Nutrition for endurance athletes: fueling long runs, carb periodization, and race-day strategy backed by science and Garmin data

Nutrition is the most undertrained skill in endurance sport. Athletes spend months perfecting their VO2max, threshold, and long-run pacing, then sabotage race performance by under-fueling, mis-timing carbohydrate intake, or experimenting with nutrition on race morning. The research is clear: proper fueling can improve endurance performance by 5–15% compared to an ad-hoc approach — a far larger marginal gain than any legal supplement or training tweak.

The energy cost of endurance. A 70 kg runner burning roughly 1 kcal per kg per km uses approximately 2,940 kcal to complete a marathon. Glycogen stores in muscles and liver hold roughly 1,800–2,200 kcal in a well-fed athlete. That leaves a deficit of 700–1,100 kcal that must be covered by fat oxidation (limited at higher intensities) and exogenous carbohydrate intake during the race. For cyclists, a 4-hour gran fondo at moderate intensity costs 2,400–3,200 kcal. The same storage math applies. Without a structured fueling plan, every athlete hits the wall — it is not a question of fitness but of substrate availability.

Daily fueling by training phase. Base phase (moderate volume, low intensity): 5–7 g of carbohydrate per kg of body weight per day. At this phase, fat oxidation handles a larger share of the energy demand, and moderate carb intake supports recovery without excess. For a 70 kg runner, that is 350–490 g of carbs daily — rice, oats, potatoes, pasta, fruit, and bread as staples. Build phase (increasing intensity and volume): 7–9 g/kg/day. Quality sessions demand glycogen-replete muscles. Timing matters: consume 1–1.5 g/kg of carbs within 30 minutes of finishing a hard session to maximise glycogen resynthesis rates. A banana with a recovery shake, or rice cakes with honey, immediately post-session. Peak and race phase: 8–12 g/kg/day in the 36–48 hours before the event (carb loading). This is the most critical nutrition window in the entire training cycle.

Carb periodization: training the metabolic engine. Carb periodization means varying daily carbohydrate intake based on the training demand of that specific day, not eating the same macros regardless of schedule. High-carb days align with quality sessions (threshold, intervals, long runs). Low-carb days align with rest days or very easy sessions. This approach — sometimes called 'fuel for the work required' — improves metabolic flexibility, enhances fat oxidation during low-intensity efforts, and ensures glycogen stores are full when they matter most. A practical weekly template for a runner doing 70 km/week: Monday (rest): 4 g/kg carbs. Tuesday (threshold): 8 g/kg. Wednesday (easy): 5 g/kg. Thursday (intervals): 8 g/kg. Friday (easy): 5 g/kg. Saturday (long run): 9 g/kg. Sunday (recovery): 5 g/kg.

Fueling long runs: the gut training protocol. The number one reason athletes bonk in races is not lack of fitness — it is gastrointestinal intolerance to consuming carbohydrates at the rate needed during prolonged exercise. The gut is trainable. Over 4–6 weeks of progressive practice, athletes can increase their tolerance from 30–40 g/hr to 60–90 g/hr of exogenous carbs during exercise. Start by consuming one gel (25 g carbs) every 30 minutes during your weekend long run. After 2 weeks, increase to one gel every 20–25 minutes. By week 4–6, aim for 60–80 g/hr using a mix of glucose and fructose (the 2:1 ratio bypasses the intestinal glucose transporter bottleneck). Always practice with the exact products you plan to use on race day.

The glucose-fructose dual transport advantage. Intestinal glucose absorption maxes out at roughly 60 g/hr through the SGLT1 transporter. Adding fructose — absorbed via the GLUT5 transporter — allows an additional 30 g/hr, bringing the theoretical max to 90 g/hr. This is why modern sports nutrition products use maltodextrin (glucose polymer) plus fructose. For athletes racing 2+ hours, the difference between 60 g/hr and 80–90 g/hr of carb intake can mean 10–15 minutes over a marathon or 20+ minutes over an Ironman bike leg. This is the single largest legal performance gain available to most age-group endurance athletes.

