The Research
Wasn't Built
For You

There's a gap at the centre of women's health and performance science, and it isn't subtle. For decades, the research that underpins training recommendations, nutrition guidelines, recovery protocols, and supplement dosing has been conducted primarily on one type of subject: men. The advice that filtered down from that research, through coaches, GPs, dietitians, and gym floors, was handed to women too. Nobody mentioned that it was never tested on them.

The numbers

Exercise physiologist and researcher Dr. Stacey Sims, whose work has spent more than two decades focused on female physiology, has put the figure plainly: approximately 94% of sports science studies are conducted on male subjects. The number has become a shorthand for a structural failure, not the result of one decision, but of decades of assumptions built into how science gets funded, designed, and published.

The reasons are not entirely cynical. Historically, researchers avoided female subjects because hormonal variation across the menstrual cycle was considered a confounding variable, something that would complicate results and require larger, more expensive study populations to control for. The logic was clean, if circular: women's hormones make the data messier, so we'll exclude women and produce cleaner data. The problem is that the data was then used to advise women anyway.

The NIH in the United States only mandated the inclusion of women in federally funded clinical research in 1993. Prior to that, the exclusion was standard practice. And even after the mandate, implementation in sports science has been slow. Funding structures, publication incentives, and study design norms all continued to favour the simplicity of all-male cohorts.

What gets built on male data

This matters because female physiology is not a minor variation on male physiology. It is a unique system that requires an equally unique approach.

Women have higher oestrogen and progesterone, hormones that fluctuate significantly across the menstrual cycle and affect muscle protein synthesis, fuel selection, cardiovascular function, joint laxity, thermoregulation, and recovery. Women have different body composition, lower oxygen-carrying capacity relative to body mass, and a different ratio of fast-twitch to slow-twitch muscle fibre. Women's stress response is mediated differently, with the HPA axis operating under distinct hormonal constraints. The menstrual cycle isn't a minor variable to be controlled for. Nor does it only serve reproduction. It's a central feature of female biology that shapes the response to almost everything: training, nutrition, recovery, immune function, mental health, metabolic health, cancer risk, and longevity.

When you build a field of science without accounting for that, you produce recommendations that not only work inconsistently for women, but can be damaging for them.

None of this was considered when the recommendations were written. Protein intake guidelines, carbohydrate fuelling strategies, creatine dosing, intermittent fasting protocols, sleep recovery benchmarks, VO2 max training zones — the vast majority were derived from studies on men and applied universally. The working assumption was that the female body would respond in broadly the same way, scaled down for body weight. It doesn't.

Where the gaps show up

Protein requirements. Standard guidelines recommend 0.8g of protein per kilogram of bodyweight per day. This figure was established through nitrogen balance studies conducted predominantly on male subjects, and was designed to prevent deficiency in the general sedentary population, not to support the repair and recovery demands of active women. Research on active women suggests the requirement is meaningfully higher, particularly in the luteal phase when progesterone increases muscle protein breakdown. Women who train and follow standard protein guidance are likely under-fuelling muscle repair.

Fuelling for training. The established wisdom around carbohydrate loading and fasted training was built on male metabolism. Oestrogen improves fat oxidation and metabolic flexibility in ways that alter how women respond to carbohydrate availability. In the follicular phase, women can draw more readily on fat for fuel. In the luteal phase, carbohydrate dependence increases. Neither of these patterns featured in the research that produced current fuelling guidelines.

Creatine. Creatine supplementation is one of the most studied ergogenic aids in sports science. The recommended dose, typically 3–5g per day, is based on studies conducted almost exclusively on men. Research by Sims and others suggests women may have a lower baseline creatine store relative to muscle mass, potentially making supplementation more impactful, yet women have historically been told the benefit applies equally at the same dose, with no reference to the sex-specific data gap.

Recovery. Sleep architecture, HRV benchmarks, and recovery timelines were all established through research on male athletes. Women's sleep quality is measurably disrupted during the luteal phase due to rising core temperature and altered progesterone metabolism. The "normal" recovery window looks different across the cycle, something that doesn't appear in recovery frameworks built without female subjects.

Intermittent fasting. The evidence base for time-restricted eating comes overwhelmingly from male studies. The hormonal response to fasting — specifically the interaction between cortisol, the HPA axis, and reproductive hormone suppression, is different enough in women that blanket IF recommendations carry real risks that the research never screened for, because the research never enrolled women in sufficient numbers to detect them.

The cost of the gap

The practical consequences for active women are significant. Women who follow male-derived training and nutrition advice often hit walls their male counterparts don't encounter: unexplained fatigue, poor recovery, stalled progress, disrupted cycles, and burnout. When they report these experiences, the response is frequently to train harder, eat less, or simply accept that they're not built for high performance.

The problem is not the women. It is that the framework was never designed for them.

Relative energy deficiency in sport (RED-S) — a condition caused by chronic under-fuelling relative to training load — disproportionately affects female athletes, in part because calorie and macronutrient recommendations were built on male data that didn't account for women's higher relative energy needs per unit of lean mass. The female athlete triad (low energy availability, menstrual dysfunction, low bone density) is a documented consequence of advice that was never tested on the people it was applied to.

The researchers changing this

Dr. Stacey Sims has spent her career building the evidence base that was missing. Her research focuses on sex differences in physiology, the menstrual cycle's effect on training adaptations and nutrition needs, and practical protocols for female athletes at every level. Her 2016 book Roar and its follow-up Next Level represent the most accessible synthesis of female-specific performance science currently available, and they are grounded in research that the mainstream sports science field spent decades not producing.

She is not alone. Researchers including Dr. Georgie Bruinvels, Dr. Kirsty Elliott-Sale, and Dr. Emma Ross have published extensively on menstrual cycle effects on performance and injury risk. The IOC issued a consensus statement in 2014 on the female athlete triad, updated to the broader RED-S framework in 2018. The field is shifting, slowly, in response to the weight of evidence that women's physiology is distinct and that distinct physiology requires distinct research.

What this means for you

It means that much of the advice you've received, on how to train, how to eat, when to rest, how much protein you need, how fasting affects you, how hard you should push, was not built with your body in mind. It doesn't mean the advice is always wrong, but it means you have every reason to question whether it applies to you, and to seek out guidance grounded in female-specific data.

It means that symptoms like persistent fatigue, poor recovery, disrupted sleep, reduced tolerance for training, and lost menstrual cycles are not failures of effort or resilience. They are often the predictable result of applying the wrong framework to a body it wasn't designed for.

And it means that understanding your own physiology, how your cycle affects your energy, your strength, your recovery, your nutritional needs, is not a luxury or a distraction. It is one of the most beneficial things you can do. Start with research that was actually built for you.

That's why SYNC exists.