Metabolic Damage Isn’t Real (But Relative Energy Deficiency Syndrome Is)

Updated: 28th December, 2025

This article is adapted from the content in the 3rd edition of our book, The Muscle and Strength Pyramid: Nutrition.

Energy Availability 

Energy availability refers to whether you have adequate energy available to maintain normal physiological function when accounting for activity. When energy availability drops too low, either severely or over a prolonged period, it can lead to Relative Energy Deficiency in Sport (REDs). REDs is a syndrome characterized by impaired physiological function, including disruptions to metabolic rate, reproductive and menstrual health, bone integrity, immune function, protein synthesis, and cardiovascular health [1]. 

Energy availability is calculated as the number of calories left over after exercise expenditure, relative to lean body mass (LBM). Mathematically, this is expressed as: 

Energy availability = (Total energy intake – Exercise expenditure) / LBM 

For example, a 100 kg (220 lb) athlete at 10% body fat has 90 kg of lean mass. If they consume 3000 kcal and burn 400 kcal per day through exercise, they’re left with 2600 kcal for basic physiological needs. Their energy availability would be:

2600 ÷ 90 = ~28.9 kcal/kg LBM/day

When energy availability falls too low, typically below 30 kcal/kg LBM/day, the risk of developing REDs symptoms increases.

Signs of metabolic and reproductive hormonal downregulation associated with REDs have been observed across a wide range of populations — from lean, male Army Rangers during intense training, to both active and sedentary women of normal weight, and even women with obesity undergoing rapid weight loss. 

These effects tend to emerge when energy availability drops below ~30 kcal/kg LBM/day (13.6 kcal/lb LBM/day), regardless of whether that deficit stems from increased exercise expenditure, reduced energy intake, or a combination of both [2, 3]. While 30 kcal/kg LBM/day is a helpful rule of thumb, it is not a universal threshold. Most physique athletes will dip below this level at some point during contest prep, and even those who manage to stay above it may still experience symptoms of REDs, or not, depending on individual variability. Baseline non-exercise activity, LBM composition, sex, and a range of physiological factors all influence how much energy availability is “enough” for each person [4].

Low energy availability also exists on a spectrum, from mildly suboptimal to severely debilitating. But at its core, it simply means there isn’t enough “leftover” energy to support normal physiological function fully. And when energy availability drops too low, REDs can affect nearly every system in the body, undermining both health and performance.

Reading this, you may wonder: “How is this different from metabolic adaptation?” Great question. While many of the physiological changes associated with metabolic adaptation also appear in REDs, the terms describe different things. Metabolic adaptation refers specifically to how the body reduces energy expenditure during and after weight loss. In contrast, REDs describes how that reduction — and overall low energy availability — impacts health and performance. 

It’s also important to distinguish energy availability from energy balance.  You can be in a state of low energy availability while at energy balance. This is because, unfortunately, metabolic adaptation can persist after weight loss, when most of your energy intake is used to support training and basic metabolic functions, leaving too little for other vital processes. 

Rosenbaum [5] followed matched trios of participants: one who had lost ≥10% of their body weight and maintained it for 5–8 weeks, one who had maintained that loss for over a year, and one who had never lost weight. Despite maintaining their weight, the weight-reduced participants had significantly lower total daily energy expenditure than those who had never lost weight, regardless of whether their weight loss was recent or long-term. This persistent downregulation, averaging ~15% below predicted levels, is a consistent finding across the literature [6].

Let’s apply this concept with a hypothetical competitor in stage condition. Using a well-established energy expenditure equation [7], we estimate the daily energy needs of a 170 cm (5’6”), 70 kg (154 lb), 25-year-old woman at 12% body fat who performs moderate exercise 4–5 times per week. Her calculated total daily energy expenditure (TDEE) is approximately 2491 kcal/day.

If she were sedentary, her estimated expenditure would drop to 2041 kcal/day, suggesting an average daily exercise expenditure of ~450 kcal (2491 – 2041).

Now, suppose she previously weighed 77 kg and lost 10% of her body weight to reach her current weight. As discussed earlier, we expect about a 15% reduction in energy expenditure due to metabolic adaptation. Applying this to her 2491 kcal/day estimate, her adjusted, actual expenditure would be closer to 2117 kcal/day.

At 70 kg and 12% body fat, her lean body mass (LBM) is:

70 × 0.88 = 61.6 kg

Now we can calculate her energy availability using the formula:

Energy Availability = (Total Energy Intake – Exercise Expenditure) / LBM

Assuming she is eating at her new maintenance (2117 kcal), her energy availability is:

(2117 – 450) / 61.6 = ~27.1 kcal/kg/LBM/day

This puts her below the commonly cited 30 kcal/kg/LBM/day threshold at which symptoms of REDs can occur, highlighting that even a competitor eating at maintenance can remain in a state of low energy availability, depending on training demands and the lingering effects of metabolic adaptation.

Indeed, not everyone experiences a 15% reduction in total energy expenditure after weight loss; some experience less, others more. But on average, most individuals cannot maintain a very low body fat after weight loss without encountering at least some symptoms of REDs. As discussed earlier, this is because everyone has a natural body fat range within which physiological function remains relatively normal. Dropping below this range, even if weight is stable, can disrupt hormonal balance, recovery, mood, and other markers of health and performance.

The primary metabolic hormone that triggers the cascade of changes associated with metabolic adaptation during dieting is leptin.

