‘Women Are Not Small Men:’ Dr. Stacy Sims and the Women’s Endurance Sports Revolution

Get to know the iconic author and scientist, Dr. Stacy Sims, and her work in revolutionizing understanding of the female athlete.

By on April 11, 2024 | Comments

“Women are not small men.”

It’s a statement that Stacy Sims, Ph.D., a decorated athlete, exercise physiologist, nutrition scientist, and author, has made for years, as she and her teams have revolutionized women’s endurance sports.

Until recently, research into female athletes and what they need to succeed has been limited to non-existent, with women athletes often being treated as small men. A leader in female sports research, Sims has dedicated her career to pioneering science-backed research in exercise nutrition and performance focused on women. Sims has spent the last two decades working to shift the paradigm for more gender equity in sports across the board, including in trail running and ultrarunning.

Sims is the author of two books, “Roar” and “Next Level,” both of which discuss women’s specific needs in endurance sports throughout the life cycle. A new edition of “Roar” was released in January 2024.

The original, which offered biohacks into the nuances of nutrition and performance for the female athlete, was published nine years ago, and Sims says, “All the physiology aspects and basic science in the first book are still valid, though science over the past five to six years has just exploded in the female athlete space.”

With nearly 100 new pages, the book’s second iteration is now more broadly athlete-oriented and less elite-focused. There are more case studies across the board from general fitness to ultrarunning. There are larger, more in-depth sections on puberty and another on the menstrual cycle, including tracking the differences between using an IUD and an oral contraceptive. There’s a huge section on biohacking athletic performance and health, including discussing current trends, like using wearables to measure heart rate variability and blood glucose levels.

Stacy Sims - lecturing

Stacy Sims, Ph.D., continues to lead the field of research on female athletes. All photos courtesy of Stacy Sims.

How Being A Competitive Athlete Dovetailed Into Science

Born in 1973 into an American military family, Stacy Sims, now 50, spent most of her formative childhood years in The Netherlands before landing in San Francisco, California, for high school. Today, Sims lives in Mount Maunganui, a town on the North Island of New Zealand.

Sims was a ballerina until age 13 when her instructor advised her to choose between ballet and running. Sims made her choice, and she joined the cross-country team at high school, where she pursued distance events alongside playing field hockey. Feeling burned out on running upon reaching college — at Purdue University in West Lafayette, Indiana — she declined a walk-on spot on the cross-country team and instead joined the rowing club.

Throughout her undergraduate years, Sims and her rowing squad ran for fitness. “We called ourselves the Gamers Club. We did things like run the Chicago Marathon, just because it was a few days after the Head Of The Charles Regatta[, a rowing race in Massachusetts.] Or we’d run all the stairs in the parking garages just trying to challenge ourselves. So, I ended up doing 20 marathons before I was 20,” said Sims.

Academically, Sims started as a political science major and French minor. “I wanted to be a translator for the United Nations and participate in political science. In elementary school, I was in French immersion. Most of my 5th and 6th grade classes were taught in French, so it was my second language,” said Sims.

There was one problem: She couldn’t stay awake through her political science classes. They were too boring for her. Her roommate pointed out that as into exercise and competitive racing as she was, and with her desire to be a chef, she might really like exercise physiology and metabolism. During her second year in college, she switched majors to pursue a career in sports science — finding exactly the niche that kept her attention — and graduated in 1995. Afterward, she moved to Springfield College outside of Boston, Massachusetts, for a master’s degree in exercise physiology and metabolism.

While Sims participated in various sports growing up, running was always a part of her training and eventually dovetailed with her scientific interests. While working on her master’s degree in Boston, she got involved with an ultrarunning group.

Sims said, “I was looking at overtraining and female endurance runners because there was so much information on male runners and overtraining. Being in that sport, I saw how women presented with overtraining and it was completely different than men. I really wanted to dig in: Are there immune differences? Are there mood differences? What’s going on with the menstrual cycle? We found vast differences between what the literature said for men and what was going on with women. That was actually one of the very first times where I was like, ‘I’m studying women and I’m studying women runners.’ In all my experiences in undergrad, I’d been told, ‘We don’t study women.’ ‘Let’s throw their data out.’ ‘Why do you want to study women?’ ‘We don’t know enough about men.'”

