What Is Energy Availability? The Number Behind Under-Fuelling
Updated: 7 hours ago
A runner eats 2,300 calories on a day she burns 900 on a long run. With 48kg of fat-free mass, that leaves her body about 29 calories per kilogram for everything else. That figure has a name.
Energy availability is the energy left for your body's basic functions after training is paid for, calculated as food intake minus exercise energy, relative to lean mass. When it drops below about 30 kcal per kilogram of fat-free mass a day, hormones, bone health, immunity and recovery start to suffer.
Nothing about her eating changed that day. Her training did.
Quick Overview
What is energy availability?
Energy availability is the share of your daily food energy left once exercise has taken its cut, expressed per kilogram of fat-free mass. It's the budget your body has for repairing tissue, building bone, regulating hormones and fighting infection.
It differs from energy balance, which compares what you eat with everything you burn and shows up on the scales. That's why an athlete can hold a steady weight for months and still be under-fuelled. The body quietly lowers its own running costs to match the shortfall, and the scales never move.
How do you calculate energy availability?
Take what you eat in a day, subtract the energy cost of your training, and divide by your fat-free mass in kilograms.
Energy availability = (energy intake − exercise energy expenditure) ÷ fat-free mass
A 75kg (165lb) cyclist with 64kg of fat-free mass eats 3,200 calories and burns 1,100 on a long Saturday ride. That leaves about 33 kcal/kg. On a rest day with the same food, he'd sit near 50.
Fat-free mass is used because lean tissue does most of the metabolic work. Around 45 kcal per kilogram of fat-free mass a day goes with energy balance and normal function in healthy young adults (Loucks et al., J Sports Sci, 2011), and that's where most athletes should spend most days.
What happens when energy availability is too low?
The body stops funding what it can postpone. Reproductive hormones, bone building, immunity and repair go first.
In regularly menstruating women, the luteinising hormone pulses that drive a normal cycle were disrupted once energy availability fell below about 30 kcal per kilogram of lean mass a day (Loucks and Thuma, J Clin Endocrinol Metab, 2003). Bone formation markers were suppressed at the same level (Ihle and Loucks, J Bone Miner Res, 2004).
Men may respond differently, but they aren't immune. In one small study, four days at 15 kcal/kg cut leptin and insulin in exercising men without significantly changing testosterone or thyroid hormone (Koehler et al., J Sports Sci, 2016).
Chronic low energy availability leads to Relative Energy Deficiency in Sport (RED-S, now often written REDs). The 2023 IOC consensus links it to stress fractures, menstrual and reproductive disruption, poorer immunity, low iron, low mood and stalled performance. It puts prevalence at 15–80% depending on the sport (Mountjoy et al., Br J Sports Med, 2023). There's more in our piece on RED-S and low energy availability.
Is 30 kcal per kilogram a hard cut-off?
No. It's a useful marker, and your body doesn't check itself against it every evening.
The figure comes from short, controlled studies, mostly in women, and individual responses vary widely (Areta et al., Eur J Appl Physiol, 2021). The IOC now separates two kinds of low energy availability. Adaptable LEA is the brief dip a body copes with during a training camp or a travel week. Problematic LEA is when deficits are large, long or frequent enough to cause harm (Mountjoy et al., Br J Sports Med, 2023).
Outside a lab, you also can't measure it precisely. Food diaries under-record intake, watches mis-estimate exercise energy, and fat-free mass needs a body-composition scan. So I don't ask athletes for their number to the decimal. I look at how their eating compares with their training, and at the warning signs:
Missed, irregular or stopped periods
Low libido, or fewer morning erections in men
Stress fractures or recurring bone injuries
Frequent colds and slow recovery
Performance that stalls despite consistent training
Persistent tiredness or low mood
Each has other possible causes, so treat them as a reason to look closer, not a diagnosis.
How do you keep energy availability high enough?
Eat for the training you're doing that day. Most of the under-fuelling I see comes from eating the same on a two-hour session day as on a rest day, far more often than from deliberate restriction.
Carbohydrate is where the gap usually opens. Endurance training of one to three hours a day calls for roughly 6–10g per kilogram of bodyweight (Thomas et al., J Acad Nutr Diet, 2016). Poha, upma, oats or a bagel before a morning session, an extra roti or more rice or pasta at lunch, and a proper snack afterwards close most of it. Our carbohydrate guide for athletes has the detail.
Don't bank calories for dinner. Long gaps on heavy days leave you in deficit for hours, and double-session days need a meal between sessions, not a coffee. No supplement fixes low energy availability. Only eating more, in a structured way, does.
If eating or training has started to feel distressing, if your periods have stopped for three months or more, or if you've had a stress fracture without an obvious cause, see a doctor or a sports dietitian experienced with RED-S.
The Fueletics Perspective
Energy availability is the number I think about most with endurance athletes, and the one they've usually never heard of. They judge their eating by the scales, and the scales can't tell you your body has started cutting costs.
Your food needs move with your training, day by day. That's why our Daily Fuelling Engine sets training-day and rest-day intake separately instead of giving you one number for the week. Where your floor sits depends on your fat-free mass, training load and schedule, and guessing it is how athletes end up under-fuelled without knowing. If you want yours worked out properly, start with a Fueletics assessment.
FAQs
What is a normal energy availability for athletes?
Around 45 kcal per kilogram of fat-free mass a day supports energy balance and normal function in healthy young adults. Below about 30 is where lab studies in women show hormonal and bone changes.
Can you have low energy availability without losing weight?
Yes. The body lowers its own energy use to match a shortfall, so weight can stay stable while hormones, bone and recovery suffer.
Is low energy availability the same as an eating disorder?
No. It's often unintentional, from not eating enough to match training. It can occur alongside disordered eating, and anyone whose eating feels driven by anxiety or rigid rules should talk to a doctor.
Do men get low energy availability?
Yes. Men may tolerate short deficits with fewer hormonal changes, but chronic low energy availability still affects bone, testosterone, recovery and performance.
Research References
Loucks AB, Kiens B, Wright HH. Energy availability in athletes. J Sports Sci. 2011;29(Suppl 1):S7–S15. doi:10.1080/02640414.2011.588958
Loucks AB, Thuma JR. Luteinizing hormone pulsatility is disrupted at a threshold of energy availability in regularly menstruating women. J Clin Endocrinol Metab. 2003;88(1):297–311. doi:10.1210/jc.2002-020369
Ihle R, Loucks AB. Dose-response relationships between energy availability and bone turnover in young exercising women. J Bone Miner Res. 2004;19(8):1231–1240. doi:10.1359/JBMR.040410
Koehler K, Hoerner NR, Gibbs JC, et al. Low energy availability in exercising men is associated with reduced leptin and insulin but not with changes in other metabolic hormones. J Sports Sci. 2016;34(20):1921–1929. doi:10.1080/02640414.2016.1142109
Areta JL, Taylor HL, Koehler K. Low energy availability: history, definition and evidence of its endocrine, metabolic and physiological effects in prospective studies in females and males. Eur J Appl Physiol. 2021;121(1):1–21. doi:10.1007/s00421-020-04516-0
Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med. 2023;57(17):1073–1097. doi:10.1136/bjsports-2023-106994
Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. J Acad Nutr Diet. 2016;116(3):501–528. doi:10.1016/j.jand.2015.12.006
