01 May 6 Signs You Might Be Underfueling as a Runner

6 Signs You Might Be Underfueling as a Runner (And What to Do About It)

Updated December 2025

If there’s one thing runners need to do to perform their best, it’s get enough fuel. Yet underfueling is incredibly common, even among runners who are training consistently and doing “all the right things.” Some research in the area has shown up to 70% of athletes are not eating enough to meet the demands of their training. 

Below are six signs you may be underfueling as a runner, why it matters, and, most importantly, what to do about it.

But First, What Does “Fuel” Mean?

Fuel is a term you’ll often see used in performance nutrition, but what does it actually mean, and why can underfueling be such a problem for runners?

Fuel describes the energy we get from the food we eat. Outside of performance nutrition, this energy is usually referred to as calories. These calories are required by our working muscles during exercise, but they’re also essential for keeping the rest of our body functioning properly.

Underfueling occurs when a runner is simply not taking in enough calories to meet the demands they’re placing on their body.

There are many reasons this can happen. Sometimes it stems from concerns about body weight or body composition. Other times it’s due to a lack of understanding of how much fuel you need as an athlete compared to the general population. And in other cases, it’s as simple as not having enough time in the day to prepare and eat enough food. I mean, we’re all busy right?

Whatever the reason, consistently underfueling can have serious consequences for both performance, and long term health.

It All Comes Down to Energy Availability

Energy availability refers to the amount of energy you have left for your body’s basic functions after accounting for the energy you use during exercise.

For runners who spend hours each week training, energy needs are significantly higher than those of sedentary individuals, people who don’t exercise. Your body requires enough energy not only to fuel your runs, but also to support things like hormone production, immune function, digestion, and bone health.

When energy availability becomes too low to sustain all of these body processes, performance will almost always suffer. Beyond that, your body’s basic needs may no longer be met. This can lead to a condition known as Relative Energy Deficiency in Sport (RED-S), which occurs when an athlete chronically fails to consume enough fuel. RED-S can have negative impacts on every body system from your digestion and gut health all the way to your weight and body composition. 

Are You Underfueling?

It can be hard to tell as a runner if you’re actually eating the right amount to support your training demands. To help, here are some signs your calorie intake may be too low to support your training and overall health, from a Sport Dietitian and Certified Running Coach.

Sign #1: You’re Sick or Injured All the Time

One of the most overlooked aspects of fueling is that every process in your body requires energy, not just physical movement.

Digestion, breathing, circulation, immune function, and tissue repair all rely on adequate fuel. When calorie intake is consistently too low, immune function can become compromised, leaving you more susceptible to frequent colds or infections.

Low energy availability also interferes with recovery from training. Running places stress on the body, and without enough fuel to repair and rebuild, recovery suffers. Over time, this increases the risk of overuse injuries, bone fractures, strains and sprains.

Common signs include:

• Frequent colds or lingering illnesses

• Recurring injuries or slow-healing aches

• Feeling “run down” despite consistent training

Sign #2: Your Sleep Quality Is Poor

Underfueling doesn’t just affect your training, it can spill over into other areas of your life, including sleep.

Sleep is a critical part of the recovery process, and low energy availability has been shown to negatively impact sleep quality. Difficulty falling asleep, restless nights, or waking frequently can all be signs that your body isn’t getting enough energy.

Poor sleep then further impairs recovery and performance, creating a cycle that often continues until fueling is addressed.

You may notice:

• Trouble falling or staying asleep

• Restless or non-restorative sleep

• Waking up in the middle of the night feeling hungry

• Feeling tired despite spending “enough” hours in bed

Sign #3: You’re Constantly Hungry — or Have No Appetite at All

This one might seem obvious, but in today’s diet-focused world that isn’t always the case.

If you’re hungry, your body is asking for food.

Yet many runners try to push past or ignore hunger signals, especially if fat loss is a goal. For athletes, this approach often backfires. Hunger is a useful signal, and consistently ignoring it can contribute to low energy availability.

On the other hand, some runners experience the opposite problem. During periods of intense training or chronic underfueling, appetite can actually decrease. Some people may also experience digestive symptoms such as nausea, bloating, or constipation around meals, because their digestive system isn’t getting the fuel it needs to function normally. Yes, even your digestive system requires calories to work properly!

If you find it easy to skip meals or forget to eat, setting a regular meal and snack schedule- even using reminders like a task in your phone can help ensure your body is getting the fuel it needs.

Signs can include:

• Constant hunger or food preoccupation

• Skipping meals unintentionally

• Low appetite despite heavy training

• GI symptoms around meals

Sign #4: You’ve Lost Your Period

For female runners, the loss of a normal menstrual cycle is a significant warning sign and should never be ignored.

The endocrine system, which regulates hormones, is highly sensitive to energy availability. When calorie intake is too low, hormone production can be disrupted, leading to missed or irregular periods.

While losing your period may seem convenient, these hormonal disruptions can have serious long-term consequences, including reduced bone mineral density, increased risk of stress fractures, and potential fertility issues.

Red flags include:

• Missed or irregular periods

• Periods stopping during training blocks

• Being told this is “normal” for runners (it absolutely isn’t)

Sign #5: You’re Struggling to Focus or Concentrate

We often think about muscles needing fuel to perform, but the brain is actually one of the largest consumers of energy in the body.

At rest, the brain uses roughly 20% of the energy we get from food, primarily in the form of glucose. When energy intake is insufficient, cognitive function can suffer.

Poor concentration, brain fog, memory issues, irritability, and mood changes can all be signs that your brain isn’t getting the fuel it needs to function optimally.

You may experience:

• Brain fog or poor focus

• Irritability or low mood

• Difficulty concentrating at work or during runs

Sign #6: Your Performance Has Declined

One of the clearest indicators of adequate fueling is performance progression. With appropriate training and recovery, runners should gradually feel stronger, fitter, and more resilient.

If you’re feeling run down, struggling through workouts, or relying heavily on caffeine to get through your runs, it may be time to take a closer look at your energy intake.

When fueling is inadequate, the body simply can’t adapt to training the way it’s supposed to.

Common performance clues include:

• Workouts feeling harder than expected

• Slower recovery between sessions

• Plateauing or declining performance

Want to find out if you’re underfueling? My free fueling audit will walk you through all of these signs and symptoms to help you find out if your nutrition is helping or hindering your running performance.

Get your free fueling audit here

The Bottom Line

Underfueling can quietly undermine both health and performance, often without runners realizing it’s happening.

Runners require significantly more energy on a daily basis than non-athletes, and meeting those energy needs is essential for long-term progress, injury prevention, hormone health, and overall well-being.

Fueling adequately isn’t about eating perfectly all the time, or even eating as clean as possible. Instead, it’s about giving your body the energy it needs to support the work you’re asking it to do.

To learn more about what to eat before and after your runs, check out my blog post on pre- and post-run fueling.

Or, if you’re interested in more science-backed, personalized support, you can learn more about working with me here.