What does underfueling feel like? The definitive guide

December 4, 2025 •

4 min read

According to one survey of athletes, over 40% of active females and 18% of males showed indicators of Low Energy Availability (LEA), the core cause of underfueling. The physical and mental signs can be subtle at first, making it difficult to pinpoint exactly what does underfueling feel like, but they are often mistaken for overtraining or simple tiredness.

Quick Summary

Underfueling, or low energy availability, presents with a wide range of symptoms affecting physical and mental well-being, including persistent fatigue, mood swings, and poor performance. These signs are often misattributed to other issues, but recognizing them is crucial for recovery and preventing long-term health consequences like hormonal imbalances and injuries.

Key Points

Subtle Onset: Underfueling symptoms often begin subtly, presenting as persistent fatigue and mood changes that are easy to mistake for overtraining or stress.
Psychological Impact: Expect psychological symptoms such as irritability, anxiety, brain fog, and a notable drop in motivation for training and daily activities.
Physical Breakdown: Physical signs include consistent lethargy, a decline in training performance, prolonged muscle soreness, and frequent illnesses or injuries.
Hormonal Disruption: Chronic underfueling can lead to significant hormonal issues, including menstrual irregularities in women and low libido in men.
Strategic Refueling: Recovery requires a targeted increase in calorie intake, particularly carbohydrates, with emphasis on proper pre- and post-workout nutrition.

The Mind-Body Connection: Psychological and Physical Signs of Underfueling

Underfueling, whether intentional or not, forces your body to adapt by conserving energy, which leads to a cascade of negative effects. When energy intake consistently fails to match energy expenditure from daily life and exercise, the body prioritizes essential functions like breathing and circulation over 'non-essential' processes such as recovery, immunity, and hormone regulation. This leads to a distinct set of physical and psychological symptoms that signal something is wrong.

Psychological Symptoms

Mood Swings and Irritability: When your brain isn't receiving enough glucose, your mood can become volatile. Small stressors can feel overwhelming, and you may find yourself feeling anxious or irritable for no apparent reason.
Brain Fog and Poor Concentration: The brain is a major consumer of glucose. A consistent lack of fuel can lead to feelings of mental fogginess, difficulty focusing, and memory issues, making daily tasks feel like a struggle.
Lack of Motivation: The drive to train or engage in normal activities can plummet. What once brought you joy now feels like a chore, and you constantly feel a sense of apathy.

Physical Symptoms

Persistent Fatigue and Lethargy: This is often one of the first and most common signs. You feel drained, even after a full night's sleep, and have a general lack of energy throughout the day that isn't solved by rest.
Poor Performance and Recovery: Your training capacity and strength decrease, and you may struggle to complete workouts that were once manageable. Your muscles feel sore for longer periods, and recovery time is extended.
Frequent Illness or Injury: Low energy availability compromises the immune system, making you more susceptible to colds, infections, and nagging injuries that just won't heal. Stress fractures are a particular risk for chronically underfueled athletes.
Gastrointestinal Distress: Underfueling can slow down the digestive tract, leading to issues like constipation, bloating, and general gut discomfort.
Hormonal Changes: In women, this can manifest as irregular or absent menstrual cycles (amenorrhea). In men, it can lead to decreased testosterone levels and low libido.
Changes in Appetite: You may experience heightened hunger cues or, paradoxically, a suppressed appetite, especially after intense exercise.
Poor Sleep Quality: Your body may wake you up to signal a need for fuel, preventing you from getting the deep, restorative sleep necessary for recovery.

Underfueling vs. Overtraining: A Comparison

It's easy to confuse the symptoms of underfueling with overtraining, but the cause and solution are different. Many athletes mistakenly believe they are just training too hard when the real problem is a lack of proper nutrition.


