Should You Work Out on an Empty Stomach? The Truth About Fasted Exercise and Your Metabolism
One of the most common questions I hear from clients, especially early morning exercisers or those following intermittent fasting is this:
Should I eat before or after I work out?
If you’ve ever rolled out of bed, skipped breakfast, and jumped straight into a workout, you’ve already tried fasted training, whether you realised it or not. But is exercising on an empty stomach a smart strategy for fat loss or metabolic health? Or does it just leave you under fuelled and under performing?
Let’s take a grounded look at the science behind fasted exercise and what it actually does to your body, metabolism, and results.
What Is Fasted Exercise?
To be clear, fasted exercise isn’t just “I skipped lunch.” It refers to working out after 10–14 hours without food, usually first thing in the morning after an overnight fast. At this point, your body has fully digested and absorbed your last meal, and you’re officially in what’s called a postabsorptive state.
Without fresh fuel in the tank, your body is forced to get energy from stored sources, mainly fat and a bit of glycogen. And that’s where fasted exercise earns its reputation.
What Happens to Your Body During Fasted Workouts?
Here’s a simple way to visualise it: think of your body as a hybrid car.
The petrol tank is your glycogen stores (from carbs).
The electric tank is your body fat.
When you’re working out fasted, your body leans more heavily on the electric tank, burning a greater proportion of fat for energy. This switch helps train something called metabolic flexibility your body’s ability to shift smoothly between carbs and fat as fuel.
But that doesn’t mean you’re only burning fat. Most workouts still rely on a mix of fuel sources. However, in a fasted state, the contribution from fat is higher, and you tend to preserve glycogen for more intense moments (like sprints or resistance work).
The Potential Benefits: Fat Burning, Insulin Sensitivity, and Flexibility
So what’s the upside of training fasted?
Increased fat use during exercise
Improved insulin sensitivity
Greater metabolic flexibility
Potential for better blood sugar control
Your body also produces more free fatty acids and ketone bodies during a fast, which can support brain function and metabolic health. Over time, these adaptations may help support long-term wellness, especially in people at risk of metabolic disorders.
But Does It Lead to More Fat Loss?
This is where things get real.
Yes, fasted workouts burn more fat during the session. But no, studies haven’t found they lead to greater overall fat loss. Why? Because fat loss depends more on total energy balance across the whole day, not just what fuel you burned in a single session.
A 2016 meta analysis found that while fat oxidation was higher in fasted cardio, there were no long term differences in body fat reduction. Another study found no fat loss advantage between fasted and fed training over six weeks.
So while fasted exercise might feel like you’re tapping into fat stores more efficiently, the bigger picture still comes down to: calories in vs. calories out.
Does Fasted Exercise Impact Performance?
Here’s where fasted training can hit a wall, your performance.
The research suggests:
Workouts under 60 minutes? You’re probably fine training fasted.
Longer or high intensity sessions? You may see a drop in performance, endurance, or output.
That’s because glycogen fuels high performance. When it’s low, you’re likely to fatigue quicker, struggle to hit higher intensities, and feel less powerful.
If you’re lifting heavy, building muscle, or racing, eat first. Fasted training won’t give you an edge here.
The Bottom Line
Fasted exercise has its place. It may help your body become more metabolically flexible and efficient at using fat for fuel, but it’s not a magic bullet for fat loss.
If your priority is performance, strength, or muscle gain, eating before you train will likely serve you better.
More important than when you eat is that you’re training at all. Regular physical activity, whether fasted or fed, is the biggest driver of health, longevity, and metabolic function.
If you’re curious to experiment with fasted training:
Start with low to moderate intensity workouts (like a walk, gentle cycle, or light strength session).
Do it after an overnight fast, around 10–12 hours is ideal.
Stay hydrated and fuel properly after the session to support recovery.
As always, listen to your body. Try it, track how you feel, and adjust. You don’t need to follow trends, you just need to find what works for you.
👟 Need help finding your ideal training routine or metabolic rhythm? I offer personalised coaching for fitness, fat loss and energy. Head to https://www.jezluff.com/application-form-online and let’s get started.