IB SEHS: Energy Systems and Nutrition

Understanding the intricate relationship between what you eat, how your body produces energy, and how you perform is a cornerstone of elite sport. For IB Sports Exercise and Health Science, mastering this topic is not just about memorizing facts; it's about learning to apply biochemical principles to optimize human performance, manage fatigue, and support long-term health. This knowledge forms the basis for effective coaching, credible athletic advising, and informed discussions on the ethics of modern sport.

Macronutrients and Their Role in Energy Systems

The body's energy currency is Adenosine Triphosphate (ATP). However, muscles store only enough ATP for a few seconds of explosive activity. For sustained exercise, three distinct energy systems—ATP-PC, glycolysis, and the aerobic system—replenish ATP using different fuels: carbohydrates, fats, and, to a minor extent, proteins.

Carbohydrates are the premier high-intensity fuel. Stored as glycogen in muscles and the liver, they are the exclusive substrate for the glycolytic (anaerobic) system, which provides rapid ATP without oxygen for activities lasting roughly 30 seconds to 2 minutes. During aerobic metabolism, carbohydrates are also efficiently broken down with oxygen. For example, a 400m runner relies heavily on the glycolytic system, depleting muscle glycogen, while a soccer player utilizes both anaerobic and aerobic carbohydrate metabolism throughout a match.

Fats, stored as triglycerides in adipose tissue and within muscle fibers, are a dense energy reserve. They are metabolized exclusively through beta-oxidation within the aerobic system, producing substantial ATP but at a slower rate than carbohydrates. Fats are the dominant fuel source during low to moderate-intensity exercise (e.g., a long, slow distance run) and at rest. The ability to efficiently use fat as fuel is a key adaptation in endurance training, conserving precious glycogen stores.

Proteins are not a primary energy source but contribute via gluconeogenesis (making new glucose) during prolonged endurance exercise, typically after 90+ minutes, when glycogen stores are critically low. Their main role is structural—repairing and building muscle tissue damaged during training. Inadequate protein intake impairs recovery and adaptation, while excessive intake does not translate to increased muscle or strength without the stimulus of resistance training.

Hydration, Electrolytes, and Thermoregulation

Hydration is fundamentally about maintaining homeostasis, particularly for thermoregulation and cardiovascular function. During exercise, sweat losses lead to hypohydration (body water deficit), which increases heart rate, reduces blood volume, elevates core temperature, and significantly impairs performance and cognitive function.

A strategic hydration plan involves more than just drinking water. It must account for electrolytes, primarily sodium lost in sweat. Sodium is critical for maintaining fluid balance, nerve impulse transmission, and muscle contraction. For sessions under 60 minutes, water is generally sufficient. For longer or more intense exercise, especially in heat, a sports drink containing carbohydrates (4-8% solution) and electrolytes (20-30 mmol/L sodium) helps maintain blood glucose, stimulate thirst, and promote fluid retention. Monitoring urine color and body weight changes before and after training are practical methods for assessing hydration status.

Nutritional Strategies: Pre-Competition, During, and Recovery

Nutrition timing manipulates fuel availability to enhance performance and accelerate recovery. Pre-competition nutrition (3-4 hours before) aims to top up liver and muscle glycogen stores with a high-carbohydrate, moderate-protein, low-fat/fiber meal to minimize GI distress. Carbohydrate loading is a specific protocol used by endurance athletes to super-compensate muscle glycogen stores by tapering training while increasing carb intake in the days before an event.

Nutrition during exercise is crucial for events lasting longer than 60-90 minutes. Consuming 30-60 grams of carbohydrates per hour (via gels, drinks, or easy-to-digest foods) helps maintain blood glucose and delay glycogen depletion, a phenomenon known as "hitting the wall" in marathon running.

Recovery nutrition focuses on the "anabolic window"—the 30-45 minute period post-exercise when muscles are most receptive to nutrient uptake. An effective recovery strategy has two key goals: refuel and repair. This is best achieved by consuming a combination of high-glycemic carbohydrates (to rapidly replenish glycogen) and high-quality protein (to stimulate muscle protein synthesis) in a 3:1 or 4:1 carb-to-protein ratio.

Dietary Supplements, Ergogenic Aids, and Ethical Issues

A dietary supplement is intended to supplement the diet, while an ergogenic aid is any substance or method that enhances performance. Many substances fall into both categories, and their efficacy, safety, and legality vary widely.

Common evidence-supported supplements include:

• Creatine Monohydrate: Increases phosphocreatine stores, enhancing power output in repeated high-intensity bursts (e.g., weightlifting, sprint intervals).
• Caffeine: A central nervous system stimulant that reduces perceived exertion, increases alertness, and can enhance endurance performance and high-intensity team sport activity.
• Beta-Alanine: Buffers hydrogen ions in muscle, delaying acidosis and fatigue during high-intensity exercise lasting 1-4 minutes.

The ethical landscape is governed by the World Anti-Doping Agency (WADA) Prohibited List. Substances are banned if they meet two of three criteria: they enhance performance, pose a health risk, or violate the "spirit of sport." This creates a clear ethical divide between legal supplements and banned performance-enhancing substances (PES) like anabolic steroids, EPO, and blood doping. A major ethical grey area exists with substances that are legal and widely used (like caffeine) but can be manipulated for ergogenic effect, raising questions about fairness and the pressure on athletes to seek any marginal gain.

Common Pitfalls

1. Prioritizing Supplements Over Whole Foods: A common mistake is investing in expensive "magic bullet" supplements while neglecting a foundational, balanced diet. No supplement can compensate for inadequate energy intake, poor carbohydrate timing, or chronic dehydration. Always optimize your basic nutrition first.

1. Overhydration with Plain Water (Hyponatremia): In long-duration events, drinking excessive amounts of plain water can dilute blood sodium levels, leading to hyponatremia, a dangerous and potentially fatal condition. The solution is to match fluid intake with sweat losses and ensure sodium intake during events lasting several hours.

1. Misapplying Protein Intake: Consuming a massive protein shake immediately after training is less effective than spreading protein intake evenly throughout the day (e.g., 20-40g per meal). The body's rate of muscle protein synthesis is saturable; excess protein at one time is simply oxidized for energy or stored as fat.

1. Ignoring Individuality in Nutrition Plans: Adopting a champion's diet without consideration for personal differences in physiology, sport demands, gut tolerance, and preferences is a recipe for failure. Effective sports nutrition must be personalized. What works for a marathoner will not work for a gymnast or a rugby forward.

Summary

• The three energy systems (ATP-PC, glycolytic, aerobic) utilize carbohydrates as the primary high-intensity fuel and fats as the dominant low-intensity fuel, with proteins playing a minor energetic role but a major structural role in recovery.
• Hydration strategies must balance water and electrolyte (especially sodium) replacement to maintain cardiovascular function and thermoregulation, preventing both hypohydration and hyponatremia.
• Nutrition is periodized: pre-competition meals maximize glycogen stores, fuel during exercise maintains blood glucose, and a recovery protocol (3:1 carb-to-protein) within 45 minutes post-exercise optimizes refueling and repair.
• While some dietary supplements like creatine and caffeine have strong ergogenic evidence, their use exists within a complex ethical framework defined by the WADA Prohibited List, separating legal aids from banned performance-enhancing substances.
• The most effective nutrition plan is individualized, prioritizes whole foods, and is grounded in the biochemical principles of human energy systems.