Do Fasted Workouts Really Burn More Fat? Here's What Science Says

Every few years, fasted exercise training becomes the latest talking point across social media.

In this context, fasted training usually means doing your workout first thing in the morning, before you have breakfast.

Supporters argue it is the most effective route to losing body fat. Critics insist it is a bad idea that could even make you put on weight. So what does the evidence actually show?

The headline answer: no clear winner for weight loss or sports performance

When you look across the research, there is no convincing “best” approach for everyone.

The current evidence does not demonstrate that fasted exercise is superior for weight loss or sports performance. At the same time, the evidence generally does not suggest fasted training is harmful in most everyday scenarios (with a possible exception in some aspects of elite athletes’ performance).

Where did the idea come from?

Advocates of fasted exercise often claim it drives bigger improvements in body composition - the balance of lean mass (muscle), bone and fat. More specifically, the promise is that training without breakfast leads to greater fat loss.

It’s worth remembering that “better” body composition can occur in several ways: you might lose fat mass while maintaining lean mass, lose fat while gaining lean mass, or even gain lean mass without any drop in body fat. Any of these outcomes can be viewed as positive, depending on the goal.

The rationale for fasted exercise comes from studies showing that metabolism behaves differently when you exercise after eating compared with exercising before you eat.

For example, doing aerobic exercise in a fasted state tends to increase how much fat you use as a fuel at that moment - what researchers call fat oxidation - when assessed at a single point in time.

From there, it was an understandable (but not guaranteed) step to assume that higher fat oxidation during a workout must lead to greater fat loss over weeks and months.

However, a 2017 systematic review produced by my team found that a fasted exercise training programme does not appear to produce meaningful long-term differences in body fat loss compared with training after eating.

This mismatch - between fat used as fuel during a workout and actual long-term change in body fat - is commonly misinterpreted.

A likely explanation is compensation. Once you eat, fat burning may drop back, and people who train hard can sometimes end up expending less total energy across the rest of the day (for instance, moving less without realising it).

More broadly, exercise science often finds that immediate responses do not reliably predict long-term outcomes.

As one example, a single bout of very intense exercise can temporarily suppress aspects of immune function, yet regular exercise over time is associated with beneficial effects for the immune system.

What does eating soon after - or just before - your workout do?

Having a meal containing carbohydrates and protein close to training is likely to support performance in the next exercise session.

But whether you eat that meal before your workout or shortly afterwards seems to make only a small difference overall.

Interestingly, research suggests that shifting a larger share of your daily food intake to the morning - and, in particular, increasing morning protein - may help improve body composition and support weight loss.

That said, this is about when you eat during the day rather than meal timing specifically around exercise.

A practical addition: hydration, caffeine and comfort

One factor often missed in online debates is hydration. Even if you skip breakfast, taking on water before and after training can improve comfort and help you feel better during exercise - especially if you wake up mildly dehydrated.

Some people also use caffeine (for example, coffee) before morning sessions. That can help perceived effort and performance for some, but it can also aggravate nausea or jitters for others - so it is best treated as optional and individual.

What about sports performance?

The evidence is fairly consistent that eating before exercise improves performance in activities lasting longer than 60 minutes, while it has little effect on shorter sessions.

This aligns with what athletes actually do. You can see it in the limited support for fasted exercise among top performers: one survey of almost 2,000 endurance athletes found that non-professional athletes were more likely to train fasted than professional athletes.

What about strength training?

So, does fasted versus fed training change outcomes for muscle strength, size and body composition when doing resistance training (such as weightlifting)? Unfortunately, the evidence base here is small and generally low quality.

What we have so far suggests there is no meaningful difference.

One recent randomised controlled trial reported no differences in strength, power or lean body mass when participants completed resistance training twice per week for 12 weeks, either after fasting or after eating.

Another consideration: who should be cautious?

While most healthy people can choose either approach, fasted sessions may be less suitable for some individuals - for example, people prone to dizziness, those with a history of disordered eating, or anyone managing conditions where blood glucose stability matters. In those cases, eating something small before exercise may be the safer and more comfortable option.

What are the potential drawbacks?

Training without eating can leave you extremely hungry afterwards, which may make it easier to reach for less nutritious options later in the day.

Some people also experience headaches or nausea when they attempt fasted exercise. This is not universal, though; plenty of people report feeling excellent when exercising while fasting.

So what should you do?

If time is tight and skipping breakfast is the difference between getting your run or workout done and missing it, then fasted training can be a perfectly workable choice - and you probably do not need to worry about major negative consequences.

On the other hand, if the thought of exercising on an empty stomach puts you off going to the gym, have breakfast first. You can be confident that eating beforehand is unlikely to undermine your goals.

Exercise fashions and wellness “hacks” will keep cycling in and out of favour, but the message supported by solid, consistent evidence is simpler: exercising matters most.

Not the time of day, not the exact workout choice, not even the precise amount - and certainly not whether you have eaten immediately before you exercise.

By Mandy Hagstrom, Senior Lecturer in Exercise Physiology, School of Health Sciences, UNSW Sydney.

This article is republished from The Conversation under a Creative Commons licence. You can read the original article on The Conversation website.