While leptin is predominantly released from body fat, its levels also respond rapidly to short-term changes in energy balance, dropping quickly at the start of a diet, even before any measurable fat loss occurs [8]. Though adipose tissue is the main source of leptin [9], it is also produced in other tissues, including the stomach, which may help signal short-term energy availability, in contrast to the long-term signal provided by fat-derived leptin [10]. Macronutrient composition and caloric intake can influence leptin levels in the short term [11], but when comparing individuals across a wide range of body fat levels, resting leptin levels are most strongly predicted by body fat percentage [12]. This suggests that the degree of metabolic adaptation, and potentially the severity of REDs symptoms, depends on the magnitude and duration of energy restriction, the amount of fat lost, and how far below your normal body fat range you go. 

Importantly, even if you’re eating at maintenance, leptin will still drop to low levels after meals if your body fat is too low. As a result, symptoms of metabolic adaptation and REDs can persist long after the diet ends. This is why bodybuilders don’t stay stage-lean year-round — doing so would compromise health, performance, and the ability to make further progress. Instead, they intentionally regain some body fat post-competition to restore hormonal function and enter a productive off-season. We’ll cover this in more detail in the Post-diet Recovery section after The Pyramid Levels.

The consequences of not regaining enough body fat after contest prep have been studied. In a case series of physique athletes [13], some competitors made only minimal increases to their calorie intake and cardio post-contest to limit fat regain, while others increased calories and reduced cardio more substantially. Even though all participants ate above maintenance during the 8–10 week post-contest period, it was the amount of body fat regained, not calorie intake, that determined how quickly metabolic adaptation reversed. Only those who meaningfully regained body fat saw improvements in symptoms of metabolic adaptation and REDs. Those who remained very lean with minimal fat regain maintained low leptin levels and suppressed metabolic rates, similar to what was measured 1–2 weeks before their shows. Ultimately, being extremely lean is not healthy.

Signs and symptoms of low energy availability can appear if you diet too aggressively, for too long, or once you get very lean. The exact thresholds for what constitutes “too hard,” “too long,” or “too lean” vary widely between individuals. For physique competitors, unfortunately, getting “too lean” is often the goal. So while we recommend staying within the suggested rates of weight loss and above ~30 kcal/kg/LBM (~13.6 kcal/lb/LBM) for women and possibly above ~25 kcal/kg/LBM (~11.4 kcal/lb/LBM) for men, as some research suggests men may tolerate lower thresholds [14], your best guide is not the numbers, but your body. Pay attention to the signs and symptoms of REDs (as shown in the figures), and adjust accordingly.

If issues arise, consider slowing your rate of weight loss or incorporating non-linear dieting strategies, which we’ll discuss in Level 4. Non-physique athletes can likely avoid REDs entirely and minimize metabolic adaptation by not exceeding the recommended rates of weight loss and by avoiding unsustainably low body fat levels. If symptoms still occur, you can reduce their severity by dieting more conservatively or cutting back on cardio.

If you have found this helpful, you might be pleased to know it is just a small section adapted from our Muscle and Strength Pyramid books.

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References

1. Mountjoy, M., Ackerman, K.E., Bailey, D.M., et al. 2023 International Olympic Committee’s (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med, 2023. 57(17): p. 1073–97.
2. Loucks, A.B. Energy balance and body composition in sports and exercise. J Sports Sci, 2004. 22(1): p.1–14.
3. Loucks, A.B. Energy availability, not body fatness, regulates reproductive function in women. Exerc Sport Sci Rev, 2003. 31(3): p144–8.
4. Burke, L.M., et al. Pitfalls of Conducting and Interpreting Estimates of Energy Availability in Free-Living Athletes. Int J Sport Nutr Exerc Metab, 2018. 28(4): p. 350–63.
5. Rosenbaum, M., Hirsch, J., Gallagher, D.A., Leibel, R.L. Long-term persistence of adaptive thermogenesis in subjects who have maintained a reduced body weight. Am J Clin Nutr, 2008: 88(4): p. 906–12.
6. Rosenbaum, M., Leibel, R.L. Adaptive thermogenesis in humans. Int J Obes (Lond), 2010, 34 Suppl 1(0 1): p. S47–55.
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9. Kasacka, I., Piotrowska, Ż., Niezgoda, M., Łebkowski, W. Differences in leptin biosynthesis in the stomach and in serum leptin level between men and women. J Gastroenterol Hepatol, 2019. 34(11): p. 1922–18.
10. Picó, C., Oliver, P., Sánchez, J., Palou, A. Gastric leptin: a putative role in the short-term regulation of food intake. Br J Nutr, 2003. 90(4): p. 735–41.
11. Izadi V, Saraf-Bank S, Azadbakht L. Dietary intakes and leptin concentrations. ARYA Atheroscler, 2014. 10(5): p. 266–72.
12. Considine, R.V., Sinha, M.K., Heiman, M.L, et al. Serum immunoreactive-leptin concentrations in normal-weight and obese humans. N Engl J Med, 1996: 334(5): p. 292–5.
13. Longstrom, J.M., Colenso-Semple, L.M., Waddell, B.J., Mastrofini, G., Trexler, E.T., Campbell, B.I. Physiological, Psychological and Performance-Related Changes Following Physique Competition: A Case-Series. J Funct Morphol Kinesiol, 2020. 5(2): p. 27.
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