Carbohydrates upper limit per hour

The limit to how quickly carbohydrates can be oxidized during exercise does not come from the muscles – they utilize fuel much faster than it can be delivered. The limit comes from the gut, and specifically from the permeability of two transporter proteins located in the wall of the small intestine.

The ceilings of the two transporters

The transporter protein that moves glucose through the intestinal wall reaches its peak speed in most adults at approximately sixty grams per hour. This figure is based on studies where glucose or maltodextrin was administered at increasing rates, and oxidation plateaued at this ceiling regardless of how much more was consumed.

The ceiling for the fructose pathway is lower, around thirty to thirty-six grams per hour for trained athletes.

When these are added together, the theoretical peak when both pathways are at full capacity is approximately ninety or ninety-six grams per hour. This is where the ninety-gram ceiling, familiar to endurance athletes, comes from.

Why the ceiling is rarely reached

Most athletes using commercial products do not come close to the theoretical ceiling because the products themselves create bottlenecks earlier in the chain. A solution with high osmolality slows gastric emptying, meaning that fuel reaches the intestinal transporters at a significantly lower rate than they could handle. A poorly chosen sugar ratio overloads one pathway while underutilizing the other – a 2:1 mixture at ninety grams per hour keeps the glucose pathway fully loaded, leaving the fructose pathway at only thirty grams. Large, infrequent doses create pulsatile loading instead of continuous saturation.

How gut training shifts the limit

Repeated high carbohydrate intake during training increases the production of these transporter proteins in the intestinal wall. The benefit is real but moderate, at best perhaps a fifteen or thirty percent increase from the baseline. It requires months of consistent effort and varies greatly between individuals.

At a practical level

With an untrained gut and unoptimized products, the practical ceiling is around fifty-five or sixty-five grams per hour. With a trained gut, the right ratio, and a low-osmolality product, the ceiling rises to approximately eighty or ninety grams. With a long-trained gut, some athletes can reach over one hundred grams.

The biggest single improvement comes from the product composition. Gut training provides significant but gradual additions on top of that foundation.

Summary

To approach the theoretical peak, four things are needed simultaneously: a low-osmolality carbohydrate source, an optimized glucose-to-fructose ratio, consistent small-dose intake, and gut training to increase transporter capacity. Each matters. Optimizing them all at once distinguishes good fueling from elite-level 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.

Rowlands DS, Houltham S, Musa-Veloso K, Barnard F, Roberts K, Hashimoto S. (2015). Fructose–glucose composite carbohydrates and endurance performance. Sports Medicine. 45(11), 1561–1576.

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