Wk 9&10 Lecture 2: Fat Mobolization

The Adventures of Sun Yi

• Introduction: Following Sun Yi, a runner on the Canadian Olympic team, as she participates in a marathon, detailing her physiological adaptations and energy sources.

• Sun Yi's current race position: two hours into a marathon, at the 32 km mark of a 42 km race.

Background on Energy Sources

• Energy Provision: Discussed in previous lectures (focus on glucose breakdown, mitochondrial oxidation, oxidative phosphorylation).

• Fat as Fuel: Exploring the switch from carbohydrates (glycogen) to fats during prolonged exercise, especially in endurance events.

• Fat Structure: Triacylglycerols (triglycerides) as efficient storage molecules composed of fat (glycerol + fatty acids).

Triacylglycerols and Fat Storage

• Composition of Fats: Composed primarily of triglycerides. Glycerol acts as a backbone with fatty acid esters linked.

• Adipose Tissue: Location of stored triglycerides, facilitating energy production via fatty acid breakdown.

• Fat Cell Structure: Large central droplet of fat surrounded by cytoplasm containing nucleus, mitochondria, etc.

  • Fat cells' role: Management of energy reserves, controlling fat use and synthesis.

Understanding Fatty Acids

• Structure of Fatty Acids: Comprising a carboxyl group and long hydrophobic chain with an even number of carbons.

• Saturated vs Unsaturated Fats:

  • Saturated Fatty Acids: No double bonds, solid at room temperature (e.g., palmitic acid, stearic acid).

  • Unsaturated Fatty Acids: One or more double bonds, typically liquid at room temperature (e.g., oleic acid, linoleic acid).

• Omega Fatty Acids: Classifications based on the position of the last double bond (omega-3, omega-6, omega-9).

Energy Sources During the Marathon

• Energy Dynamics in Sun Yi: High reliance on glycogen reserves initially, transitioning to fat breakdown as she continues running.

• Glycogen Depletion: Entered the race with 235 grams but has already used 60 grams by the 32 km mark.

• Fatty Acid Utilization: 40 grams of body fat are being broken down to meet energy demands at 32 km.

• Shift in Energy Source: Transition from ~80% glycogen usage to a more even 60% carbohydrates and 40% fats.

Biochemical Responses to Exercise

• Hormonal Control: Adrenaline increases during exercise, enhancing glycogenolysis (glycogen breakdown) in muscle, while glucagon supports glucose release in the liver.

• Metabolism in Muscles vs. Liver: Muscle primarily metabolizes glycogen for its own needs, while liver exports glucose to maintain blood glucose levels.

• Energy Yield: Fats provide 9 kilocalories per gram, while carbohydrates provide 4 kilocalories per gram.

Lipolysis and Fat Mobilization

• Mechanism of Lipolysis: Breakdown of triglycerides into free fatty acids and glycerol via lipase enzymes.

• Hormonal Activation: Exercise-induced levels of adrenaline and glucagon stimulate fat mobilization in adipose cells.

• Caffeine's Role: Inhibits phosphodiesterase, prolonging cyclic AMP activity which favors fat mobilization.

Fatty Acid Activation

• Process of Activation: Fatty acids must be converted to acyl-CoA for metabolism—this requires ATP and CoA, performed by acyl-CoA synthetase.

• Energy Investment: Overall, two ATP equivalents are required for the formation of one molecule of fatty acyl CoA.

Transport of Fatty Acids into Mitochondria

• Carnitine Shuttle: Fatty acyl-CoA cannot penetrate the inner mitochondrial membrane; instead, they are converted to acyl-carnitine by carnitine acyltransferase and transported across.

• Importance of Regulation: The carnitine shuttle offers control over the rate of fatty acid oxidation, necessary during transitions between exercise states.

Key Takeaways for Endurance Athletes

• Glycogen vs. Fat: Awareness of energy use differentiation and management important for sustained performance, especially in endurance events like marathons.

• Hydration and Heat: Significant water loss during the race necessitates hydration to avoid heat-related issues.

• Nutritional Strategy: Pre-race meal choices can influence performance, including carbohydrates and coffee for endurance support.

• Control Mechanism of Fat Utilization: Emphasizes the critical balance and strategic use of energy reserves during prolonged physical activity.