Paradigm Shift: Sims’ Scientific Research Supports Female Athletes

Years later, after earning her Ph.D. from the University of Otago in New Zealand and holding several research positions at institutes including Stanford University and the University of Waikato, Sims’ pursuit of scientific research to support female athletes, women’s sports, and running has never waned.

When asked about the next frontier for understanding and supporting female athletes, Sims says, “There’s almost no research done in sports science. We’re looking outside of sports science to get more collaborative work, mixed methods, and pulling the lived experiences in to really understanding the sociocultural drive of what sport is for women, how we can train women, and how we can make really good judgment calls as coaches and practitioners — that’s where we’re seeing the push going in the next year and a half or so.”

There’s more to training women than just adjusting workouts. Sims explains that in some cultures, when a person is on their period, they do not submerge themselves in cold or hot water after sports practice, and they also do not discuss their period, so a coach might think that the athlete is being belligerent. To improve the sporting environment and the ability to coach, important variables include why an athlete might be physically faltering during an intense workout, their family background, and how they grew up, including what foods they eat.

From a physiological standpoint, Sims has been studying menstrual fluid to determine if there are biomarkers for indicating if an individual has endometriosis, polycystic ovary syndrome (PCOS), or other health implications that can affect overall health and athletic performance. “We have just finished some really cool pilot tests [comparing] venous blood versus menstrual fluid and finding very statistically significant correlations between the two,” said Sims. Ultimately, if a female can use menstrual blood for such a test, they can avoid needing to go to a physician for a blood test, which only captures symptoms from a single point in time.

For athletes, applying such a tool can answer questions about their ovulation timeframe, if they have a high inflammatory marker, or if they are at risk for relative energy deficiency in sport (RED-S). Beyond the athlete population, this tool could be used for rural health. Sims says it could be a viable option “where people don’t have access to doctors or are afraid to go to them. They can do these at-home kits with a natural occurrence every 28 to 40 days.”

Events and Coaches: How Trail Running and Ultrarunning Can Be Inclusive to Women Athletes

When examining the history of sports and culture, it’s apparent women have been required to adapt to a male lens. For example, the menstrual cycle is an inherent part of women’s physiology and has often been considered a sign of fallibility.

Sims says, “We need to take a pause and rethink: How are we supporting our female athletes? What is their physiology and training history? Then we can put in appropriate protocols.”

First and foremost, a low-touch step is to provide sanitary products at race aid stations and to normalize the acceptance of menstrual cycles, especially since athletic events can be a common time for athletes’ menstrual cycles to occur. Sims says, “Most often, an athlete’s period comes around their ultra [race], because they’ve tapered, eaten more, and had a reset. All of a sudden, boom, your period comes on race day. You might be out there for 100 miles, and you’re like, ‘What do I do?'”

Women also need different fueling than men. Sims says, “Women have more gastrointestinal (GI) distress than men attributable to sex differences in gastrointestinal transit time, sensitivity, and the specific effects of estrogen and progesterone on gut function. Women need more mixed and macronutrient food and some carbohydrates — not just the quick hits of simple sugar or only gels. Their gut can’t handle as much fructose or maltodextrin as men — that causes a lot of pressure in women’s intestines.”

As a result of eating too many simple sugars, the body pulls water from other spaces into the intestines, causing dehydration, muting the appetite, and increasing nausea. Additionally, the mucosal lining — which lines the intestines and prevents the endotoxin release that causes GI issues — erodes faster in women. For digestion, women have a slower gastric emptying rate and intestinal motility rate than men. In longer endurance races, women often have a more difficult time keeping food down in the second half when compared to men.

Sims emphasizes it’s really important to understand that women’s physiology is unique. “When I’m talking about women, I mean female, as in cisgender women. And if we’re talking about that, then there are sex differences from birth, where women who have XX [sex chromosomes] are born with more oxidative fibers. Men are born with more glycolytic or those anaerobic fibers.” Sims says that as a result, “Women already have really robust mitochondria, mitochondrial function, and mitochondrial respiration. We have a greater ability for burning free fatty acids: Our body defaults when we’re in low energy to burn fat, preferentially. When we aren’t in low energy, it goes through blood glucose and a little bit of liver muscle glycogen and then into free fatty acid or amino acid.”

