The surprising science behind women’s hormones, metabolism, and exercise
Summary
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Women aren’t “smaller men”—your body plays by different rules.
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Most fitness advice ignores how hormones affect metabolism and recovery.
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Fasted workouts and calorie-cutting can backfire for women.
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Your menstrual cycle holds the key to smarter training and nutrition.
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With a few strategic changes, you can feel stronger, leaner, and more in control.
Menstrual Cycle-Based Routine
| Period | Body State | Training Focus | Nutrition Focus |
Days 1–6
Menstrual | Energy low, immune down | Walk, stretch, gentle yoga Prioritize rest | Warm, iron-rich foods
Protein + hydration |
Days 6–13
Follicular | High energy, max output | PR lifts, HIIT, speed work | Protein + carbs
Appetite easier to manage |
Days 14–20
Ovulation–Mid Luteal | Temp ↑, fatigue ↑ | Moderate intensity Focus on form over load | Extra hydration
More protein, low-GI carbs |
Days 21–28
Late Luteal | Mood drops, sleep declines | Light cardio Recovery work | Omega-3, magnesium
Smart snacks (sweet potato, dark chocolate) |
This post is a comprehensive summary of the video “The Truth About Exercise On Your Period!” from The Diary of a CEO featuring Dr. Stacy Sims. It explains why women often struggle with mainstream fitness advice and offers cycle-based training and nutrition strategies rooted in real science. All insights are drawn from Dr. Sims’ 20+ years of research in exercise physiology and female health.
Why is this happening to me?
You train hard. You eat clean. But your boyfriend drops fat while you bloat and stall. Why? Dr. Stacy Sims explains: “We’re not biologically built the same. So why are we training the same?” Most fitness advice is blind to the fact that women’s bodies shift dramatically across the month—and across life stages. It’s not your fault. You’ve been following a system that wasn’t made for you.
Science wasn’t made for you
Did you know that most exercise studies are based on men aged 18–22? Not only that—when Dr. Sims submitted her lab results as a student, they were thrown out. Why? “Because they didn’t match the men’s.”
“We don’t study women because of the menstrual cycle.”
That’s what she was told. So the protocols, diets, and recovery advice we all follow? They’re generalized from male physiology—and that’s costing women their performance and health.
Your body plays by different rules
Women’s bodies follow a monthly hormonal rhythm, not a static baseline. This affects:
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Metabolism: A woman’s body uses sugar from food differently depending on where she is in her monthly cycle. Sometimes it stores energy well, other times it doesn’t—and that can change how tired or hungry she feels.•
Joint stability: Around the middle of the cycle (when a woman can get pregnant), a hormone called estrogen makes her joints a little looser. That can make things like knees more wobbly and easier to hurt.•
Oxygen capacity: Women have smaller lungs and less oxygen-carrying blood than men. That means during exercise, their muscles get tired a little faster, especially in high-speed or high-power sports.•
Injury risk: Because women usually have wider hips, their legs don’t line up in a straight line like men’s do. This puts more pressure on the knees and makes them 3 to 4 times more likely to tear something called the ACL, especially in sports like soccer.•
Temperature tolerance: Women’s bodies don’t handle hot or cold the same way as men’s. For example, an ice bath might feel too cold to be helpful, and it can take longer for women to start sweating when it’s hot.You're not just “different.” You're strategically complex. And when you work with those patterns, you gain control.
Fast doesn’t always mean fit
The fitness world loves calorie-cutting, fasting, and keto. But these often backfire for women.
Here’s why:
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Hypothalamus sensitivity: A part of the brain called the hypothalamus keeps track of how much energy the body has. In women, it’s extra sensitive. So when there’s not enough sugar (energy) in the blood, it thinks something is wrong—and starts slowing things down to “save power.”•
Fasted training: If a woman exercises without eating first, her body might not burn fat—it might start breaking down her muscles instead. That means she can lose strength instead of weight.•
Extreme diets: When women eat too little or cut out too many foods, it can confuse their body. Their period might stop, sleep can get worse, and important systems (like the thyroid) may stop working properly.“When energy drops, the female body doesn't burn fat—it protects it.”
This evolutionary mechanism once protected fertility during famine. Now? It stalls progress and harms well-being.
Train With, Not Against, Your Cycle
Instead of working harder, work smarter—with your hormones, not against them. Women thrive when training and nutrition are synced with their menstrual cycle. Here's your 28-day roadmap:
Menstrual Cycle-Based Routine
| Period | Body State | Training Focus | Nutrition Focus |
Days 1–6
Menstrual | Energy low, immune down | Walk, stretch, gentle yoga Prioritize rest | Warm, iron-rich foods
Protein + hydration |
Days 6–13
Follicular | High energy, max output | PR lifts, HIIT, speed work | Protein + carbs
Appetite easier to manage |
Days 14–20
Ovulation–Mid Luteal | Temp ↑, fatigue ↑ | Moderate intensity Focus on form over load | Extra hydration
More protein, low-GI carbs |
Days 21–28
Late Luteal | Mood drops, sleep declines | Light cardio Recovery work | Omega-3, magnesium
Smart snacks (sweet potato, dark chocolate) |
Baseline Nutrition Rules (Cycle-Aware)
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No fasted training: Always have 15g protein + 30g carbs (e.g., shake + banana)•
Refuel within 45 minutes post-exercise: Especially important for muscle synthesis in women•
Eat dinner early (6–7 PM): Helps manage cortisol and improve sleep•
30 plant types/week: Boosts gut microbiome and hormone detox Recommended Supplements
Supplement | Why | How to Take |
Creatine | Muscle, brain, gut support | 3–5g daily with breakfast |
Vitamin D3 | Immunity, mood, recovery | 1000–2000 IU with lunch |
Omega-3 | Inflammation control | 1000mg with fat-rich meal |
Iron (if ferritin < 50) | Energy, focus, cycle support | Every other day with Vitamin C, on empty stomach |
Bottom line:•
Align training intensity with hormone phase
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Fuel before, during, and after workouts
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Recovery is just as important as stress
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Small nutrient tweaks = huge benefits
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Listening to your cycle = long-term consistency
6. Small Tweaks, Big Results
You don’t need a radical diet. You need alignment. Most women who apply even a few of these changes report better sleep, more consistent energy, and measurable strength gains. And no—this isn’t about being “delicate.” It’s about being deliberate. Your biology is a blueprint for performance. Understand it, and you unlock what’s already yours.
7. The Future Is Female—And Informed
For too long, fitness has been built for one kind of body. But times are changing. More women are reclaiming their data, their rhythms, and their results. With researchers like Dr. Sims leading the charge, the fitness world is finally catching up. You don’t need to work harder. You need to work with your body. And now, you finally can.