The “Fasted Cardio” Debate: Is It Right for Women?

Fasted cardio—an approach where you hit the gym on an empty stomach—has been touted as a quick route to torching fat. But what’s the truth?

Some research does suggest that fasted cardio can be effective for fat loss, but it’s not so cut-and-dried, especially for women. Women, with complex hormonal cycles, don’t always respond optimally to fasted cardio. Here’s what you need to know, with insights from obé instructor and registered nutritionist Kat B.

What is Fasted Cardio, and Why Do People Do It?

“Fasted cardio is when you work out on an empty stomach,” says Kat. “A fasted state typically requires you to not eat any food for about 10-12 hours.” Usually, that means you hit your workout class in the morning before you eat breakfast.

But what about hunger?! How are you going to fuel those gains? That’s the point. In theory, working out in a fasted state can lead to greater fat loss by forcing your body to use stored fat for energy (also known as fat oxidation). Instead of burning through your breakfast, you’ll be mobilizing fat. 

A 2016 study published in the British Journal of Nutrition reviewed studies and found that in 27 separate studies, people who performed fasted cardio experienced a significant increase in fat oxidation compared to people who had eaten beforehand. A review of studies, also known as a meta-analysis/systematic review, is considered the “gold standard” of research. So it’s pretty safe to say that fasted cardio is effective at burning fat.

But before we close our books and conclude that fasted cardio is right for everyone, let’s take a deeper dive into the mechanisms of how it works and whether it’s actually suitable in every scenario.

The Downsides of Fasted Cardio

“There have been no major findings that fasted cardio is better for overall health,” says Kat. When you consider whether to start a new health intervention, it’s important to look at the full picture. While fasted cardio may burn fat, it doesn’t mean that it’s objectively better than other types of exercise or other ways of approaching exercise. 

For example, you may burn more fat during the workout, but how much fat do you burn over the course of 24 hours? When you fuel your workouts, you can push harder and potentially create excess post-exercise oxygen consumption (EPOC).

With EPOC, you continue burning fat for up to 24 hours post-workout. But in order to create EPOC, you need to work really, really hard. And most people can only do that if they have sufficient energy (aka have enough fuel in their bodies).

“Not eating before a workout can lead to decreased performance and the increase of muscular breakdown,” warns Kat. In other words, skipping that pre-workout snack might leave you feeling sluggish, causing you to cut your session short or lift lighter than you otherwise could. Over time, that can actually undermine muscle gains and overall metabolic health. 

Factor in a woman’s unique hormonal cycles and the added stress from fasted training, and you can easily tip the scales from “healthy challenge” to “hormonal havoc.” That’s why looking at the broader context—your mood, energy levels, and recovery—is crucial before committing to a fasted regimen.

And while a systematic review is the gold standard of research, you’ve got to take a deeper look into each study that was used.

For starters: How many men vs. women were used in each study? For the systematic review mentioned, 80% of the studies only analyzed men, while just 13% analyzed women, and 6% analyzed both sexers. Why does this matter? Men and women have different biological makeups, corresponding to unique hormonal patterns and responses.

Why Men and Women Respond Differently to Fasted Cardio

Men and women have distinct hormonal rhythms. While men follow a mostly 24-hour pattern, women’s bodies operate on the infradian rhythm (their monthly menstrual cycle), meaning that they have different levels of certain hormones and certain points in the day and month.

Fasted cardio layers two stressors—fasting (nutritional stress) and exercise (physical stress)—onto this already complex hormonal landscape. Because cortisol is naturally higher in the morning, working out on an empty stomach can keep levels elevated longer.

This can disrupt menstrual cycles, alter thyroid function, and promote fat storage, especially in women who are more sensitive to these fluctuations due to estrogen and progesterone’s direct influence on stress hormone regulation.

By tapping into glycogen stores too aggressively, fasted workouts may also trigger the body to interpret a “famine” scenario, lowering thyroid hormone activity and leading to fatigue, irregular cycles, or difficulty maintaining muscle.

Kat concludes: “When women engage in fasted workouts it can disrupt blood sugar balance, decrease thyroid activity, and promote fat storage.”

While occasional fasted cardio may be fine for some, women’s hormones often respond more intensely, making it easier to tip from healthy hormetic stress into imbalance.

READ MORE: Intermittent Fasting for Women—Yes or No? A Nutritionist’s Ultimate Verdict

Better Options for Women

Curious about using fat as fuel without risking hormonal chaos? Kat suggests steady-state (zone 2) cardio:

• What It Looks Like: Jogging, incline walking, or moderate dance cardio.
• Intensity Level: 60–70% of your maximum effort—enough to break a sweat but not so intense that you can’t chat with your workout buddy.
• Why It Works: This pace encourages fat oxidation without pushing cortisol through the roof.

READ MORE: The 5 Heart Rate Zones, Explained—Plus Aerobic vs. Anaerobic Fitness

If you’re aiming for fat loss while staying mindful of hormonal health, this approach can be a win-win. You get a solid cardio session without the potential downsides of fasted training—like compromised energy or elevated stress hormones.

The Bottom Line

Fasted cardio can be alluring, especially when buzzwords like “fat oxidation” come into play. But while it may benefit some people, the science suggests that for most women, the potential hormonal disruptions might not be worth the gamble.READ MORE: Stop Watching “What I Eat in a Day” Videos—Here’s Why