Training on an empty stomach: benefits, risks, and how to do it right

The first thing many people do upon waking up is slip on their sneakers and go for a run, or head straight to the gym before breakfast. Some do it because they're not hungry that early, others because they've read it helps burn more fat. But is there scientific basis for this? Or is it just another fitness myth that sounds good but doesn't hold up?

The honest answer is: it depends. And that uncomfortable answer is the most useful, because fasted training is neither good nor bad in and of itself. It's a tool. Like almost everything in nutrition and sports, it works for some people in certain circumstances, and may be counterproductive for others.

In this article, we'll look at exactly what happens in your body when you work out without eating, what the current scientific evidence says, for whom it makes sense, and how to implement it if you decide to give it a try.

## What Happens in Your Body When You Train Fasted

When you go several hours without eating — typically 8 to 12 hours since your last dinner in the morning — your blood glucose and insulin levels are low. Muscle glycogen isn't completely depleted, but your liver has fewer reserves than usual.

In that context, when you start exercising, your body has to look for energy somewhere. With less circulating glucose available, it turns more readily to fatty acids. In other words, it mobilizes fat for fuel.

This is real. It's well documented. An analysis published in *Nutrients* based on data from 51 studies confirmed that fasted aerobic exercise, especially at low and moderate intensities, significantly increases fat oxidation. Another study reported increases of up to 50% in fat oxidation during the 24 hours following exercise compared to the same workout performed after eating.

However, burning more fat during a workout is not the same as losing more fat in the long term. This nuance is crucial and we’ll discuss it further on.

## The Real Benefits of Fasted Training

**Greater Mobilization of Fat as Fuel**

This is the most documented benefit. At low and moderate intensity — below 65% of your VO2 max — a fasted body reaches for fatty acids more quickly. This is useful for endurance athletes who want to train their metabolism to use fat efficiently, which is especially relevant for long races where glycogen is scarce.

**Improved Metabolic Flexibility**

Regular fasted training can improve your body's ability to switch between carbohydrates and fats depending on demand. Long-distance runners and cyclists have been using this resource for decades in their easy morning sessions, precisely to avoid relying solely on glycogen.

**Insulin Sensitivity**

Exercise itself already improves insulin sensitivity. Doing it fasted may boost this effect, since the body is processing energy without high insulin present. For people prone to insulin resistance, this could be a relevant factor.

**Practicality and Digestive Comfort**

Not everything needs a molecular basis. Many people simply perform better without eating beforehand. It makes the workout lighter, with no half-digested meals or feeling of heaviness. And if fasted training is what lets you show up consistently, that's already a huge benefit.

## The Risks No One Fully Tells You About

**Muscle Loss: Myth or Reality?**

This is the most widespread fear, and recent evidence clarifies it quite a bit. The 2025 review published in Scielo concluded that protein catabolism does not significantly increase during moderate-duration and moderate-intensity fasted exercise sessions. In other words: with a reasonable session, you're not going to "eat up your muscles."

The problem arises with long sessions, high intensity, or when fasting is prolonged too much. If you've gone 16 hours without food and do a demanding strength session, the risk goes up. Also if your usual diet is already low in protein.

The practical solution: if your goal is to maintain or build muscle, opt for fasted sessions of less than 60 minutes at moderate intensity, and be sure to eat quality protein immediately afterwards.

**Hypoglycemia and Dizziness**

For most healthy people, the body perfectly regulates blood sugar during fasted exercise. But some individuals, especially those more sensitive to glucose changes, may feel dizzy, weak, or even become hypoglycemic.

If you've never trained fasted before, start with short sessions and see how your body reacts. It's nothing to be scared of, but definitely something to monitor.

**Cortisol and Accumulated Stress**

There's a lot of noise about this on social media. Yes, cortisol is higher in the morning (that's normal, part of the circadian rhythm) and exercise itself also raises it. Fasted training can cause a slightly higher cortisol spike.

The real problem isn't that one-time spike, but when it's combined with chronic stress, little sleep, and a high frequency of intense training. In that context, fasted training can be an unnecessary extra load. But for a well-rested person, without excessive stress and doing moderate sessions, it shouldn't be an issue.

**Important Contraindications**

People with type 1 diabetes, reactive hypoglycemia, eating disorders, or who are pregnant should not train fasted without medical supervision. Nor should those with a history of anemia or frequent drops in blood pressure.

## Who It Works For—And Who It Probably Doesn't

It works well for people doing low to moderate intensity morning cardio, endurance athletes looking to optimize fat metabolism, those practicing intermittent fasting who fit their workout into the fasting window, and people who simply feel better not eating beforehand.

It's probably not ideal for people doing high-intensity training (HIIT, CrossFit, heavy strength sessions), athletes already on an aggressive caloric deficit, those who struggle to perform without breakfast, and people with the aforementioned medical conditions.

## Practical Protocols According to Your Goal

This is the part most articles don't flesh out: there’s no single protocol. It depends on why you train.

**If Your Goal Is Fat Loss**

Low to moderate intensity fasted cardio (brisk walking, easy cycling, slow-paced swimming) for 30-45 minutes can be useful as a complement. It's not a miracle tool, but it can accelerate fat mobilization if the rest of your diet is controlled. After training: have a meal with protein (20-30g) and moderate carbs to prevent catabolism and replenish glycogen.

**If Your Goal Is Endurance**

Endurance athletes can do their long, easy-paced sessions fasted 1-2 times per week. The goal isn't to lose weight but to teach the muscle to work with less available glucose. Quality sessions (intervals, tempo runs) should always be fueled.

**If Your Goal Is Strength or Muscle Gain**

Here, fasted training makes less sense. Strength performance depends greatly on muscle glycogen and circulating amino acids. If you have to choose, it's better to eat something light beforehand — even just a banana and coffee — than to do a hard workout totally fasted. If you still want to try, keep sessions short (less than 45 minutes) and recover immediately with protein.

## What to Eat Afterward to Maximize Benefits

This point really changes the equation and is where most people mess up. If you're going to train fasted, your post-workout meal is more important than usual.

What works: fast-digesting protein (scrambled eggs, Greek yogurt, protein shake, about 25-35g), moderate glycemic index carbs (oats, fruit, rice) to replenish glycogen without causing a huge insulin spike. And avoid extending the fast for several hours after exercise: your body is in a catabolic state and the longer that lasts, the more muscle could be compromised.

A real-life example: María, a running coach, does her easy morning runs fasted three times per week. When she gets home, she has 3 scrambled eggs with avocado and toast for breakfast. She’s been doing this for two years, has improved her marathon time, and has perfectly maintained her muscle mass. The key: sessions no longer than 50 minutes, controlled intensity, and immediate breakfast afterward.

## Conclusion: Useful Tool, Not Dogma

Fasted training has real scientific basis. It's not a myth, but it's also not the ultimate solution for fat loss or peak performance. It's a tool that, when used correctly, can provide specific metabolic benefits and fit well with some lifestyles.

If you want to try it, start slowly. Short sessions, moderate intensity, and pay attention to how your body responds. If you feel good, performance is stable, and you’re not feeling dizzy: go for it. If you struggle to concentrate, run out of energy halfway, or end up much hungrier than normal, maybe it’s just not for you.

And that's perfectly fine. There’s nothing inherently superior about training fasted compared to eating breakfast first. What matters is that the training happens, that it’s consistent, and that it fits your real life.