High-Carb Fueling for Trail Running: The Science Behind the Revolution

Learn how high-carb fueling can transform your trail running performance. Science-backed intake targets, gut training protocols, and race-day strategies.

Why High-Carb Fueling Is Rewriting Trail Running Nutrition

For most of the past two decades, the trail running community ran on fat and faith. The low-carb, fat-adapted approach dominated: train your body to burn fat, preserve glycogen, go long. It made intuitive sense. Then 2024 happened. David Roche broke a 19-year course record at Leadville 100 fueling at 120-140 g of carbohydrates per hour. Rod Farvard dominated the Canyons 100k and nearly won Western States on a high-carb strategy. Records across all of endurance sport started falling, and the same variable kept showing up: athletes eating far more carbohydrates than conventional wisdom said they should. This article covers the science behind high-carb fueling for trail running, how to build toward higher intake rates without GI meltdowns, and what the research actually recommends for different race distances and intensities.

The Science: Why Carbohydrates Drive Performance

Glycogen availability at all exercise intensities is the bedrock claim here, and it's not seriously disputed in the sports science literature. Podlogar and Wallis (2022) reviewed contemporary carbohydrate research for endurance athletes and concluded that high carbohydrate availability for prolonged intense exercise remains a top priority, with meaningful advances in recommended type and quantity before, during, and after hard efforts.

The older ceiling of 60 g/h was set by a single bottleneck: the SGLT1 intestinal transporter, which maxes out glucose absorption at roughly 60-67 g/h. The key insight that changed everything is that fructose uses a separate transporter, GLUT-5. Mix glucose and fructose together at approximately a 1:0.8 ratio, and you can clear up to 90 g/h through the gut, sometimes more with training. A 2022 study by Podlogar et al. found that trained cyclists ingesting 120 g/h at a 0.8:1 fructose-to-maltodextrin ratio showed higher exogenous carbohydrate oxidation rates compared to 90 g/h protocols, sparing more endogenous glycogen in the process.

The most recent data pushes this further. A 2026 review published in the Journal of Nutrition (van Loon et al., 2026) found that contemporary evidence supports raising the effective upper limit from 90 to 120 g/h for trained participants, with both exogenous and whole-body carbohydrate oxidation rates increasing at these higher doses. That translates to better fuel delivery, preserved glycogen stores, and delayed fatigue in events lasting 2.5+ hours, which covers nearly every trail race worth showing up for.

What David Roche's Approach Actually Means for You

Roche's Leadville performance has attracted outsized attention, and rightly so: 120-140 g/h over 100 miles at altitude is a number that would have seemed absurd five years ago. But the more useful takeaway from his work isn't the specific gram count. It's two connected principles.

First, most trail runners are severely underfueling during training, and that limits both performance and adaptation. Roche outlines a practical framework where easy/moderate long runs, including most ultras, warrant 75-90 g/h regardless of duration. Most trail long runs involve some climbing and variable intensity, which means glycogen is being burned throughout. Underfueling those runs doesn't build fat adaptation in any meaningful way; it just blunts the training stimulus.

Second, the ability to absorb 90-120+ g/h is a trainable skill, not an innate capacity. Roche describes his own progression from conventional intake toward 150 g/h through systematic gut training during workouts. This matters because the biggest barrier most trail runners face isn't the science, it's the nausea and GI distress that shows up the first time they try to push intake from 40 g/h to 90 g/h on a long run.

If you're planning for an upcoming goal race, Trailia's Race Predictor can help you estimate finish time and structure your nutrition targets around expected pace and time on feet.

Gut Training: The Skill Nobody Practices

The gastrointestinal tract responds to repeated fueling stimulus like any other system: it adapts. A 2023 systematic review by Martinez et al. in Sports Medicine examined eight studies on gut training and feeding challenge protocols. Two weeks of repetitive carbohydrate feeding at 30-90 g/h reduced gut discomfort by an average of 47% and cut carbohydrate malabsorption by 45-54%. That's a meaningful physiological change, not just an attitudinal one.

