Why fueling matters
Your muscles store enough glycogen for roughly 90 minutes to 2 hours of hard running. In a marathon, many runners run short of it after about 30 kilometers or 20 miles, the infamous wall: the pace drops sharply and every step feels heavy. Taking in carbohydrate during the race and starting with full stores delays or prevents it.
Carbohydrate during the race
Sports nutrition guidelines recommend:
| Duration | Carbohydrate per hour |
|---|---|
| under 1 hour | none needed, or a mouth rinse |
| 1 to 2.5 hours | 30 to 60 g |
| over 2.5 hours | up to 90 g, from glucose and fructose |
The gut can absorb only about 60 g of glucose an hour; adding fructose, which uses a different transporter, lets well-trained runners absorb up to about 90 g.
How the calculator works
- Gels needed = carbohydrate target × (race time − 30 minutes) ÷ carbohydrate per gel, because most runners start fueling after 20 to 30 minutes.
- Gel interval spreads these gels evenly over the rest of the race.
- Fluids and sodium are totaled from your hourly plan.
- Carb loading uses 10 g per kilogram of body weight per day, the middle of the 8 to 12 g/kg recommended for the 36 to 48 hours before a race longer than 90 minutes.
Example
A runner aiming for 4 hours with a target of 60 g an hour and 22 g gels needs about 10 gels, one every 21 minutes after the first half hour. At 500 mL an hour, they drink about 2 liters over the race, with about 1,000 mg of sodium. A 150-pound runner would eat about 680 g of carbohydrate a day during carb loading.
Practical tips
- Train your gut: practice your race-day fueling during long runs, with the same products.
- Take gels with water, not with sports drink, to avoid an overly concentrated mix.
- Know the course: check where the aid stations are and what they offer.
- Drink to thirst or to a plan based on your sweat rate; do not drink so much that you gain weight, which risks hyponatremia.
- Breakfast: eat 1 to 4 g of carbohydrate per kilogram 1 to 4 hours before the start.
Limits
Needs vary with body size, pace, heat and individual tolerance. Some runners cannot tolerate more than 30 to 40 g an hour without stomach trouble. Adjust the plan in training, not on race day.