Undereating Symptoms: How Athletes Can Spot and Avoid


As an athlete, you need to eat enough calories to meet your energy needs for your level of physical activity. Otherwise, you may end up with undereating symptoms, which can have serious consequences for your health, mental health and sports performance.

In this post, we will discuss the top 10 undereating symptoms and also how they can affect your athletic performance.

We will also provide some tips on how to prevent and respond to undereating and RED-S.

What is Relative Energy Deficiency in Sport (RED-S)?

Undereating can lead to a syndrome called Relative Energy Deficiency in Sport (RED-S), which is defined by the International Olympic Committee (IOC) as

“impaired physiological functioning caused by relative energy deficiency, and includes but is not limited to impairments of metabolic rate, menstrual function, bone health, immunity, protein synthesis, cardiovascular health and psychological health.” (1

RED-S can affect both male and female athletes of any age, sport or level of competition. 

However, it is often under-diagnosed and overlooked in male athletes, who may lack one of the clearest and best-known underfueling symptoms: irregular or missing periods.

Top 10 undereating symptoms

Athletes may experience one or more of these undereating symptoms when they under fuel and do not eat enough calories to support their activity. Some of the most common symptoms include the following.

1. Fatigue

Feeling tired and sluggish during and after exercise is one of the most common and obvious signs of undereating. This is because your body does not have enough energy to fuel your muscles, brain and other organs.

With these low energy levels, you may also experience reduced endurance, strength, power and speed.

2. Poor recovery

When you are undereating, you may have trouble recovering from training and competition. This means that you may not be able to adapt to the stress of exercise and improve your physical activity, fitness and performance.

You may also be more likely to have injuries, infections and inflammation.

3. Low mood

Your mental health can also suffer if you are undereating. Symptoms like feeling depressed, anxious, irritable or apathetic are not uncommon. Also, you may lose interest in your sport or other activities that you enjoy.

You may also have difficulty concentrating, making decisions or coping with stress.

4. Hormonal imbalances

Undereating can disrupt your hormonal balance, which can have various effects on your body.

For female athletes, this can cause menstrual irregularities or amenorrhea (absence of periods). For male athletes, this can cause low testosterone levels, which can affect muscle mass.

You may even experience hair loss if your calorie intake dips too low.

5. Low bone mineral density

Your bones may become weaker and more brittle if you are undereating. This can reduce your bone mineral density (BMD), which is a measure of how strong and healthy your bones are.

Low BMD can increase your risk of developing osteoporosis (brittle bones) and stress fractures (broken bones) .

6. Gastrointestinal problems.

You may experience gastrointestinal problems such as nausea, bloating, constipation or diarrhea if you are undereating. This can affect your appetite, digestion and absorption of nutrients. It can also make you feel uncomfortable and impact your performance.

7. Reduced immune function

Your immune system may not work properly if you are undereating. This can weaken your ability to fight off infections and diseases. This can make you more susceptible to getting sick or having prolonged illnesses .

8. Decreased cognitive function

A change in your cognitive function is a symptom of underfueling. This is your ability to think clearly, concentrate and process information. This can affect your memory, learning, attention span and reaction time .

9. Impaired thermoregulation

Your thermoregulation may be impaired if you are undereating. This is your ability to maintain a stable core body temperature in different environments. This can make you more sensitive to heat or cold stress and affect your sweating and hydration.

10. Reduced growth and development

If you are a young athlete who is still maturing physically and mentally, undereating can affect your growth and development. This can affect your ability to maintain a healthy weight, height, pubertal development and overall health.

How to Prevent and Treat Undereating

The main cause of undereating is low energy availability (LEA), which is the amount of energy that is left for your body functions after subtracting the energy that is used for exercise.

LEA can be caused by various factors such as:

  • Eating too little or skipping meals
  • Exercising too much or too intensely
  • Having a poor diet quality or variety
  • Having disordered eating behaviors or an eating disorder
  • Having a high stress level or psychological distress
  • Having a lack of knowledge or awareness about nutrition

To prevent and treat undereating and RED-S, you need to ensure that you have adequate energy availability (EA), which is the amount of energy that meets both your exercise and body needs.

EA can be improved by various strategies such as:

  • Eating enough calories to match your energy expenditure
  • Eating frequently throughout the day (every 3-4 hours)
  • Eating a balanced diet that includes carbohydrates, protein, fat, vitamins, minerals and fluids
  • Eating before, during and after exercise to optimize performance and recovery
  • Seeking professional help from a sports dietitian or nutritionist if you have any dietary concerns or issues
  • Seeking professional help from a sports psychologist or counselor if you have any mental health concerns or issues
  • Monitoring your weight, body composition, menstrual cycle, bone health and blood tests regularly
  • Adjusting your training load and intensity according to your energy status and goals
  • Taking rest days and recovery weeks to allow your body to heal and adapt
  • Educating yourself and others about the risks and consequences of undereating
Signs of undereatingStrategies to improve energy availablity
FatigueEat enough calories
Poor recoveryEat frequently, include protein
Low moodEat a balanced diet rich in color
Hormonal imbalancesEat before/during/after exercise
Low bone mineral densitySeek professional help
Gastrointestinal problemsMonitor weight/body composition/menstrual cycle/bone health/blood tests
Reduced immune functionAdjust training load/intensity
Decreased cognitive functionTake rest days/recovery weeks
Impaired thermoregulationEducate yourself/others about issues


Undereating is a serious problem that can affect any athlete regardless of their gender, age, sport or level of competition. It’s important to recognize undereating symptoms since the impact can affect health and performance that may last for a long time.

However, undereating is also preventable and treatable with proper nutrition education, intervention and support.

If you suspect that you or someone you know may be undereating, do not hesitate to seek help from qualified professionals who can help you achieve optimal EA, health and performance.

Also, for athletes who may be struggling with low energy availabillity should seek expert help from eating disorder specialists if needed.

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