Energy Availability

After discussing the framework of this series last week, we will now focus on one of the most important parameters in sports nutrition in this first part: energy availability.

When it comes to calories, weight management, and nutrition, most of us still think of the classic energy balance: calories in minus calories out. If the balance is negative, you lose weight. If it is balanced, you maintain your weight.

On paper, this is absolutely correct. If you want to get lighter, you have to eat less than you burn. But what might suffice for the average office worker who goes to the gym twice a week is simply too basic for an ambitious endurance athlete. The starting point and the needs of your body are far more complex.

Why? Because your body doesn't care at all if the balance hits "zero" at the end of the day if it was completely under-fueled for hours in between. If you burn 1,500 kcal on the bike in a fasted state in the morning and only balance this deficit at 8:00 PM with a huge meal, your body was in a massive, catabolic (muscle-breaking) state of stress for hours. During this time, it couldn't focus on adaptation and recovery for the next session, but was purely fighting for "survival" and ramping up cellular emergency mechanisms—very few of which are performance-enhancing.

In endurance sports, we therefore look less at the pure energy balance and much more at Energy Availability (EA).

What is Energy Availability?

Energy availability describes the amount of calories remaining for your body to keep all life-sustaining systems running after deducting your training expenditure. This includes your immune system, hormone production (like testosterone or estrogen), bone density, and cell repair.

Scientifically, this is expressed in the following formula:

EA= (EI-EEE) / FFM

EI (Energy Intake): Your energy intake from food (kcal).
EEE (Exercise Energy Expenditure): The energy you actively burn during training (kcal).
FFM (Fat-Free Mass): Your fat-free body mass in kilograms (body weight minus fat mass).

The result shows you how many calories per kilogram of fat-free mass your physiological system still has left for its actual work.

The Magic Numbers: 35 kcal and the Physiological Abyss

So where is the optimal range for energy availability? Science provides clear thresholds here:

Optimal (> 45 kcal/kg FFM/day): Here you are perfectly fueled. Hormonal balance, rapid recovery, and a strong immune system. Your body is ready to convert training stimuli into real performance gains.
The Gray Zone (30 - 45 kcal/kg FFM/day): This is the range for athletes who want to specifically reduce their weight. 35 kcal/kg FFM/day is considered the critical benchmark for weight loss phases. If you drop below this, the body already starts making its first subtle cutbacks—for example, recovery time lengthens. You should not consistently fall below this value when aiming for a deficit.
The Abyss (< 30 kcal/kg FFM/day): This is where it gets clinically relevant. If energy availability falls below this threshold for prolonged periods, the body literally pulls the plug. Reproductive hormones drop (loss of periods in women, massive testosterone drop in men), bone density decreases (stress fractures threaten), and performance crashes. We are talking about RED-S (Relative Energy Deficiency in Sport) here—an enormously important topic that we will explore in depth later in this series.

The Time Factor and "Within-Day Deficiency"

Energy availability is all well and good, but ultimately this approach does not solve the initially mentioned scenario with the fasted training in the morning and the big meal in the evening. Standard daily models simply fall short here.

Because our body simply doesn't have a 24-hour clock. It doesn't do an accounting check just before midnight to see if enough energy was supplied. Your brain, more specifically the hypothalamus, scans your energy status in real-time.

Imagine your energy account like a checking account: If you burn 1,500 kcal in training on an empty stomach in the morning, your account goes massively into the red (into the "overdraft"). Even if you eat a huge meal in the evening and balance the account again at 8:00 PM, you were insolvent all day. The nasty thing about this analogy is: it unfortunately hits the nail on the head. Just like with finances, you have to pay expensive interest for this loan—and you suffer from this hormonal "debt" much longer afterward than the brief moment of "getting the loan" (the fasted training) brought you.

How does this work specifically in the body?

If you remain in an energy deficit of more than 400–600 kcal for several hours (even with an ultimately balanced daily total), it triggers a cascade that brings long-term problems:

Hormonal Emergency Brake: The hypothalamus registers the energy shortage and immediately throttles the release of hormones responsible for reproduction and metabolism (e.g., LH and fT3). In men, testosterone levels drop; in women, the menstrual cycle stalls.
Catabolic Stress: To keep blood sugar levels stable for the brain, the body releases more cortisol. This stress hormone actively breaks down muscle protein to generate energy from it. Simply put, during your "hunger hole," you are destroying exactly the structures you painstakingly tried to build in training.
Economy Mode: The body lowers its metabolic rate. You feel sluggish, get cold faster, and your recovery slows down enormously.

Scientific studies (including Fahrenholtz et al., 2018) show that athletes with such regular "within-day deficits" exhibit the same health damage as athletes who generally eat too little—even though their calorie balance mathematically adds up in the evening. So the athletes do not get lighter, but still have the same problems as a significant kcal deficit.

Timing is Everything

That is why it is extremely important to understand the model of energy availability. However, Within-Day Deficiency shows us how complex our nutrition is and that timing is often much more important than the pure balance. Here are three classic examples where timing plays a crucial role:

The "Open Window" (Metabolic Window): Directly after intense exercise, muscle cells are extremely receptive to nutrients (increased insulin sensitivity). Anyone who waits 1 to 2 hours here misses the most effective phase to replenish glycogen stores and initiate the switch from the catabolic to the anabolic (building) state.
Overnight Protein Synthesis: Your body repairs itself during sleep. A slow-digesting protein source (e.g., casein from quark or a protein shake) shortly before bedtime supplies the organism with essential amino acids for hours. If this building block is missing, overnight recovery fizzles out.
Carboloading: A competition is not decided by breakfast on race day. To maximally fill muscular glycogen stores, the body needs 36 to 48 hours. Anyone who only starts eating pasta the evening before the marathon will stand at the starting line the next morning with a half-empty tank.

Practical Takeaway: Creating Awareness Without Calorie Obsession

So what do we take away from this first post? That in the end, eating is just as important as your training!

In the bodybuilding scene, there's a saying: "Abs are made in the kitchen." For us endurance athletes, the same applies, even if it might not be quite as strikingly visible in the mirror, but rather hides within the training data and performance development:

The 300W FTP or the 4:00 min/km at the anaerobic threshold are made in the kitchen! If you don't give your system the energy it needs in time, your body will not be able to initiate the necessary adaptation processes. On the contrary: the probability of negative health consequences increases enormously.

In the next parts of the series, we will address the other elementary building blocks of nutrition. Starting with the role and importance of macronutrients (carbohydrates, fats, and proteins) to weight management, down to detailed questions: Which blood markers should be checked regularly and which supplements truly have scientifically proven performance-enhancing effects?

References

Areta, J. L., Taylor, H. L., & Koehler, K. (2021). Low energy availability: history, definition and evidence of its endocrine, metabolic and physiological effects in prospective studies in females and males. European Journal of Applied Physiology.
Fahrenholtz, I. L., et al. (2018). Within-day energy deficiency and metabolic response in male endurance athletes. International Journal of Sport Nutrition and Exercise Metabolism.
Loucks, A. B., Kiens, B., & Wright, H. H. (2011). Energy availability in sports. Clinics in Sports Medicine.
Mountjoy, M., Sundgot-Borgen, J. K., Burke, L. M., et al. (2018). IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. British Journal of Sports Medicine.
Stellingwerff, T., Heikura, I. A., Meeusen, R., et al. (2021). Overtraining Syndrome (OTS) and Relative Energy Deficiency in Sport (RED-S): Shared Pathways, Symptoms and Complexities. Sports Medicine.