For about a decade now, endurance athletes have been told to fuel with up to 90 grams of carbohydrates per hour. This guideline is based on limitations to how much glucose and fructose the gut can absorb, and is the maximum amount recommended in the Nduranz Fueling Calculator.
However, there is a trend of athletes fueling with 120 grams per hour. Many elite athletes are going even higher. Tudor Pro Cycling riders, for example, regularly aim for 135 g/h during races.
Is it time to update fueling guidelines from 90 g/h to 120 g/h?
Let’s break down whether fueling with over 90 g/h actually improves performance, the risks, and if you should adjust your strategy.
Absorption Is Not the Same as Utilization
Even though current fueling guidelines say athletes can absorb “up to 90 g/h,” this amount is not universal. There is substantial variability in carbohydrate absorption, with some athletes able to absorb more than 90 grams per hour, whereas others absorb less.
In theory, higher absorption should translate into higher exogenous carbohydrate oxidation, which in turn would then lead to more carbohydrate availability during exercise. This sounds beneficial because more fuel should mean better performance.
But this overlooks a critical point: performance is not limited by carbohydrate availability alone. It is also limited by how the body uses different fuel sources, particularly muscle glycogen.
Muscle Glycogen: The Real Performance Limiter
To understand whether fueling with 120 g/h is beneficial, we need to distinguish between the two key carbohydrate pools: liver glycogen and muscle glycogen
Liver Glycogen:
Its role is to keep blood glucose levels stable and supply glucose to your brain. If liver glycogen depletes, blood glucose levels drop, leading to brain fog, dizziness, blurred vision, muscle tremors, and extreme weakness. In short, you bonk.
Bonking is largely preventable. Even fueling with 60-80 g/h is enough to keep blood glucose levels stable and spare liver glycogen. So long as athletes maintain adequate fueling, liver glycogen isn’t a limiting factor in most endurance events.
Muscle Glycogen:
Muscle glycogen is stored locally in muscle fibers and used directly to power muscle contractions, particularly at high intensities. Unlike blood glucose, it cannot be used elsewhere in the body or replenished quickly.
When muscle glycogen runs low, fatigue sets in. Your muscles won’t be able to perform at intensity, which is why muscle glycogen is considered the key performance-limiter in long endurance events.
Does 120 g/h Actually Spare More Muscle Glycogen?
Logically, one might think that increasing carbohydrate intake would spare muscle glycogen. However, research doesn’t support this. Regardless of how much glucose is circulating in the blood, muscles still mostly rely on their own stored glycogen during higher-intensity work.
In other words, increasing carbohydrate intake beyond 90 g/h isn’t likely to spare more glycogen. You might prevent hypoglycemia more effectively, but you wouldn’t meaningfully preserve the internal fuel that actually limits performance.
There is some newer research that shows that fueling might spare muscle glycogen in some situations. So far, this is mostly only seen in runners. Likewise, some limited research shows fueling can also replenish glycogen during exercise. Intensity wasn’t a factor in the study but, in theory, this probably only happens during periods of low-intensity. Think a calm, downhill section of a race or while drafting in a peloton.
But, when glycogen sparing is observed, it is not dose-dependent. In other words, increasing intake from 90 g/h to 120 g/h is not likely to preserve more muscle glycogen.
In fact, in other scenarios consuming large amounts of carbs may have the opposite effect — prompting the body to burn more glycogen rather than preserve it.
High Fueling May Increase Muscle Glycogen Oxidation
Counterintuitively, instead of preserving muscle glycogen, some research shows increasing carbohydrate intake may actually increase muscle glycogen oxidation.
One theory as to why this happens is that elevated blood glucose levels stimulate glycogenolysis within the muscle. Another is that the body shifts toward a “carbohydrate-dominant” metabolic state when carbohydrate is abundant, increasing reliance on both circulating glucose and stored glycogen. Interestingly, this effect usually occurs along with gastrointestinal (GI) issues, suggesting incomplete absorption.
