Nutrition Basics: Fueling Your Training

Endurance nutrition revolves around three macronutrients scaled to body weight and training load: carbohydrates (3-12 g/kg/day depending on training volume), protein (1.2-2.0 g/kg/day), and fat (minimum 0.8-1.0 g/kg/day). Pre-workout fueling means consuming 1-4 g/kg of carbohydrates 1-4 hours before training. During exercise, carbohydrate needs range from zero (for sessions under 60 minutes) to 60-90 g/hour for sessions over 2.5 hours, using dual-source glucose-fructose products to maximize absorption rates. Post-workout recovery requires 0.3 g/kg protein plus 1.0-1.2 g/kg carbohydrates within 60 minutes. For races over 90 minutes, carbohydrate loading (10-12 g/kg/day for 1-3 days) can increase glycogen stores by 50-100%. Hydration targets 400-800 ml/hour with 500-1000 mg sodium per hour for efforts exceeding 2 hours.

Macronutrients: The Three Pillars

Carbohydrates: Your Primary Training Fuel

Carbohydrates are stored as glycogen in muscles (approximately 300-500g) and the liver (approximately 80-100g), providing the dominant fuel source for moderate-to-high intensity exercise. Unlike fat stores (which are virtually unlimited), glycogen stores are limited and can be depleted in 60-120 minutes of hard exercise.

The ACSM/AND/DC joint position statement (Thomas et al., 2016) provides carbohydrate guidelines scaled to training load:

For a 70 kg triathlete training 10 hours per week with one session on most days, daily carbohydrate intake on training days would be approximately 350-490g (5-7 g/kg). On rest days, this drops to 210-350g (3-5 g/kg). Quality carbohydrate sources include rice, oats, potatoes, sweet potatoes, pasta, bread, fruit, and legumes.

Protein: Building and Repairing

Protein serves two critical functions for endurance athletes: repairing exercise-induced muscle damage and supporting the structural adaptations (mitochondrial biogenesis, enzyme synthesis) driven by training. The ISSN position stand (Jager et al., 2017) recommends 1.4-2.0 g/kg/day for athletes, which is meaningfully higher than the general population recommendation of 0.8 g/kg/day.

Key protein principles:

• Daily total: 1.2-2.0 g/kg/day (84-140g for a 70 kg athlete)
• Distribution matters: Spread intake across 3-4 meals with 0.25-0.40 g/kg per meal (18-28g for a 70 kg athlete). Protein synthesis has a ceiling per meal -- eating 60g in one sitting is not twice as effective as 30g.
• Leucine threshold: Each protein-containing meal should provide approximately 2-3g of leucine, the amino acid that triggers muscle protein synthesis. This is easily achieved with 25-30g of high-quality protein (dairy, eggs, meat, fish, soy).
• Pre-sleep protein: 30-40g of casein-rich protein (such as Greek yogurt or cottage cheese) before bed supports overnight recovery.

Fat: Essential but Often Misunderstood

Dietary fat is essential for hormone production (including testosterone and estrogen), absorption of fat-soluble vitamins (A, D, E, K), cell membrane integrity, and as a fuel source during low-intensity exercise. However, fat should not displace carbohydrates during heavy training periods.

• Minimum intake: 0.8-1.0 g/kg/day (56-70g for a 70 kg athlete)
• Recommended range: 20-35% of total energy intake
• Quality sources: Olive oil, avocado, nuts, seeds, fatty fish (salmon, mackerel), eggs
• Caution: Low-fat diets (<15% of energy from fat) impair hormone production and are not recommended for athletes

Timing: Before, During, and After Training

Pre-Workout Nutrition

The goal of pre-workout nutrition is to top off liver glycogen (which depletes overnight during fasting), maintain blood glucose availability, and avoid gastrointestinal distress during exercise.