Protein for endurance athletes: the overlooked macronutrient. Endurance athletes need 1.4–1.8 g of protein per kg of body weight per day — more than the general population but less than strength athletes. The key is distribution: 0.3–0.4 g/kg per meal, spread across 4+ eating occasions, maximises muscle protein synthesis rates. Post-session, combine protein with carbohydrate: 20–30 g of protein plus 1–1.5 g/kg of carbs within 30 minutes. This accelerates both glycogen resynthesis and muscle repair. Sources: lean meat, fish, eggs, dairy, legumes. For plant-based athletes, combining legumes with grains ensures complete amino acid profiles.

Hydration and electrolytes: the Garmin Body Battery connection. Dehydration of 2–3% body weight impairs endurance performance by 5–10%. But over-hydrating (hyponatremia) is equally dangerous and increasingly common in recreational marathon runners. The practical approach: weigh yourself before and after a representative training session to calculate your sweat rate. Most athletes lose 500–1,200 ml/hr depending on heat, humidity, and intensity. Replace 80% of losses during exercise — complete replacement is neither practical nor necessary. Sodium is the critical electrolyte: 500–1,000 mg per litre of fluid consumed during exercise, more in hot conditions. Garmin Body Battery recovery patterns correlate with hydration status — athletes who are chronically under-hydrated show slower overnight recovery and lower morning Body Battery scores. At CoachUpFit, we use this signal to flag hydration issues before they become performance problems.

Race-day nutrition protocol: the 48-hour sequence. T-minus 48 hours: begin carb loading at 8–10 g/kg/day. Reduce fibre, fat, and protein to make caloric room. White rice, pasta, bread, pancakes, jam, sports drink, dried fruit. T-minus 24 hours: continue loading. Final large meal at lunch, then smaller meals. No alcohol, no high-fibre foods, no unfamiliar items. Race morning (3–3.5 hours before start): 1–2 g/kg of easily digestible carbs — white toast with honey, a banana, porridge with jam. Coffee if habitual (150–200 mg caffeine). Sip 400–500 ml of fluid until 45 minutes before start. T-minus 15 minutes: one gel or 200 ml of sports drink. During race: 60–90 g/hr carbs from gels + sports drink at aid stations, starting at km 5 (or 15 minutes into a cycling event). Do not wait until you feel depleted — by then you are 20–30 minutes behind.

Common nutrition mistakes in endurance athletes. Chronic under-fueling: training in a caloric deficit day after day suppresses adaptation, increases injury risk, and leads to relative energy deficiency in sport (RED-S). If you are losing weight unintentionally during a training block, you are under-fueling. Fat phobia: endurance athletes need 20–30% of calories from fat for hormone production, cell membrane integrity, and absorption of fat-soluble vitamins. Cutting fat below 15% impairs testosterone, cortisol regulation, and recovery. Supplement over-reliance: no supplement replaces adequate calories, carbohydrate timing, and sleep. The only supplements with strong evidence for endurance performance are caffeine (3–6 mg/kg, 30–60 minutes pre-race), sodium bicarbonate (0.3 g/kg for events 1–7 minutes), and potentially beetroot juice (400 mg nitrate, 2–3 hours pre-race for events under 40 minutes). Everything else is marginal at best.

Tracking nutrition with training data. At CoachUpFit, we do not prescribe rigid meal plans — we teach athletes the framework and help them calibrate intake to training demand using objective signals. Garmin Training Load tells us the energy cost of each session. Body Battery recovery slopes tell us if overnight refueling and hydration are adequate. HRV trends flag when chronic under-fueling or over-training is suppressing the autonomic nervous system. The combination of nutrition periodization and real-time wearable data creates a feedback loop that self-managed athletes rarely achieve on their own. That is what elite coaching adds: not just a training plan, but the metabolic intelligence to fuel it correctly.