Most ultra DNFs start with gel rejection at hour 6–8. The runner stops being able to swallow another sweet syrup, stops fueling, and the cascade begins. The fix: pick gels that work for the long miles, train your gut on them, and rotate flavors so nothing becomes intolerable.
The picks
Maurten
Maurten Gel 100
The hydrogel that changed gel science. 25g of high-concentration carb in a non-sweet, non-sticky formula. Most marathon and ultra elites race on these. Your gut will tolerate them when nothing else works.
- 25 g carb per gel · 100 cal
- Hydrogel — bypasses gut absorption issues
- No artificial flavor, no caffeine
- Used by Eliud Kipchoge, Courtney Dauwalter
Spring Energy
Spring Energy McRae Gels
The real-food gel. Made in Boulder from rice, salt, ginger, lemon. Tastes like savory soup; agrees with your stomach at hour 12 when sweet flavors stop working.
- 120 cal per gel · ~28g carbs
- Real-food ingredients (rice, salt, ginger)
- Savory profile — pairs with sweet later
- Made in Boulder, CO
SiS
SiS Beta Fuel Gel
40g of carbs per gel — the most concentrated formula on the market. Great for runners targeting 80+ g carbs/hour. Bigger 80ml package size than typical gels.
- 40 g carb per gel · 165 cal
- 1:0.8 maltodextrin:fructose ratio (max absorption)
- No caffeine version + 100mg caffeine version
- Larger gel size = more carb per stop
Huma
Huma Chia Energy Gel
Made with chia seeds and real fruit puree. Slow-release carbs, gut-friendly, no thick syrup feel. Perfect for runners whose stomachs reject standard gels at hour 6.
- 100 cal · 22g carbs · chia + fruit puree
- Slow-release energy — no spike/crash
- Best on sensitive stomachs
- Real-fruit flavor (raspberry, mango, apple cinnamon)
Clif
Clif Bloks Energy Chews
When gels feel too thick to swallow, switch to chews. 33 calories per Blok, 6 per package. Salt + caffeine variants for mid-race. Most-tolerated chew on the market.
- 6 chews per package · ~200 cal
- Gummy texture — easier than gel at hour 8
- Caffeine + non-caffeine flavors
- Includes salt-only version (90mg per Blok)
Carbs-per-hour math
The current sport-science target for endurance fueling is 80–120 g carbs per hour. Most ultrarunners can absorb 60–90 g/hr after gut training. Gel-only math:
- Maurten 100 — 25g carbs → need 3+ gels/hour for upper range
- SiS Beta Fuel — 40g carbs → 2 gels/hour hits 80g
- Spring Energy — 28g carbs → ~3 gels/hour
- Huma Chia — 22g carbs → 3–4 gels/hour
For most ultrarunners, the realistic strategy is gel + chew + drink. 1 gel + 1 small handful of chews + 1 bottle of Tailwind per hour gets you to 80g+ carbs without overloading any single source.
The rotation rule
Don't take the same gel for 12 hours. Sweet flavors (lemon, raspberry, vanilla) become nauseating after hour 6. Salty/savory (Spring's McRae rice, Maurten unflavored) extend tolerance. Plan a gel rotation:
- Hours 0–4: Standard gels (Maurten, SiS) — caffeine OK if usual
- Hours 4–10: Mix in Spring Energy real-food gels and chews
- Hours 10+: Switch to savory (Spring McRae, broth at aid stations, real food)
Train your gut, not just your legs
Long runs are gut-training as much as leg-training. From week 9 of any ultra plan, eat at race fueling pace on every long run — same gels, same intervals, same flask mix. The gut adapts the same way the legs adapt: slowly, with consistent stimulus. Most ultrarunners under-fuel in training and then expect race-day fueling to work. It usually doesn't.
Quick test: if you can't tolerate 60g carbs/hour on your 22-mile training run, you can't tolerate it on race day either. Train it forward.
What we don't recommend
Honey Stinger Organic Energy Gel: beloved by many, but the honey-base gets thick at cold temps and the gut tolerance is poor for runners not already adapted to honey.
GU Original Gel: a classic for marathon fueling, but the maltodextrin-only formula can be hard on stomachs at ultra distance. Switch to GU Roctane or upgrade to Maurten for ultras.