Symptom	Underfueling (Low Energy Availability)	Overtraining (Excessive Stress)
Cause	Calorie and nutrient intake does not meet energy demands.	Excessive training volume or intensity with inadequate rest.
Fatigue	Persistent and pervasive fatigue not resolved by rest.	Fatigue directly linked to intense training periods.
Performance	Performance steadily declines over time, often plateauing or getting worse.	Performance initially plateaus, then declines. Can be mistaken for a lack of effort.
Recovery	Muscles are sore for extended periods; delayed recovery.	Delayed recovery, but often tied specifically to the heavy training load.
Hormones	Directly causes hormonal disruptions (e.g., amenorrhea, low testosterone).	Can contribute to hormonal stress responses, but underfueling is a primary cause of imbalance.
Solution	Increase overall calorie intake, especially carbohydrates, and prioritize meal timing.	Reduce training load, incorporate more rest, and improve recovery methods.

How to Recognize and Respond to Underfueling

Increase Energy Intake: This is the most crucial step. Focus on increasing your overall calorie consumption with nutrient-dense foods. Don't fall into the trap of severely restricting any macronutrient, especially carbohydrates, which are vital for fueling high-intensity activity.
Prioritize Meal Timing: Eating before and after workouts is key. Consume easily digestible carbohydrates in the hour before training and a combination of carbs and protein within an hour after to replenish glycogen stores and aid muscle repair.
Listen to Your Body's Cues: For many, the ability to recognize hunger and fullness cues is disrupted. Start by setting a regular meal schedule to retrain your body and pay attention to subtle signals like a grumbling stomach or food cravings.
Sleep and Stress Management: Proper fueling supports better sleep and emotional regulation. Ensure you are getting adequate, high-quality sleep and actively managing your stress levels, as these factors are deeply intertwined with energy balance.
Seek Professional Help: If symptoms are severe or persistent, consult a registered sports dietitian. They can help create a personalized fueling plan to restore your body's functions and get you back to optimal health and performance. The effects of chronic underfueling can be serious and may require a multidisciplinary approach. For a deeper dive into the relationship between underfueling and training, consider resources like the Trail Runner Magazine article.

Conclusion: The Path Back to Feeling Fueled

Understanding what underfueling feels like is the first and most important step toward recovery. It's a complex issue with intertwined physical and psychological symptoms that can be easily confused with other problems like overtraining. By recognizing the signs—from chronic fatigue and mood swings to poor performance and frequent illness—you can begin to take proactive steps. The path to recovery involves increasing your energy intake, especially carbohydrates, prioritizing meal timing around exercise, and actively listening to your body's needs. Whether you are a dedicated athlete or someone simply trying to live a healthy, active life, recognizing and addressing underfueling is essential for unlocking your full potential and ensuring long-term well-being.

Frequently Asked Questions

While the symptoms of underfueling and overtraining often overlap, the root cause is different. Underfueling stems from an insufficient calorie and nutrient intake to support your energy expenditure, while overtraining is caused by excessive training load without adequate rest.

Underfueling can lead to mood swings, heightened irritability, and even feelings of anxiety or depression. This is because a lack of glucose, the brain's primary fuel source, impairs proper brain function and emotional regulation.

When the body is underfueled, it conserves energy by suppressing non-essential functions, including the immune system. This makes you more vulnerable to infections and illnesses. Similarly, a lack of energy and nutrients impairs muscle and bone repair, increasing the risk of injuries like stress fractures.

Yes, it can. While some people experience constant hunger, others, particularly those who are chronically underfueled, may experience a suppressed appetite or a lack of interest in food. This is another way the body adapts to low energy availability.

RED-S stands for Relative Energy Deficiency in Sport and is a syndrome caused by chronic underfueling. It encompasses a wide range of health issues, including hormonal disruptions, bone density loss, and impaired metabolic function, and affects both male and female athletes.

Begin by increasing your overall energy intake, focusing on a balanced mix of macronutrients, especially carbohydrates. Prioritize eating before and after workouts, listen to your body's hunger cues, and ensure you get adequate sleep. Consulting with a sports dietitian can provide a structured plan for recovery.

Yes. Intense exercise can sometimes suppress appetite, but it is critical to refuel within an hour after your workout to replenish glycogen stores and aid muscle recovery. A liquid or gel form of carbohydrates and protein can be an effective and palatable option.