5 Signs You Might Be Underfueling as a Runner
If your runs are getting harder, your recovery is slower, or you’re dragging through the day despite solid sleep—it might not be your training plan that’s the problem. It might be your fueling.
In marathon training, nutrition is as important as kilometers. But while most runners obsess over splits, pace charts, and long-run distance, many forget that what you eat (and how much) fuels every step, every recovery, and every personal best.
Underfueling doesn’t just mean not eating enough food—it means your body isn’t getting the carbs, protein, fats, and micronutrients it needs to perform and repair. This can happen unintentionally, especially for busy runners who train hard but skip meals, underestimate their needs, or rely on low-calorie “healthy” snacks that don’t actually cover the workload.
Here’s how to spot the five most common underfueling symptoms in runners, why they matter, and how to fix them so you can train stronger, recover faster, and hit your marathon goals.
Want a fueling plan that’s as personalized as your training plan?
Inside my Endurance Essentials Program, I’ll show you exactly how to fuel for marathon training, prevent underfueling, and run your strongest race yet.
Sign #1: You’re Always Tired (Even When You’ve Slept Well)
What it could look like:
You’re getting 7–9 hours of sleep but still waking up foggy. Coffee feels like your lifeline. By 3 PM, you’re mentally and physically done—even on rest days.
Why it might be connected to underfueling:
Without enough carbohydrates and micronutrients, your body shifts into energy conservation mode—slowing recovery, reducing training quality, and making everyday life feel harder.
What you can do:
Eat protein + carbs at breakfast to refill glycogen and stabilize energy.
Always fuel before runs, even short ones.
Have a recovery snack (protein + carbs) within 30–60 min post-run.
Learn your true carb and calorie needs for your mileage and training load.
Sign #2: Your Skin, Hair, or Nails Have Changed
What it could look like:
More hair in the drain. Nails that split easily. Skin that looks dull or breaks out more often.
Why it might be connected to underfueling:
Your body will protect vital systems over cosmetic ones. Without enough protein, iron, B vitamins, zinc, and healthy fats, your skin barrier, hair follicles, and nail beds take the hit.
What you can do:
Include protein at every meal to support tissue repair.
Eat healthy fats from avocado, nuts, seeds, and olive oil to nourish skin and hair.
Track food intake for a week to spot nutrient gaps, you may need a boost by supplementing to get your body back on track.
Not just the Macronutrient
A balanced diet will also get you the micro-nutrients you need to maintain a strong, healthy body
Sign #3: You Struggle to Focus or Feel Foggy
What it could look like:
You lose track of conversations. You reread sentences because they don’t “stick.” You feel mentally slow at work or in daily tasks.
Why it might be connected to underfueling:
Your brain runs on glucose, aka CARBS. When carbs are low, or nutrients like iron and B vitamins are lacking, your mental clarity and reaction time suffer.
What you can do:
Add a carb-rich snack before mentally demanding tasks or workouts.
Check your iron levels—especially important for female endurance athletes.
Hydrate consistently; even mild dehydration impacts focus.
Notice when brain fog hits—often it’s tied to long gaps between meals.
Sign #4: Workouts Feel Harder Than They Used To
What it could look like:
Runs that used to feel easy now feel like slogs. Your feet feel like bricks. Paces drop. Recovery takes days. Even yoga feels like too much.
Why it might be connected to underfueling:
Low glycogen, reduced oxygen delivery, and poor muscle recovery make every workout feel like a grind. Without enough carbs, your body starts breaking down muscle for fuel.
What you can do:
Eat a high-carb meal 2–3 hours before key workouts.
Use mid-run fuel for any session over 90 minutes.
Log workouts alongside nutrition notes to see patterns.
Replenish with carbs + protein immediately post-run.
Working out over 60 minutes?
Don’t forget to take your nutrition with you. If you’re going to be out for more than 60 minutes, plan to take some form of simple carb to keep those energy stores up.
Sign #5: You’re Dealing with Frequent Headaches or Body Aches
What it could look like:
Persistent low-grade headaches. Muscle soreness that never fully goes away. Achiness that feels “normal” because it’s constant.
Why it might be connected to underfueling:
Low glycogen, dehydration, electrolyte imbalance, and chronic inflammation are common when marathon runners aren’t fueling enough.
What you can do:
Hydrate with electrolytes during and after long runs.
Spread meals evenly across the day instead of skipping.
Include anti-inflammatory foods: berries, fatty fish, leafy greens, sweet potatoes.
Increase carbs around training sessions to speed recovery.
Just ask!
I love helping clients develop meal plans that pack all the nutrients that they need into meal plans and recipe books!
You Don’t Have to Guess What Your Body Needs
Underfueling doesn’t make you tough—it makes you tired. These symptoms are your body’s way of asking for more support, not more willpower.
When you fuel your marathon training properly, your energy, pace, recovery, and mood all improve.
💬 Ready to find your sweet spot for carbs, protein, and total calories so you can train without burning out? My Endurance Essentials Program will help you get there.
Learn more about my program here.
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