How to Optimize Endurance Training for Female Athletes

There has historically been little research into how female athletes should train to best suit their physiology. That’s changing—and women are fast learning how to get the most from their bodies.

Most of us are aware of the key physiological differences that exist between men and women. The most notable include and impact cardiovascular capacity, metabolism, hormones, bone density, and body composition. Yet as familiar as we are with these differences, when it comes to training, performance, and recovery in endurance sports, the approach has historically focused on male data and research. Training models and best practices have been modeled on this research with women training and fueling in much the same way as men. It’s only in the last few years that a spotlight has been shone on the unique physiology of women—and the significant impact this can have on training, nutrition, performance, and recovery. In short, training for endurance sports shouldn’t be a one size fits all approach and what’s right for men can be far from optimal for women. 

Work with Your Physiology—Not Against It

Dr. Stacy Sims is an exercise physiologist and sports nutritionist who specializes in female athlete health and performance. She says: “Women are not small men. Stop eating and training like one.” She has led the charge for greater awareness, research, and education around the gender differences that exist and the impact they can have for active women. She urges women of all ages to adapt their training, recovery, and nutrition to work with their physiology versus against it. 

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A key part of this is understanding how female hormones can impact how a woman feels and responds to training at different times in her cycle. As Sims says: “Female sex hormones can alter immunity, autonomic nervous system, glucose regulation and metabolism, appetite, fluid balance, and body composition.” This helps to explain why a woman might find a workout easy one week yet two weeks later the same workout can feel much harder (or the same metrics might require greater perceived exertion). 

Understanding the Menstrual Cycle 

Understanding the menstrual cycle and its impact is an important first step for female athletes wanting to optimize their training. The average menstrual cycle lasts 28 days and is broken down into three key phases:

1. The follicular phase: Begins on the first day of menstruation and lasts until ovulation (approximately days 1-14). The pituitary gland releases follicle-stimulating hormone (FSH), stimulating the growth of ovarian follicles. Estrogen levels rise as the follicles mature. Some women may experience increased energy and improved mood during this phase. It’s often referred to as the low-hormone phase.

2. The ovulatory phase: Occurs around the midpoint of the cycle and lasts 24-48 hours (typically around day 14 in a 28-day cycle). A surge in luteinizing hormone (LH) triggers ovulation, where a mature egg is released from the ovary. Some women may experience symptoms like mild cramping.

3. The luteal phase: Follows ovulation and lasts until the start of menstruation (approximately days 15-28). Progesterone levels rise. This is where PMS (premenstrual symptoms) can occur. It can be referred to as the high-hormone phase.

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Of course, it’s important to emphasize that every woman is different and menstrual cycles can vary from 21 days up to 40 days. Symptoms and impacts on training can also vary widely, which is why Sims strongly recommends every female athlete tracks their cycle against training and recovery, comparing their data month on month. 

She says: “Female athletes have unique and unpredictable hormone profiles, which influence their physiology and nutritional needs across their lifespan. We begin by recommending that female athletes of reproductive age track their hormonal status against training and recovery to determine their individual patterns and needs. 

“Once you have established your own patterns and understand how you may be affected (or not) by your hormone profile, you can dial in specifics for your body and your sport.”

Research shows that key performance indicators such as VO2 max and lactate threshold remain constant throughout a woman’s cycle, so women shouldn’t rule out a PR if race day happens to fall during their period. As Sims says: “In reality, your hormones are favorable for performance once your period starts.” 

That said, there are common patterns and trends many women notice during these phases, some of which are: 

Follicular Phase

• Training can feel easier during the low-hormone phase of the cycle (which starts on day 1 of bleeding). One study conducted on swimmers found that women clocked their fastest times during menstruation and their slowest during the premenstrual phase. That said, for women who suffer with heavy cramping or discomfort with the onset of bleeding, the physiological benefits of the low-hormone phase can be diminished.

