The High-Carb Fueling Revolution

David: All around the world, across endurance sports, records are falling. Is it better equipment? Better training? While those factors play a role, I think that the primary driver is something much simpler: athletes are eating way more carbs to fuel racing and training.

Back in the early 2010s, when my co-coach Megan and I started coaching trail and ultra runners, we had an advantage. Our pre-race email used to say: “eat as many carbs as you can, ideally totaling 300-450 calories per hour.” That advice was higher than most publicly available recommendations. There were some physiological theories floating around about glycogen availability, but few studies, and only whispers about extremely high-carb fueling approaches being used in the real world. We miss that advantage! It made coaching so much easier when our athletes could constantly stuff carbs into their muscles while their competition tried to get by with a gel every 45 minutes. 

But something happened to that advantage.

Gradually and then all at once, it seemed like high-carb fueling hit the mainstream. Now, athletes in the pro cycling peloton are pushing 90-120 grams of carbs per hour, with some going far beyond our initial cap of 450 calories per hour. Runners are fueling heavily in events as short as 1 hour. Showing up at a group long run without lots of fuel is seen as what it is: amateur hour. 

And records are getting blown away. In road marathons, the shoes may have made a 4% difference on race day. Our theory is that fueling like a pro in training and racing puts those shoe gains to shame.

What caused the seismic shift in how the endurance world talks about fueling? Like most parts of exercise science, we think the broader theoretical agreement relied on stone-cold, dispassionate results. In cycling, athletes started experimenting with 90-120 grams of carbs per hour, and they excelled at the top level. The rumors spread, spurred by physiologists working with cycling teams. It ushered in an era defined by “fatigue resistance” (also called “durability”), where the best athletes with the most carbs could put out a high fraction of their maximal power later in events. In recent races, journalists have spied fueling cheat-sheets on the top tube of some bike teams. They turned fueling into a formula, and in the process, they turned peak performance into a near certainty.

Simultaneously, there was convergent evolution across sports, with ultra running and marathon running having similar experimentation. Jim Walmsley came onto the scene and dominated, with articles quoting absurdly high mid-race caloric intake. Some of the athletes we coached were at the forefront of that process too, like Clare Gallagher fueling with 450 calories per hour. Gone were the days of more widespread “fat adaptation” – in narrow-margin races where speed mattered, the winners knew what the cyclists knew: 

If you fuel enough in training, you put out more power while fatigued and recover faster, leading to higher power in racing. If you fuel enough in racing, that higher power you earned in training fades less as the event goes on. What many athletes thought was a training or endurance limitation was actually a fueling limitation.

Those results coincided with a growing body of scientific literature backing up what we were seeing in the real world. Decades of research pointed toward the possible benefits of high carb fueling, primarily focused on glycogen availability, intracellular processes related to muscle contractions, and cognitive function. A 2020 study on elite mountain runners found that 120 grams of carbs per hour led to less muscle damage and lower perceived exertion. Several studies on cyclists found better fatigue resistance with higher carb intakes. There are some nuances, particularly related to GI system distress, but even the GI system can be trained. 

Then I’d say to invest in Netflix. Priorities.

Companies filled the niche at the cutting edge of high carb fueling, creating options that were easy to ingest like hydrogels, containing multiple transportable carbohydrates to avoid overloading gut transporters. Precision Fuel and Hydration, Science in Sport, Maurten, and others made it so that high carbs became as simple as a 1-second slurp, rather than the 30-second gag of some previous generations of fueling options.

You can see why records are falling. The previous generations trained hard (maybe harder, since they were breaking down more from underfueling). They were amazing athletes. But they had no margin for error, less knowledge of the high-carb physiology, and few options for actually putting it into practice, even if they wanted to. That shift was exemplified in a personal experience that Megan and I had at the Crown King Scramble 50k this year. 

It was the 32nd edition of the race, which was net uphill with 7000 feet of climbing. It was the perfect laboratory, since the uphills later on in the event could mirror some of the studies evaluating fatigue resistance. Here were the fueling guidelines we used, which we recommend to athletes we coach as well, from beginners to some of the top pros in the world:

1. Aim for 75-90 grams of carbohydrates per hour at baseline, increasing to 90-110 grams of carbs per hour with more experience, combining liquid calories with hydrogels (or other fuel sources you practice with at high levels)

2. In the first hour, count any calories taken in the 30 minutes prior to the start

3. In addition, dial in hydration needs and sodium intake for your personal physiology, usually around 16-24 oz fluid per hour and between 600-1000 mg sodium per 32 oz fluid consumed, with high variance among individuals (hydration is beyond the scope of this article, so make sure you test yourself for your needs, which vary a lot more than fueling needs)

We kept our fueling strategy simple with this approximate total:

• Hour one: 40g carb Science in Sport Beta Fuel gel taken 20 min pre-race, plus 20g carb in Precision drink mix. Another 40g carb gel at 45 min, plus 20g carbs from Precision (120g total)

• Hour two and every hour after: alternate 30g Precision Gel with 40g SiS Gel every 25 min, plus 30g Precision drink mix, with the option to space a gel out if needed (~114 g per hour)

Megan won by 30 minutes in the 2nd fastest time ever. That makes sense. She’s a beast.

My story is a bit more interesting, because I dealt with a knee injury from a tumble on the stairs for some of the winter. My training volume was lower, with a lot of time on the bike. Plus, I’m just not the superstar that Megan is. And I broke the course record by 3 minutes.

Some of the best athletes in trail running have done this race. No offense to myself, but based on my background, I’m not quite at that level. While my training and talent might not be record breaking, I am a superstar at slurping. 

But that’s not where the story ends. Plot twist! I didn’t actually win the race. On the final climb, I was stomped by Ryan Raff, an emerging trail star, who went on to beat me by 3 minutes and the course record by a full mile. I was awestruck as he danced away up the steep grade, looking like he was unaffected by fatigue and gravity. After the race, I looked at his Strava to see how the race was won. And in his description:

“415 cal/hr. 97g carbs/hr”

Damn. I wonder if he listens to our podcast.

High-carb fueling used to be an advantage. Now, it’s a prerequisite. 

Photo: Mountain Outpost