Learning from Stacy Sims
The scientist who proved "women are not small men"—and changed how female athletes train and eat
The Story
For decades, sports science studied men and told women: "Just do what the guys do, but less."
Stacy Sims knew that was wrong.
As both an elite athlete (Ironman competitor, UCI cyclist) and a scientist, she experienced firsthand how nutrition and training advice designed for men failed women. Her menstrual cycle affected her performance. Her recovery needs were different. The standard protocols didn't work.
So she dedicated her career to figuring out why—and what to do about it.
Her research has revolutionized how female athletes think about training, nutrition, and hormones. Her mantra has become a rallying cry:
"Women are not small men."
Who is Stacy Sims?
| Credential | Detail |
|---|---|
| Role | Senior Research Associate, Auckland University of Technology; Adjunct Faculty, Stanford Lifestyle Medicine |
| Known For | Female-specific exercise physiology; menstrual cycle and performance; menopause and athletes |
| Background | Elite triathlete and cyclist; PhD in Exercise Physiology |
| Publications | ROAR and Next Level — essential reading for female athletes |
Sims has competed at the Ironman World Championships in Kona and the Xterra World Championships. She knows what it feels like to be a female athlete—not just what the data says.
What ISP Students Learn
Lesson 1: Women Are Naturally Better Fat Burners
Women have more Type I (slow-twitch) muscle fibers, higher mitochondrial density, and better metabolic flexibility than men.
Translation: Women are naturally better at burning fat for fuel.
This means:
- Women don't need as much "Zone 2" training to build fat-burning capacity—they already have it
- Women benefit MORE from high-intensity training (which stimulates pathways they're less naturally developed in)
- The classic "long slow distance" approach is less critical for women than for men
What this means for young female athletes: Don't just copy male training plans. Your physiology may need more intensity and less "junk miles."
Lesson 2: The Menstrual Cycle Is a Vital Sign
Sims views a regular menstrual cycle as a health indicator, not an inconvenience.
If your period disappears (amenorrhea), it's a red flag that something is wrong—usually under-eating relative to training (Relative Energy Deficiency in Sport, or RED-S).
The two phases:
| Phase | Hormones | Training Implications |
|---|---|---|
| Follicular (Days 1-14) | Low estrogen/progesterone | Best time for high-intensity work, heavy lifting, peak performance |
| Luteal (Days 15-28) | High estrogen/progesterone | Higher body temp, harder to hit top gears, increased protein breakdown |
The 2025 update: Sims now emphasizes that these are guidelines, not rules. Individual variation matters. Track your cycle alongside how you feel—some women perform great in their luteal phase.
What this means for young female athletes: Know your cycle. Use it as information, not an excuse—but recognize that it affects performance.
Lesson 3: Never Train Fasted
Sims strongly opposes fasted training for women.
The reason: Women's brains are more sensitive to energy availability. When you train without eating, cortisol spikes. High cortisol:
- Promotes fat storage (the opposite of what you want)
- Breaks down muscle
- Disrupts hormones
"Fasted training in women tells the body: 'We're starving. Conserve energy.'"
Her recommendation: Always eat before training. Even a small protein + carb snack (like Greek yogurt with berries) is enough to signal "fed" status to your brain.
What this means for young female athletes: That "burn more fat by training hungry" advice? It's wrong for you.
Lesson 4: Protein Needs Are Higher (Especially Around the Cycle)
Because progesterone (high in the luteal phase) promotes muscle breakdown, women need MORE protein to maintain muscle—especially in the second half of their cycle.
Sims' recommendation:
- 1.6–2.2 g/kg body weight daily (significantly higher than the standard RDA)
- 30-35g protein within 30-45 minutes after training
- Higher protein during the luteal phase to counteract progesterone's catabolic effect
The leucine trigger: Women need to hit the leucine threshold (~2.5-3g per meal) to turn on muscle building. This is harder in the luteal phase because estrogen inhibits the pathway.
What this means for young female athletes: Prioritize protein at every meal—especially after training and during the second half of your cycle.
Key Takeaways
| Lesson | One-Liner |
|---|---|
| Natural fat burners | Women don't need as much Zone 2; they benefit more from intensity |
| Cycle is a vital sign | Missing periods = red flag for under-fueling |
| Never train fasted | Fasting spikes cortisol and promotes fat storage in women |
| Protein needs are high | Especially in the luteal phase when muscle breakdown increases |
How This Shows Up at ISP
Stacy Sims' research shapes female athlete education in the Bio Skill Tree:
- Menstrual cycle tracking is taught as a performance tool
- Nutrition education includes female-specific protein needs
- "Fasted training = fat burning" myths are explicitly debunked
- Girls learn that their physiology is different—and that's okay
When ISP teaches female athletes, we teach them as female athletes—not small men.
The Evolution of Cycle-Syncing
Sims' earlier work emphasized strict cycle-syncing (train hard in follicular, easy in luteal).
Her 2025 position is more nuanced: Use the cycle as context, not prescription.
- Some women feel great in their luteal phase
- Stress, sleep, and illness matter more than cycle phase on any given day
- Track multiple metrics (HRV, mood, sleep) alongside your cycle
The goal: understand YOUR patterns, not blindly follow averages.
Learn More
- How ISP Works →
- De Souza & Loucks — RED-S and Energy Availability →
- Nancy Clark — Making Peace with Food →
"Women are not small men. Train like a woman. Eat like a woman. Recover like a woman."