Ride or Die is a bold ultra-endurance challenge, with Clarissa as one of its participants. Her journey serves as a powerful example of what the female body is capable of when physiology meets performance. This article explores when and how women can train fat oxidation safely and effectively—through a science-based, cycle-aware approach.
Clarissa is a participant of Ride or Die – an ultra-endurance event that’s anything but ordinary. The route spans over 1000 kilometers, from Basel to Barcelona – by bike, alone, without support, and without a fixed place to sleep. She carries everything she needs with her: food, gear, and unwavering motivation.
On Monday, April 14, she sets off. With nothing but her bike, a clear goal, and a relentless determination to push beyond limits. Each day, she covers around 300 kilometers – battling headwinds, heavy storms, flooded roads, and deep puddles. Pain and exhaustion ride with her, yet she presses on. After just three and a half days, she reaches her destination: Barcelona. Soaked, worn out – but triumphant.
What she experienced on the road, how fast she was, and how the other participants fared – you’ll find out this fall in a multi-part YouTube series. But this journey was more than just a physical challenge.
Clarissa’s ride represents a question that many women in endurance sports still face:
Should women train fat oxidation – or is it hormonally risky?
It’s a topic often misunderstood. Many training models are based on studies conducted with male subjects. As a result, they often overlook the specific needs of the female body – especially regarding metabolism, hormonal fluctuations, and energy utilization.
Fat oxidation in women is not only possible – it can be a powerful strategy when done right.
Modern sports science makes one thing clear: the female body does not function like the male body – especially when it comes to hormone regulation, energy use, and fat metabolism.
Most traditional training models are based on studies conducted on male participants. For women, this can be problematic – particularly with strategies like fasted training or low-carb protocols.
We often hear things like:
“Women shouldn’t train fat oxidation.”
“Train low doesn’t work for women.”
But these claims are overly simplistic. They ignore cyclical hormone shifts – and overlook the fact that ultra-endurance events like Clarissa’s require a very different physiological preparation than a 60-minute interval session.
Dr. Stacy Sims is one of the world’s leading experts in female athlete physiology. In her work, she consistently challenges the one-size-fits-all, male-centric approach to training. She’s especially vocal on the topic of fasted workouts.
Her widely cited statement:
“Women can train with low carbohydrate availability – but they must NOT train empty.”
In other words: Yes, women can train with lower carb availability.
But no, they should never train completely fasted – the body needs at least a minimal energy supply in the form of protein for protection.
Unfortunately, this nuance is often lost in practice. Many interpret her stance as a blanket rejection of fat oxidation training for women – but that’s not the case. Sims doesn’t oppose it – she differentiates. And that distinction is key if women want – and need – to train fat oxidation effectively.
Fat oxidation training, often referred to as “train low,” means deliberately training with low glycogen availability to teach the body to use fat as a primary fuel source.
In ultra-endurance sports, this ability is crucial. Glycogen stores typically last only 2–3 hours. Beyond that, the body must switch efficiently to burning fat. Training this adaptation helps preserve energy, spare glycogen, and sustain long-duration performance.
But women respond differently to this stimulus than men. Why?
Throughout the menstrual cycle, different hormones dominate at different times, directly affecting how the body metabolizes fuel:
Estrogen is dominant. The body is anabolic, insulin-sensitive, and more efficient at utilizing fat. This is an ideal window for fat oxidation training – as long as protein is consumed beforehand.
Progesterone rises. It suppresses gluconeogenesis (the creation of glucose from other sources), increases resting energy needs, and amplifies cortisol response. The body becomes more vulnerable to stress.
Fat oxidation training – especially without adequate fueling – should be avoided in this phase.
One thing is clear across the literature: Women should not train completely fasted.
Before any session with reduced carbohydrate availability, women should consume at least 15 grams of protein – for example, a whey shake, plant-based EAAs, or a protein snack.
