The 90g carbs per hour recommendation has become gospel in endurance sports, backed by research showing elite cyclists can oxidize up to 90 grams of mixed carbohydrates hourly. But blindly applying this cycling research to ultra running is causing more DNFs than it’s preventing. Here’s why 90g carbs per hour might be destroying your race—and what to do instead.
The Cycling Research That Started It All
The 90g carbs per hour guideline comes from groundbreaking studies on Tour de France cyclists using 2:1 glucose-to-fructose ratios. These studies are scientifically sound—for cycling. The problem? Running creates fundamentally different physiological demands that dramatically reduce carbohydrate absorption capacity.
Why Running Changes Everything
During cycling, blood flow to the gut remains relatively stable. During running, especially on technical terrain or climbs, vertical displacement and impact cause:
- 40-60% reduction in splanchnic blood flow
- Increased gut permeability (“leaky gut”)
- Reduced gastric emptying rates
- Higher risk of exercise-associated GI distress
Research in the Scandinavian Journal of Medicine & Science in Sports shows that runners can typically absorb only 60-75g carbs per hour compared to cyclists’ 90g, with significant individual variation based on gut training and running intensity.
The Individual Tolerance Reality
The biggest misconception about 90g carbs per hour is that it’s universally achievable. Studies reveal massive individual variation in carbohydrate absorption capacity during running:
Untrained gut: 30-40g carbs per hour Moderately trained: 50-60g carbs per hour Well-trained gut: 60-75g carbs per hour Elite with extensive gut training: 75-90g carbs per hour
If you’re attempting 90g carbs per hour without months of progressive gut training, you’re setting yourself up for the GI disaster that ends most ultra races between miles 40-60.
Warning Signs You’re Overconsumimg
- Bloating or sloshing feeling in stomach
- Nausea that worsens with continued fueling
- Urgent bathroom needs every 30-60 minutes
- Vomiting or inability to tolerate any nutrition
- Energy crash despite consuming adequate calories
Finding Your Personal Carbohydrate Ceiling
The solution isn’t abandoning high carbohydrate intake—it’s discovering your individual absorption threshold through systematic testing.
The 4-Week Gut Training Protocol
Week 1: Start at 40g carbs per hour during long runs Week 2: Increase to 50g carbs per hour Week 3: Progress to 60g carbs per hour Week 4: Test 70g carbs per hour
Advance to the next level only if you complete the previous week’s long run with zero GI distress. Most ultra runners plateau at 60-70g carbs per hour—and that’s perfectly sufficient for peak performance.
Ultra Distance Requires a Different Approach
Even if you can absorb 90g carbs per hour early in an ultra, maintaining this rate for 12+ hours is unrealistic. Research on 100-mile finishers shows successful fueling strategies average:
- Hours 1-6: 60-75g carbs per hour
- Hours 7-15: 45-60g carbs per hour
- Hours 15+: 30-50g carbs per hour
This natural decline matches reduced running intensity and accumulated GI fatigue. Fighting this pattern by forcing 90g carbs per hour throughout your race typically leads to complete nutritional shutdown.
Key Takeaways
- 90g carbs per hour research is based on cycling, not the gut-jarring impact of ultra running
- Most ultra runners can only absorb 60-75g carbs per hour maximum, even with gut training
- Individual absorption capacity varies 3x based on training, genetics, and running intensity
- Progressive gut training over 8-12 weeks is required to increase absorption capacity safely
- Late-race fueling naturally decreases to 30-50g per hour in successful 100-mile efforts
Your Personalized Fueling Strategy Starts Here
Stop chasing the mythical 90g carbs per hour target and start discovering your body’s actual absorption capacity. Begin your next training block with conservative carbohydrate intake around 40-50g per hour, then progressively increase by 10g every 2-3 weeks while monitoring GI response.
Your optimal rate might be 55g per hour or 75g per hour—the key is finding YOUR ceiling through systematic testing, not following guidelines designed for cyclists sitting in an aerodynamic tuck position.
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