As the window before exercise shrinks, food choices should shift toward simpler, lower-fiber, lower-fat options to speed gastric emptying. Avoid high-fat and high-fiber meals within 2 hours of exercise, as these slow digestion and increase GI distress risk.

During Exercise: Fueling on the Move

Jeukendrup (2014) established the definitive framework for carbohydrate intake during exercise:

The 60-90 g/hour range for long efforts requires specific attention. The intestinal transporter for glucose (SGLT1) saturates at approximately 60g/hour. To exceed this rate, you must also use the fructose transporter (GLUT5) by consuming a mix of glucose and fructose. Most commercial sports nutrition products for long-course racing now use this dual-source approach.

Gut training is mandatory. The gut is a trainable organ. Athletes who practice race-day nutrition during training sessions increase intestinal absorption capacity and reduce GI distress. Start with 30g/hour in training and gradually work up to target race rates over 4-6 weeks.

Post-Workout Recovery

The post-workout window is about replenishing glycogen stores and initiating muscle repair. While the "anabolic window" is not as narrow as once believed, consuming recovery nutrition within 60 minutes of training is still optimal, particularly when the next session is within 24 hours.

• Protein: 0.25-0.40 g/kg (18-28g for a 70kg athlete) -- approximately 20-30g of high-quality protein
• Carbohydrate: 1.0-1.2 g/kg (70-84g for a 70 kg athlete) -- critical for glycogen replenishment
• Practical examples: Chocolate milk (a well-researched recovery option), Greek yogurt with granola and fruit, rice bowl with chicken, recovery shake with protein and banana

If your next training session is more than 24 hours away, the urgency of immediate post-workout nutrition decreases -- normal meals throughout the day will accomplish the same glycogen replenishment. However, the protein component remains valuable for stimulating muscle repair regardless of the next session timing.

Race Fueling: Carbohydrate Loading

For races lasting longer than 90 minutes, carbohydrate loading increases muscle glycogen stores by 50-100% above normal levels (Burke et al., 2011). The modern protocol is simpler than the old depletion-supercompensation method:

• 1-3 days before race: Increase carbohydrate intake to 10-12 g/kg/day (700-840g for a 70 kg athlete)
• Reduce training volume: Taper normally; the combination of high carb intake and reduced training maximizes glycogen storage
• Choose familiar foods: This is not the time to experiment. Stick to white rice, pasta, bread, potatoes, and other well-tolerated carbohydrate sources
• Reduce fiber and fat: These fill you up without contributing to glycogen storage and can cause race-day GI issues

Hydration and Electrolytes

Hydration needs are highly individual and vary with body size, sweat rate, temperature, and humidity. General guidelines:

• During exercise: 400-800 ml/hour, adjusted based on sweat rate and conditions
• Sodium: 500-1000 mg/hour for efforts exceeding 2 hours (approximately 1.0-2.0g of salt per hour)
• Monitoring: Body weight before and after training can estimate sweat rate. Aim to limit weight loss to less than 2% of body weight during exercise.
• Hyponatremia risk: Drinking too much plain water during long events can dilute blood sodium to dangerous levels. Always include sodium in your hydration strategy for events over 2 hours.

Example: Daily Nutrition Plan for a 70 kg Triathlete Training 10 Hours/Week

Training day (1.5 hours of training: morning swim + evening run)

Rest day

On rest days, reduce carbohydrates to 3-4 g/kg (210-280g), maintain protein at 1.4-1.6 g/kg (100-112g), and keep fat at 0.8-1.0 g/kg (56-70g). Total calories would be approximately 2,100-2,300 kcal. The main adjustment is reducing portion sizes of rice, pasta, and bread at meals while keeping protein portions consistent.

Practical Rules for Training Nutrition

1. Scale carbohydrates to training load, not to a fixed number. A 3-hour ride day requires significantly more carbohydrates than a rest day. Periodize your nutrition alongside your training.