The mechanisms are concrete: repeated carbohydrate exposure increases SGLT-1 transporter expression in the intestinal lining, reduces intra-gastric pressure sensitivity, speeds gastric emptying, and improves the gut's tolerance to volume. Think of it as building a larger, faster pipeline for carbohydrate delivery during racing.

Practically, this means fueling workouts with similar carbohydrate intake to what you plan on race day, across training runs of similar intensity and duration. Starting at 60 g/h and building toward 75-90 g/h over 6-8 weeks is a reasonable progression for most runners. Longer ultras (50-100+ miles) may justify pushing toward 90-120 g/h, but only if gut training has preceded the attempt.

Mixture type matters here too. Single-source carbohydrates (glucose or maltodextrin alone) are capped at roughly 60 g/h by the SGLT-1 transporter. To exceed that, you need multiple transportable carbohydrates: glucose/maltodextrin paired with fructose. Most contemporary sports gels and drink mixes use a 2:1 or 1:0.8 glucose-to-fructose blend for exactly this reason.

Race-Specific Carbohydrate Targets

Intake recommendations scale with duration and intensity, not body weight or training status, according to the research.

Efforts Under 90 Minutes

At this duration, glycogen depletion is not the limiting factor. 30-60 g/h is adequate for most hard sessions. The main benefit here is maintaining blood glucose and reducing perceived exertion in the back half of the workout, not replacing muscle glycogen in real time.

90 Minutes to 4 Hours (Trail Marathons, Mountain 50ks)

This is where the data for 75-90 g/h becomes most compelling. A 2020 study by Urdampilleta et al. compared 60, 90, and 120 g/h intake across elite trail runners during a mountain marathon. The 120 g/h group showed less neuromuscular fatigue and better high-intensity run capacity recovery at 24 hours post-race compared to both lower intake groups.

For a typical 50k taking 5-7 hours, consistent 75-90 g/h intake across the whole effort should be the baseline target. Some runners can achieve 100-120 g/h once gut training is established.

6+ Hour Events (50-Mile and 100-Mile Ultras)

This is where food variety and palatability become real constraints, and where solid foods typically enter the picture alongside gels and drinks. The carbohydrate target stays at 60-90+ g/h, but runners should plan for appetite suppression in the later stages (common from mile 50 onward in 100-milers) and front-load intake while eating is easier. Aid station food that mixes glucose sources (potatoes, rice balls, fruit) with gel intake can help maintain intake without taste fatigue.

At altitude, Roche specifically noted that high-carb fueling becomes harder. Altitude suppresses appetite and increases nausea risk. Planning for reduced intake and compensating with higher-density carbohydrate sources (gels over solids) can help.

The Train-Low Question

One counterargument worth addressing directly: doesn't training with less fuel improve fat oxidation and mitochondrial adaptation? The "train low, compete high" model has real molecular support: low glycogen states do activate AMPK and drive mitochondrial biogenesis. This is not in dispute.

The dispute is over whether it produces performance benefits that exceed those from a consistently well-fueled training approach. A 2024 randomized trial by Prieto-Bellver et al. put 17 highly trained cyclists into either a carbohydrate periodization group or a high-carbohydrate group for five weeks and found no significant differences in performance outcomes between the two groups. Earlier reviews have found that only about 37% of train-low studies show clear performance advantages over adequately-fueled training.

The most sensible synthesis: strategic low-carb sessions (easy recovery runs, some morning sessions at low intensity) may offer some metabolic upside, but any quality session, long run, or race-day effort should be well-fueled. Habitually underfueling training to chase fat adaptation at the cost of quality and adaptation is a poor trade.

If you're not sure whether your training load is matching your current fitness and recovery capacity, Trailia's Race Readiness tool can help you assess where you stand before committing to a fueling strategy change.

The Practical Start: A Four-Week Gut Training Block

Here is a simple framework for shifting toward higher carbohydrate intake without wrecking yourself on the first long run:

  • Weeks 1-2: Target 60 g/h on all runs over 60 minutes. Use a glucose/fructose blend (2:1 ratio). Practice eating on the run, including climbs and technical terrain where swallowing is less automatic.
  • Weeks 3-4: Push toward 75 g/h on runs over 90 minutes. Introduce a third gel per hour on long days. Log any GI symptoms and note which products or timing strategies correlate with discomfort.
  • Pre-race: In the final 2-3 weeks before a goal race, simulate race-day intake on your longest training runs. If the race target is 90 g/h, rehearse 90 g/h. The gut has to practice the protocol, not just be told about it.

