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:

Muscles run on glycogen

Muscles run on glycogen

When glycogen runs out, fatigue sets in

When glycogen runs out, fatigue sets in

We therefore have to maximise glycogen reserves and carbohydrate intake.

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.

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........

© The Conversation