What Happens if You Don't Eat Before a Race?

November 17, 2025 •

4 min read

According to sports dietitians, arriving at the start line under-fueled is one of the most common mistakes athletes make. The consequence is often a significantly compromised performance, making it crucial to understand what happens if you don't eat before a race and why a proper pre-race meal is so important.

Quick Summary

Skipping your pre-race meal can lead to fatigue, lightheadedness, and drastically reduced performance, especially for events lasting over 60-90 minutes. It results in depleted glycogen stores, forcing your body to rely on less efficient fat reserves for fuel, which causes the sensation known as 'hitting the wall.'

Key Points

Performance Diminishes: Skipping a meal before a race, especially for events over 60-90 minutes, will lead to reduced performance and premature fatigue due to a lack of readily available glycogen.
Risk of Bonking: Inadequate pre-race fuel significantly increases your risk of 'bonking' or hitting 'the wall', a sudden and complete loss of energy that forces you to slow down drastically.
Mental Fog and Dizziness: Low blood sugar from not eating can negatively impact brain function, causing mental fog, poor concentration, and lightheadedness during your race.
Long-Term Health Consequences: Repeatedly running under-fueled can disrupt hormones, lead to muscle breakdown, increase injury risk, and weaken your immune system over time.
Strategic Fueling is Key: The best strategy involves a carb-rich meal 2-4 hours prior to the race and a small, easily digestible carb snack 30-60 minutes before the start.
Practice Race Day Fueling: Always practice your race day nutrition strategy during long training runs to ensure your stomach can tolerate the food and avoid unpleasant surprises.

Why Your Body Needs Fuel for a Race

Your body operates much like a car; it needs fuel to perform effectively. During exercise, your primary energy source is glycogen, which is stored in your muscles and liver. Glycogen is derived from the carbohydrates you consume. When you train or race, your body taps into these reserves to power your movements. Without a pre-race meal, you start with an empty or half-empty tank, setting yourself up for potential failure long before the finish line. For intense or long-duration races (over 60-90 minutes), this lack of readily available energy is particularly detrimental.

The Immediate Consequences of Not Eating

Early Fatigue and Sluggishness: When glycogen levels are low, your body has to work harder to convert other sources like fat into energy. This process is less efficient, leading to a noticeable drop in performance and a feeling of lethargy much earlier than anticipated.
Dizziness and Lightheadedness: Glucose is not just for your muscles; it's also the primary fuel for your brain. Low blood sugar (hypoglycemia) can occur, causing dizziness, mental fog, and poor concentration. This can be a serious safety risk during a race, where clear focus is essential for navigating the course and avoiding injury.
The Dreaded 'Bonk': This is the most infamous consequence of inadequate pre-race fueling. Hitting 'the wall' is a sudden and complete loss of energy that leaves your muscles feeling like lead. At this point, your body is out of easy-access glycogen and you are forced to dramatically slow down.
Increased Perceived Exertion: A run on empty will simply feel much harder than it should. Your body is already under stress from the race itself, and adding nutritional stress significantly increases the effort required to maintain pace, making a planned easy run feel like a sprint.

Long-Term Effects and Health Risks

For athletes who regularly skip meals before training or racing, the consequences can extend beyond a single poor performance. This practice can disrupt hormone levels, including elevating stress hormones like cortisol, which can increase muscle protein breakdown. Over time, this can lead to muscle wasting, weakened recovery, and a compromised immune system. This is a form of relative energy deficiency in sports (REDs), which can also lead to more severe issues like increased injury risk, including stress fractures.

Comparison: Fed vs. Fasted Performance


Aspect	Fueled (Ate Before)	Fasted (Didn't Eat)
Primary Energy Source	Readily available glycogen from recent carbohydrate intake.	Stored body fat and depleted glycogen reserves.
Energy Efficiency	High efficiency. Muscles have immediate access to their preferred fuel source, allowing for sustained high intensity.	Low efficiency. Body struggles to convert fat to energy quickly, forcing a slower pace.
Endurance	Enhanced. Full glycogen stores allow for longer, higher-intensity effort before fatigue sets in.	Reduced. Glycogen depletion leads to early fatigue and 'bonking,' hindering prolonged effort.
Mental Focus	Maintained. Steady blood glucose levels ensure the brain is adequately fueled for concentration.	Impaired. Low blood sugar can cause mental fog, dizziness, and poor decision-making.
Recovery	Quicker. Proper fueling supports faster glycogen replenishment and muscle repair post-race.	Slower. The body is in a greater deficit, prolonging the recovery process.
Risk of 'Bonking'	Low. A consistent stream of fuel prevents glycogen depletion during the race.	High. Depleted energy stores can lead to a sudden and complete loss of power.

Strategic Pre-Race Fueling

The best strategy is not to show up on an empty stomach, but to fuel wisely. For races over 60 minutes, a carb-rich breakfast 2-4 hours before the start is recommended to top off glycogen stores. This can be followed by a small, easily digestible carbohydrate snack 30-60 minutes before the race starts.

Example Pre-Race Meal Plan

3-4 hours before: A balanced meal with easily digestible carbohydrates, a little protein, and low fat. Oatmeal with a banana and nut butter, or a bagel with cream cheese and honey are common examples.
1 hour before: A simple, fast-acting carb source to top off energy. A banana, energy gel, or a small handful of pretzels are great choices.

For an excellent resource on fueling strategies and why carbs are the athlete's master fuel, consult REI's expert advice on the topic.

The Conclusion: Fuel is Non-Negotiable

In short, intentionally not eating before a race is a gamble with your performance and well-being. While it might be tempting for perceived weight loss or due to pre-race jitters, the physiological reality is that your body requires carbohydrates to perform at its best during endurance events. The risk of bonking, fatigue, and injury far outweighs any perceived benefit. By practicing a strategic, carbohydrate-focused fueling plan during training, you can ensure your race day goes smoothly and you cross the finish line feeling strong and accomplished, not depleted.

Frequently Asked Questions

While some studies suggest fasted training increases fat oxidation, overall calorie burn and long-term weight loss results are similar to training when fueled. The performance detriment from running empty often leads to fewer calories burned overall.

'Bonking' is a term for the sudden and extreme fatigue that happens when your body's glycogen stores are fully depleted. It results in a feeling of hitting a wall, with heavy legs and a dramatic loss of pace.

For a large, carb-heavy meal, aim for 2-4 hours before the race. For a smaller, quick snack, 30-60 minutes is sufficient. This allows enough time for digestion without causing stomach issues.

Focus on easily digestible carbohydrates and avoid foods high in fat, fiber, and protein. Good options include oatmeal, bananas, white toast with jam, or energy gels.

If pre-race anxiety affects your stomach, stick to very simple, low-fiber carbs like half a banana, a small energy gel, or diluted juice. Practice this during training to find what works best for you.

Carb loading isn't necessary for short distances like a 5K. However, a light snack like a banana about an hour before can prevent hunger and provide a mental boost without causing digestion issues.

If you are used to it, coffee can boost alertness and performance. However, avoid excessive amounts as it can cause stomach upset and the need for frequent bathroom stops. Never try a new coffee routine on race day.