In simple terms: when you consume more carbohydrates, the body may choose to burn more carbohydrate across all sources – including muscle glycogen.
High Fueling May Also Suppress Fat Oxidation
High carbohydrate intake also has predictable effects on substrate utilization.
Typically, as exercise continues and glycogen stores deplete, the body will start to utilize more fat for energy. However, when you fuel with high amounts of carbohydrates, this shift is blunted. The body continues to rely heavily on carbohydrates and less on fat.
Performance Implications
We established that fueling with high amounts (120+ g/h):
- Does not necessarily spare more glycogen
- May increase muscle glycogen use
- Reduces fat oxidation and increases glycogen utilization
Whether this potentially improves or harms performance depends on the duration of the event.
Shorter Endurance Events (up to ≈2.5 hours)
In shorter endurance events like marathons, biathlon, or cycling criteriums, performance is limited more by power output than by muscle glycogen.
Fueling with 120 g/h or more can be advantageous in this context. Carbohydrates are a faster and more oxygen-efficient fuel than fat, meaning your muscles can produce more energy per unit of oxygen consumed. Since higher carbohydrate intake may increase carbohydrate oxidation, it could support sustained power output. This would marginally improve performance.
The higher fuel intake may increase muscle glycogen oxidation. However, this isn’t the main limiting factor at these durations.
Takeaway: During shorter endurance events, athletes likely benefit from consuming as many carbohydrates as they can tolerate, as it may help sustain intensity and improve performance.
Longer Endurance Races (2.5+ hours)
In longer events, such as triathlon, ultra trail, or longer cycling races, muscle glycogen depletion becomes a major performance limiter. Winning strategies are built around preserving muscle glycogen at all costs.
Fueling with 120 g/h or more might seem appealing in this context, especially during lower-intensity sections where exogenous carbs could theoretically spare or even restore glycogen. However, this effect doesn’t seem to be dose-dependent, and there’s no practical way to know if glycogen sparing is actually happening during a race.
Instead, there is a risk that high carbohydrate intake could cause you to burn through muscle glycogen stores faster. High carbohydrate intake could also cause your body to burn more carbs and less fat, leading to premature glycogen depletion and fatigue.
Takeaway: During longer endurance events, fueling with 120 g/h is risky because it can cause the body to burn through glycogen stores faster. Fueling with 90 g/h is a safer, better-tested strategy.
Practical Risks of Fueling with High Amounts
Beyond metabolic considerations, there are clear practical risks associated with very high carbohydrate intake.
Gastrointestinal distress is the most immediate concern. As carbohydrate intake rises, the likelihood that some carbs remain unabsorbed in the gut increases, potentially causing bloating, cramping, or diarrhea. While gut training can improve tolerance over time, it cannot eliminate this risk entirely.
Energy surplus and weight gain is another important factor. Consuming large amounts of carbohydrates can push an athlete into an energy surplus, leading to weight gain. In one-day races, this might be a major concern. However, during stage races, repeated over-fueling can add up.
To compensate for over-fueling and prevent weight gain, the athlete might have to restrict carbohydrates after the race or even skip dinner. Ironically, this can impair glycogen replenishment, leaving athletes under-fueled for the next stage and potentially harming performance.
Verdict: Should You Aim for 120 g/h?
The current evidence does not support a universal shift from 90 g/h to 120 g/h.
Higher carbohydrate intake may benefit some athletes in specific situations, especially shorter events where maximizing carbohydrate oxidation supports performance, but only if the athlete can tolerate high amounts without GI issues.
For longer races, the risks often outweigh the potential benefits. Increased glycogen use and suppressed fat oxidation could accelerate fatigue, and high intake increases the likelihood of gut issues or unwanted energy surplus.
Luckily, fueling science is improving rapidly. Personalized oxidation testing can help athletes determine their optimal carbohydrate intake to support performance without the associated risks.
However, these tests are not yet widely available. Until they are, 90 g/h remains the well-supported benchmark — not because it is a hard limit, but because it balances effectiveness, gut tolerance, and safety.