The term ‘energy availability’ refers to whether or not you have adequate energy to maintain not only the energy demands of exercise or sport but also of normal physiological function.
You can be at energy balance, maintaining a stable body mass, but be in a state of ‘relative energy deficiency’ where reproductive and metabolic function are downregulated to maintain energy balance. This often occurs in physique competitors and weight-class restricted strength athletes in the process of dieting or if they attempt to maintain too lean of a physique. Metabolic adaptation occurs in response to dieting, and some of the adaptive reduction in energy expenditure includes down-regulation of ‘non-essential’ physiological functions, which can persist post-diet if an adequate amount of body fat isn’t regained. Prolonged, low energy availability can have long-term negative health consequences 1.
Essentially, energy availability in sport refers to the “leftover” caloric intake for an athlete after training is subtracted for physiological function. This calculated value is expressed relative to lean mass. As an example, a 10% body fat, 100 kg (220 lb) athlete (90 kg or 198 lb of lean mass) consuming 3000 kcal and expending 400 kcal on average in training (2600 kcal “leftover”) is said to have an energy availability of 28.9 kcal/kg (2600 kcal divided by 90 kg), or 13.1 kcal/lb (2600 kcal divided by 198 lb). More on the math in a bit, but for now let’s talk concepts.
The original studies on energy availability showed that when energy intake was maintained while exercise energy expenditure was increased, metabolic and reproductive function were negatively affected. Subsequently, when energy intake was raised, the negative effects were reversed 2. In reading this, it may just seem like energy availability is another term for energy balance. You might be thinking, “I already know an energy deficit can cause downregulation of reproductive and metabolic hormones.” That’s correct; an energy deficit does result in decreased production of metabolic and sex hormones in both men and women, but what is critical to understand is this can occur at energy balance (caloric maintenance, where weight is maintained) as well.
For example, in a 2017 Finnish study on female physique competitors, not all women had regained their menstrual cycle despite regaining body weight at 3–4 months post-competition 3. A more extreme example is the case study of a 26–27 year-old, drug-free figure competitor whose menstrual cycle did not return until 71 weeks after competition, despite a regain of her body weight a year prior 4. But this is not isolated to women, and indeed signs and symptoms of low energy availability have been observed in male, drug-free bodybuilders as well 5.
A lean individual will often be in a state of low energy availability when in a caloric deficit, but the state of being at energy balance while exhibiting these signs and symptoms is known as ‘relative energy deficiency’ ¹.
Athletes who remain in a state of low energy availability can experience negative effects on performance; negative effects on the endocrine, cardiovascular, immune, metabolic, reproductive, and gastrointestinal systems; the loss of menstrual function; and a reduction in bone health in women specifically.
While these detrimental effects don’t always coincide with disordered eating or a negative body image, they often do. The behaviors and self-regulation necessary to maintain low body fat and energy intake can lead to psychological stress due to the push and pull of the opposing desires to both maintain and relinquish control, as personal and athletic goals come into conflict with biologically driven desires. Low energy availability, menstrual dysfunction, and bone loss are known as The Female Athlete Triad, and the broader dysfunctions that can occur as a result of low energy availability (in men or women in sport) are collectively termed Relative Energy Deficiency in Sport or RED-S ¹.
The takeaway is that even if you don’t expend a ton of calories in training, if you maintain a leaner physique than your body “wants” (which often requires a lower energy intake than your body needs to maintain full functionality), there can be health and performance consequences.
But how do you know if you have low energy availability? Let’s get back to the math for a second to partially answer this question.
A large body of work addresses the concept of an energy availability threshold of 30 kcal/kg in women (~13.6 kcal/lb)—although low energy availability can occur in both sexes.
When energy availability drops below this point, signs and symptoms of metabolic and reproductive (in women) downregulation have been observed. However, a hard-line cut off at 30 kcal/kg is scientifically problematic. Conceptually, there is no reason the body “sees” energy expended from exercise any differently than non-exercise activity. The original research establishing the 30 kcal/kg threshold was done in a homogeneous, sedentary group of women, so the threshold likely applies in many cases, but in those with higher (or perhaps lower) non-exercise activity levels, it doesn’t.
For many women, symptoms of low energy availability may or may not occur to various levels of severity within the range of 30-45 kcal/kg (or perhaps lower). And in men, it seems the “threshold” is speculated to be closer to 20-25 kcal/kg as observed among dieting bodybuilders . Meaning, the male in the example I gave at the start of this section at 28.9 kcal/kg might fair far better in terms of the severity of hormonal and metabolic downregulation compared to a female at the same relative energy availability. An equivalent example would be a 154 lb (70 kg) woman at 18% body fat (57.4 kg or 126.5 lb of lean mass) consuming 2100 kcal and expending 440 kcal (1660 kcal “leftover”) on average in training; she would also have an energy availability of 28.9 kcal/kg (1660 kcal divided by 57.4 kg) or 13.1 kcal/lb (1660 kcal divided by 126.5 lb) like the male, but would have a higher probability of experiencing hormonal and metabolic downregulation compared to him.
For all of these reasons, it might be more appropriate to assess not only the mathematical relationship but more importantly, the signs and symptoms associated with low energy availability 6.
So yes, you probably want to try to stay above 30 kcal/kg/LBM (~13.6 kcal/lb/LBM) in women and ~25 kcal/kg/LBM (~11.4 kcal/lb/LBM) in men, but I think paying attention to signs of symptoms of low energy availability is more important. If maintaining a certain level of leanness or body mass—and subsequently a restricted energy intake—results in:
increase your calories and consider that it may prove more optimal in the long run to maintain a higher body mass (which we discuss more in the Recovery Diet section of our book).
If you have found this helpful, you might be pleased to know it is just a small section taken from our Muscle and Strength Nutrition Pyramid book. The second edition, along with the Training companion book, was released this January 3rd, 2019.
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Thank you for reading. Questions welcomed in the comments.
– Eric, Andy, and Andrea