Resting metabolic rate is the energy you burn just being alive. Add training and you get TDEE, total daily energy expenditure. Subtract exercise energy from intake, divide by lean mass, that’s energy availability, the single most important number in endurance sports nutrition. Calculate all three. Catch RED-S before it catches you.
Pick a method based on what you know, if you have body fat % from calipers or DEXA, use Wang or Cunningham (best for athletes). Otherwise Mifflin or Harris-Benedict.
The single most important number in endurance sports nutrition. Most under-fueled athletes don’t know they are.
EA = (intake − exercise kcal) / FFM kg
It’s the energy left over for daily living, basal function, and recovery after training is paid for. Per kilogram of metabolically-active tissue.
≥ 45 kcal/kg FFM, optimal. Adaptation, recovery, hormones intact.
30–45, sub-optimal. Adaptations slow, mood/sleep can suffer, but tolerable in short blocks.
< 30, RED-S risk. Reproductive, bone, immune, metabolic systems start to compromise.
Day-to-day variation matters. EA averaged over 3 weeks is more meaningful than a single day. And acute deficits during a race or hard session aren’t the same as chronic under-fueling, context always.
Relative Energy Deficiency in Sport (RED-S, formerly female athlete triad) shows up everywhere: stalled adaptation, frequent illness, stress fractures, lost period, low libido, mood crashes, frozen weight loss. Endurance athletes are extraordinarily susceptible, the calorie cost of training scales fast, and many athletes self-restrict on top.
Three patterns to watch in yourself or your athletes:
If you’re consistently < 30 kcal/kg FFM, eat more, not later, today. If symptoms are present (HR/HRV trending wrong, training feels heavy for no reason, persistent niggles), pause hard sessions until EA is restored. The body will outwork you back to fitness if it’s actually fueled.
Four equations, different inputs, slightly different answers. Pick by what you know.
500 + 22 × FFM. Built on athlete data, leans on lean mass. Most accurate when your FFM is well-measured. Tends to predict 5–10% higher than Mifflin for trained athletes.
21.5 × FFM + 407. Similar in spirit to Cunningham. Slight differences from organ-tissue physiology assumptions; useful as a cross-check.
10W + 6.25H − 5A ± 5/161. Sex-specific. Validated in mixed populations; under-predicts athlete RMR by 5–10%. Better than Harris-Benedict for general use.
Men: 88.36 + 13.40W + 4.80H − 5.68A. Women: 447.6 + 9.25W + 3.10H − 4.33A. The original 1919 formula. Slight over-prediction in modern populations; included for reference.
Mask + gas analyser, 20-min fasted measurement. Population formulas are estimates; lab measurement gives your number. Worth doing if you’re building a precise fueling plan or chasing weight goals.
Indirect calorimetry at the lab measures your actual oxygen uptake and CO2 production at rest, your RMR, not a population guess. Pair with body composition and you’ve got the foundation of a real fueling plan.
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