans obtain energy by eating such plants or animals that consume plants

Macronutrients as Energy Sources

• Carbohydrates, fats, and proteins are the primary energy substrates

  • These are broken down to → release stored energy through chemical pathways

Bioenergetics

The process of converting food into energy at the cellular level

Metabolism

The sum of all chemical reactions in the body

Measurements of Energy

Measured in calories (cal) = heat required to raise 1g of water by 1°C

• Kilocalories (kcal): 1 kcal = 1,000 cal

  • But … kilocalorie and calorie are used synonymously, thus when one reads that someone eats or expends 3,000 cal per day, it really just means the person in ingesting or expending 3,000 kcal per day

Energy Substrates

• Energy is stored in chemical bonds of → macronutrients (carbon, hydrogen, oxygen, nitrogen)

  • The breakdown of these bonds (catabolism) results in energy being released and stored as ATP

Macronutrient Energy Sources

• At rest: energy comes equally from carbohydrates and fats (WRONG?) → should rely mostly on fats as there is no increase in demand for energy

• Intense, short-duration exercise: relies primarily on → carbohydrates

• Prolonged, lower-intensity exercise: uses a mix of → carbohydrates and fats

• Proteins → mainly used for enzymes and structural roles but contribute little to energy metabolism

Primary Fuel Source: Carbohydrates

• Carbohydrates → converted to glucose (six-carbon monosaccharide) transported in the blood

• Storage form: stored in the cytoplasm of muscle cells as glycogen in → muscles and the liver

  • Muscle glycogen: used for ATP production during exercise

  • Liver glycogen: converted to glucose (excess) and released into the bloodstream for energy

• Limits to glycogen stores

  • Depleted during prolonged, intense exercise

  • Requires dietary carbohydrate intake for replenishment

Importance of Carbohydrates for Brain Function

• Brain tissue → relies exclusively on glucose for energy

  • Severe carbohydrate depletion can → impair cognitive function and muscle performance

Fuel/Energy Source: Fats

• Primary fuel source during → prolonged, low-intensity exercise + at rest?

• Possess larger energy reserves compared to carbohydrates

• Stored as triglyceride → broken down into → glycerol and free fatty acids (FFAs) for ATP production

Limitations of Fat Metabolism

• (1) Slower rate of release compared to carbohydrates

• (2) Cannot fully meet the high energy demands of intense exercise

Comparison of Fat and Carbohydrate Energy Yield

• Fat: 9.4 kcal per gram

• Carbohydrate: 4.1 kcal per gram

• Fat → provide more than double the energy per gram compared to carbohydrates

  • But … the energy release is too slow to meet all of the energy demands on intense of muscular activity

Fuel/Energy Source: Protein

Only plays a minor role in energy production

• Not a primary energy source under normal conditions

• Used mainly when → carbohydrate and fats stores are → low

  • e.g., starvation, prolonged intense exercise

• Can contribute up to → 10% of total energy during endurance exercise

Gluconeogenesis

Process where conversion of protein or fat gets converted into glucose for energy

Lipogenesis

Process where conversion of protein into fatty acids for storage or energy use

Energy Yield of Proteins

• Proteins like carbohydrates → provide 4.1 kcal per gram

• Only amino acids, the building blocks of proteins, can be used for energy

Factors Controlling the Rate of Energy Production

• (1) Substrate availability

• (2) Enzyme activity

• (3) Rate-limiting enzymes and negative feedback

• (4) Factors affecting enzyme function

Substrate Availability

• Body tends to use more of the substrate that is → readily available

  • e.g. if carbohydrate stores are high, the body relies more on carbohydrate for energy rather than fat or protein

  • This is known as the → mass action effect

Enzyme Activity

Enzymes play a key role in controlling energy production by → speeding up reactions (catalysis) through:

• Lowering activation energy, making reactions occur faster

• Not changing the total amount of energy produced → only the speed at which it is released

• Enzymes usually have names ending in “-ase”

  • e.g., ATPase breaks down ATP to release energy

Rate-limiting Enzymes and Negative Feedback

Metabolic pathways involve multiple enzyme-catalyzed steps

• Rate-limiting enzyme: a specific enzyme that control the → overall speed of the pathway

• These enzymes are regulated through → negative feedback

  • What does this mean?

    • Too much product accumulation → enzyme slows down

    • This prevents excessive energy production and maintains balance

Factors Affecting Enzyme Function

• (1) Temperature:

  • Increased temperature → speed up enzyme activity (to a point)

• (2) pH levels

  • Changes in acidity can enhance or inhibit enzyme function

• (3) Cofactors

  • Some enzymes require addition helper molecules → cofactors to work properly