This small amount is enough to protect muscle tissue, buffer the cortisol response, and maintain hormonal balance during training.
Two key factors determine carbohydrate needs: cycle phase and training intensity.
Thanks to estrogen, the body can rely more on fat stores at lower intensities (e.g., Zone 2, <90 minutes). Carbohydrates are optional – as long as protein is present.
Progesterone changes the game: it increases energy demands and reduces stress resilience.
This makes it essential to add 10–15 grams of carbohydrates before training – a small banana or rice cake is often enough to stabilize hormonal responses.
For women with a body weight of approximately 60 kg, the following practical guidelines apply.
Fat oxidation training is only suitable for low-intensity endurance work. As soon as intervals, sprints, or VO₂max sessions are involved, carbohydrate demand spikes – regardless of cycle phase.
In the luteal phase especially, stress reactivity is heightened and energy needs increase.
Any kind of Train Low – whether through carb restriction or fasted sessions – becomes risky here.
Instead, focus on low-intensity sessions, targeted strength training, and full fueling support.
Train Low isn’t a lifestyle strategy. It’s a targeted tool. And like any tool, it works only when used at the right time, in the right context – especially for women.
The follicular phase is the optimal window: the body is primed for metabolic adaptation and less vulnerable to stress – as long as protein is included and energy balance is respected.
In contrast, the luteal phase demands energy availability, recovery, and hormone support.
Women training for long-duration, carbohydrate-depleting events – such as multi-day cycling races, ultramarathons, long-distance triathlons, or alpine traverses – absolutely need a well-developed fat metabolism.
In these races, glycogen stores simply don’t last. The ability to efficiently tap into fat is a cornerstone of endurance and performance.
For women like Clarissa taking on Ride or Die, strategically implemented fat oxidation training isn’t a liability – it’s a long-term advantage.
Fat oxidation isn’t off-limits for women. But it’s also not a universal solution.
The current scientific evidence is clear:
Women can train fat oxidation – but they must do it with intention.
✅ In the follicular phase
✅ Never fasted – always with protein
✅ Only at low intensity
Clarissa’s journey shows what’s possible when women train with their physiology, not against it.
Not less – just different.
Not controlled – but understood.
Not restrictive – but strategic.
‘Train Low’ isn’t a limit. It’s a tool. And used wisely, it becomes a strength.
Carter, S. L., Rennie, C., & Tarnopolsky, M. A. (2001). Substrate utilization during endurance exercise in men and women after endurance training. American Journal of Physiology-Endocrinology and Metabolism, 280(6), E898–E907.
Mountjoy, M., Sundgot-Borgen, J., Burke, L., et al. (2014). The IOC consensus statement: Beyond the Female Athlete Triad—Relative Energy Deficiency in Sport (RED-S). British Journal of Sports Medicine, 48(7), 491–497. https://doi.org/10.1136/bjsports-2014-093502
Sims, S. T., Kerksick, C. M., Smith-Ryan, A. E., Janse de Jonge, X. A. K., Hirsch, K. R., & Arent, S. M. (2023). International society of sports nutrition position stand: Nutritional concerns of the female athlete (Article 2204066). Journal of the International Society of Sports Nutrition. https://doi.org/10.1080/15502783.2023.2204066
Sims, S. T., & Heather, A. K. (2018). Myths and Methodologies: Reducing scientific design ambiguity in studies comparing sexes and/or menstrual cycle phases. Experimental Physiology, 103(10), 1309–1317. https://doi.org/10.1113/EP086797
Sims, S. T., & Yeager, S. (2016). ROAR: How to Match Your Food and Fitness to Your Unique Female Physiology for Optimum Performance, Great Health, and a Strong, Lean Body for Life. Rodale Books.
Tarnopolsky, M. A. (2008). Sex differences in exercise metabolism and the role of 17-beta estradiol. Medicine and Science in Sports and Exercise, 40(4), 648–654. https://doi.org/10.1249/Mss.0b013e31816212ff
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