The High-Carb Revolution
Humans have been running for longer than we’ve been talking, but for most of history, we ran for just one of two reasons:
1. We were chasing something
2. We were being chased by something
The era of 9-5 workers donning a pair of overpriced Nikes and shuffling around the city is only about 70 years old, and the practice of carbo-loading ahead of long races is a similar age.
Before I jump into the high-carb revolution, let’s take a quick detour to explain what carbohydrates are, and how we use them when running.
What are carbohydrates, and why do we carbo-load?
Carbs are one of the three main macronutrients (along with proteins and fats) that provide energy to your body. At their most basic level, they are sugars: simple sugars like glucose and fructose, double sugars like sucrose and lactose, and complex carbohydrates like starch, glycogen, and fibre, are all carbs.
When you eat carbohydrates, your digestive system breaks them down into glucose, which enters your bloodstream and gets transported to your muscles and organs. Your body stores this glucose as glycogen (think of it as your internal fuel tank), primarily in your muscles and liver. An average person can store roughly 300-600 grams of glycogen, which translates to about 1200-2400 calories worth of readily available energy.
For runners, carbohydrates are particularly important because they're your body's preferred fuel source during moderate to high-intensity exercise. While your body can burn fat for energy (and does, especially during easier efforts), fat burning requires more oxygen and takes longer to convert into usable energy. When you're pushing the pace, your muscles demand quick energy, and carbohydrates deliver it faster than any other aerobic fuel source.
The catch? Unlike fat stores, which are virtually unlimited in most people, your glycogen reserves are finite. Depending on your fitness level, body size, and running intensity, you will deplete these stores in roughly 90 minutes to 2 hours of running. In short, this is why we carbo-load: we’re trying to top up the fuel tank as much as possible so that we can have the most amount of carbohydrate-supplied energy for the longest amount of time.
The High-Carb Revolution
In 2019, Eliud Kipchoge became the first human to run a marathon in under two hours, which he achieved on a course in Vienna perfectly designed for the attempt. Had you asked a sports nutritionist ten years ago if the course was perfect, they likely would have taken one look at his fuelling plan (>100g of carbs/hour) and pointed out what would have seemed like a glaring flaw: a lack of porta-potties every kilometre.
Distance runners live by an unofficial collection of holy commandments: Thou shalt not run with an Apple Watch. Thou shalt be generous with kudos. Thou shalt not consume more than 60 grams of carbs per hour.
While the first two items on the list I just made up to suit this article are perhaps less scientific than I would like, for decades, commandment #3 was straight out of a lab. Research had shown that glucose transporters in our intestines max out at about one gram per minute. Push beyond that and you’d pay the price in stomach cramps and bathroom breaks.
But, as we learnt above, glucose is far from the only carbohydrate out there, a fact that earlier studies didn’t consider. While glucose does hit a ceiling at about 60 grams per hour, adding fructose, for example, opens up a separate absorption pathway. Think of it like a highway: if all the cars are trying to use the same exit ramp, you get a traffic jam. But if some cars can use a different exit, then there’s no problem. Researchers discovered that by combining glucose and fructose, athletes could effectively add extra lanes to their nutritional highway.
This revelation coincided with another discovery: the gut, like muscles, can be trained. Athletes who gradually increased their carbohydrate intake over weeks and months found their tolerance improving. The human digestive system is like a trainable muscle that can adapt to handle far more than anyone imagined.
By 2020, the transformation had spread throughout elite endurance sports. Professional cyclists were routinely consuming 120 grams per hour, and ultra-runners were maintaining intakes that would have been considered reckless just years before. The old ceiling wasn't really a ceiling at all, but more like a speed limit sign that everyone had been obeying without question, never realising the road could handle much more.
The true benefit, however, does not lie in race day performance. Athletes who train with consistently high carbohydrate availability enjoy faster recovery between sessions, reduced muscle damage, and the ability to maintain quality across consecutive hard days. Careers are lasting longer and training loads are increasing without fear of injury.
Runners are learning a lot from cyclists, who are at the forefront of the high carb revolution
Parting Thoughts
The hotter the fire, the more it burns. If you’re regularly exercising at a high intensity, you need to ensure you’re also fuelling the flames properly – in this case, it often means eating the types of food straight out of a ten-year-old’s birthday party (apple cinnamon scrolls are my go-to for a post-workout snack, thanks to their high glucose and moderately high fructose content, but there are far healthier options out there if you’re so inclined!).
While this area of research is very much still in its infancy, the positive impact that high-carb fuelling has on endurance performance for athletes at all levels is undeniable. This is perhaps best illustrated by the elites: in 2024, 17 distance running world records were broken, from the 1500m all the way up to the 6-day (yes, that’s a real thing! Stine Rex covered 913.6km in 6 days around a 1.4km loop near her house). What links these opposite ends of the spectrum? The need to adapt to hard training week after week, a feat best accomplished by high-carb fuelling.
I want to leave you with a quote from Leadville 100 course record holder David Roche. David and his wife Megan run the SWAP website and podcast and are the main reason I started looking into high-carb fuelling. Both are far more knowledgeable on this subject than I am, so if you’re interested in David’s 8 Guidelines for High-Carb Fueling, or any of the couple’s other work, be sure to check out those hyperlinks.
““High-carb allows athletes to push harder during races without burning through their fuel stores. For example, previous Leadville record holder Matt Carpenter is better than me on all fitness metrics like VO2 max and lactate threshold, but high-carb intake allows me to push harder relative to my (lower) fitness levels. But the biggest change is that higher carb intake improves adaptation and recovery. Every day in training, we’re playing a high-stakes game with our health and stress levels. High-carb tilts the table in our favour. I am 36 going on 22, and that’s because carbs let me push harder on the day and then adapt faster than ever before.””