For many years, sports nutrition was rooted in a simple metaphor: the body is an engine, glycogen (the body’s quick-release carbohydrate reserve) is its fuel, and fatigue occurs when the tank runs low.
Under this logic, nutrition strategy seemed quite obvious: eat lots of carbohydrates, fill the tank, and if possible, keep topping it up while exercising. More carbs = better performance.
But the physiology of exercise isn’t really this simple. A review published in January 2026 looked at over 160 studies on the intake, metabolism and use of carbohydrates. It suggests that the “engine” approach doesn’t add up. This doesn’t mean carbohydrates are somehow useless, but it does mean their main role is not what we thought it was.
Muscles, energy and assumptions
The traditional model focuses almost exclusively on muscle, and rests on three assumptions:
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Muscles run on glycogen
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When glycogen runs out, fatigue sets in
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We therefore have to maximise glycogen reserves and carbohydrate intake.
This approach became popular in the 1960s, when muscle biopsies allowed scientists to measure glycogen levels before and after exercise. They observed that athletes with higher levels could withstand longer periods of moderate to intense exertion, and thus the near-universal recommendation of “carb-loading” before exercise was born.
However, one detail was generally overlooked: what happened to blood glucose levels and the central nervous system when these athletes were nearing maximum capacity.
Blood, liver, brain
The new review shines the spotlight on something much smaller than a muscle, but much more important: the small reserve of glucose that circulates in the blood, and the role of the liver in keeping it stable.
At any given moment, the blood pumping through our veins contains just a few grams of glucose – more of a small puddle than a fuel tank. But this glucose is vitally important because the brain depends on its continuous flow. When sustained physical exertion causes blood glucose levels to drop and the liver cannot produce enough to keep up, the body perceives a threat; if glucose levels keep dropping, there’s a risk of hypoglycemia-induced brain damage.
The nervous system’s response is to slam on the brakes. The brain recruits fewer motor neurons, lowers strength levels and forces us to slow down or stop, even when a muscle could theoretically keep on contracting.
Viewed in this way, fatigue isn’t a case of an “empty fuel tank”, but a protection system that limits performance to prevent accident or injury.
‘Hitting the wall’
Anybody who has run a marathon or completed a similar endurance event will be familiar with “the wall”. Traditionally, this phenomenon has been chalked up to a simple question of energy. The glycogen has run out, game over.
But more recent approaches have added nuance to this picture. Most studies that analyse carbohydrate intake and performance observe a pattern: in the group that doesn’t take carbohydrates, blood glucose levels steadily fall to low levels. In the group that does take them, this fall slows or stops, and performance can be maintained for longer.
The benefit of carbohydrates therefore seems to lie less in “feeding the muscle”, and more in keeping blood sugar levels within a safe range, protecting the nervous system. “The wall” is, for the most part, the body activating its emergency brake.
There are strong arguments for this interpretation, as when a muscle truly runs out of ATP molecules (the “molecular unit of currency” that allows cells to function), what we see is extreme stiffness, similar to rigor mortis. But this isn’t how a tired athlete experiences it – what they feel is a gradual drop in performance, not a total mechanical failure.
How much carbohydrate do we really need?
If the main role of carbohydrates in exercise to keep glucose levels stable and avoid hypoglycemia, the question ceases to be “how much can we get?”. Instead, we should be asking “what is the minimum amount that we need in any given situation?”
Many older recommendations have recently been called into question. It was previously thought that prolonged efforts tend to need high intakes of 60-90g of carbohydrates per hour (or even higher).
These amounts may still be useful in very specific contexts, such as very long competitions and elite sports, but current evidence shows something unexpected. In many situations, much smaller amounts produce similar effects.
In fact, the new review suggests that 15-30g per hour during prolonged exercise provides a performance boost comparable to much higher doses. The key difference is not so much the amount, but avoiding a dangerous drop in blood glucose. Once that mission is accomplished, increasing the dose doesn’t always do much to help.
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This completely changes nutrition strategies. Instead of pursuing increasingly higher doses (which can lead to gastrointestinal discomfort, psychological dependence on carbohydrate gels and unnecessary expense), the practical approach becomes more refined. By finding the minimum point where glucose stabilises, performance can be sustained.
Furthermore, increasing the dose above certain ranges can have unexpected effects: it reduces fat oxidation, raises insulin and, in some studies, depletes muscle glycogen instead of conserving it. These are all the exact opposite of what athletes want.
In short, carbohydrates act as a glucose buffer, not as an unlimited fuel source. If we have already slowed the decline, adding more does not yield a corresponding benefit.
Low-carb athletes
Something else that challenges the prevailing carbohydrate dogma is the performance of athletes who follow low-carb diets. Very high fat oxidation rates have been measured in these athletes, remaining high even at significant intensities (above 85% of maximum oxygen consumption or VO2 max). In some contexts, their performance levels are similar to those of athletes on carbohydrate-rich diets.
This doesn’t mean that fat is always better fuel, but it does mean that the body can adapt and use it even under intense exertion. The old mantra that “at high intensity, you only burn carbohydrates” is not as universal as previously thought.
The concept underpinning this new understanding is metabolic flexibility, the ability to switch from one fuel to another depending on demand and availability. A chronically high-carbohydrate diet, without periodisation, could reduce signals to use fat, promote a feeling of dependence on energy gel and make the metabolism less flexible.
The alternative is not zero-carb, but learning to periodise. That means using training that forces the body to use more fat, with carbohydrates as a momentary, strategic backup.
The cult of carbs
So what does all this mean for people who train or compete in athletic events?
First of all, it demystifies things. Carbohydrates should neither be worshipped nor shunned – they are a tool. Instead of “the more the better”, we should be asking:
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What am I trying to achieve today? Is it immediate, maximum performance, or improving metabolic flexibility in the longer term?
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How will my body respond? Hunger, slumps, “hitting the wall”, digestive discomfort and dependency are all things to watch out for.
Second, it helps us understand that performance limits are not determined solely by muscle fibres. The brain, through its constant monitoring of glucose and other fuels, acts as a higher regulator, and when it senses an imbalance, it lowers power output. When used properly, carbohydrates can delay the activation of this brake, but this is mainly done by keeping glucose levels stable, not by endlessly refilling the tank.
And third, we have to remember that general recommendations are only a starting point. People with diabetes, a tendency towards hypoglycemia or who take medication need careful, tailored guidance, ideally supervised by a professional.
The future of sports nutrition does not lie in an increasing dependence on sugar, but in training metabolisms to use whatever is available at any given moment. Carbohydrates will continue to play an important role, but increasingly as a minimum effective dose for the brain, not an unquestionable source of “muscle fuel”.
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