Cyclists: The Hidden Reason You Only Bonk Early on Race Day

 

 

Most endurance athletes spend months fine-tuning their fueling plans. Yet, on race day many still bonk, sometimes shockingly early. Although it’s easy to blame poor pacing or a missed gel, there’s often a hidden reason you burn through energy so fast: the surge of adrenaline and the body’s stress response that ramp up your carbohydrate use before you even start.

The Physiology of Race Day Stress

When you line up for a race, your body doesn’t always distinguish between excitement and threat. To your nervous system, there isn’t much difference between chasing a PR, running away from a bear, or just feeling keyed up on the start line.

This stress response releases a surge of the hormones epinephrine (adrenaline) and norepinephrine, which:

• Raise your heart rate and accelerate your breathing
• Dump extra sugar into your bloodstream
• Signal your muscles to start breaking down stored glycogen right away

Even if you don’t feel anxious, that sense of being “amped” or highly focused can flip these metabolic switches.

Evidence from Athlete Tests

I see this phenomenon in the CTS physiology lab frequently. Just the idea of a VO2 max or lactate threshold “test” can create enough stress to raise an athlete’s blood lactate, a marker of anaerobic effort, to levels I normally see near lactate threshold or Functional Threshold Power (FTP). Even during easy stages of an incremental test, intensities in Zone 2 for most athletes, I sometimes see respiratory exchange ratio (RER, which shows how much carbohydrate you’re burning versus fat) climb over 1.0. An RER of 1.0 indicates that nearly 100% of energy expenditure is coming from carbohydrate use, which is a scenario that typically only appears during efforts at or above threshold intensity. That’s a clear signal that stress response is pushing the athlete to burn through carbohydrates much faster than they would just from the training intensity itself.

You may not need lab data to spot it. If you notice the following signs during training or competition, it may indicate an elevated stress response and more rapid carbohydrate burn rate:

• Your heart rate running 10–15 beats higher than normal during warm-up
• Feeling jittery or flushed
• Breathing harder than you expect at easy effort
• You start sweating before you even start working hard
• You feel the urge to take in carbs earlier than usual

You don’t have to feel anxious, just being excited and ready to go on race day can trigger this.

How Stress Burns Through Carbohydrate Faster

Normally, your body uses a mix of fat and carbohydrates to fuel endurance exercise. But when adrenaline and stress hormones kick in, your muscles can shift to burning mostly carbohydrates, even at easy to moderate efforts.

A healthy stress response can be great if you need a burst of power or you’re trying to get away from a bear, but it also means you’ll drain your limited glycogen stores much earlier than you realize. In my experience, athletes with elevated stress responses can burn through glycogen 30–50% faster in the first hour of a race or high-pressure ride than they do in low-stress training rides.

If you’re doing a longer race or event, this faster drain doesn’t necessarily mean you’ll bonk in the first hour, especially if you’re fueling well. But it does mean you’ll reach that point of low reserves much sooner if you don’t adjust. Even in shorter events of 1–2 hours, if you go in under-fueled or rely on minimal carbs, you can find yourself hitting the wall surprisingly early.

Think of it like draining your battery faster: you won’t run empty right away, but you’ll end up low much sooner than you planned if you don’t pace and fuel accordingly.

How Stress Response Changes During Longer Events

The good news is that stress hormones don’t stay maxed out for your entire race.

• 0–60 min: Stress hormones are typically highest. Heart rate is elevated, breathing is faster, and carbs may be the primary fuel or at least burning at a more rapid rate than normal.
• 1–2 hours: You settle into rhythm, adrenaline tapers, and your body shifts back toward a more normal mix of fat and carbohydrate usage. At a sustainable endurance pace under lactate threshold, athletes typically burn about 50/50 carbohydrate and fat, with fluctuations +/- 10% in response to short term efforts.
• After 2 hours: If you’ve paced evenly, your system calms further. But any surge, attack, or new stress can reignite the adrenaline response.

Think of adrenaline like having afterburners on for takeoff. They help you launch, but you can’t run them forever.

How to Stop Burning Carbohydrate So Fast

Your stress response is responsible for ramping up your carbohydrate burn rate, so it’s important to take steps that lessen the surge of stress hormones. These steps include:

1. Practice Race Simulations
   Include workouts that feel like real events: harder starts, group rides, or training races where you feel nervous or excited. Use these sessions to notice how your body responds (like higher heart rate or faster fatigue) and to practice staying calm and sticking to your nutrition plan under pressure.
2. Fuel Early and Consistently
   Don’t wait until you feel tired or hungry! Start taking in carbs early, even if you had a good pre-race meal. In longer events, aim to start within the first 30-45 minutes. In shorter races, you still may need more carbohydrates than you might expect, since adrenaline can make you burn through energy reserves surprisingly fast. Just remember, this doesn’t mean taking in huge amounts all at once. Just stick to the fueling strategy you’ve practiced and spread intake evenly.
3. Start Easier Than You Think You Should
   Don’t always trust your perceptions during the first hour. The stress response can make everything feel effortless, but your body is basically lying to you. You’re often burning through energy reserves far faster than you realize. Start conservatively so you don’t drain your carbohydrate tank too soon.
4. Manage Pre-Race Nerves
   Arrive early, do a familiar warm-up, and practice calming techniques like deep breathing. Avoid big hits of sugar right before the start.

“Should I Slam a Carbohydrate Gel Before the Start?”

Taking in a gel or sugary sports drink just before a race can help if you’re event is going to start with an all-out effort (e.g., cyclocross, short-track MTB, criterium). But if there’s going to be a delay at the start line or a gentle roll-out for more than about 10–15 minutes before you start riding hard, the combination of adrenaline and fast carbs can spike your blood sugar and lead to a sudden drop once you ramp up energy demand. A pre-race gel isn’t automatically bad, especially for races under an hour. For events lasting 90 minutes or longer, where pacing varies and you may have more steady riding before crunch time, steady fueling once you’re moving is usually a safer, more reliable approach.

The Bottom Line

Stress isn’t just mental, it’s metabolic.
Even if you train smart and fuel generously, adrenaline on race day can still shift your metabolism toward faster carbohydrate use. Whether your event is 90 minutes or nine hours, the key is to start early, stick to a plan you’ve practiced, and fuel consistently. If you haven’t accounted for this faster burn before, recognizing the signs and adjusting your approach can help you finish stronger.

References

1. Burke, L.M., Hawley, J.A., Wong, S.H.S., Jeukendrup, A.E. (2011). Carbohydrates for training and competition. Journal of Sports Sciences, 29(sup1), S17–S27.
2. 2. Jeukendrup & Killer (2010): Pre-exercise carbohydrate ingestion timing influences glucose and insulin; when carbs are consumed 15–75 min pre-exercise, transient hypoglycemia may occur at onset.
3. Hackney AC. Stress and the neuroendocrine system: The role of exercise as a stressor and modifier of stress. Expert Rev Endocrinol Metab. 2006;1(6):783–792. doi:10.1586/17446651.1.6.783
4. Romijn JA, Coyle EF, Sidossis LS, et al. Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. Am J Physiol Endocrinol Metab. 1993;265(3):E380–E391. doi:10.1152/ajpendo.1993.265.3.E380