Half Marathon Race-Day Fueling: Do You Need Gels During a 21K?

Introduction

“Is a gel really necessary for a half marathon, which is only 21 km?” This is one of the most common questions from runners preparing for their first half marathon. The answer is: it depends. Compared with a full marathon (42 km), the energy expenditure of a half marathon is relatively manageable. However, if your finish time exceeds 90 minutes, timely carbohydrate intake can help sustain your power output in the latter stages, preventing a “slowdown” or even a “bonk.” This article provides customized half-marathon fueling strategies for runners targeting different finish times.

Calculating Half-Marathon Energy Needs

Running energy expenditure is roughly 1 kcal per kilogram of body weight per kilometer. For a 60 kg runner completing a half marathon:

• Total expenditure is approximately 60 × 21 = 1260 kcal
• The body’s muscle glycogen stores are about 400–500g (1600–2000 kcal)
• In theory, muscle glycogen should not be completely depleted during a half marathon

But this calculation has a caveat: muscle glycogen must be fully loaded before the race, and the intensity cannot be too high (higher intensity relies more on glycogen). If you’re running a near-all-out half marathon, muscle glycogen may indeed run low in the second half.

Fueling Strategy by Finish Time

Key principle: After 75–90 minutes of exercise, blood glucose begins to drop. Even if muscle glycogen isn’t depleted, exogenous carbohydrates can still help maintain your pace.

How to Use Gels Correctly

Many runners fuel incorrectly, which leads to gastrointestinal discomfort:

• Take gels with water: Gels must be consumed with water (100–150 ml); otherwise, the high sugar concentration can irritate the stomach
• Don’t wait until you’re tired: Gels take 10–15 minutes to absorb, so take them early while you “still feel fine”
• Use brands you’ve tested before race day: Don’t try unfamiliar gel brands on race day to avoid stomach issues
• Caffeinated versions work better: Caffeine can reduce perceived fatigue; consider using a gel with 50 mg of caffeine after the 15 km mark

Aid Station Layout at Taiwan Half Marathons

Taking common large-scale half marathons in Taiwan as an example, aid stations are typically set up as follows:

• 5 km: Water (some races offer sports drinks)
• 10 km: Water + sports drink + food (bananas, salt candies)
• 15 km: Water + sports drink
• 18 km: Water (final aid station)

Since the food options provided at aid stations are limited, if you need gels, it’s recommended to carry your own, tucked into your shorts waistband or using a hydration belt.

Pre-Race Carbohydrate Loading (The Most Important Fueling Strategy)

Rather than relying on gels on race day, it’s better to fully fuel up beforehand:

• 3 days before the race: Consume 8–10g of carbs per kg of body weight daily (rice, noodles, sweet potatoes, bananas)
• Dinner the night before: High-carb, low-fiber (white rice or pasta; avoid beans and high-fiber vegetables)
• Breakfast on race day (2–3 hours before the start): 200–400g of carbs (toast with peanut butter + banana + sports drink)
• 15–30 minutes before the start: A small gel or 100 ml of sports drink to top off blood glucose

Hydration Strategy

Sweat loss during a half marathon varies with temperature:

• Taiwan autumn/winter (15–20°C): Sweat rate is about 0.5–1L/hr; drink 100–150 ml at each aid station
• Taiwan spring (25–30°C): Sweat loss increases significantly; drink 150–200 ml at each station and consider electrolyte replacement
• Don’t stop at aid stations for more than 10 seconds to drink; practice “drinking while walking” or “drinking while jogging”

Practical Tips

• Do a “gel practice” the week before the race: Simulate fueling during a 15–18 km long run to confirm your stomach’s tolerance
• Carry 2 gels (1 as backup); they’re lightweight (about 30g each) and won’t affect your running
• For Taiwan summer half marathons (e.g., March–April), heavy sweating due to high heat means hydration takes priority over gels
• If you have a sensitive stomach, switch to “energy chews,” which are easier to consume in portions

Conclusion

There’s no one-size-fits-all fueling strategy for a half marathon, but there is a consensus: for runs exceeding 90 minutes, proactively consuming carbohydrates helps sustain power output. Rather than guessing on race day, test repeatedly during training to find a fueling plan that suits your stomach and taste, and on race day you’ll only need to execute it.

Related Reading

• Full Marathon Fueling Strategy: Timing for Gels, Electrolytes, and Energy Bars
• Half-Marathon Race Fueling: Energy and Hydration Plans for Two Aid Stations
• Fueling by Distance: An In-Race Energy Map from 5K to Full Marathon
• Ultramarathon Nutrition Strategy: Energy Plans for 24-Hour Races