Fasted cardio is exactly what it sounds like: doing cardiovascular exercise—usually in the morning—before you’ve eaten anything. The thinking is simple: if you train without food in your system, your body will burn more fat for fuel, helping you lean out faster.

For men, especially those who are metabolically healthy, this approach can sometimes support fat oxidation and insulin sensitivity. But for women, fasted training taps into a far more complex physiological network—one shaped by hormonal cycles, energy availability, and stress responses that differ dramatically from men’s.

As respected female physiology researcher and scientist Dr Stacy Sims puts it: “Women are not small men.” And fasted training is one area where that distinction can have profound implications.

How female physiology responds differently to fasted training

The female body is not only wired differently from the male body—it’s wired to protect reproductive function, maintain hormonal balance, and respond differently to stress. Training without fuel can act as a significant stressor, and how a woman’s body reacts depends on where she is in her menstrual cycle, whether she’s peri- or postmenopausal, and her overall health and training status.

The cortisol conundrum

Fasting and intense training both elevate cortisol, the body’s primary stress hormone. In women, especially those in their reproductive years, chronically high cortisol can trigger a cascade of effects: menstrual irregularities, thyroid suppression, increased fat storage (especially abdominal fat), and even bone density loss.

According to Dr Sims, fasted training often leads to greater cortisol spikes in women, which in turn reduces the effectiveness of the training session and can impair recovery. Over time, this may hinder progress rather than enhance it.

When we start talking about fasted training, women are already maximally capable of burning fatty acids from sex differences in the muscle cell, and also estrogen exposure encourages fatty acid utilisation.

Dr Stacy Sims

Menstrual cycle matters

Hormonal fluctuations across the menstrual cycle mean women’s bodies shift how they use energy. During the follicular phase (day 1 to ovulation), estrogen levels rise and insulin sensitivity improves. This is the best window for higher-intensity training, and some women may tolerate low-level fasted cardio better here.

But during the luteal phase (post-ovulation), progesterone climbs, insulin sensitivity drops, and the body tends to rely more heavily on carbohydrates for fuel. Training fasted in this phase is more likely to spike cortisol, reduce performance, and negatively impact recovery.

This cyclical fuel usage makes a one-size-fits-all fasted training plan ineffective—and potentially harmful—for many women.

What about body composition and fat loss?

Let’s address the elephant in the room: does fasted cardio help women lose more fat?

The short answer: not significantly—and not sustainably.

Most well-controlled studies show that, when calories and activity are equal, fasted and fed cardio result in similar fat-loss outcomes. What matters more is overall energy balance, quality of nutrition, sleep, stress levels, and consistency.

But in women, especially active women, the margin for error is smaller. If fasted cardio leads to under-fuelling, reduced training quality, or elevated cortisol, the long-term results may be worse, not better.

Dr Sims explains that, for many women, fueling before training—especially with some carbohydrate and protein—can improve workout intensity, recovery, and lean mass retention, all of which support better body composition over time.

Is there ever a place for fasted cardio?

Yes—with caveats. Fasted training might be appropriate for some women if:

• The intensity is low (e.g. zone 1 walking)
• The session is short (30 minutes or less)
• They’re in the follicular phase
• They’re well-fuelled and not under stress
• It’s done occasionally, not daily

For example, a morning walk before breakfast might help enhance insulin sensitivity and support circadian rhythm health. But for moderate to intense sessions—especially strength or HIIT workouts—eating something beforehand is usually the smarter choice.

Dr Sims recommends a small snack with 15–20g of carbs and 10g of protein, such as:

• A small banana with a spoon of peanut butter
• A rice cake with cottage cheese
• A small protein shake with half a banana

These simple additions can blunt cortisol, improve performance, and enhance post-workout recovery.

What the critics say

Not everyone agrees with Sims’ take. Some coaches argue that fasted training teaches the body to use fat more efficiently, and that it can be psychologically beneficial for people who feel nauseous eating before a workout.

There’s also evidence that endurance athletes—especially those training for long races—can benefit from occasional fasted sessions to improve metabolic flexibility.

But these are niche cases. For the majority of women, especially those balancing careers, families, stress, and real-life training goals, performance, recovery, and hormonal health outweigh any marginal gain in fat burning.

The bottom line

Fasted cardio is not a magic bullet for fat loss. For women, it may carry more risks than rewards—especially when done frequently, intensely, or during certain phases of the menstrual cycle.

Dr Stacy Sims’ work highlights a bigger truth: women deserve training and nutrition guidance that respects their physiology. Fasted training can work in some contexts—but it’s not the default, and it’s not without trade-offs.

The smarter approach? Fuel appropriately, train in sync with your cycle, prioritise recovery, and listen to your body.

To watch Dr Stacy Sims talk exclusively to Unfiltered about fasted cardio for women, click here.