• Energy Balance vs. Energy Availability
• RED-S: When the Body Doesn’t Have Enough Left
• How the Body Stores Energy and What Weight Fluctuations Really Mean
• The Bottom Line

In modern sports nutrition, an important shift in perspective has taken place in recent years. For a long time, everything revolved around one simple question: are you consuming more calories than you burn, or fewer? But this model falls short. The concept of energy availability shows why two people with identical calorie balances can be in completely different physiological states and why that matters deeply for your health.

§Energy Balance vs. Energy Availability

The classic energy balance recognises three states:

Isocaloric: intake equals expenditure — body weight stays stable. Hypercaloric: more in than out — you gain weight. Hypocaloric: less in than out — you lose weight.

That sounds straightforward. The problem is that this view says nothing about what’s happening inside the body. An athlete can have a mathematically balanced calorie intake and still be suffering from a serious physiological deficit.

This is exactly where energy availability (EA) comes in. It answers the more important question: how much energy is left for the body’s vital processes (such as hormone synthesis, immune function, and cell repair) after sport has taken its share?

The Formula

Why divide by fat-free mass (FFM) rather than total body weight? Simple: fat tissue is a massive energy store, but at rest it consumes very little energy. The metabolically active tissues (muscles, heart, liver, and brain) all reside in the fat-free mass. These are the tissues that need to be fuelled. Relating EA to FFM is therefore far more meaningful than using total body weight.

How Much Energy Do I Actually Need?

Before we can meaningfully assess EA, we need to know the actual energy requirement. That’s not as straightforward as it sounds and choosing the right calculation method makes a real difference.

Resting Metabolic Rate: Three Formulas Compared

The resting metabolic rate (RMR) is the energy your body uses at complete rest, essentially to keep your heart, lungs, and all other systems running. There are several formulas for calculating it:

Harris-Benedict (1919) — A classic that works well for people of average weight, but becomes inaccurate at very low or very high body weights because it doesn’t account for actual body composition.

Mifflin-St. Jeor (1990) — Currently the most well-validated method for most people. It reflects modern lifestyle conditions and is particularly accurate for those who are overweight (margin of error around ±10%).

Müller et al. (2004) — This formula distinguishes between BMI categories and is especially relevant for clinical applications. It is explicitly recommended in the German S3 guideline for the prevention and treatment of obesity — a clear indicator of its strength at the extremes (severe under- or overweight).

Calculating Total Energy Needs: The PAL Factor

The resting metabolic rate alone isn’t enough. It needs to be multiplied by the Physical Activity Level (PAL) to arrive at the actual daily requirement. The PAL factor reflects how active someone is throughout the entire day — work, leisure, and sleep all included.

For people who train 3 to 5 hours per week, an addition of approximately 0.3 PAL units on top of the leisure value is recommended.

Practical tip: Anyone wanting to objectively assess their activity level should look at step-counter data. Fewer than 5,000 steps per day is considered sedentary (very little movement). A genuinely active lifestyle only starts at 10,000 steps. Subjective assessments — “I’m actually pretty active” — are notoriously unreliable.

§RED-S: When the Body Doesn’t Have Enough Left

Relative Energy Deficiency in Sport (RED-S) is a condition that develops when the body is consistently left with too little energy over a prolonged period. It isn’t simply about losing weight. RED-S is a complex medical syndrome that affects almost every body system at once. First symptoms can appear after as little as 2 to 4 weeks of chronically low energy availability.

The Key Thresholds

Important to understand: the 30 kcal threshold is not a magic safety line. Some people develop symptoms at values well above 30 kcal/kg FFM, depending on individual stress factors, sex, and the duration of the deficit.

What Happens in the Body During a Chronic Energy Deficit?

When the body consistently receives too little energy, it switches into “conservation mode”. This is a deeply ingrained evolutionary survival strategy: it strictly prioritises. Vital functions like heartbeat and brain supply are maintained, at the expense of systems the body temporarily classifies as “dispensable”: reproduction, bone metabolism, and immune defence. Smart in the short term, dangerous in the long run.

Which Systems RED-S Attacks

The insidious thing about RED-S is that symptoms often develop gradually, are diffuse, and affect many different body systems at once, which makes diagnosis difficult.

Hormonal System In women, it frequently leads to menstrual irregularities up to and including amenorrhoea (absence of the period). In men, testosterone deficiency (hypogonadism) can develop. The thyroid and the hunger-and-satiety hormone leptin also fall out of balance.

Bone Density Declining bone density is one of the most critical aspects of RED-S — because it can be partially irreversible. The risk of stress fractures is permanently elevated. Bone that wasn’t adequately built up during adolescence cannot be fully recovered later in life.

Digestion and Immune System Gastrointestinal complaints and frequent infections are often among the first warning signs — and are commonly misattributed to stress or training load. The immune system is simply running on reserve.

Athletic Performance Less strength, poorer endurance, weaker concentration, delayed recovery — RED-S damages performance on every level. Those who think eating less will make them more efficient often end up with the opposite result.

Mental Health and Social Life Depressive moods, irritability, and social withdrawal are not uncommon. The emotional impact of a chronic energy deficit is frequently underestimated.

§How the Body Stores Energy and What Weight Fluctuations Really Mean

To better understand energy deficiency, it’s worth looking at the body’s internal storage systems.

Fat and Glycogen Stores

Fat tissue provides around 7,000 kcal per kilogram, by far the largest energy store in the body. It also serves as mechanical protection and as a hormone-producing organ.

Glycogen (stored carbohydrates) is found in the liver (80–120 g) and in the muscles (250–600 g). Crucial to know: 1 g of glycogen binds approximately 3 g of water. Someone who increases their carbohydrate intake (for example, after a diet phase) may gain 1–3 kg in the first few days (not fat, but water).

Understanding this mechanism removes the fear many people have about refilling their stores.

Thermogenesis: What’s Actually Burned During Digestion

Not every calorie on your plate ends up in your body. Some is lost as heat during digestion. This so-called diet-induced thermogenesis (DIT) varies by macronutrient:

Protein has the highest thermogenesis: 20–30% of the energy consumed is used directly for digestion and further processing.

Fat is stored extremely efficiently: only 2–4% is lost as heat.

Converting carbohydrates to fat (de novo lipogenesis) is energy-intensive and uses around 24% of the energy. What does this mean in practice? An extremely low-fat diet combined with low energy availability can actually increase metabolic stress, because the body has to spend more energy converting carbohydrates.

§The Bottom Line

Anyone serious about healthy nutrition and sport can no longer ignore the concept of energy availability. The question isn’t just “how much am I eating?” but “how much is left for my body after exercise?”

Moderate calorie deficits can be sensible and manageable. But staying chronically below critical thresholds risks irreversible damage to bones, the hormonal system, and athletic performance, often without noticing it at first.

(All thresholds and formulas are based on current scientific recommendations and clinical guidelines, including the German S3 guideline for the prevention and treatment of obesity, as well as international consensus positions on RED-S.)