• Research suggests that some people may adapt better to HIIT (high-intensity interval training) and strength training in this phase. If this is the case, aim to work more of these sessions into this phase for maximum gains. 

• Increases in inflammation during this phase may mean it takes longer to recover from workouts, so focusing on recovery is key. 

• If doing a longer endurance effort (60+ minutes), consider increasing your intake of carbs before your session.

RELATED: Hormonal Imbalance Symptoms Female Athletes Don't Want To Ignore

Ovulatory Phase

• Estrogen levels drop as ovulation occurs and some people may experience pain related to this and other symptoms such as a slight increase in body temperature or disrupted sleep. Once estrogen and progesterone start to rise, this can lead to slightly higher resting heart rate and breathing rate. 

Luteal Phase

• During the high-hormone phase before a period starts, training can feel harder. There are higher rates of muscle breakdown during hard efforts during this phase, making it harder to make and maintain muscle. 

• Metabolism is impacted too, with the rise in estrogen and progesterone shifting reliance on carbs, which means women will need more carbs for high-intensity activities.

• This can be a good time to focus on lower-level intensity training, good nutrition, and add in more recovery time if needed. 

• Blood sugar levels can be more unstable during this time, resulting in cravings and mood changes. Opt for meals and snacks that contain complex carbs and protein. 

Adequate Fueling for Training

With hormones having such a significant impact on a female athlete’s response to training, adequate fueling is all the more important. 

Sims says: “When it comes to nutrition, the very first need for all athletes, regardless of age, hormone profile, or sport, is getting enough energy in to meet the demands of life. We are seeing such an uptick in low energy availability (LEA) across all levels of athletes, it is paramount to understand what LEA is, and how it can contribute to Relative Energy Deficiency in Sport (REDs).”

RELATED: The Rise in REDs and Why Athletes Should Be Aware of It

LEA is defined as having limited energy to support your normal body functions once energy expended through exercise is subtracted from your total dietary energy intake. As Sims says: “You’re not eating enough to support both your training and your basic biological needs. This is what makes LEA particularly insidious. You may be able to run, swim, bike, and lift, at least for a while, but your body doesn’t have enough calories and nutrition left over to keep your organ systems operating at optimal levels.” 

Do this for too long and it can lead to a host of problems such as stress fractures, depression, brain fog, GI issues, anemia, and amenorrhea (loss of periods). 

In her book ROAR, Sims cites a 2022 study of more than 200 female endurance athletes that reported 65% were at risk of LEA, 23% were at risk of exercise addiction, and 21% had disordered eating behavior. 

She says: “Many women athletes are chronically underfed, whether it’s because they’re afraid to eat; they’re in a constant state of trying to lose weight, or they are unknowingly underfueling themselves because they just don’t realize how much energy and nutrition they need.”

For endurance training, Sims recommends 4.5 to 7 grams of carbohydrates per kilogram of body weight a day. Women doing moderate training and/or short intense days (like CrossFit training) still demand about 3.5 to 5 grams of carbs per kilogram of body weight per day.  

And after exercise, she recommends female athletes focus on rapid consumption of at least 1.2 g of carbs with a minimum 0.38g protein per kilogram of body weight in order to restore spent muscle glycogen.

RELATED: Understanding Relative Energy Deficiency in Sport (RED-s)

Recommended daily protein intake is about 1.2 to 2 grams of protein per kilogram of body weight, while fat should make up about 20-30% of daily calories. 

Remember, too, that it’s the timing of fuel that’s very important for active women, i.e., fueling adequately before, during, and 30-45 minutes after your workout.  

Sims adds: “Though I don’t generally advocate rigidly counting calories and/or tracking macros, it is worth it to keep a tally for three to five days to see how much energy you’re actually taking in versus how much you think you’re taking in. 

“The next step is increasing your intake around your training, and then across the day, to see how you feel and perform when you’re hitting the mark. So many of my athletes reach their optimum performance potential once they finally achieve their energy needs.”