Is it bad to eat junk food before a race?

December 2, 2025 •

4 min read

According to a study published in the Journal of the American College of Nutrition, high-fat and high-sugar diets significantly impair athletic performance. This makes it clear that the answer to 'Is it bad to eat junk food before a race?' is a resounding yes, and understanding why is crucial for any serious athlete.

Quick Summary

Consuming junk food before a race can cause gastrointestinal distress, energy crashes, and slow digestion due to its high fat, sugar, and fiber content. Prioritizing simple, nutrient-rich carbohydrates ensures optimal performance and endurance.

Key Points

Impairs Athletic Performance: Junk food, high in unhealthy fats and sugar, leads to energy crashes and sluggishness during a race, reducing overall performance.
Causes Digestive Distress: High fat, fiber, and sugar content in junk food can lead to bloating, cramps, nausea, and diarrhea, as it is difficult to digest during intense exercise.
Disrupts Energy Regulation: Refined sugars cause blood sugar spikes followed by dramatic crashes, which depletes your energy stores and hinders endurance.
Slows Digestion: The body diverts energy to the digestive process for heavy, fatty foods, taking power away from your muscles and making you feel weighed down.
Best to Choose Complex Carbs: Optimal pre-race fuel consists of easy-to-digest complex carbohydrates, consumed several hours before, for sustained energy and performance.
Avoid on Race Day: Never experiment with new foods on race day; stick to tried-and-true fueling strategies to avoid unpleasant surprises and optimize results.

The science behind junk food and athletic performance

Understanding how your body processes food is key to grasping why junk food is so detrimental before a race. Junk foods are typically high in unhealthy fats, simple sugars, and excessive sodium, while being low in essential nutrients like vitamins, minerals, and complex carbohydrates. This nutritional imbalance sets the stage for a poor athletic performance.

Your body relies on glycogen—a form of stored carbohydrate—for energy during exercise. While junk food may provide a quick sugar spike, it lacks the complex carbohydrates needed for sustained energy release. This leads to a rapid blood sugar crash, causing fatigue and reduced endurance. Moreover, the high fat and fiber content in many junk foods slows down digestion significantly, diverting blood and energy to the stomach rather than the working muscles. This can lead to a feeling of sluggishness and discomfort that directly hinders your performance.

The negative effects on your digestive system

One of the most immediate and unpleasant consequences of eating junk food before a race is gastrointestinal (GI) distress. The combination of high fat, fiber, and sugar creates a perfect storm for digestive issues. During physical activity, your blood flow is diverted away from the digestive tract and towards your muscles. When you consume foods that are difficult to digest, this process is disrupted, and the undigested food jostles around, causing problems.

Here’s a breakdown of the specific GI issues caused by junk food:

Bloating and cramping: The high fiber and fat content takes longer to process, leading to bloating and uncomfortable stomach cramps.
Diarrhea and nausea: Excessive sugar intake can have a laxative effect, while the overall heavy composition of junk food can trigger nausea and the infamous "runner's trots".
Heartburn: Spicy or acidic junk foods can irritate the stomach lining, increasing the risk of acid reflux and heartburn during your run.
Slow digestion: A large, fatty meal sits in your stomach for hours, leaving you feeling full and heavy rather than light and energized.

The impact on your energy levels and endurance

Beyond immediate digestive woes, junk food wreaks havoc on your body's energy regulation. The refined sugars in items like soda, candy, and pastries cause a sharp increase in blood glucose, followed by a sudden drop. This dramatic fluctuation can lead to a performance-killing energy crash, leaving you feeling drained and tired when you need power the most. A study found that eating a fast-food diet led to reduced motivation and impaired performance in animal subjects. For sustained endurance, your body needs a steady supply of energy from complex carbohydrates, not the empty, volatile calories found in junk food.

Comparison Table: Pre-Race Junk Food vs. Optimal Fuel


Feature	Junk Food (e.g., burger, fries)	Optimal Pre-Race Meal (e.g., oatmeal, banana)
Carbohydrate Type	Simple, refined carbs causing energy spikes and crashes.	Complex carbs providing a slow, sustained release of energy.
Digestion Speed	Slow due to high fat and fiber content.	Fast and easy to digest, with minimal GI distress.
Nutrient Value	Low in essential vitamins, minerals, and antioxidants.	High in beneficial micronutrients to support performance.
Fat Content	High in unhealthy saturated and trans fats.	Low in fat, or contains healthy, unsaturated fats in moderation.
Hydration Effect	High sodium content can lead to dehydration.	Promotes proper hydration, especially when paired with water/electrolytes.

Long-term consequences of poor race nutrition

Repeatedly relying on junk food before races or intense training sessions can lead to more than just a single bad performance. It can set a pattern of poor habits that negatively impacts your long-term health and athletic potential. A high-fat, high-sugar diet weakens the immune system, slows muscle recovery, and can lead to weight gain, making consistent and effective training more difficult. For more information on the long-term effects of diet on fitness, consider exploring resources from reputable sports nutrition organizations, such as the Academy of Nutrition and Dietetics.

The best pre-race fueling strategy

Instead of junk food, focus on nutrient-dense, easy-to-digest carbohydrates in the hours leading up to your race. The timing of your meal is just as important as its contents. A larger meal should be consumed 3 to 4 hours beforehand, while a small snack can be eaten 30 to 60 minutes prior.

Good pre-race options include:

Complex carbs (2-4 hours before): Oatmeal, whole wheat toast, or a bagel with a little jam.
Small, simple carbs (30-60 minutes before): A banana, rice cakes, or a simple energy bar.
Hydration: Water and electrolyte drinks are critical for maintaining fluid balance and preventing cramps.

In conclusion, while the temptation for a quick, comforting bite of junk food can be high, the negative consequences for your race performance and digestive comfort are not worth the risk. Prioritizing proper fueling with healthy, easily-digestible foods is a critical component of race day preparation that can make all the difference in achieving your goals.

Frequently Asked Questions

Junk food is often high in fat, fiber, and sugar, which are all difficult for the body to digest. During a race, blood flow is directed towards your muscles, away from your digestive system, leading to uncomfortable cramps as the undigested food sits in your stomach.

For a substantial meal, you should eat 3 to 4 hours before the race starts to allow for proper digestion. If you need a small energy boost closer to the event, a light snack of simple carbohydrates can be consumed 30 to 60 minutes beforehand.

Eating too much simple sugar before a race can cause a rapid increase in your blood glucose, followed by a sharp crash. This leads to fatigue and depleted energy levels, which can significantly hinder your performance.

While not recommended, if you are highly conditioned and have a very 'iron stomach,' some runners can tolerate small amounts of simple carbs found in certain less-processed 'junk' items. However, for most, the risk of GI distress and an energy crash far outweighs any potential benefit.

Excellent pre-race alternatives include easy-to-digest complex carbohydrates such as oatmeal, a banana with a small amount of nut butter, or plain white pasta. These provide sustained energy without upsetting your stomach.

Yes. A consistent diet high in processed fats and sugars can weaken the immune system, slow muscle recovery, and contribute to weight gain over time. This makes it harder to train effectively and achieve your full athletic potential.

While fiber is healthy, it is difficult and slow to digest. Consuming high-fiber foods like beans, whole grains, or certain vegetables before a race can cause bloating, gas, and an increased need for a mid-run bathroom break.