Fasted vs. Fed Training: What’s Best for Your Body and Hormones?

Fasted cardio is having a moment. But like these things do in the nutrition world, there are just as many people screaming “DON’T DO IT!” as there are touting its benefits.  It’s being touted by influencers promising fat loss and debated among biohackers chasing metabolic flexibility, and it’s leaving many people—especially women—wondering: Should I eat before I work out or not?

As a Doctor of Clinical Nutrition and functional medicine expert, my team and I have guided thousands of individuals toward personalized wellness strategies. And like most things in functional medicine, the answer to whether fasted or fed workouts are better isn’t one-size-fits-all. It depends on your goals, hormonal health, and type of exercise.

This post unpacks what the research says about fasted vs. fed training, how your body responds in each state, and who should choose which strategy—and when.

What Is Fasted Training?

Fasted training typically refers to exercising without consuming calories beforehand—usually after an overnight fast of 8-12 hours. For most people, this means working out in the morning before breakfast. The theory behind fasted workouts is that, without readily available glucose, your body will dip into stored fat for energy.

What Is Fed Training?

Fed training involves consuming food—typically a meal or snack with carbohydrates, fat, and/or protein—before your workout. The idea is that providing fuel to your body will optimize performance, support recovery, and prevent stress-related metabolic consequences.

These two states affect how your body sources energy, recovers from workouts, and adapts over time.

The Science: Does Fasted Training Burn More Fat?

Let’s start with the popular claim that fasted training increases fat burning. There is some truth to this. A 2013 study published in the British Journal of Nutrition found that participants who performed moderate-intensity cardio in a fasted state burned more fat during the workout than those who exercised after eating. A 2018 review and meta-analysis concluded that fasted training leads to beneficial metabolic adaptations.  And a 2016 British Journal of Nutrition study shows that fasted training promotes acute fat oxidation, especially during aerobic exercise of moderate intensity.

However, the keyword here is acute. While fat oxidation may increase in the short term, multiple studies have shown that body composition changes over time (i.e., fat loss) are not significantly different between fasted and fed groups, as long as total calories are equated.

Translation: Fasted training may burn more fat during the workout itself, but this doesn’t necessarily lead to greater long-term fat loss.

Still, for those with metabolically stable bodies and balanced hormones, fasted workouts—especially low to moderate intensity ones—can help improve fat adaptation and metabolic flexibility.

The Performance Piece

Fasted workouts might be beneficial for some individuals with stable metabolism and hormone levels, but they often come at the expense of performance. A 2018 meta-analysis showed no difference in exercise performance for workouts less than 60 minutes, but that 54% of people improved performance when in a fed state and working out for longer than 60 minutes. So I recommend that when workouts exceed 45–60 minutes or involve higher intensity (think weight lifting, intervals, CrossFit, or long-distance running), performance may increase with feeding before.

Hormonal Impacts: Women vs. Men

Here’s where things get more nuanced—and more important. Much of the research on fasted training has been conducted on men, who have relatively stable hormone patterns day to day. Women, especially those of reproductive age or in perimenopause, experience cyclical hormonal fluctuations that significantly impact how the body responds to stress, including exercise stress.

According to Dr. Stacy Sims, an expert in female physiology and sports nutrition, fasted training can increase cortisol levels in women more significantly than in men, particularly when combined with other stressors like caloric restriction or poor sleep. Elevated cortisol over time can impair thyroid function, disrupt sex hormone balance, and increase fat storage—especially in the abdominal area.

This is especially true for women in perimenopause, or those with existing adrenal or thyroid dysregulation. While fasted cardio might initially seem like a “fat-burning hack,” it can actually backfire, putting the body into a conservation mode and leading to unwanted weight gain, fatigue, or hormonal maladaptation.

The Case for Personalization

So what’s the right choice? Like most things in functional medicine, the answer depends on you—your goals, your hormones, your stress levels, and your metabolic health.

Here’s how I break it down:

Fasted Training Might Be Appropriate If:

• You’re male or a postmenopausal female.
• You’re doing low to moderate intensity workouts (like walking, gentle cycling, or yoga).
• Your workout is under 60 minutes.
• You have good blood sugar stability and aren’t experiencing symptoms of adrenal or thyroid dysfunction.
• Your goal is to improve fat adaptation and metabolic flexibility
• You’re doing light morning movement like walking, yoga, or zone 2 training

The key is context: if you’re under-eating, overtraining, or in a state of hormonal imbalance, fasted training can do more harm than good.

Fed Training Is Better If:

• You’re a woman in perimenopause or with known hormonal imbalances.
• You’re doing high-intensity or long-duration training (>60 minutes).
• Your main goal is improving workout performance.
• You struggle with fatigue, poor recovery, or disrupted sleep.
• You’re intent to not lose muscle, or have noticed muscle catabolism lately.
• You’ve experienced plateaued weight loss or worsening body composition despite consistent effort.

Protein Timing: Post-Workout Priority, With Some Flexibility

Contrary to the old myth that there’s a tiny “anabolic window” for muscle gains, newer research shows that muscle protein synthesis (MPS) remains elevated for several hours post-exercise.

In most cases, getting 20–40g of high-quality protein within 30–60 minutes post-workout is the top priority for muscle repair and growth.

That said, for those working out in a fasted state—or who can’t eat soon after—a small amount of protein before the workout can help. Ingesting protein or amino acids beforehand increases circulating amino acids during the workout, which can reduce muscle protein breakdown and aid recovery.

Bottom line: Prioritize protein after your workout, but if you’re delaying your next meal, a small pre-workout protein dose can help bridge the gap.

What About Essential Amino Acids (EAAs) for Fasted Workouts?

If you’re training fasted but want to protect lean muscle mass, essential amino acid (EAA) supplements can be a smart tool.

EAAs include the nine amino acids your body can’t make on its own—including leucine, which is a key driver of MPS. Research shows that taking EAAs before a workout can stimulate MPS even in the absence of whole food protein.  When I do fasted training, I use Kion aminos (you can get a discount with code LYONS here).

Benefits of EAAs pre-workout:

• Reduce muscle catabolism
• Support muscle retention, especially during weight loss or fasted training
• May help improve focus and energy without breaking a fast in a traditional sense

Typical dose: 5–10g of EAAs, taken 15–30 minutes before training

This can be especially beneficial for women in perimenopause, individuals with low muscle mass, or athletes combining fasted cardio with strength training.

Final Thoughts and Key Takeaways

• Fasted workouts can enhance fat oxidation but aren’t a magic bullet for long-term fat loss.
• Fed workouts support better performance, hormone balance, and muscle retention—especially important for women and high-intensity training.
• Protein after your workout is essential; pre-workout protein or EAAs can help in certain cases.
• As always, the best approach depends on your unique metabolic and hormonal landscape.

The fasted vs. fed training debate doesn’t have to be an either/or. Both can be useful tools when applied appropriately. What’s more important is listening to your body, respecting your unique hormonal rhythms, and fueling in a way that supports—not sabotages—your long-term health.

If you’re unsure which approach is right for you, book a free consultation – our team can guide you through bioindividual strategies based on your goals and health history.

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️ Want to hear about this topic in audio format? → Check out the podcast episode here!