You ate. You drank. You kept the carbs coming. You finish your long ride with tired legs and the comforting feeling that you got your fuelling right.
But being well fuelled doesn’t mean you’ve escaped every effect of a long session.
A small cycling study offers an interesting example: carbohydrates helped maintain blood glucose, but didn’t prevent changes in certain rapid eye movements.
Here’s what that means, and where we need to be careful.
🔬 The study in plain English
Nine healthy men each completed two three-hour cycling sessions at 60% of their maximum aerobic capacity.
During one session, they drank a carbohydrate solution providing 0.7 g per kilogram of body weight per hour. For a 70 kg rider, that works out to 49 g of carbohydrate an hour. During the other, they received a placebo.
The study used a randomised, double-blind crossover design: each participant tried both conditions, with the treatment concealed.
Researchers measured blood glucose and tested eye movements and several aspects of visual function before and after exercise.
📊 What changed during long rides, and what didn’t
Carbohydrates made a clear difference to blood glucose. At the end of the ride, average glucose was 5.2 mmol/L with carbohydrate, compared with 3.2 mmol/L with placebo.
However, certain rapid eye movements, called saccades, which shift your gaze between targets, became slower after exercise in both conditions.
That finding needs context: measured visual attention, visual acuity, binocular function, and global motion perception remained unchanged.
So this wasn’t evidence that everyone’s vision had deteriorated or that the riders had become generally less alert. It was a specific change in eye-movement control. Read the study abstract on PubMed.
⚖️ What this cycling study can, and can’t, tell us
Nine participants, all men, make this a small study with limited scope.
The researchers suggest that the changes in eye movement may involve a central nervous system mechanism. That is a possible explanation, not proof of what exactly caused them.
The findings also don’t establish that participants experienced “brain fog”, became unsafe to drive, or needed a particular amount of recovery time. The abstract reports no driving assessment or validated post-ride waiting period.
And the carbohydrate dose was part of the experiment, not a universal fuelling prescription.
🚴♀️ What I’d take into my next long ride
The practical message is to keep fuelling and give recovery some attention too.
- Keep your fuelling plan. This finding isn’t a reason to cut carbohydrates.
- Make room for recovery. Avoid squeezing a demanding ride into a schedule that leaves you rushing straight into your next responsibility.
- Treat sleep and cooling as separate priorities. Don’t expect your gels to do every job.
- Pay attention to how you feel afterward. Feeling tired or unwell calls for a change of plans.
These are broader practical considerations, rather than interventions this study tested.
You can’t out-gel a bad night’s sleep.
🩷 Doing your long rides indoors?
Three hours on the trainer goes a lot better when the basics are sorted before you clip in: fan, towel, bottles, fuel within reach, and a setup that doesn’t fight you. The Indoor Cycling Starter Pack is my no-fluff checklist to get you there.
🚗 What about driving after a long ride?
There’s no evidence here for a blanket “wait 30–60 minutes” rule.
The sensible principle is simpler: don’t start driving when you’re tired. If you’re not fit to drive, delay the trip or arrange another lift. A snack and a short break shouldn’t be treated as automatic clearance. That is consistent with general road-safety guidance on fatigue.
💡 The takeaway
Carbohydrates supported blood glucose during these long rides. They didn’t prevent every measured effect of exercise.
That’s an interesting finding, with limits. It’s not a reason to fear your next endurance session or rethink everything in your bottles.
Fuelling supports the ride. Recovery deserves attention too.
Source: Connell CJW, Duncan S, Black JM, Kuhn G, Thompson B & Gant N (2026). Fuel Doesn’t Fix It: Oculomotor Control Declines During 3 Hr of Exercise Irrespective of Carbohydrate Availability. International Journal of Sport Nutrition and Exercise Metabolism. doi.org/10.1123/ijsnem.2025-0231
This article is for general information only and isn’t medical advice. If you regularly feel dizzy, unwell, or unusually wiped out after long rides, chat to your doctor or a sports dietitian.
🩷The Endurance Diaries🩷
