Women are NOT Small Men! – Dr. Stacy Sims
Women Are Not Small Men: Why Fitness Advice Is Failing Half the Population
Most of what we know about exercise and nutrition comes from research done on men, then generalized to women. That’s the blunt starting point exercise physiologist Dr. Stacy Sims makes in her conversation with Dr. Mark Hyman on The Dr. Hyman Show. Sims has spent her career studying female physiology specifically, and her message is simple but overdue: women aren’t just smaller men, and training or eating like one is quietly working against them.
The hormonal chaos of midlife is real, and it’s not inevitable
Sims explains that women sail through their twenties and early thirties with the benefit of stable sex hormones and naturally high endurance capacity. But things shift in the late thirties and accelerate through the forties, as ovulatory cycles become less frequent, progesterone drops, and estrogen fluctuates unpredictably. This hormonal shift also changes the gut microbiome, favoring bacteria linked to fat storage and cravings for simple carbs, which in turn affects neurotransmitters like serotonin and dopamine.
At the muscle level, something specific happens too. Women experience dysfunction in the fast-twitch muscle fibers responsible for power and speed, partly independent of hormones and partly worsened by declining estrogen. That’s why so many women in midlife notice they suddenly feel weaker or slower, even if they haven’t changed their routine. It’s not in their heads. As Sims puts it, women aren’t going crazy, they’re just being measured against the wrong baseline, one built from male physiology.
Heavy strength training protects more than muscle, it protects the brain
One of the most striking points in the conversation is the link between heavy resistance training and brain health. Sims cites a large randomized controlled trial showing that only heavy lifting, not moderate weights with higher reps, stimulated improved neural connectivity in the prefrontal cortex, the brain region most responsible for cognition. Lighter, higher-rep training still benefits other brain areas, but the heavy end of the spectrum appears to be uniquely protective against cognitive decline and may help slow the risk of Alzheimer’s and dementia.
Importantly, Sims stresses it’s never too late to start. Research on people in their seventies and eighties shows the same heavy-lifting benefits for bone, muscle, and brain health. The advice to start light and stay light as you age, she argues, is exactly backwards.
“Skinny fat” is a hidden risk, even for people who look healthy
Sims describes a pattern she sees often in lean, active women: a DEXA scan reveals high visceral fat and surprisingly low bone density, despite a clean diet and consistent cardio. The explanation is usually the same, not enough strength training and not enough food. Without adequate fueling, the body doesn’t have what it needs to repair and build tissue, so it pulls from bone reserves instead, while a separate hormonal shift redirects circulating fat toward visceral storage. The result can look lean on the outside while carrying real metabolic risk on the inside.
Cardio needs to change too, not just disappear
Because women are naturally more endurance-capable at a cellular level, Sims argues that long, slow “zone two” cardio isn’t the most valuable use of training time in midlife. Instead, she recommends true high-intensity interval training, and specifically distinguishes it from the moderate-intensity classes marketed toward this exact demographic (think spin, orange theory, or boot camp style classes). Genuine high intensity means working at roughly 80% effort for repeated intervals, with real recovery between them, something most popular “intense” classes don’t actually achieve.
This kind of training does double duty: it improves the muscle’s ability to absorb glucose without relying heavily on insulin, and it produces lactate, which serves as an efficient fuel source for the brain and may help reduce the risk factors linked to amyloid and tau plaque buildup.
Nutrition should follow the body’s clock, not a trend
Rather than debating keto, intermittent fasting, or the latest diet trend, Sims returns again and again to circadian rhythm. Women have a distinct cortisol awakening response tightly linked to appetite hormones, and if that morning cortisol spike isn’t met with food within about an hour of waking, hunger and satiety signals become dysregulated for the rest of the day. That often shows up later as cravings, “hangry” afternoons, and uncontrolled snacking at night, which then disrupts sleep.
Her recommendation isn’t extended fasting windows, but rather front-loading calories earlier in the day and avoiding the popular delay-breakfast approach many diet trends promote. She distinguishes this from strict intermittent fasting, which she says can leave women under-fueled and missing key micronutrients and fiber.
Protein needs are higher than most people think, especially with age
Sims is direct about protein: the standard RDA reflects the minimum needed to avoid malnutrition, not what’s optimal for building or maintaining lean mass. She points to research suggesting an ideal range of roughly 0.8 to 1 gram of protein per pound of body weight. As women age, they become more resistant to using that protein efficiently, meaning both quantity and timing matter more, not less. She recommends women aim for 35 to 40 grams of protein within a short window after strength training, since delaying that intake mutes the body’s repair signals. Even 10 to 15 grams before a workout can help improve the body’s post-exercise response.
Small, consistent habits beat perfect plans
Perhaps Sims’ most practical piece of advice is the simplest: block off small, non-negotiable chunks of time, even just ten minutes a few times a week, as a “wellness meeting” with yourself. From there, build outward: strength training two to three times a week, jump or hard-landing training to stimulate bone density, a modest dose of creatine (three to five grams daily), and consistent protein and fiber at every meal.
The takeaway
The throughline of Sims’ message is that female physiology isn’t a smaller, slightly adjusted version of male physiology, it’s fundamentally different, and the mismatch between generic fitness advice and women’s actual biology is what’s been failing so many people for so long. The fix isn’t more restriction or more cardio. It’s heavier strength training, properly timed protein, food that respects circadian rhythm, and the understanding that getting older doesn’t have to mean getting weaker, if the training actually fits the body doing it.
This article is for informational and educational purposes only. It does not constitute medical advice, diagnosis, or treatment, and is not a substitute for a conversation with a qualified healthcare provider. Individual health needs vary. If you have questions about your health, we invite you to schedule a consultation with one of our providers.