Why most trail runners DNF: the fueling math most people get wrong

Most DNFs are fueling failures

Ask any ultramarathon race director what most droppers say after a DNF. It's not injury. It's not undertrained legs. It's "I just fell apart" — which almost always means "I ran out of fuel and couldn't recover."

The research on this is consistent. At distances above 50K, fueling execution determines your finish more reliably than your VO2 max. You can be 10% undertrained and still finish a 100K if your fueling is right. You can be race-ready and drop at mile 35 if it's wrong.

The numbers most runners miss

At race pace, trail runners burn roughly 500-700 calories per hour depending on elevation and body weight. Your body can store approximately 90 minutes of glycogen at that intensity. After that, you're running on what you eat.

The recommended carbohydrate target is 60-90g per hour for efforts over 2.5 hours. Most runners who haven't planned their nutrition explicitly are eating far less — sometimes half that amount.

Aid station math is harder than it looks

The problem isn't knowing the target. It's translating 60-90g/hour into what to actually pick up at aid stations that are unevenly spaced, sometimes 8 miles apart and sometimes 2 miles apart.

A flat caloric target doesn't help if your next aid station is after a 3,000-foot climb. You'll burn more, have less appetite, and need to pre-load before the ascent.

This is why generic nutrition guides fall apart for trail running. The course shape changes everything.

What a personalized plan actually looks like

A good fueling plan does four things: accounts for each leg's elevation and expected time, adjusts for conditions (heat adds ~150ml/hour hydration needs), personalizes carb targets for your body weight, and gives you specific quantities for each aid station pickup rather than general hourly targets.

That's exactly what Aerostride generates. Import your course, set your profile, and get an aid-station-by-aid-station plan you can actually follow.