According to TodoMountainBike, a study published September 10, 2026 in the International Journal of Sport Nutrition and Exercise Metabolism, titled "Fuel Doesn't Fix It: Oculomotor Control Declines During 3 Hr of Exercise Irrespective of Carbohydrate Availability," put nine healthy men through two 180-minute cycling sessions at 60% of maximal aerobic capacity.

After three hours, saccadic eye movement velocity dropped by 13%, movement amplitude fell by roughly one degree, and latency — the delay before the eyes responded to a new target — rose by 25 milliseconds. All three changes were statistically significant.

The counterintuitive core of the finding is that carbohydrates made no difference to the eye-movement changes.

In the carbohydrate trial, riders consumed 0.7 grams per kilogram of body weight per hour — about 52.5 grams hourly for a 75 kg cyclist — and held blood glucose at an average of 5.2 mmol/L, versus 3.2 mmol/L in the placebo condition.

Yet saccade speed, amplitude and latency shifted comparably either way.

The researchers suggest the changes may reflect mechanisms within the central nervous system rather than insufficient glucose availability, though TodoMountainBike notes the experiment did not directly establish which neurological processes were responsible.

Perhaps the most important distinction concerns what researchers did not find: visual acuity, binocular function, visual attention and global motion perception showed no statistically significant deterioration.

Riders' eyes moved differently, but their measured ability to see clearly and process visual information held.

TodoMountainBike cautions the study does not establish that these changes impair obstacle detection, slow reaction time on the bike, or increase crash risk, and the measurements were taken in a controlled laboratory before and after exercise, not during real-world riding with terrain, speed and decision-making demands.

The nine male participants also limit how far results generalize to women, other age groups and training backgrounds.

For endurance athletes, the research adds a neurological dimension to fatigue beyond power output, heart rate and glycogen — the legs and blood glucose can look fine while aspects of nervous system function still shift after sustained exertion.

It also reinforces how much deliberate fueling still matters: TodoMountainBike stresses the results are not evidence against carbohydrate supplementation, which supports endurance performance and energy availability.

The broader stakes of structured exercise are underscored elsewhere this week — Health and Me reports that in the RESET for Remission trial published in The Lancet Regional Health – Americas, 54% of young adults with early-onset type 2 diabetes achieved remission after 24 weeks on an intensive diet-and-exercise programme, versus 4% on usual care, with resistance training preserving muscle during weight loss.