Fasted Cardio: The Red Flags To Be Aware Of

For the sprightly early risers among us, those who are fans of intermittent fasting (or simply aren’t hungry first thing in the morning), or those who are training with fat loss goals in mind, it’s common to wonder whether exercising fasted or fed is the way to go.

Many people believe that if you work out on an empty stomach, your body turns to fat as a source of fuel, but it’s not quite that black and white. When it comes to nutrition and exercise habits that will have you feeling your best, fasted exercise does come with a few red flags - especially for women - and giving yourself even a small pre-workout snack can help tremendously.

It’s true, fat-burning may increase when fasted

Yes, fasted exercise can increase fat burning when your body doesn’t have recently consumed carbohydrates to use as fuel in the form of glycogen. According to the Australian Institute of Fitness, this increased fat oxidation process is also thought to happen during fasted exercise because your insulin levels are lower, allowing for greater ulitlisation of your fat stores for energy and improved metabolic health.

But… and there are a few buts!

The exercise intensity matters

A 2016 review of 27 studies confirmed that low-to-moderate intensity exercise in a fasted state can induce an increase in fat oxidation, with the intensity of the movement being a key call-out. Fat oxidation when fasted tends to be higher when the exercise intensity is lower - think between 40 and 65% of your VO2 max. So if that’s the aim of the game for you, a HIIT workout or heavy lifting session doesn’t fit the bill. We’re talking about something like a morning walk or a leisurely bike ride.

A 2020 review also highlighted that more research needs to be done to understand the effects on overall physical performance and health, with researchers advising that fasted high-intensity training should be avoided.

Overall, your energy balance matters more

At the end of the day, if losing fat is your goal, increased fat-burning during the time that you’re exercising won’t make a difference if your overall energy balance (in other words, how much you’re eating!) doesn’t leave you in a calorie deficit. Likewise, you could avoid including any fasted exercise in your routine and still lose fat if your nutrition is on point.

A 2014 study aimed to investigate the changes in fat mass following four weeks of fasted vs fed aerobic exercise, with 20 healthy young females assigned to either a fasted or fed training group where they would complete an hour of steady-state aerobic exercise three times a week while following a nutrition plan designed to induce a calorie deficit. Both groups showed significant fat loss, but there were no significant differences between the groups.

Your energy, power and focus can take a hit

As the Australian Institute of Fitness points out, exercising on an empty stomach isn’t recommended if you want to perform at your best during your workouts (which you should!). Without any fuel in your tank, you may notice you have less energy, strength, stamina and focus, or that you fatigue much quicker.

Aside from your performance being impacted, fasted training can also be a recipe for lightheadedness or injury, especially if you’re working out with intensity. Exercising with low blood sugar levels can impair your cognitive function, focus, reaction time and coordination too, all of which you want to be switched on when you’re in the middle of a workout.

You run the risk of muscle breakdown and injury

Although training on an empty stomach can mean your body turns to fat as fuel, there is another alternative that you definitely don’t want - where your body breaks down muscle tissue for energy instead - the very tissue you want to maintain and build. When you don’t have proper fuel and train at a high intensity, this can happen!

You should consider your overall stress profile, longevity and goals

We’ve said it many times and we’ll say it again, there are plenty of better reasons to exercise than burning fat or calories. As soon as you reframe your Sweat sessions to be about building strength, increasing power, making progress, feeling empowered, improving your fitness and performing at your best, the idea of training with no gas in your tank actually seems counterintuitive. And if losing fat is one of your goals, rest-assured, you can still achieve it while including some pre-workout nutrition. Food isn’t something you need to earn.

We want you to focus on longevity and creating lifelong habits that help you to thrive, and for most women, regularly training on an empty stomach just ain’t it. It can leave you exhausted, underperforming, or with your body in a higher state of stress in the long run.

The research often leaves much to be desired, especially for women

One of the most important things to note is that although there is plenty of research around fasted exercise, a huge amount of it is based on male subjects, who have completely different physiology than women. Some health advice is universal for both men and women (such as eat more vegetables, include strength training in your routine and get plenty of sleep), but fasted training does not fall into that category. Women have completely different reproductive and hormonal systems, and different training protocols are better suited to female physiology.

Researchers also note that care should be taken when prescribing or recommending fasted exercise, as all the current data looks at the acute effects and can’t speak to the long-term or holistic health effects.

If you DO want to work out first thing in the morning

For those who love to exercise upon waking, there are a couple of things you can do to optimise your routine and performance. Firstly, make sure you’re hydrated, as dehydration can increase the risk of injury, fatigue and lightheadedness. Drink plenty of water the day before, have some before you start, and maintain good hydration during and after your session.

Secondly, if you’re planning on doing an intense training session, make time to have a small snack when you wake up and a good dinner the night before. If you’re not in the mood for something like a banana or piece of toast, a small protein shake with your choice of milk is also a great option to give your body a mix of carbohydrates and protein. If you’re planning on doing a low-intensity cardio session (such as a walk), cap it at 60 minutes.

Thirdly, regardless of your training style or intensity, make sure you refuel yourself afterwards with a proper meal containing both carbs and protein.

Above all, listen to YOUR body

Your body is incredibly intelligent, so during your workouts (and beyond), try to tune into the cues your body is giving you. If you’re not sure where to start, here are a few questions that can help you guide you.

• When doing fasted exercise, do you feel like you have enough energy to perform at your best in your workouts?

• When doing fasted exercise, do you ever experience fatigue or lightheadedness?

• When doing fasted exercise, do you feel tired but wired afterwards? How would you describe your stress levels, energy and mental focus?

• When you include multiple fasted exercise sessions in your routine, do you finish the week feeling energised and strong or fatigued and burnt out?

• If you have a small snack or protein shake before your workout, do you notice a difference in how you feel during and after your workouts?

Fuel up to feel strong

Strength, energy, power and performance are four words that go hand-in-hand with fuel. Even if you love an early morning workout, experiment with different pre-workout snack options and prepare to level up your Sweat sessions even more. We want to see you thrive.