For much of modern sports science, there’s been an unspoken assumption: if it works for men, it works for women. Training protocols, nutrition guidelines, supplement recommendations, recovery models—they’ve all been built around male physiology.

And in doing so, women have been underserved at best, and actively harmed at worst.

As Dr. Stacy Sims famously says:

“Women are not small men.”

It’s a rallying cry, but also a research-backed correction to decades of one-size-fits-men science. And it’s not about equity—it’s about accuracy. Because when women train, eat, and recover according to data that doesn’t apply to them, the outcomes don’t just stall—they can be counterproductive.

In this article, we explore what makes the female body fundamentally different, why that matters for training and nutrition, and how to future-proof female performance.

The gender data gap in science

Let’s start with the problem. Women have historically been excluded from exercise physiology and medical research. Why?

Because menstrual cycles were considered “too complex” to account for. In fact, as recently as 2021, women made up just 6% of participants in sports science research that controlled for hormonal fluctuations. That leaves the vast majority of recommendations based on the male model: flat hormone profiles, predictable energy systems, and responses that don’t match how female biology actually works.

It means we’re missing vital data on how women adapt to stress, build muscle, burn fat, absorb nutrients, and recover. And when that gap isn’t addressed, women are left working with guesswork.

How female biology changes the game

Women’s physiology differs from men’s in key ways, especially when it comes to hormones. Here are the primary systems where the divergence matters most:

1. Hormonal fluctuations

The menstrual cycle has profound effects on how the body performs. Across a typical 28-day cycle:

• Follicular phase (Day 1–14): Rising estrogen enhances insulin sensitivity, reduces inflammation, and supports higher-intensity efforts.
• Ovulation (~Day 14): Peak estrogen and a brief testosterone bump can make this an ideal time for strength and power gains.
• Luteal phase (Day 15–28): Progesterone rises, increasing body temperature and fluid retention, decreasing heat tolerance, and altering fuel usage (greater reliance on carbohydrates).

These shifts affect everything from endurance to motivation, hydration to glycogen storage. Ignoring them can leave women under-recovered, overtrained, or simply not progressing.

2. Fuel metabolism

Women tend to oxidise more fat and conserve glycogen more efficiently during endurance training—especially in the follicular phase. This means they don’t need to rely on the same carb-loading strategies that work for men, and may benefit more from stable, frequent fuelling than intermittent fasting or low-carb plans.

But in the luteal phase, progesterone increases resting metabolic rate and promotes a shift back toward carbohydrate usage. That’s why cravings go up—and why restricting food during this phase can backfire.

3. Thermoregulation and hydration

Progesterone increases core temperature and reduces plasma volume, making women more prone to overheating and dehydration in the second half of the cycle. This changes sweat response, fluid needs, and tolerance for hot-weather training.

4. Neuromuscular control

Studies show that joint laxity increases around ovulation due to hormonal changes, which may raise the risk of ACL injuries—particularly in female athletes who train intensely during this phase without adjusting movement mechanics.

These aren’t subtle shifts. They influence strength, endurance, injury risk, mood, and motivation. Yet most training plans for women still ignore them.

What happens when we ignore these differences?

When women train and eat like men—ignoring cycle phases, energy needs, and stress responses—there are common consequences:

• Under-recovery: Female athletes may feel constantly fatigued or overtrained, especially if they train intensely during high-hormone phases.
• Relative Energy Deficiency in Sport (RED-S): Low energy availability can disrupt hormonal function, menstrual regularity, and bone health.
• Plateaus: Many women see progress stall or even reverse because they’re using protocols not designed for their biology.
• Increased injury risk: Joint instability and poor hydration planning can raise the risk of soft tissue injury, especially around ovulation.

And beyond the physical, there’s the psychological toll of being told your body isn’t responding because you’re not “disciplined” enough—when the real issue is that the plan was never designed for you in the first place.

Where Dr. Sims’ approach shifts the paradigm

Dr. Sims advocates for training and nutrition that work with the female body—not against it. Her key principles include:

1. Cycle syncing

Adjusting training volume and intensity based on menstrual phase to align with hormonal strengths and vulnerabilities. For example:

• Heavier lifts and HIIT in the follicular phase
• More recovery, mobility, and moderate training in the luteal phase

2. Pre-fuelling and recovery

Women are more sensitive to low energy availability. Sims recommends fuelling before training (even a small snack) and prioritising protein and carbs within 30 minutes after workouts—especially during high-hormone phases.

3. Temperature and hydration strategies

Cool-down protocols, hydration plans with sodium, and adapting training in hot environments are especially important during the luteal phase.

4. Empowering female athletes with knowledge

Sims doesn’t just offer training tips—she reframes the conversation. Women’s bodies aren’t problematic or unpredictable. They’re complex, powerful, and capable. But they need training frameworks that reflect their design.

Are there valid counterpoints?

Some researchers argue that the variability of individual menstrual cycles makes broad generalisations tricky. And not all women experience dramatic fluctuations—or even regular cycles. For example, women on hormonal birth control may have different responses to training that don’t follow a typical hormonal pattern.

Additionally, elite female athletes sometimes train hard across all phases of the cycle with success. But these cases are often highly individual, carefully monitored, and not really generalisable.

Still, the majority of emerging science supports Dr. Sims’ thesis: when women understand their physiology, they can adapt their training—and see better results with less frustration.

The future of female-centred fitness

More research is being done on female-specific protocols. Elite athletes like the US women’s soccer team now track and adjust training based on cycle phase. Digital tools like Wild.AI and FitrWoman are helping women log symptoms, plan training, and make better-informed decisions.

But education remains the key. Most women still haven’t been taught how their bodies work—let alone how to train for them.

That’s why “Women are not small men” isn’t just a catchphrase. It’s a turning point in how we think about female health and fitness.