Comparatively, due to men having more glycolytic fibers, which are anaerobic sugar-burning fibers, they go through their blood sugar first, followed by the glycogen stored in their liver, and then their muscle glycogen stores, all before they get into fat-burning. From an endurance standpoint, training routines for male runners need to include more low-intensity and base-building training in zone two to get metabolically flexible and to be able to tap into burning more body fat.

“By the nature of being born with XX chromosomes, you are already there — women are already super endurance. If we consider estrogen’s components — that encourage sparing carbohydrate and encourage the use of free fatty acids — then there’s another layer above and beyond those XX factors that allow women to be very successful at endurance,” says Sims.

Stacy Sims headshot

Stacy Sims, Ph.D., was one of the first to insist that when it comes to training and competing, women are not small men.

Sims advises that for training and to prevent injury and illness, it’s important to have woman-tailored training plans that balance volume with intensity, plus one-legged exercises for control, stability, and overall running technique. “For our women to remain uninjured and keep progressing, we need to rethink about the volume. We need some time on the feet, but we don’t need to do the significant amount of volume that’s ingrained in all of the running programs, because those are all based on male data.”

Equally important is to have a long-term vision for training plans for women athletes. “From early on, we need to do intensity across the board to teach the brain and the muscles how to use that high intensity, to be able to polarize a bit. Then that improves running economy and running speed, and you already have the ability to do endurance.”

The fastest-growing group in ultra-endurance sports is women who are over 45 years old, which is when they are reaching perimenopause or menopause. Sims says it’s when “their hormones drop, and they’re really good at going long and slow.”

Understanding Women Are Not Small Men

For two decades, Sims has said, “Women are not small men.” She’s been paramount in encouraging more research, and the space is finally growing and the culture shifting.

“I love the fact that women feel comfortable talking about menstrual cycles and menopause, which is really good in the sports space from grassroots to elite athletes and hasn’t quite reached the greater world yet — it’s still a little bit taboo. But it’s good to see people acknowledge the fact that women have reproductive qualities that can really help or hinder, depending on how people perceive it, their ability to perform and reach their potential.”

Once accepted and studied, experts can “help women to improve mental capacity and mental acuity, understand where they’re coming from, and help them optimize their bodies wherever they are in their menstrual cycle. When they get to perimenopause and postmenopause, they need to change things up as well to keep optimizing their potential with the way their body is changing into a new biological state. Even four years ago, I wouldn’t have been able to say that — so, that’s really exciting,” says Sims.

In September 2023, Dr. Stacy Sims, alongside a handful of colleagues, advocated in Washington, D.C., about the need for more research about women in military spaces. President Joe Biden’s office was in attendance, responded in agreement, and provided funding. Sims says, “That was a pretty cool, identifying moment in my career. Not only were we able to get the U.S. Department of Defense (DOD) to listen — in the fact that we have to look at taking care of our women if we want to recruit more women and keep women to be able to lead — but also pushing in a wider fashion saying we need more research across the board in all healthcare. That’s really exciting.”

Sims explains the DOD sees a huge incidence of injury in female recruits, from cadets onward, and data such as body-max index (BMI) charts are archaic. “If women are considered overweight on the BMI, then they get put on restrictions. But we know in today’s society — where you have practices like CrossFit, Olympic lifting, and heavy lifting — that women’s bodies are getting very strong and need to carry the packs, do tactical skills, and go on missions. But the guidelines are not reflective of those changes.”

The result is a high incidence of RED-S and eating disorders, as well as women aging out of the military because they hit perimenopause and postmenopause. Sims says the DOD is “really pushing hard of how are we going to change the system to be able to pull more women in and take care of them.”

Sims also used the opportunity in Washington, D.C., to talk about the need to improve healthcare as a whole for women, including finding noninvasive measures to identify and help the reproductive aspects of PCOS and endometriosis instead of reverting to birth control as a default. Sims is also concerned about the state of artificial intelligence and the male biases that are inherently built into it from the majority of engineers being men.

Providing Women-Focused Resources

In June 2024, Sims is launching a youth course targeted toward coaches, parents, and educators to help them understand female physiology between puberty and age 20.

“We’re trying to explain how we should be changing training, what we should be looking at, and how we should support girls in staying active and in sports.” The five-week, 70-hour online course is self-led. By the end of 2024, Sims aims to create a course for teenagers to study.

