Formula, in endurance

Female Athlete Formula Suite

Female-specific training math accounts for energy availability (EA — IOC 2018 RED-S framework) and menstrual cycle phase. EA = intake minus exercise calories per kg fat-free mass. Below 30 kcal/kg FFM is clinically low and disrupts menstrual function and bone health. Cycle phase modulates training response. Standard Mifflin underrates lean female athletes.

4 variables. Published May 14, 2026. By AI Fit Hub.

Education, not medical advice. The result comes only from the published formula and your inputs. How we check our formulas.

Formula

energy_availability = (energy_intake_kcal − exercise_energy_kcal) / fat_free_mass_kg
   EA < 30 kcal/kg FFM: clinically low, RED-S risk
   EA 30-45 kcal/kg FFM: suboptimal
   EA ≥ 45 kcal/kg FFM: adequate

cycle_phase_modifier:
   Follicular (Day 1-14): training capacity baseline
   Ovulatory (Day 13-15): peak strength + power
   Luteal (Day 15-28): −5-8% high-intensity capacity, +slight aerobic capacity
   Late luteal (Day 25-28): GI symptoms, fluid retention — schedule deload

Variables

energy_intake_kcal

Daily energy intake

Total kcal eaten. Track for 7-14 days for accuracy (food labels + USDA + cooked-weight measurements).

exercise_energy_kcal

Exercise energy expenditure

Calories burned in structured training (above NEAT baseline). Use the food-to-exercise formula or a calibrated HR-based tracker. Don't double-count BMR.

fat_free_mass_kg

Fat-free mass

Total mass minus fat mass. Requires accurate body-fat % (DXA, BIA, skinfolds). Critical input — EA is reported per unit lean mass, not total mass.

cycle_phase

Menstrual cycle phase

Phase relative to first day of menstruation. Track 3+ cycles to identify personal pattern; cycle lengths vary 21-35 days normal. Hormonal contraceptive users have suppressed natural cycle; flat hormone profile across the month.

Step by step

1

Measure fat-free mass. DXA is gold standard; calibrated BIA scale is acceptable (±5% error).

DXA shows 18% body fat at 60 kg → FFM = 60 × (1 − 0.18) = 49.2 kg.
2

Track 7-day average daily energy intake.

Week average: 2,100 kcal/day.
3

Compute exercise energy. Use MET × time × mass for sessions, or HR-based tracker (de-bias by 15-20% if a wrist wearable).

5 sessions/week × 350 kcal = 1,750 kcal/week → 250 kcal/day average.
4

Compute energy availability: EA = (intake − exercise) / FFM.

EA = (2,100 − 250) / 49.2 = 1,850 / 49.2 = 37.6 kcal/kg FFM/day.
5

Classify. If <30 + amenorrhea or stress fractures, address before any performance work. If 30-45 + sub-optimal performance, increase intake 200-400 kcal/day for 2-4 weeks and re-assess.

EA 37.6: suboptimal. Add 300 kcal/day (carbs + healthy fats), re-evaluate after 3 weeks. Expect: training quality up, menstrual regularity if disrupted.

Worked example

Recreational runner (60 kg, 18% BF) training 5x/week, suspects under-fueling

Body mass

60 kg

Body fat %

18%

Fat-free mass

49.2 kg

Daily intake

2,100 kcal

Exercise energy

250 kcal/day avg

EA = (2,100 − 250) / 49.2 = 37.6 kcal/kg FFM/day Classification: 30-45 = SUBOPTIMAL

EA 37.6 indicates suboptimal energy availability. Symptoms to watch: irregular cycle, declining HRV, frequent illness, fatigue not relieved by rest, plateaued or declining performance. Intervention: increase daily intake 300 kcal (~75g carbs) for 4 weeks. Re-evaluate cycle regularity + training response. If symptoms persist or cycle disrupted (oligo/amenorrhea), refer to sports dietitian + GP — RED-S requires medical oversight.

Common variations

Female Athlete Triad (Nattiv et al. 2007): pre-IOC framework focused on three components — energy availability, menstrual function, bone density. RED-S 2014/2018 expanded to include cardiovascular, immune, GI, psychological effects.

Mountjoy 2018 IOC consensus: refined the EA framework with the LEAF-Q screening tool. Practical implementation in clinical sports medicine.

Hormonal contraceptive use suppresses natural cycle. The Sims & Yeager 2016 review notes contraceptives mask RED-S menstrual signal — markers shift to bone density (DXA) and clinical symptoms.

Perimenopausal athletes (35-50): declining estradiol shifts substrate utilization toward higher fat oxidation; protein requirements may rise to 2.0-2.4 g/kg (Sims 2022). Training response slows; recovery extends.

Low EA is more common than reported: roughly a quarter to nearly half of female endurance athletes screen positive on LEAF-Q across published cohorts. Standard population BMR formulas (Mifflin) systematically underestimate energy needs in lean female athletes by ~150-300 kcal/day.

Run the numbers next

Nutrition TDEE Calculator
Estimate your daily energy expenditure with Mifflin-St Jeor + activity factors.
Nutrition BMR Calculator
Estimate basal metabolic rate and maintenance calories using Mifflin-St Jeor assumptions.
Nutrition Macro Calculator
Convert calorie targets into protein, carbs, and fat grams for your goal.

Sources

Mountjoy et al. (2018). IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. , British Journal of Sports Medicine — IOC 2018 RED-S framework
Mountjoy et al. (2014). The IOC consensus statement: beyond the Female Athlete Triad — Relative Energy Deficiency in Sport (RED-S). , British Journal of Sports Medicine — original RED-S framework
Nattiv et al. (2007). American College of Sports Medicine position stand. The female athlete triad. , Medicine & Science in Sports & Exercise — original ACSM Triad position
Sims & Yeager (2016). Reframing female fueling and recovery: the next gen sports nutrition for women. , Current Sports Medicine Reports — practical female-athlete fueling
Screening for Relative Energy Deficiency in Sport: Detection of Clinical Indicators in Female Endurance Athletes , Medicine & Science in Sports & Exercise (2025) — Wasserfurth et al.