Carbohydrate Metabolism in Sports and Exercise

Relevance of Carbohydrates in Sports and Exercise

• Carbohydrates (CHO) are a critical energy source for performance.

• The right dose of carbohydrates is essential, as excess can cause damage and disease.

  • Paracelsus (1493-1541): "All substances are poisons; the right dose differentiates a poison and a remedy."

Relevance of Carbohydrate for Metabolism

• Instantaneous energy source: 4 kcal/g, with or without oxygen.

• Protein sparing: Preserves protein by providing an alternative energy source.

• Stored as glycogen in muscle and liver.

  • Liver glycogen stores are about 5x higher than in skeletal muscle.

• Can be converted to fat for energy storage.

• Keeps the gastrointestinal (GI) tract healthy.

Importance of CHO for Exercise

• CHO is the most important energy source for athletic performance.

• Increasingly important in competition at >85% VO2max.

• CHO fuels about 70% of exercise at this intensity.

Four Steps of Carbohydrate Metabolism

• Step 1: Glycolysis

• Step 2: The Link Reaction

• Step 3: The Krebs Cycle

• Step 4: The Electron Transport Chain (ETC)

Step 1: Glycolysis

• Breakdown of glucose to two molecules of pyruvate.

• Occurs in the cytoplasm.

Step 2: The Link Reaction

• Breakdown of pyruvate to Acetyl CoA.

• Pyruvate is transported into the mitochondria.

Step 3: Krebs Cycle

• Hans Krebs: Pioneer of cellular respiration (Nobel Prize in 1953).

• >75% of the original energy in glucose is still present in two molecules of pyruvate.

• If oxygen is present, pyruvate is completely metabolized to CO_2.

• Takes place in the mitochondria.

• Acetate from acetyl CoA combines with oxaloacetic acid (OAA) to form citrate (citric acid).

• Each turn of the cycle produces:

  • 1 ATP by substrate-level phosphorylation

  • 3 NADH

  • 1 FADH_2

• Purpose is to 'steal' electrons (with H) from carbon molecules to NADH and FADH_2.

Krebs Cycle Summary (Per Pyruvate )

• 3 CO_2

• 4 NADH + H^+

• 1 FADH_2

• 1 ATP

Krebs Cycle Summary (Per Glucose)

• 6 CO_2

• 8 NADH + H^+

• 2 FADH_2

• 2 ATP

Step 4: Electron Transport and Oxidative Phosphorylation

• Most ATP comes from the energy of electrons carried by NADH and FADH_2.

• Energy in electrons powers ATP synthesis.

• The electron transport chain has thousands of copies in the cristae of the mitochondria.

Step 4 Sub-Processes

• Electron transport

• Chemiosmosis

Step 4i: Electron Transport

• NADH and FADH_2 donate electrons to the ETC.

• Electrons are passed along the ETC in a series of oxidation-reduction (redox) reactions.

  • Oxidation: Loss of electrons (LEO)

  • Reduction: Gain of electrons

• NADH excites complex 1, pumping H^+ into the intermembrane space.

• Complex 1 passes the electron to CoQ.

• FADH_2 hands its electron to complex 2; no H^+ pumping.

• Complex 2 passes electrons to CoQ.

• Complex 3 pumps H^+ ions into the intermembrane space.

• H^+ ions flow down their concentration gradient through ATP synthase (complex 5) via chemiosmosis.

Step 4ii: Chemiosmosis

• Peter Mitchell: Nobel Prize in Chemistry in 1978.

• Energy from electron transport is used to pump H^+ across the inner mitochondrial membrane.

• This creates a proton concentration gradient.

• Backflow of H^+ through ATP synthase synthesizes ATP from ADP and Pi.

• Chemiosmosis converts a H^+ gradient into ATP, which is critical for fueling muscle contractions during extended periods of exercise.

• Uses potential energy of H^+ to induce backflow through ATP synthase.

• ATP synthase is a molecular machine converting released energy into ATP.

Complexes and Supercomplexes

• Electron transport complexes are not simply arranged in series

• They form supercomplexes where individual complexes associate with each other.

Measuring Changes in Carbohydrate Metabolism

Respiratory Quotient (RQ) and Respiratory Exchange Ratio (RER)

• RER of 0.7 indicates burning fat; RER of 1.0 indicates burning 100% CHO.

• CHO RER = 1.0 because:

  • 6 molecules of O2 are used in glucose oxidation producing 6 molecules of CO2.

  • C6H{12}O6 + 6 O2 6 CO2 + 6 H2O + 38 ATP

• Fat is oxidized with an RQ/RER of 0.70.

  • 23 molecules of O2 are used for fat oxidation, producing 16 molecules of CO2 (16/23 = RQ = 0.70).

  • C{16}H{32}O2 + 23 O2 \rightarrow 16 CO2 + 16 H2O + 130 ATP

Control of Blood Glucose and Glucose Uptake

Pancreas and Liver Control

• Alpha-cells: Glucagon (stimulates liver breakdown of glycogen to release glucose into blood).

• Beta-cells: Insulin (stimulates muscle and fat cells to absorb glucose).

Insulin vs. Contraction-Induced Glucose Uptake

• Insulin uses a PI3k-dependent mechanism to induce GLUT-4 translocation.

• Exercise acts independently of PI3k, likely through Ca^{2+} release.

• Exercise-stimulated glucose uptake is preserved in insulin-resistant muscle.

• Therefore, good therapy in type II diabetes.

Adaptations to Carbohydrate Metabolism with Training

Substrate Utilization

• With training, there is a rightward shift of the CHO utilization curve.

• CHO is spared at higher exercise intensities (both relative and absolute intensities).

• More use of fats.

Molecular Adaptations

• Endurance training increases aerobic enzymes.

Molecular Adaptations Examples

• Swimming training:

  • Significant increase in Citrate Synthase activity (mmol .kg-1. min-1) and Succinate dehydrogenase activity (\mu$$mol/g) with training distance (m/day) and duration (months).

• Aerobic vs. Anaerobic Training:

  • Significant increase in select aerobic enzymes (Succinate dehydrogenase, Malate dehydrogenase) with Aerobic Training.