Daily carbohydrate intake matters too. A 2024 review confirmed that 7-10 g/kg/day is needed to fully replenish glycogen within 24 hours. A 65 kg runner needs 455-650 g of carbohydrate daily during heavy training. Most trail runners who self-report as "high carb" are significantly below this range when they actually track intake.

Key Takeaways

  • Multiple transportable carbohydrate blends (glucose + fructose) are essential for intakes above 60 g/h, removing the main physiological bottleneck to high-rate fueling.
  • Contemporary evidence supports a ceiling closer to 120 g/h for trained athletes, not 90 g/h, particularly for efforts over 2.5 hours.
  • Gut training is trainable: 2 weeks of consistent fueling practice can reduce GI discomfort by up to 47% and cut carbohydrate malabsorption by 45-54%.
  • Most trail runners benefit from 75-90 g/h on long runs and key quality sessions; underfueling those efforts blunts adaptation without meaningful fat-adaptation benefits.
  • Front-load intake in ultras: appetite suppression gets worse the longer you're out there.

Conclusion

The shift toward high-carb fueling in trail running isn't a fad, and it's not exclusive to elite athletes who can run 15-hour 100-milers. It's a direct consequence of better understanding of intestinal physiology, better product formulation, and athletes willing to treat gut training as a real part of race prep. The research now supports intake rates that would have seemed impossible a decade ago, but only if the gut has been prepared for them.

Start with consistency at 60 g/h, build toward 75-90 g/h over 4-6 weeks, and replicate your race-day fueling plan on your longest training runs. The performance gains aren't theoretical. They show up on the clock.

Citations

  1. Podlogar T, Wallis GA. New Horizons in Carbohydrate Research and Application for Endurance Athletes. Sports Med. 2022;52(Suppl 1):S5-S16. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9734239/
  2. van Loon LJC et al. From Metabolism to Medals: Contemporary Perspectives and Revisiting Carbohydrate Guidelines for Fueling Endurance Athletes during Exercise. J Nutr. 2026. https://www.sciencedirect.com/science/article/pii/S002231662600091X
  3. Martinez IG, Mika AS, Biesiekierski JR, Costa RJS. The Effect of Gut-Training and Feeding-Challenge on Markers of Gastrointestinal Status in Response to Endurance Exercise: A Systematic Literature Review. Sports Med. 2023;53(6):1175-1200. https://pubmed.ncbi.nlm.nih.gov/37061651/
  4. Podlogar T, Bokal S, Cirnski S, Wallis GA. Increased exogenous but unaltered endogenous carbohydrate oxidation with combined fructose-maltodextrin ingested at 120 g/h versus 90 g/h at different ratios. Eur J Appl Physiol. 2022. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9560939/
  5. Urdampilleta A, Arribalzaga S, Viribay A, et al. Effects of 120 vs. 60 and 90 g/h Carbohydrate Intake during a Trail Marathon on Neuromuscular Function and High Intensity Run Capacity Recovery. Nutrients. 2020;12(7):2094. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7400827/
  6. Costa ML et al. The importance of carbohydrate intake for high-performance athletes in endurance exercises. Research, Society and Development. 2025. https://rsdjournal.org/rsd/article/view/48597

Frequently Asked Questions

How many grams of carbohydrates per hour should trail runners aim for during a race?

It depends on race duration and intensity. For efforts under 90 minutes, 30-60 g/h is sufficient. For trail marathons and 50ks lasting 3-6 hours, 75-90 g/h is the current evidence-backed target. For 50-100 mile ultras, trained athletes with gut adaptation may benefit from pushing toward 90-120 g/h. These numbers require practice in training, not just race day execution.

What is multiple transportable carbohydrate (MTC) fueling and why does it matter?

