Blog
Endurance Athlete Fueling
Stephen M. Walker II • February 16, 2026
Endurance training depletes glycogen at 1 to 3 g per minute. A well-fueled athlete stores approximately 400 to 500 g total. A two-hour long run at race pace can drain over 250 g. When glycogen runs low, the body shifts to fat oxidation, which cannot sustain the same power output. The practical consequence is simple: endurance performance is constrained by fuel availability, and fueling strategy must scale to training demands.
Carbohydrate Targets by Training Load
The 2024 International Consensus Conference on Optimizing Elite Athletic Performance (Copenhagen, 29 leading scientists, Bangsbo et al., Scandinavian Journal of Medicine and Science in Sports) explicitly recommended scaling carbohydrate intake to the demands of specific training sessions.
| Training load | Carbohydrate target | Example for 70 kg athlete |
|---|---|---|
| Rest day or light activity | 3 to 5 g/kg/day | 210 to 350 g |
| Moderate session (60 to 90 min at moderate intensity) | 5 to 7 g/kg/day | 350 to 490 g |
| Hard session (90 to 120 min or high intensity) | 6 to 8 g/kg/day | 420 to 560 g |
| Very hard or multiple sessions | 8 to 10 g/kg/day | 560 to 700 g |
| Race day or extreme volume | 10 to 12 g/kg/day | 700 to 840 g |
The difference between a rest day and a hard training day for a 70 kg runner is 140 to 350 g of additional carbohydrate. That is 560 to 1,400 additional calories from carbs alone. Ignoring this scaling means either chronically under-fueling hard days or over-fueling easy days. Both compromise the training block.
Day-Before Fueling
The 24 hours before a hard session matter as much as the session itself. Glycogen resynthesis takes 24 to 48 hours with adequate carbohydrate intake. Starting a hard session with partially depleted stores because you under-ate the day before means you begin the workout already compromised.
If Wednesday calls for a threshold workout, Tuesday evening needs a carbohydrate-rich meal that prioritizes glycogen loading. A dinner of pasta, rice, or potatoes with a moderate protein source sets up the next day's session.
During-Session Fueling
For sessions exceeding 60 to 90 minutes, during-session carbohydrate becomes a meaningful performance variable.
| Session duration | Carbohydrate target | Delivery method | Gut training required |
|---|---|---|---|
| Under 60 min | Mouth rinse or none | N/A | No |
| 60 to 90 min | 30 to 60 g/hr | Sports drink, single gel every 30 to 45 min | Minimal |
| 90 to 150 min | 60 to 90 g/hr | Glucose and fructose combination (dual transport) to maximize absorption | Yes. Practice in training before race day. |
| 150+ min | 80 to 120 g/hr | Mixed sources with practiced protocol. Gels, chews, drink, and real food for ultra-distance. | Yes. Gut tolerance at this volume must be trained systematically. |
The glucose-fructose combination exploits separate intestinal transport pathways (SGLT1 for glucose, GLUT5 for fructose), allowing higher total carbohydrate absorption than glucose alone. Athletes who attempt 90+ g/hr on race day without having practiced it in training frequently experience gastrointestinal distress. Gut training for race nutrition is the difference between having the right target on paper and being able to hold that target deep into the event.
Protein During High-Volume Blocks
Endurance athletes tend to under-prioritize protein because the caloric demands of training crowd it out. When you need 3,000 to 4,000 cal/day and most of the additional energy needs to come from carbohydrate, protein can drift below optimal levels unless you actively protect it.
| Parameter | Target | Rationale |
|---|---|---|
| Minimum daily intake | 1.6 g/kg/day (70 kg athlete = 112 g minimum) | Supports eccentric damage repair, immune function, and structural adaptation |
| Distribution | 30 to 40 g per meal across 3+ meals | Sustained amino acid supply for repair throughout the day |
| Source priority during high-carb phases | Lean sources: chicken, fish, egg whites, low-fat dairy | Delivers protein without the caloric density that competes with carbohydrate needs |
Race-Week Protocol
| Day | Training | Nutrition focus |
|---|---|---|
| Monday (6 days out) | Normal easy run | Normal intake |
| Tuesday (5 days out) | Light session | Begin increasing carbs to 7 to 8 g/kg |
| Wednesday (4 days out) | Easy or rest | 8 to 10 g/kg carbohydrate |
| Thursday (3 days out) | Shakeout run | 10 to 12 g/kg carbohydrate |
| Friday (2 days out) | Rest | 10 to 12 g/kg, familiar foods only |
| Saturday (1 day out) | Rest or light activation | 10 to 12 g/kg, early dinner, hydrate |
| Sunday (race day) | Race | Pre-race meal 3 to 4 hours before start (1 to 4 g/kg carbohydrate from familiar, well-tolerated foods). During-race fueling per practiced protocol. |
The loading phase prioritizes low-fiber, easily digestible carbohydrate sources to minimize gastrointestinal risk on race day. White rice, pasta, pancakes, and fruit juice are common choices. Nothing new on race day. If your fueling plan includes aggressive carbohydrate targets, rehearse the exact breakfast, fluid timing, and gel schedule in training rather than improvising on the start line. The full progression is covered in Gut Training for Race Nutrition, and the full 48-hour execution model lives in How to Set Up a Race-Week Nutrition Plan.
REDs Risk Signals
Relative Energy Deficiency in Sport (REDs), recognized by the IOC consensus statement, develops when energy availability (total intake minus exercise expenditure) falls too low for too long.
| Signal | Threshold or pattern | Action |
|---|---|---|
| Energy availability below 30 kcal/kg fat-free mass/day | Calculate from intake minus exercise expenditure | Reduce training volume or increase intake immediately |
| Menstrual irregularities | Any change from baseline | Screen for low energy availability |
| Recurring illness during training blocks | More than 2 episodes in 8 weeks | Assess energy availability and total stress load |
| Unexplained performance decline | Persistent fatigue, mood changes despite adequate training | Medical evaluation. Do not assume it is just training stress. |
The risk is highest when training volume increases without a corresponding increase in energy intake. An athlete who adds 20 km per week without eating more can slide into low energy availability without any conscious intention to restrict. The symptoms develop gradually and are often attributed to training fatigue rather than inadequate fueling. For the female endurance athlete version of that problem, including menstrual changes, ferritin drift, and the intake patterns that usually cause it, read Low Energy Availability in Female Endurance Athletes.