From a research standpoint, she’s excited about research that her Ph.D. students are wrapping up on concussions. “Looking at across the menstrual cycle, there are different outcomes depending on when you get hit. In the low-hormonal phase, you tend to recover a lot faster and not have as much symptomology. High-hormone phase, you end up with a longer duration of concussion recovery, up to maybe six months of fatigue,” she said.

On her website, Sims offers a range of online courses, like Menopause 2.0 and Women are Not Small Men. She also has microlearning courses, training programs, books, and a blog. “We have a downloadable sheet of how to tailor a program for marathoning all the way through ultrarunning,” she notes.

Sims also mentions that there are several other good sources for science-backed information on women’s health available. One is the Stanford FASTR Program, which offers female athlete education via its website. It provides handouts, videos, blog and journal articles, Instagram posts, podcasts, and recipes. The team behind the program includes Lead Researcher Megan Roche, M.D., Ph.D., and Program Director Emily Kraus, M.D., and it offers reputable information on a range of topics like nutrition and mental health.

Sports medicine physician Kathryn Ackerman, M.D., MPH, who teaches and researches at the Mass General Research Institute at Harvard University and specializes in sports medicine and endocrinology, offers dependable resources including articles and blogs.

The Australian Institute of Sport dietetics site offers a range of information about fueling and recovery, such as recipes and guidance regarding supplements and energy availability.

Stacy Sims - interacting with students

Through lectures, research, and seminars, Stacy Sims, Ph.D., continues to advance education and research into how women’s bodies respond to training differently than men’s.

Fitness for Adventure and the Future

After many years spent as an elite athlete, Sims no longer aims to compete at the top end of sports.

“I’ve done the whole gamut. Raced Ironman Kona, the Maui Xterra World Championships. I’ve raced high level on the [road] bike and been to world cups, and you must have all of that high-level dedication and self-involvement to be at that level. I don’t have it in me anymore, because I have a daughter who is 11 years old, and I really am passionate about helping all women in my career. Now, I love going to the gym and lifting. I really like the feeling of being strong. I do ocean swimming. I still love gravel riding. Trail running. So, a little bit of everything to keep fit and healthy.”

On Monday, Wednesday, and Thursday, Sims takes a strength development class at her gym: squats, deadlifts, push presses, bench presses — all functional movements. Sims and her husband have a gym date every Friday for a longer, higher-intensity workout. On the weekend, she rides her bike. She might also hit a few swim squad meetups in the morning or do an ocean swim during the week.

Sims enjoys focusing on well-rounded fitness, allowing her to partake in life adventures and accept invitations to join friends on trips. This September, she’ll visit the Cook Islands in Polynesia to swim between the isles, around 5 kilometers every day for three days. She is also doing an obstacle race in New Zealand with her husband in mid-April. “I’ve never done one before. It’s called the Ultimate Athlete. We live in a beach town, so it’s on the beach, on soft sand, and it has 40 different obstacles like tire flipping, cargo rope climbing, under-overs, and wall climbs. That’s my challenge — being completely different from anything else.”

Ultimately, Sims is more motivated than ever to keep chipping away at her lifelong career in women’s sports and research.

Sims says, “I’m just really happy that people are having these conversations about women in sport. I think about when I was doing sports and what it was like for women — training like men, trying to be like men, trying to keep up with them, and always getting injured or sick. No one would talk about having their period unless they were talking about how awful they felt. It’s really cool that people are talking about it, because it helps empower women across the board.”

Call for Comments

  • Has Stacy Sims’ work informed how you train and compete?
  • Have you read Sims’ books “Roar” and “Next Level?”
  • How can trail running and ultrarunning continue to support the specific needs of women runners?
Morgan Tilton

Morgan Tilton is the WeRunFar columnist of iRunFar and a Staff Writer for GearJunkie and AllGear Digital. Morgan has covered outdoor industry news, adventure travel, and human endurance for nearly a decade. Aside from iRunFar, Morgan has written for more than 70 publications, including recent contributions to Outside, Forbes, Trail Runner, Runner’s World, Bicycling, and NewsBreak. She’s a recipient of more than a dozen accolades for her travel writing from the North American Travel Journalists Association. Based in Crested Butte, Colorado, Morgan enjoys mountain running and exploring the high alpine in the summer when she’s not splitboarding or mountain biking.