AT A GLANCE
The argument in five lines
- "Do not eat below BMR" is the rule you learned. The number that actually tracks with stalled progress is Energy Availability, not BMR.
- Energy Availability is what is left of your food once training has taken its share, expressed per kilogram of fat-free mass. It is the budget your body has to run recovery, hormones, and adaptation.
- The published bands were built on studies of young women. Treat them as a working framework, not a universal constant. The 2023 IOC consensus now describes low availability as a spectrum, not a fixed line.
- The trap for lean, trained people is that they almost never eat below BMR. They eat below availability on the hardest training days, week after week, and call it discipline.
- KEXBI never hands a flat daily target. It calibrates the availability floor to your sex and body composition, then moves intake with each session's actual energy cost so the number your body needs to build stays where it should be.
You have probably read that dropping below your Basal Metabolic Rate is where the damage starts. Thyroid slows, leptin falls, recovery goes. It is a useful rule of thumb, but it is not the physiology that actually governs whether you build or stall.
The number that does is called Energy Availability. It is what is left of your food once training has taken its cut, the budget your body has to run everything that is not exercise: repair, hormones, adaptation, the work of turning training into progress. Under sustained load, it matters far more than BMR ever did.
Energy availability, not BMR, is the number that tracks with stalled progress, poor recovery, and the frustrating "I am doing everything right and nothing is moving" plateau.
Chapter 01 · The formula nobody shows youEnergy availability, defined
Energy availability is not TDEE minus intake. It is a specific calculation, the one used across the REDs and sports-nutrition literature.
Two numbers you have probably never tracked, your fat-free mass and the actual energy cost of your session, decide whether you are fuelling recovery or quietly borrowing against it.
Chapter 02 · Why BMR is the wrong lineThe two days that clear BMR and diverge anyway
Here is an illustrative example. A 107 kg trained male with a fat-free mass of 91 kg has a BMR of about 2,335 kcal, calculated with the Cunningham equation, which accounts for lean mass, rather than a formula that ignores composition [3].
Look carefully. The training day eats more food, and comfortably more than BMR. BMR says he is fine. Energy availability says he spent that day with far too little left over for his body to build on.
This is the trap lean, trained people fall into. They almost never eat below BMR. They eat below availability, on the hardest training days, week after week, and call it discipline.
One necessary caveat
The strongest evidence for what happens below roughly 30 kcal per kg FFM comes from studies in women, where the reproductive and thyroid axes down-regulate below that region [4]. In men the hormonal picture appears more resilient, and a precise male floor has not been established [5,6], so we do not claim a male athlete's testosterone collapses after a single low day. What is well established across both sexes is quieter and more relevant to your training: an energy deficit measurably reduces muscle protein synthesis, the process that turns a session into adaptation [8]. That is the cost that shows up as a plateau.
Chapter 03 · What sustained low EA actually doesOne shortage, several systems
Run low availability once and you recover. Stack it for weeks and the body reorganises around the shortage rather than adapting to your training.
None of this shows up as "you ate below BMR." It shows up as stalled progress and recovery that never quite arrives.
Chapter 04 · What KEXBI does about itA floor calibrated to you, not a borrowed constant
Most calorie apps end at one number, your daily target, and treat eating under it as success. KEXBI never hands you a single flat number. It works out a target intake that moves with each session's actual energy cost, so the energy left for your body stays where it should be.
And it does not defend a one-size line. The 30 kcal / kg figure came from young women; men and leaner or heavier bodies sit differently. KEXBI calibrates the floor to you, to your sex and your composition, rather than enforcing a borrowed constant.
Before the day starts
Session energy cost, in kcal.
Intake budget that protects your energy availability.
Protein distribution, to support muscle protein synthesis.
Carbohydrate timing, to fuel the session itself.
Fat-free-mass drift, to keep the BMR estimate honest over time.
Chapter 05 · The long viewAnyone can have one under-fuelled day
The difference is whether you stack them. Run availability into the low band on a Tuesday and you recover on Wednesday. Do it every heavy session for three months and the two lines diverge.
Chapter 06 · The pointTrack the number that decides
"Do not eat below BMR" is a shortcut people learned because the real number was invisible. It does not have to be. Energy availability is the number that decides whether your training becomes progress, and it is the one your calorie app is not watching. KEXBI tracks it, calibrates it to you, and adjusts your intake and timing so you keep building instead of quietly borrowing against tomorrow.
References
- Loucks AB, Kiens B, Wright HH. Energy availability in athletes. J Sports Sci 2011;29(S1):S7 to S15.
- Mountjoy M, et al. 2023 IOC consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med 2023;57(17):1073 to 1097.
- Cunningham JJ. A reanalysis of the factors influencing basal metabolic rate in normal adults. Am J Clin Nutr 1991;54(6):963 to 969.
- 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 to 311.
- Koehler K, 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 to 1929.
- 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 to 21.
- Rosenbaum M, Leibel RL. Adaptive thermogenesis in humans. Int J Obes 2010;34 Suppl 1:S47 to S55.
- Areta JL, et al. Reducing resting skeletal muscle protein synthesis is rescued by resistance exercise and protein ingestion following short-term energy deficit. Am J Physiol Endocrinol Metab 2014;306(8):E989 to E997.