Multiple transportable carbohydrates refers to combining glucose (or maltodextrin) with fructose in a single product. Glucose uses the SGLT-1 intestinal transporter, which maxes out at roughly 60-67 g/h. Fructose uses GLUT-5, a separate transporter. By combining both, absorption capacity increases to 90 g/h or beyond. Any gel or drink mix targeting intakes above 60 g/h should use a glucose/fructose blend at approximately a 2:1 or 1:0.8 ratio.

Can average trail runners actually absorb 90+ grams of carbs per hour, or is that only for elites?

The gut adapts to carbohydrate load regardless of athletic level. A 2023 systematic review found that just two weeks of consistent high-carb fueling practice reduced GI discomfort by 47% and cut malabsorption by 45-54% across a range of athletes. The key is progressive exposure: start at 60 g/h, build over 4-6 weeks, and use glucose/fructose blends. Elite speeds are not a prerequisite.

What did David Roche's Leadville 100 performance prove about high-carb fueling?

Roche broke a 19-year course record at Leadville in 2024 fueling at 120-140 g of carbohydrates per hour, then improved his own record in 2025. His approach demonstrated that very high carbohydrate intake is achievable during 100-mile mountain efforts and can support performance that previous nutrition frameworks would have considered impossible. He has explicitly stated that many endurance limitations he experienced before high-carb fueling were actually fueling limitations in disguise.

Should I do fasted runs to improve fat adaptation for ultras?

Occasional easy runs in a fasted state may offer some metabolic adaptation benefits, but the evidence for train-low strategies producing performance advantages over well-fueled training is limited. A 2024 randomized trial found no significant performance differences between carbohydrate periodization and consistently high-carbohydrate training in highly trained athletes. Key sessions, long runs, and all race-effort work should be well-fueled. Reserve low-carb sessions for easy recovery runs only.

What is gut training and how do I start?

Gut training means systematically exposing your gastrointestinal tract to higher carbohydrate intake during workouts so it physiologically adapts: better gastric emptying, higher SGLT-1 transporter expression, lower GI symptom severity. Start by consistently hitting 60 g/h on every run over 60 minutes using a glucose/fructose blend. After 2 weeks, increase toward 75 g/h. Progress toward your race-day target over 6-8 weeks. The goal is that your race-day nutrition plan has been rehearsed repeatedly before you need it to count.

How do I fuel a trail 50k differently from a 100-miler?

A 50k lasting 4-7 hours calls for consistent gel/drink intake at 75-90 g/h throughout, with limited need for solid food. A 100-miler requires planning for appetite suppression in later miles, taste fatigue, and potential nausea. Front-load intake while eating feels easy, use a variety of carbohydrate sources (gels, aid station food, drink mixes), and have a fallback plan for when your preferred foods stop appealing. Drop bag strategy becomes a real nutrition tool at this distance.

What is the best carbohydrate ratio for trail running gels and drinks?

For intakes up to 60 g/h, a 2:1 glucose-to-fructose ratio is well-supported by research. For higher intakes targeting 90-120 g/h, a ratio closer to 1:0.8 glucose-to-fructose may be more effective at clearing carbohydrates through the gut without accumulation and discomfort. Most modern performance gels and isotonic drinks use one of these two ratios. Check labels for maltodextrin, glucose, and fructose as primary carbohydrate sources.

How much carbohydrate do I need daily to support heavy trail training?

Research consistently recommends 7-10 g/kg/day to fully replenish glycogen stores within 24 hours during heavy training. A 65 kg runner needs 455-650 g of carbohydrate daily. Most trail runners who describe themselves as eating high-carb fall well below this when they actually track intake. Prioritize carbohydrate-dense meals around your key sessions: oats, rice, bread, fruit, and pasta are the staples. This daily baseline matters as much as in-run fueling for adaptation and recovery.

What should I do if I experience nausea or GI problems during long races?

GI distress is the most common reason high-carb strategies fail on race day, and it almost always traces back to insufficient gut training beforehand. In-race, if nausea hits: slow the intake rate, switch to liquid carbohydrates if solids are causing the problem, ensure adequate hydration (GI issues worsen with dehydration), and avoid high-fat foods that slow gastric emptying. For future races, replicate your exact race-day fueling protocol on long training runs at least 3-4 times before the event.