Carbohydrates — Training Dictionary

How many carbs per hour do endurance athletes need on race day?

Carbohydrate fueling during endurance events is not based on total calories burned. Instead, it is constrained by the body’s ability to absorb and oxidize carbohydrate, and by the need to preserve glycogen over the duration of the race. For long events, small hourly deficits compound and often determine late-race performance.

Why carbs, not calories?

Endurance athletes burn far more energy than they can replace during a race. At typical race intensities, fat oxidation supplies a large portion of total energy, while carbohydrate becomes the rate-limiting fuel for sustained power, surges, and late-race pacing.

Attempting to “fuel to calories” would require carbohydrate intakes far beyond what the gut can absorb. As a result, race fueling targets are set by carbohydrate absorption and oxidation limits, not by energy expenditure.

ELI5: Why carbs matter

Think of your body like a hybrid car. Fat is the big battery that lets you keep moving all day, but carbs are the gas pedal that let you push harder, climb, and surge. You can’t replace everything you burn during a race, but eating carbs slows the drain on your limited fast fuel so you don’t run out when it matters. Fueling is about preserving carbs, not matching calories. You want to take on as much as you can, up to a limit (which is when you start to feel not so great).

Carbohydrate Absorption Limits

Laboratory and field research shows clear ceilings on carbohydrate oxidation:

~60 g/hr from glucose alone
~90 g/hr using multiple transportable carbohydrates (glucose + fructose - most popular carb mixes / gels contain both of these)
~100–120 g/hr in well-trained athletes with gut adaptation

These limits define the upper bound for usable carbohydrate intake during exercise¹,². It is possible for athletes to absorb more than 120g/hr: look no further than Cameron Jones, winner of Unbound 2025, who reportedly took upwards of 180g/hr. This is a jaw dropping amount of carbs for that race but this takes a ton of time and practice to get your body ready to handle this sort of intake.

Race Duration and Fueling Strategy

As race duration increases, preserving glycogen becomes more important and under-fueling carries higher risk. Observational data from elite and sub-elite endurance athletes suggests the following planning ranges:

Carb needs by length

Race Duration	Typical Carb Target
<2 hours	30-60g/hr
2-4 hours	60-90g/hr
4-6 hours	80-100g/hr
6-10+ hours	90-120g/hr (or more)

For long events, athletes should plan near the upper end of their personal tolerance, recognizing that actual intake often falls below plan due to terrain, intensity, and logistics. It is hard to remember to eat with everything else going on, but it can make or break your race day if done well (or not).

What sets your target?

Carbohydrate targets can be summarized as

Target Carbs/hr = min(Gut Tolerance, Race Duration Needs, On-Course Practicality)

Gut tolerance is individual and must be trained
Duration needs increase with event length
Practicality accounts for terrain, technicality, and feeding opportunities

Planning at the upper bound provides insurance against missed intake and late-race under-fueling.

Common Failure Modes

Fueling based on calories instead of absorption limits
Planning at the minimum instead of the maximum tolerable rate
Missing intake early and trying to “catch up” later
Reduced intake late due to fatigue, GI distress, or task saturation

In long races, consistent moderate under-fueling often produces a sudden and severe late bonk.

Practical Takeaways

Carbohydrate targets are rate-based, not calorie-based (Your body can only absorb and use carbs at a certain speed (grams per hour), no matter how much you eat.)
Long races require aggressive but realistic planning
The goal is not perfect execution, but minimizing cumulative deficit
Athletes should train both gut tolerance and feeding under load
- Treat training rides as your test ground for fueling needs
- Experiment with products to see what you like best - flavor, texture, type (drink mix vs. gels) as well as concentration of carbs, etc.