Energy Availability in Active Individuals: Why Undereating Impacts Hormones

“You’re working out consistently… so why do you suddenly feel exhausted, moody, weaker, or stuck?”

For many active individuals, the assumption is:
train harder = better results

But sometimes, the problem isn’t training too little.
It’s eating too little for the amount of energy the body is using.

This is where the concept of energy availability becomes important -  one of the most researched topics in modern sports and performance nutrition.

What Is Energy Availability?

Energy availability refers to the amount of energy left for the body’s normal physiological functions after exercise energy expenditure is accounted for.

In simple terms:

The body first pays for movement and exercise.
Whatever energy remains is used for:

• hormone production

• metabolism

• recovery

• immune function

• reproductive health

• cellular repair

When energy intake stays too low for too long, the body begins conserving resources.

And hormones are often the first systems affected.

Undereating Isn’t Always Intentional

Many active people unintentionally underfuel.

This commonly happens due to:

• busy schedules

• appetite suppression from intense exercise

• aggressive fat-loss goals

• skipping meals

• “clean eating” becoming too restrictive

Sometimes individuals are eating “healthy” - but still not eating enough.

And over time, the body notices.

The Hormonal Response to Low Energy Availability

Research shows that chronic low energy availability can disrupt several hormonal systems.

Reproductive Hormones

In women, low energy intake may reduce estrogen production and disrupt menstrual cycles.

In men, studies show prolonged underfueling can reduce testosterone levels, affecting recovery, mood, strength, and muscle maintenance.

Thyroid Hormones

The thyroid helps regulate metabolic rate and energy expenditure.

When energy availability becomes too low, the body may reduce thyroid hormone activity as a protective mechanism to conserve energy.

This can contribute to:

• fatigue

• lower recovery capacity

• reduced metabolic efficiency

Cortisol and Stress Response

Undereating while training hard increases physiological stress.

Research shows this may elevate cortisol levels - a hormone involved in stress adaptation.

Chronically elevated cortisol may negatively influence:

• recovery

• sleep quality

• muscle preservation

• appetite regulation

Relative Energy Deficiency in Sport (RED-S)

One of the major frameworks used in sports science today is RED-S (Relative Energy Deficiency in Sport).

Initially studied mainly in female athletes, research now shows RED-S can affect:

• women

• men

• recreational exercisers

• endurance athletes

• strength athletes

RED-S is associated with:

• hormonal dysfunction

• impaired recovery

• decreased bone health

• weakened immunity

• reduced performance

This highlights that nutrition is not just about body composition - it directly affects physiology.

Why Performance Often Plateaus

Many people believe eating less while training more automatically improves results.

But research shows prolonged low energy availability may actually reduce:

• muscle protein synthesis

• recovery quality

• exercise performance

• training adaptation

In simple terms:
the body cannot build efficiently when it feels under-supplied.

This is why some active individuals feel:

• constantly sore

• unusually fatigued

• mentally drained

• stuck despite consistency

The Body Prioritizes Survival First

From an evolutionary perspective, the body interprets chronic underfueling as a potential threat.

So instead of prioritizing:

• muscle growth

• performance

• optimal hormone production

it prioritizes:

• survival

• energy conservation

• basic physiological maintenance

The result is often a body that feels “stressed,” even when the goal was to become healthier.

Signs That Energy Availability May Be Too Low

Some common signs include:

• persistent fatigue

• poor recovery

• disrupted menstrual cycles

• low libido

• frequent injuries

• sleep disturbances

• reduced performance

• irritability or low mood

These signs are often overlooked because they develop gradually.

The Bigger Shift in Fitness Nutrition

Modern performance nutrition is slowly moving away from:
“eat as little as possible”

toward:
“fuel appropriately for the demands placed on the body.”

Because long-term health and performance depend on adequate energy support.

The Takeaway

Training places stress on the body - but nutrition is what allows the body to adapt positively to that stress.

When energy intake becomes chronically too low, hormone systems, recovery, metabolism, and performance begin compensating behind the scenes.

Sometimes the issue isn’t lack of discipline.
It’s lack of fuel.

Because the body cannot perform, recover, or thrive when it constantly feels underpowered.