A friend ran alongside me for five to ten minutes during my first 50-miler. Called my name multiple times. I never looked at them. Never knew they were there until the very last moment. That's one way running out of fuel can look. Your awareness and mental acuity just quietly disappear. I’ve also been so depleted I could barely walk due to cramping with every step. After this I knew something had to change. I've been thinking about that race a lot since I went deep on two studies for this week's issue. Because the research explains, almost exactly, what happened to me that day and why this year's 50-miler felt completely different.
ARTICLES BELOW:
The Research
Two studies this week. The first is a systematic review and meta-analysis published in Critical Reviews in Food Science and Nutrition that pulled together 136 studies examining carbohydrate intake during endurance exercise. The second is a randomized trial published in Nutrients in 2020 testing three different carbohydrate doses during a mountain marathon in elite runners: 120 g/h, 90 g/h, and 60 g/h. That third number, 60 g/h, is what most endurance athletes are actually consuming during their races.
What They Found
The research suggests these are the key takeaways
🟢 Strong evidence: the meta-analysis includes 136 studies and is among the most replicated findings in sports nutrition.
🟡 Moderate evidence: The 120 g/h muscle damage finding is moderate evidence given the elite population and small sample, but effect sizes were large and biologically consistent.
Carbohydrate intake significantly improved endurance performance across all 136 studies. This isn't a preliminary finding. This is about as close to settled as sports science gets.
The longer the event, the bigger the benefit. Duration was the strongest moderating variable. The inflection point starts around a half marathon distance. Beyond two hours, the effect grows substantially.
Time to exhaustion improved more than time trial performance. In other words, carbohydrates extend how long you can keep going even more than they speed up a fixed distance. For ultra runners, this finding is enormous.
RPE was significantly lower with carbohydrate intake. Runs feel easier at the same effort. That means better training quality, better recovery, and better performance, just from fueling.
Less trained athletes benefited more than highly trained athletes. More on this in a moment.
In the mountain marathon RCT, 120 g/h significantly reduced muscle damage markers at 24 hours post-race — creatine kinase, LDH, and GOT — compared to both the 90 g/h and 60 g/h groups. Internal exercise load was also roughly 19% lower in the high-carbohydrate group. The 60 and 90 g/h groups? Essentially identical results.
The Finding Most Runners Have Backwards
The meta-analysis found that less trained athletes benefit the most from carbohydrate intake during exercise.
Think about who is most aggressively fueling during endurance events right now. It is the competitive runners, the fast runners, the people who have read the research and take their nutrition seriously. These are also the athletes who, physiologically, have the most chronic metabolic adaptations from years of training, their bodies are more efficient at using fuel.
The recreational runner who is out there for five or six hours is often doing the opposite. They are under-fueling, sometimes barely taking anything in, sometimes operating on the belief that they are going slow enough that they do not need as much. The research says this is backwards. The less trained you are, the more your performance depends on exogenous carbohydrate. You are not too slow to need fuel. You are too slow to get away with not fueling.
I had a version of this thinking during my first 50-miler. When I started cramping and slowing down, I told myself I did not need as much fuel anymore because I was not running that fast. That was exactly wrong. The slower I went, the longer I was out there, the more I needed it. And I kept pulling back. And I kept slowing down. It was a very long, painful day.
The Gut Training Piece, You Cannot Skip This
Here is where most fueling advice falls short: it tells you what to take without telling you that your gut has to be trained to handle it.
When you dramatically increase carbohydrate intake, the body shifts blood flow toward the digestive system to help process it, and away from active musculature. In an untrained gut, this means GI distress, bloating, and potentially decreased performance. In a trained gut, the body has adapted to handle high carbohydrate loads without that shift.
This is also why the research on glucose-fructose combination products is relevant. Single-source carbohydrate, pure glucose, maxes out at roughly 60 g/h absorption because intestinal transporters get saturated. Adding fructose, which uses a completely different transporter, allows absorption above 60 g/h. This is the physiology behind products like Maurten, SiS Beta Fuel, and high-carbohydrate hydration blends. The science behind them is real even if the marketing overstates things.
The practical implication: do not show up on race day with a 100 g/h fueling plan if you have been training with 30 g/h. Start lower than you think you need, fuel every long run, and build the dose gradually over weeks. Your gut is trainable the same way your legs are.
What I Did Differently This Year
My second 50-miler, I went in with a plan designed to get me between 100 and 110 g of carbohydrates per hour, plus close to a liter of water per hour. Three sources, all time-regimented:
One scoop of The Feed high-carb mix in each bottle, 30 g of carbohydrates. One SiS Beta Fuel gel every 75 minutes. One Clif Shot Blok cube every 20 minutes. Precision hydration electrolyte tablets in every bottle.
Beverage, gel, solid. Every 20 minutes there was something happening. I did not wait until I felt like I needed it. I just executed the plan.
It worked for about five and a half hours until some nausea and GI distress started — which I credit to not training at that level. My training had been around 75 g/h, partly because fueling at 100 g/h during every long run is genuinely expensive. My gut was not fully trained for race-day doses.
But the difference was dramatic. I ran 2 hours faster than the year before. I never cramped. I RAN the entire time. I walked well after the race. I was sore for two days and back to running quickly. Compare that to my first 50-miler where I did not know a friend was running next to me for ten minutes.
That difference is fueling.
🏆 How To Apply It
Start fueling earlier than you think you need to and do it on every long run, not just races. Here is a practical framework by event length:
For runs or races under 60 minutes, carbohydrate intake during exercise provides minimal benefit. Water is fine.
For runs or races between 60 and 90 minutes, 30 to 45 g/h is a reasonable starting point. One gel or equivalent per hour.
For runs or races between 90 minutes and 2.5 hours, target 45 to 60 g/h. Practice this on every long run at this distance.
For anything over 2.5 hours, target 60 to 90 g/h using a glucose-fructose combination source. This is where the research shows the largest performance and recovery benefits.
For ultramarathons and events over 4 to 5 hours, 90 to 120 g/h is supported by the evidence — but only with a trained gut. Build to it over weeks, not days.
If you’re looking to push higher levels of carbohydrate intake during events 90 minutes - 2.5 hours then you HAVE to train your gut. So the recommendations above might be slightly off and some days you’re going to have to go BIG and push the metric up to the goals on race day.
Whatever dose you choose, use multiple carbohydrate sources and practice your exact race-day fueling protocol on your long runs. Your gut adapts with practice. Give it practice.
Hit reply and tell me: what does your fueling actually look like right now on long runs?
-Keep running happy, healthy, and strong
Spencer
Fueling showed benefits in Time to Exhaustion tests....so next week we are back to strength training
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