When it comes to fitness, health, and nutrition, the gender gap is real—and not in the way you might think. Dr. Stacy Sims, an exercise physiologist and nutrition scientist, has dedicated her career to addressing this disparity. Her groundbreaking research focuses on empowering women to work with their physiology rather than against it. Let’s unpack the key insights she shared in her recent interview.

The Flawed Foundation of Sports Science

For decades, sports science research has been predominantly based on male data. This oversight has trickled down into generalized guidelines for training, nutrition, and recovery—guidelines that often don’t work for women. “Most sports science research is done on 18- to 22-year-old college-aged men,” Sims explains. “Women’s unique physiological factors—like menstrual cycles, pregnancy, and menopause—are rarely accounted for.”

Sims recounts a pivotal moment in her early academic career. As a second-year university student participating in a metabolism lab, her results were discarded because they didn’t align with the “norm” established by male participants. The reason? Hormonal fluctuations during her menstrual cycle. “It’s easier to study men because they don’t have hormone fluctuations,” she was told. That dismissal ignited a lifelong mission to change the narrative.

Female Physiology: A Unique Blueprint

Women are not simply smaller versions of men, a point Sims stresses repeatedly. From muscle composition to fat storage, heart and lung size, and even oxygen-carrying capacity, women’s bodies are built differently. For example, women have more endurance-oriented muscle fibers, a higher percentage of body fat, and smaller hearts and lungs relative to their body size. These differences profoundly impact how women respond to training, nutrition, and recovery protocols.

One of the most significant distinctions is hormonal variability. For men, hormonal levels are relatively consistent. Women, on the other hand, navigate constant shifts due to menstrual cycles, pregnancy, perimenopause, and menopause. These hormonal changes influence everything from energy metabolism to injury risk and even how the body responds to heat and cold.

Why Women Train Differently

A common frustration Sims hears from female clients is that they don’t see the same results as their male counterparts despite following identical training plans. The reason lies in biology. For instance, women are more prone to losing lean muscle mass if they train in a fasted state. Men, in contrast, can benefit from fasted workouts without the same risk.

“If a woman comes to me and says she’s doing fasted training to lose weight, I’ll tell her she’s burning through her lean muscle mass,” Sims says. Instead, she advises women to consume a small amount of protein and carbohydrates before exercise to stabilize blood sugar and protect muscle tissue.

The Menstrual Cycle: A Training Ally

Understanding the menstrual cycle can be a game-changer for female athletes. In the low-hormone phase (days 1-14 of the cycle), women have greater access to stored carbohydrates and can tolerate higher-intensity workouts. During the high-hormone phase (days 15-28), the body relies more on fat as a fuel source, and heat tolerance decreases. Tailoring training intensity and recovery strategies to these phases can optimize performance and prevent overtraining.

Addressing the ACL Epidemic

Women are up to four times more likely than men to suffer ACL injuries, a statistic that has garnered increasing attention in sports. The culprit? A combination of wider hips (leading to a greater Q-angle at the knee), quad dominance, and hormonal influences on ligament laxity.

Sims advocates for targeted strength training to address these issues. Building hamstring and glute strength can balance the forces exerted on the knee, reducing injury risk. Incorporating lateral movements, single-leg exercises, and proper landing mechanics into training routines is also critical.

The Role of Nutrition

Sims dismantles the myth that “calories in, calories out” works equally well for everyone. Women’s hypothalamus—the brain’s control center for appetite and metabolism—is more sensitive to nutrient availability. Caloric restriction or extreme fasting can backfire, triggering the body to store fat and conserve energy.

“For women, fasting strategies need to align with their physiology,” Sims explains. An overnight fast—stopping food intake 2-3 hours before bedtime and eating within 30 minutes of waking—is beneficial for women. Longer fasts, however, can lead to elevated stress hormones, poor blood sugar control, and disrupted sleep.

Supplements and Recovery

Creatine isn’t just for bodybuilders. Sims highlights its benefits for women, including improved muscle recovery, cognitive function, and gut health. The key is dosing: women need only 3-5 grams daily, compared to the higher doses often marketed to men. Vitamin D3 also gets a nod for its role in muscle function, immune support, and overall health, especially during winter months when sun exposure is limited.

Cold Plunges and Saunas: Not One-Size-Fits-All

Cold plunges and saunas have taken the wellness world by storm, but Sims points out that women’s responses differ significantly from men’s. For cold water immersion, women achieve optimal benefits at higher temperatures (around 15°C or 59°F), as lower temperatures can cause excessive vasoconstriction. Saunas, on the other hand, require longer sessions for women to achieve the same cardiovascular and hormonal adaptations as men.

Empowerment Through Education

Dr. Stacy Sims’ work is reshaping how women approach health and fitness. By understanding their unique physiology, women can optimize their training, nutrition, and recovery for better results and long-term well-being.

As Sims aptly puts it, “Women are not small men.” It’s time the fitness and health industries fully embrace that truth—and empower women to harness their biology to thrive.

Dr Stacy’s blog