2. Hit your protein target every day -- including rest days. Muscle repair happens around the clock, not just during the post-workout window. Distribute 1.2-2.0 g/kg across 3-4 meals.

3. Never try new nutrition on race day. Every gel, drink, and solid food you plan to use in a race should have been tested multiple times in training. GI distress has ended more races than fitness deficits.

4. Train your gut. If your race plan calls for 60-90g of carbohydrate per hour, practice this rate in training sessions starting 6-8 weeks before race day. Begin at 30g/hour and increase by 10-15g/hour per week.

5. Do not restrict calories during heavy training. Under-fueling impairs recovery, increases injury risk, and can lead to Relative Energy Deficiency in Sport (RED-S). If you need to lose weight, do so during lower-volume training phases.

6. Use the dual-source approach for long events. For any effort over 2.5 hours, choose products containing both glucose (or maltodextrin) and fructose to maximize carbohydrate absorption beyond the 60g/hour glucose ceiling.

7. Prioritize sodium in hot conditions. For events over 2 hours, especially in heat, consume 500-1000 mg sodium per hour. This prevents hyponatremia and supports fluid absorption.

8. Carbohydrate load before long races. For events over 90 minutes, increase carbohydrate intake to 10-12 g/kg/day for 1-3 days before the race. Combined with a taper, this maximizes glycogen stores.

9. Post-workout nutrition matters most when sessions are close together. If you train twice in one day or have a key session the next morning, prioritize 1.0-1.2 g/kg carbs plus 0.3 g/kg protein within 60 minutes post-training. If your next session is 24+ hours away, normal meals are sufficient.

10. Do not fear fat -- but do not displace carbohydrates with it. Fat is essential for health and hormone function. Minimum intake of 0.8-1.0 g/kg/day from quality sources. However, during heavy training blocks, carbohydrate availability is the primary performance limiter.

Evidence Base

The ACSM/AND/DC joint position statement (Thomas, Erdman & Burke, 2016) represents the most authoritative consensus on sports nutrition. It synthesized decades of research to provide evidence-based macronutrient ranges scaled to body weight and training load, replacing the older percentage-of-calories approach. Their carbohydrate recommendations (3-12 g/kg/day depending on activity level) and protein recommendations (1.2-2.0 g/kg/day) form the foundation of current practice.

Jeukendrup (2014) provided the definitive framework for carbohydrate intake during exercise, building on his extensive research into intestinal absorption mechanisms. His key finding -- that glucose absorption via SGLT1 saturates at approximately 60g/hour, but adding fructose (absorbed via GLUT5) allows total carbohydrate oxidation rates of up to 90g/hour -- revolutionized fueling strategies for ultra-endurance events. His work also established the concept of "gut training," showing that the intestine adapts to higher carbohydrate loads with repeated practice.

Burke et al. (2011) consolidated the evidence on carbohydrate loading and daily carbohydrate needs for athletes. Their work demonstrated that the modern abbreviated carbohydrate loading protocol (1-3 days of 10-12 g/kg/day without prior depletion) is as effective as the older, more complex depletion-supercompensation protocol, and far more practical for athletes. They also provided the sliding-scale model for daily carbohydrate intake that aligns intake with training demands rather than using a single fixed recommendation.

Jager et al. (2017) established the ISSN position on protein and exercise, synthesizing evidence that optimal protein intake for athletes (1.4-2.0 g/kg/day) is substantially higher than the general RDA (0.8 g/kg/day). Their work emphasized the importance of protein distribution across meals, the leucine trigger concept for muscle protein synthesis, and the role of pre-sleep protein intake in supporting overnight recovery. These findings shifted the conversation from total daily protein to the quality and timing of protein distribution throughout the day.

Taken together, these position statements and reviews provide a robust, evidence-based framework for endurance athlete nutrition that replaces guesswork with precision. The key principle throughout is individualization: macronutrient needs are not fixed but scale dynamically with training load, body composition goals, and competition demands.