Two doors for fuel in the intestine

The gut is not a passive tube. It is an active border crossing—and like every border crossing, it has its own gates for different types of travelers. Understanding what gates exist and how to use them both is the difference between 60 grams and 90–120 grams of absorbed carbohydrates per hour.

The first door: the glucose gate

The gut has its own entrance for glucose and for carbohydrates that break down into glucose, such as maltodextrin. Think of it as a gatekeeper that only lets in glucose, and only when there is enough sodium present to open the gate. This gate is active and requires energy. It also has a limited permeability.

Studies put the ceiling for this gate in most adults at around 60 grams of glucose per hour. If glucose is received faster than this, it starts to accumulate in the gut, drawing water with it and triggering the bloating, nausea, and stomach problems that every long-distance runner who has fueled hard recognizes.

The second door: the fructose gate

There is a separate gate for fructose, which operates completely independently. It does not require sodium, does not share workers with the glucose gate, and is not located at the same point in the intestinal wall. Most importantly, even if the glucose gate is full, the fructose gate remains open. These two pathways do not compete with each other.

What happens when both doors are used simultaneously

When you consume a carbohydrate mixture containing both glucose-based and fructose-based sugars in the correct ratio, both doors work simultaneously. The glucose pathway handles maltodextrin. The fructose pathway handles fructose independently. The combined absorption rate is their sum.

This is precisely why studies show that glucose-fructose mixtures can support oxidation rates of 1.5–1.75 grams of carbohydrates per minute—or about 90–105 grams per hour. With glucose alone, the figure is about 1.0 grams per minute, or 60 grams per hour.

The difference is not small. In a five-hour race, the higher rate delivers 150 grams more carbohydrates to the working muscles. That’s about 600 extra kilocalories that otherwise would have remained in the bag.

Can these doors be widened with training?

There is strong evidence that the gut adapts. Repeated high carbohydrate intake during training increases both the density and activity of glucose and fructose transporters in the intestinal wall. Athletes who regularly fuel hard absorb more in competition than those who do not train their gut.

Two practical implications follow from this. Firstly, if you are new to high-performance fueling, start at the lower end of the recommendations (about 60 grams per hour) and build up slowly. Secondly, the upper limit is not a fixed wall. With regular gut training, it expands.

Summary

These two doors are not just a physiological detail. They are a practical limit to how much fuel you can deliver to your muscles during a long race. Understanding that they are independent—and that the right sugar mixture opens both simultaneously—is the basis of evidence-based endurance fueling.

References

Jeukendrup AE. (2010). Carbohydrate and exercise performance: the role of multiple transportable carbohydrates. Current Opinion in Clinical Nutrition and Metabolic Care. 13(4), 452–457.

Cox GR, Clark SA, Cox AJ, Halson SL, Hargreaves M, Hawley JA, et al. (2010). Daily training with high carbohydrate availability increases exogenous carbohydrate oxidation during endurance cycling. Journal of Applied Physiology. 109(1), 126–134.

Jeukendrup AE. (2017). Training the gut for athletes. Sports Medicine. 47(Suppl 1), 101–110.

Vist GE, Maughan RJ. (1995). The effect of osmolality and carbohydrate content on the rate of gastric emptying of liquids in man. Journal of Physiology. 486(Pt 2), 523–531.