HIIT and Fasted Cardio

25 Jun, 2019

Picture it…it’s 5am on a cold, crisp, Autumn morning. You wake up to the sound of your alarm and gaze over to the small pile of workout clothes you’d left out the night before, ready for your enthusiasm the next morning to get out on that run you’d said you would do. Except…the enthusiasm just isn’t there.


The draw of fasted cardio for fat loss was pretty high when I first got into fitness and still pops up every now and again. I got asked if fasted cardio and HIIT worked on a recent post I did on social media, so here I will go into both.

What I want to do is take a look into each, before we consider how they might be combined.


First up, let us take a look at fasting.


Fasting in itself has received a lot of attention, whether that is from the perspective of long term health or body composition or both. In a 2017 paper, Trepanowski et al., concluded that energy balance was the most important determinant for weight management. Further, the alternate day fasting group in the study had results similar to the normal caloric restriction provided caloric intake was matched.


So basically, it works if it works for you when it comes to body composition, so long as calories are in the right place to meet the goal. I.e. that phrase that is now so over used on social media = a calorie deficit. No calorie deficit = no fat loss, fasting or not.


To take this further, we can look at some conclusions drawn through research on people fasting through Ramadan to assess how fasting might affect metabolism. Lessan and Ali (2019) found that fasting did not increase energy expenditure and in fact was related to fewer daily steps. Fasting only led to weight loss during calorie restriction.

In short, if fasting makes you slow down and move less through the day, it could well lead to harder fat loss because you aren’t moving around as much.

The big question with fasting is really this: will it work for your lifestyle right now? If you aren’t sure you can make it work all the time, it probably isn’t for you. Equally, it might be something you use more flexibly to allow you to maybe go out to dinner with friends a little more often.

Just because some people do it all the time, doesn’t mean you need to. Remember, the most important thing is energy balance (how many calories you consume over the day/weeks/months) and if fasting sometimes helps you manage your daily calories then it could be a valuable tool. I will quite often skip a meal in the morning on a weekend if I know I am going out for a big meal with my girlfriend. The reason it works isn’t some magic metabolism trick, but just sensible calorie management.


Now…onto HIIT


‘HIIT’ stands for high intensity interval training. What this means is that you would perform an exercise at a very high intensity for a short period of time, followed by a period of rest before the next bout of that exercise. The exercises that are usually used are cyclical in nature, so think sprints up a hill, on a bike, on a rower, done with bodyweight or a kettle bell swing perhaps.

Although it has become very popular, I see lots of people doing what they think is HIIT really end up doing something quite different. To really achieve a high intensity for the interval a few things need to happen.

First it needs to be something relatively low skill so it is safe to push as hard as you possibly can. So we’re talking bike sprints rather than clean and jerks.

Secondly, there needs to be proper recovery between intervals so you can actually achieve the highest intensity possible. So, something like a tabata (where you go hard for 10 seconds and rest for 20) or your typical HIIT class in a commercial gym (where it’s usually even less rest and longer work) simply just doesn’t give you enough rest between intervals to work at a high enough intensity to get the job done. It usually ends up with being absolutely knackered and form getting sloppy 10 minutes in.

So, if you want to do some HIIT, make sure to give yourself adequate rest between sprints or bouts of an exercise so you can actually give it all you’ve got instead of dragging yourself through 30 minutes of non stop burpees. The way I like to organise this would be something like a 20s all out effort, followed by 1 minute 40s active rest for 6-10 rounds per block of work. If you were to do an entire workout you might do 3×6-10 minute blocks like that with a 2-3 minute complete rest between each one. 


Time for the big question on cardio…does HIIT burn any more calories than steady state?


Here’s where it gets a bit more personal. The answer is, it depends.

With steady state, you’ll roughly burn 4 calories per minute. When it comes to HIIT you’ll roughly burn 9 calories per minute and 3 calories per minute on rest minutes.

But, we must break this down further. First off, those numbers above are very general and don’t account for individual differences. Someone who has always had a high amount of muscle mass and low amount of body fat will likely use energy in a different way to someone who has a high amount of body fat and low amount of muscle mass. This could be due to being more or less efficient with movement as well as how much total tissue they carry (fat, muscle and organ tissue all demand energy, as well as a multitude of processes that happen every minute of every day to keep our bodies alive and well!)

Also, the pace required to get to truly high intensity is really hard to sustain as suggested in the section above. I’d argue very few people could sustain that for an entire minute, let alone 30. Whereas steady state cardio is generally done at a pace that is very easy to sustain for the duration.

There are a few considerations to keep in mind: firstly is what happens for the rest of the day. If you do a HIIT session in the morning, but for the rest of the day are so knackered your daily activity decreases then it might be worth reconsidering. Your time constraints may play into this too, because one takes less time than the other. Lastly, is your ability to recover from HIIT. If you are just adding stress to stress, something is gonna break eventually.

So, much like fasting, if it works within your lifestyle and you are making progress doing HIIT then crack on. But there is nothing magic about it.


Lastly, what about doing cardio in a fasted state?


As has been the theme of much of this, it really depends on what you can manage, your preferences for when you eat first in the day and what works for your daily life. If it works in your day and you feel like you can get the most possible out of a session first thing in the morning with no food in you…go for it. But remember, it isn’t anything magic.

When it comes to doing fasted cardio, my advice to clients is always pretty simple…if you enjoy it, go for it. Personally I like to walk my dog if I have the time before work. Sometimes I have breakfast before, sometimes just a coffee, sometimes a banana and a coffee. Does it make a difference? Not really, I just go for whichever one I feel like on the day.

You see, fasting while you do cardio isn’t magic. If you aren’t in a calorie deficit over a sustained period of time, you won’t change the amount of body fat you have regardless if you do any cardio fasted or fed. Fasted cardio isn’t a magic switch that fires up your metabolism, although that is certainly something I have thought in the past. For more about metabolism…read this: https://jackedpersonaltraining.co.uk/your-metabolism-isnt-the-problem/


If we were to look at it pure and simply as a set of pros and cons here’s how I would see it:

PRO: Gets done first thing so it’s out of the way. Might be better for those who can’t stomach food first thing in the morning. Could work for those with young kids or who have busy mornings, so getting up early may be the only option. You might love doing it fasted.

CON: Can’t put as much effort into it if you have low energy in the morning. You may not have much time in the morning. You might hate doing it fasted. You might offset the activity you usually do for the rest of the day because you are simply knackered, making your total energy expenditure drop and therefore pushing you away from your original intention (less energy used in the day in total, despite doing cardio).


This brings me on to the very last and most important point.


Doing HIIT/fasted cardio will do absolutely nothing for you if you aren’t in a calorie deficit. My advice for cardio and fat loss is to pick a mode you enjoy and can sustain if you are going to do any. BUT…neither fasting or HIIT are magic fat loss bullets, nor expend enough energy to ‘out-expend’ eating too many calories.

If you want to do fasted HIIT cardio, be my guest. But I’ll take brekkie in my dressing gown with my dog curled up next to me every single time and save the cardio for when I’ve got more energy.