Why exercising on an empty stomach won’t necessarily burn more fat, and other weight-loss myths


The past year and a half has been challenging for everyone, with a new level of stress that many of us have never experienced before.

A common response to stress is to snack, to reach for the comfort of a delicious bag of salty chips or a block of rich, sweet chocolate.

Combine that with lockdowns and your jeans might be feeling just a little tighter, or if you’re like me, you’re living in track pants anyway.

One day, maybe soon, you might want to put those jeans back on.

But don’t get tricked by untruths on the internet about burning fat through exercise.

Let’s unpack three common myths around exercise and weight loss.

First up: what is fat?

Body fat takes many forms, but visceral fat, which sits around your organs, and subcutaneous fat, found just under your skin, are the two that many people want to reduce through diet or exercise.

Pink and yellow blobs seen under a microscope
Almost the entire volume of each fat cell or adipocyte consists of a single droplet of fat.(

Getty Images: Steve Gschmeissner/Science Photo Library


Your fat deposits grow when you consume more energy than you use and, conversely, shrink when you consume less energy than you use.

To deplete these stores, you burn or “oxidise” fat. This provides energy for your body to do things such as move your muscles during exercise and repair your body tissues afterwards.

Myth 1: I should exercise in the ‘fat-burning zone’ to burn the most fat

If you’ve even jumped on a treadmill or cross-trainer at the gym, you may have spied a setting that tells you how hard you should exercise if you want to burn fat.

The premise of the “fat-burning zone” is based on the idea that, if you exercise at a particular intensity, your body will preferentially use fat as fuel.

While these zones may vary a little, they tend to suggest you should exercise at between 60 and 70 per cent of your maximum heart rate (which you can roughly calculate if you subtract your age from 220) to burn the most fat.

For most people, this feels like a pretty easy workout.

But is it too good to be true?

Row of people running on treadmills in gym
Should you step outside the “fat-burning zone” to keep the kilos off in the long run?(

Getty Images: Erik Isakson


If your goal is the lose fat in the long term, I have a bit of bad news for you.

While it is correct that you will burn more fat while exercising at the “fat-burning” intensity level compared with a much higher or lower level, it is only part of the picture.

This misunderstanding comes down to extrapolating data from research that tells us what’s happening at a certain point, like a snapshot, compared with monitoring changes over weeks or months, that mirror real life.

What happens when you stop exercising counts too.

For example, when you exercise at a higher intensity, you use more of other fuels, such as carbohydrates stored in muscle.

But when you finish exercise, you then use additional energy to replenish or restore these levels to normal.

You may also use a little more energy in recovery from high-intensity exercise — a phenomenon known as the oxygen debt — and your muscles may also need a little more again to recover from greater levels of damage during higher intensity exercise.

The result? Little difference in overall fat loss between training in the “fat-burning zone” and training at a higher intensity.

And there are notable benefits to high-intensity training, and it’s not just because it’s more time-efficient (because you’re burning the same amount of energy in a shorter period).

For instance, you retain more muscle and potentially even grow more muscle with high-intensity exercise, something that doesn’t happen with long-duration low-intensity exercise, particularly if you are eating less and exercising more in an attempt to lose body fat.

This doesn’t mean that you shouldn’t do low-intensity exercise, or exercise within the fat-burning zone. You may be recovering from an injury, having a light exercise day, or easing into a new exercise program.

It just means that you should know that doing so isn’t necessarily best for “fat loss”.

Myth 2: Exercise on an empty stomach for maximum fat loss

Have you heard this one? Exercising first thing in the morning, before you eat breakfast, will help you lose body fat.

The idea is if you’ve not eaten, fat is the only available fuel for your body to use. That pesky fat will melt away — at least, that’s what the internet tells us.

Like the previous myth, while there’s a kernel of truth here, it’s not that simple.

When you exercise in a fasted state — after a night’s sleep and before you eat breakfast — then you do actually use more fat as a fuel source during exercise.

Woman sitting on bed tying her shoe laces
Eating breakfast before exercising won’t undo all your hard work. (

Getty Images: Geber86


But research from my team has shown that performing exercise in a fasted or fed state does not have any meaningful impact on body fat in the medium to long term.

So why is this? If you exercise on an empty stomach, your body might compensate by burning less fat after you finish exercising and eat a meal. This effectively balances out the overall levels of fat you use as fuel.

Interestingly, eating before exercise seems to increase the amount of energy you use after exercise.

However, there is a caveat here too. Again, most of this research looked at short-term effects. We don’t know if that translates to you losing more fat in the long term.

So, if exercising on an empty stomach is something you like to do, then go ahead and do it.

But don’t do it thinking that you are getting added benefits.

Myth 3: If I want to lose fat, I need to do cardio

Another common conversation among exercisers is that if you are trying to lose body fat, then you need to focus on cardiovascular exercise.

Well …

The biggest impact on fat loss comes from better eating and drinking habits, rather than exercise — cardio or otherwise.

Programs that combine “nutritional change” with consistent exercise seem to have the most benefit.

Young woman taking a break after exercising outdoors
It’s not all about cardio when it comes to torching fat. (

Getty Images: PeopleImages


Of course, exercise is part of a healthy lifestyle, regardless of weight-loss goals. It just may not be having the effect on your waistline that you think.

And you may not have to go with cardio if you are trying to lose fat.

In research that’s yet to be published, my team found strength training had a modest fat-loss effect in people who were not actively trying to lose weight.

The amount of fat loss we measured was similar to that found in studies looking at fat loss in continuous aerobic exercise — or even high-intensity interval aerobic exercise.

So what should I do?

The key to successful fat loss is not through exercise alone, but also creating a meaningful caloric deficit over time.

In other words, energy in, energy out — which is a pretty good overall summary of how to go about fat loss.

And the takeaway message: there’s no quick fix.

Fat burned during exercise is only part of the complex puzzle of metabolism and weight loss — but there are many ways you can reach your goals.

Just find the one that works for you, and don’t get sucked in by everything you read online or what you overhear at the gym.

Dr Mandy Hagstrom is an exercise scientist at the University of New South Wales.

She is also one of the ABC’s Top 5 scientists for 2021.


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