If you’re an endurance athlete—whether you’re training for a marathon, triathlon, or just pushing your limits—you probably think you’re eating enough to fuel your body. You log the miles, hydrate, and maybe even track your macros.
What if I told you that you might still be under-fueling?
It is one of the biggest mistakes I see among endurance athletes: they train hard but eat too little—sometimes without even realizing it. They chalk up their fatigue, slow recovery, or stalled progress to “training just being hard,” but the truth is, their body is running on empty.
Underfueling is incredibly common, yet it’s often overlooked. It doesn’t just affect how you feel during workouts—it impacts your performance, recovery, and long-term health. In this blog, we will break down why so many endurance athletes aren’t eating enough, how it’s holding them back, and, most importantly, how to fix it.
The Silent Epidemic: Why Endurance Athletes Are Underfueling
Underfueling happens when your energy intake (food) doesn’t meet your energy expenditure (training + daily activity + basic bodily functions). Your body is like a high-performance race car—it needs the right fuel to run efficiently. But too many endurance athletes run on fumes without realizing it.
Signs You Might Be Underfueling (Without Knowing It)
Feeling constantly tired or sluggish, even outside of training.
Struggling with plateaus despite consistent training.
Craving junk food or having extreme hunger swings.
Frequent injuries, stress fractures, or prolonged muscle soreness.
Mood swings, low energy, or feeling mentally drained.
If any of these sound familiar, you might be under-fueling—and your performance is suffering because of it!
Why Are So Many Athletes Not Eating Enough?
Here’s the crazy part—most endurance athletes don’t realize they’re under-fueling because:
1. They Think They’re Eating “Healthy” (But Not Enough)
Many endurance athletes focus on “clean eating” but end up cutting too many calories, carbs, or fats. They fear gaining weight, so they avoid calorie-dense foods, thinking it will help them stay “lean.”
But here’s the deal: lean doesn’t always mean strong.
Under-fueled = Under-perform.
💡 Fix It: Instead of worrying about the dichotomy of “clean” versus “bad” foods, it is far more beneficial to concentrate on the concept of fueling for performance and outcome. Understanding that your body is a complex system that requires various nutrients to function optimally is crucial.
When considering carbohydrates, for instance, the source of these carbs—whether they come from rice, fruit, or a bagel—holds less significance than the fact that your body needs carbohydrates for energy, especially when engaging in physical activities or demanding workouts. Carbohydrates serve as a primary energy source, and the body breaks them down into glucose, which is essential for fueling muscle contractions and maintaining stamina.
Different carbohydrate sources offer varying benefits; for example, fruits provide natural sugars along with vitamins, minerals, and fiber, while rice is a more concentrated source of carbohydrates that can provide sustained energy. A bagel, on the other hand, may offer quick energy but lacks the micronutrients found in whole fruits. Rather than labeling foods as “good” or “bad,” it is more pragmatic to think about the role each food plays in your overall diet and how it can contribute to your performance goals.
This approach encourages a more balanced and flexible mindset towards eating, allowing you to enjoy a wider variety of foods without the guilt that often accompanies restrictive eating patterns. Ultimately, prioritizing the quality of your overall diet, ensuring you consume a diverse array of nutrients, and listening to your body’s needs will lead you to make choices that support your performance effectively. By focusing on how different foods can fuel your workouts and recovery, you can create a nutrition plan that empowers you to reach your goals, enhances your physical capabilities, and promotes long-term health and well-being.
2. They Underestimate How Many Calories They Burn
Long training sessions burn way more calories than people realize, and if you’re not actively replenishing them, you’re slowly digging yourself into a hole.
Let’s say you run 15 miles on a Saturday. That could be anywhere from 1,500 to 2,000 calories burned—just in your workout. If you don’t intentionally eat more that day, you’ll be in a deficit, even if you’re eating “normal” portions.
💡 Fix It: After long training sessions, prioritize refueling immediately with an appropriate combination of carbs, proteins, and fats (think: a recovery shake, a big smoothie, or even a sandwich with lean protein).
3. They Fear Carbs
I cannot stress this enough—carbs are not the enemy. Yet, so many endurance athletes still fall into the low-carb trap, thinking it’ll make them lighter, faster, or leaner.
In reality, cutting carbs leads to:
❌ Poor recovery
❌ Lower endurance capacity
❌ Increased injury risk
❌ Decreased energy availability
Carbs are your body’s primary fuel source for endurance training. Without them, you’re running on fumes—literally.
💡 Fix It: If you’re training hard, you need carbs. A good rule of thumb is 8-10g of carbs per kg of body weight on heavy training days.
That might mean eating more carbs than you’re comfortable with—but that’s exactly what your body needs.
The Science of Underfueling and Performance
Underfueling doesn’t just make you feel tired—it has real physiological consequences.
🏃 Low Energy Availability (LEA): This occurs when there’s not enough energy left after exercise to support basic bodily functions. A study in The Journal of the International Society of Sports Nutrition found that many endurance athletes fall into LEA without realizing it.
⚠️ Increased Injury Risk: Underfueling leads to weaker bones and higher stress fracture risks. Studies show that athletes in a caloric deficit have increased injury rates due to compromised bone density and muscle breakdown.
⚡ Impaired Recovery: Without enough fuel, muscle repair slows down, meaning you stay sore longer and struggle to recover properly.
How to Fix Underfueling (A Simple Framework)
Now that we’ve covered the why, let’s talk about the how. If you suspect you’re underfueling, here’s how to fix it starting today.
1. Calculate Your Energy Needs
Use an energy calculator or consult a sports nutritionist to estimate your total daily energy expenditure (TDEE).
Factor in training volume, body weight, and recovery needs.
✅ Example: A 150 lb runner training 6x per week might need 3,000+ calories per day.
2. Prioritize Carbohydrates
Pre-Workout: 3-4 hours before training → A high-carb meal (oatmeal, toast, or rice)
During Training: For workouts over 60 min → 60-90 g of carbs per hour (sports drink, gels, or bananas)
Post-Workout: Within 30 minutes → Carbs + protein + enough calories for optimal recovery
3. Track Your Recovery Metrics
Are you constantly sore?
Do you feel sluggish during workouts?
Are you recovering well between sessions?
These are signs that you need to fuel better! Utilizing a diary can help with consistency, identify patterns, and keep you accountable to the process.
4. Stop Fearing Weight Gain
Underfueling will hurt your performance more than a few extra pounds will. Being strong, energized, and well-fed will always beat being underfed and depleted.
Final Thoughts: Fueling = Performance
Endurance training is demanding. If you want to perform at your absolute best, you need to start eating like an athlete.
💡 Remember:
Fuel first, train second.
Carbs are your friend, not the enemy.
Underfueling is holding you back—even if you don’t realize it.
If you’re ready to start fueling smarter and training stronger, let’s work together to create a nutrition and training plan that fuels your success.
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