Should you exercise on an empty stomach? What to know about fasted workouts

The alarm goes off, it's barely light, and the question comes up again: eat something first, or head straight out the door? Fasted workouts have a loyal following, often with the promise that you'll burn more fat. This week, ABC's What's That Rash podcast worked through the evidence and landed in a similar place to the research: there's something to it, but it isn't clear cut.

Here is what fasted exercise means, what the research says, who should eat first, and a simple way to work out what suits you, plus how the Thrive Health Band helps you see how food and timing shape your workouts.

What fasted exercise means

Fasted exercise usually means a workout first thing in the morning, before breakfast, after the overnight gap since your last meal. A walk, a jog, a swim or a gym session at 6am with only water or a black coffee counts.

It isn't a special technique. It's simply exercising before you've eaten.

What the research says

Fuel use. When you exercise fasted, your body tends to use a little more fat for fuel during the session, because there's less carbohydrate from a recent meal to draw on. That's where the fat loss claims come from.

Over weeks, not one session. Reviews that compare fasted and fed exercise over several weeks generally find little difference in body composition when overall eating is similar. What you eat across the whole day matters more than whether breakfast came before or after the workout.

Performance. For longer or harder sessions, eating beforehand tends to help. Research reviews suggest that for prolonged aerobic exercise, being fed is linked with better performance, while for short sessions the difference is small.

How you feel. Some feel lighter and more comfortable training on an empty stomach. Others feel flat, light-headed or ravenous later, and end up eating more than they planned.

The NSW Institute of Sport sums it up for athletes: it depends. It suggests not training fasted for sessions longer than an hour, high intensity or power work, when performance matters, when you're trying to build muscle, or if fasting leaves you so hungry later that you overeat. A short, easy session with a good breakfast planned straight after is where fasted training is most likely to suit you.

If you'd rather eat first

Sports Dietitians Australia suggests choosing familiar foods that sit well, are higher in carbohydrate and lower in fat. Try different timings and snacks in training, not on race day. If nerves or an upset tummy are an issue, a liquid or low fibre option can help.

Easy options before a morning session:

• half a banana or a whole one
• a slice of toast with honey or jam
• a small tub of yoghurt
• a glass of milk or a small smoothie

Start well hydrated too, especially as the weather warms up.

Who should check before training fasted

If you live with diabetes, take medicines that affect your blood sugar, are pregnant, or have had a difficult relationship with food, talk with your GP or an Accredited Practising Dietitian before trying fasted exercise. The same goes if you've felt faint or shaky exercising before breakfast.

A simple way to find what suits you

Rather than following a rule, test it on yourself:

1. Pick one easy session you do regularly, such as a 30 minute walk or jog.
2. For two weeks, do it before breakfast. For the next two weeks, have a small snack first.
3. Keep the session the same, and notice how it felt, your energy through the morning, your hunger later and how you slept.

How Thrive helps you see what suits you

The Band has no screen, so there's nothing to check mid-run. Wear it day and night, and the Thrive by Optihealth app brings food, workouts and how you feel together.

• Meals and Food Fit. Log meals in Nutrition, scan barcodes or use Food Fit. Thrive learns your usual first meal time and eating window, so fasted mornings are easy to spot.
• Workouts and heart rate zones. Each workout records your heart rate and time in each zone, so you can compare how hard the same session felt fasted or fed. See heart rate zones explained.
• Recovery insights. After a session, Thrive gives an estimate of how long until you're back to your usual, such as "you need 5 hours rest".
• Check-ins. Log mood, stress and energy, and add your own factor such as "trained before breakfast" so it shows up in your patterns.
• Experiments. Set up the four-week test above as an experiment and review the outcomes at the end.
• Ask Thrive. Try "How did my energy check-ins compare on mornings I trained before breakfast?" or "Was my workout heart rate different on fasted days?"

If you work with a trainer, Thrive Loops lets you share selected information with a participating trainer, health professional or someone you trust. You decide what to share and with whom, and you can change or revoke access at any time.

Thrive also learns what is usual for you across nine areas of your health, from what the Band measures and what you log, and keeps track right across your health, including heart, temperature, stress and mood, meals and Food Fit, blood tests, medications and supplements and your medical history.

Before you buy

You'll need a compatible iPhone or Android phone, because the Thrive app is on the Apple App Store and Google Play, in Australia and eight other countries. The app stores are only for downloading the app, though; they aren't where you buy the Band or a Plan. You can buy at thrivehealthband.com, or at a participating World Gym Australia location or health and wellness clinic.

The Thrive Health Band is A$660 for the first year under the launch offer. That includes the Band, which is yours to keep, and 12 months of the Thrive by Optihealth app, My Health on the web, Health Fingerprint, Ask Thrive, Thrive Loops and your data, paid once upfront. Prices are in AUD and include GST. Free Australia-wide shipping during the launch offer (usually A$14.99).

Nothing renews automatically. After your first year, you can get a new band plus 12 more months for A$660, or keep your band and renew the Thrive Connected Plan for A$240 a year, or A$29 a month with no lock-in.

You're also covered by a 12-month Thrive Care Warranty, and consumer guarantees under Australian Consumer Law always apply. See Returns if something is wrong with your purchase.

Common questions

Is it better to work out on an empty stomach?

Not for everyone. Fasted exercise can suit short, easy sessions, but eating first tends to help longer or harder workouts. Over weeks, overall eating matters more than timing.

What should I eat before a morning workout?

Something small, familiar and higher in carbohydrate, such as a banana, toast with honey or yoghurt, with water.

Can Thrive show how food affects my workouts?

Yes. Log meals and add a check-in factor, then Ask Thrive can compare your energy and workout heart rate on fasted and fed mornings.

For more answers, see Questions.