Fuel Doesn't Fix It: Oculomotor Control Declines During 3 Hr of Exercise Irrespective of Carbohydrate Availability

This study investigates whether carbohydrate supplementation during 3 hr of cycling attenuates postexercise changes in oculomotor control and visual performance.

Within a double-blind, randomized, placebo-controlled, repeated-measures, crossover design, nine healthy males completed two 180-min cycling trials at 60% of maximal aerobic capacity, ingesting either a carbohydrate (0.7 g·kg-1·hr-1) or placebo solution. Saccadic eye movements (rapid, ballistic gaze shifts used to redirect the fovea to targets of interest), visual acuity, binocular function, and global motion perception were assessed before and after exercise. Heart rate, perceived exertion, blood glucose, and serum insulin levels were monitored throughout exercise.

Blood glucose diverged significantly between conditions across the 180-min exercise protocol, reaching hypoglycemic levels (<4 mmol/L) in placebo from 150 min (placebo: 3.2 ± 0.4 vs. carbohydrate: 5.2 ± 0.7 mmol/L at 180 min). Postexercise saccade velocity, amplitude, and latency changed equivalently across both conditions (-13% velocity, -1° amplitude, +25-ms latency; all p < .01). Visual attention, global motion perception, visual acuity, and binocular function were unchanged by exercise or intervention.

Postexercise reductions in saccade velocity and amplitude, and increased latency, were observed regardless of glycemic state, reflecting impaired oculomotor output independent of carbohydrate availability, while visual performance was preserved. These findings point toward a centrally mediated neural mechanism rather than carbohydrate substrate limitation.