HIIT is awesome for fat loss, so I’ve put together this comprehensive guide to answer all the most common HIIT related questions!

hiit explained

Written by Russ Howe PTI, and most recently updated 1 day ago.

17 min read

HIIT cardio is awesome for fat loss.

I’ve put together this comprehensive guide to answer the most common questions about HIIT so you can get the most from it in the gym.

Here’s the table of contents:

high intensity interval training explained


High intensity interval training is the bigger, cooler brother of aerobic cardio.

Instead of climbing on a treadmill and performing 30 minutes of jogging, you will be alternating between bursts of all-out effort and recovery.

Performing your cardio in this “up & down” style can lead to a significantly higher calorie burn, but this is just one of the many benefits HIIT offers. It can be structured in various ways to increase fat loss, boost endurance, and even improve sporting performance.

There’s over 30 years of research showing the effectiveness and the pitfalls of HIIT. Today we will be taking a look through the best of that research so you can build a HIIT routine which works perfectly for you.

what is hiit



It rose to prominence in the early 2000’s, but it’s been around for decades. Syvester Stallone famously used it in preparation for Rambo III in 1988, and the first scientific studies on HIIT go back even further to 1985.

It didn’t really catch on at the time, though, with most gym-goers dismissing it as the latest fad.

It hit the headlines again in early 1992, when elite swimmers and sprinters spoke about implementing it into their training for the Olympic Games.

Unfortunately, it failed to gain any traction again. This was the same Games where coaches discussed the usage of creatine monohydrate, which stole all the media thunder and went on to become the best-selling bodybuilding supplement of all time.

It would take about another 10 years before HIIT reached the popularity it has now.

does hiit burn more calories than normal cardio


Indeed it does.

It may have taken a while to be acknowledged, but most of the world’s top coaches now agree that HIIT is superior to aerobic cardio for burning fat while maintaining lean muscle tissue.

So what kind of difference are we talking about here? Research suggests improvements of 50-60%.

Let’s look at the science.

A 1994 study from researchers at Laval University, Quebec, was the first to present startling facts on just how effective HIIT can be. This study compared the fat loss results of trainees performing bursts of HIIT versus steady state cardio, and reported their findings to Metabolism Journal.

The group who performed HIIT lost 9 times more fat (not a typo).

Perhaps even more amazing is the fact that the HIIT group trained less frequently and for shorter durations (more on that later).

hiit for fat loss


Let’s say you’re performing HIIT for fat loss. How should you structure your workout for the best results?

In my opinion, the best and most comprehensive study on HIIT for fat loss was conducted by Dr. Peter Lemon and his team at the University of Ontario in 2011. (3)

They found that participants were able to DOUBLE their usual calorie burn with the following setup:

  • SIX ROUNDS of 30 second bursts.
  • FOUR MINUTES of recovery time between each burst.
  • 36 MINUTES total training time.

One of the interesting things to come out of this study is the importance of the recovery time taken between each burst. Four minutes might seem like a long time, but the researchers concluded this was paramount to the results they achieved because it allowed trainees to fully recover, ensuring that each HIIT burst they performed was at true max output.

It appears that one of the key components of a successful HIIT routine (for fat loss purposes at least) is heart rate manipulation. We need to skyrocket it during each burst, then let it return to normal before the next one. The time it takes to do this will be different for most people as it depends on your fitness level, but you get the idea.

How many times have you met people in the gym trying to do 30 seconds on, 30 seconds off, and complaining about a lack of results? Now you know why.

Tabata HIIT


I know this sounds weird, given that HIIT is often linked with shorter workouts and less training frequency, but it CAN be used to improve endurance!

The most famous example of this is the Tabata protocol.

In this 1996 trial, Dr. Tabata and his team worked with a group of elite athletes to develop an 8 minute HIIT workout that looked like this:

  • 20 SECONDS at max intensity.
  • 10 SECONDS recovery time.
  • 8 MINUTES training time.

By implementing this short HIIT workout alongside their existing routine, trainees were able to increase their VO2 MAX by a crazy 28%!

VO2 MAX is the best measure of cardiovascular fitness, and the participants were already elite athletes, which means these results are off the f**king hook! (4)

Consider how difficult it is for a bodybuilder to gain an inch to his/her already bulging quads, or for a sprinter to shave .1 of a second from his/her already great personal best.

This shows that shortened recovery between rounds, coupled with max output intervals, are great for highly trained aerobic individuals like endurance athletes. In recent years we’ve seen a number of marathon runners implement the ideas of this trial into their training, and they’re right to do so.



Let’s say you partake in a sport which requires explosive action for long periods of time, and you’d like to use HIIT to improve your game.

This could refer to rowers, rugby players, footballers, etc.

A study from Matthew Driller and his team at the University of Tazmania looked deeper into this aspect of HIIT, and concluded that we can maximize results with the following setup:

  • 3 MINUTES at high intensity.
  • RECOVER until HR returns to 70% max.
  • 40 MINUTES total training time.

When they applied this protocol with a group of highly trained rowers they noticed significant improvements to muscular power output, VO2 Max and sporting performance. (5)

does hiit burn muscle


Quite the opposite, in fact.

One of the initial reasons many bodybuilders were sceptical of including HIIT in their training routine is the theory that performing explosive cardio may lead to muscle loss.

But a 2011 study published in the Journal of Strength and Conditioning Research got to the bottom of this issue once and for all.

In this trial they compared the effects of adding either an aerobic program or a HIIT program alongside heavy weight lifting, and they concluded that HIIT had no negative impact on strength or hypertrophy. Meanwhile, the aerobic cardio group reported a 15% reduction in strength and a 30% reduction in hypertrophy. (7)

does hiit burn muscle



So why does HIIT burn so many more calories than regular cardio?

To answer this, let’s go back to the 1994 Laval study I mentioned in the opening section of this article. In this study, researchers concluded that participants using HIIT burned significantly more calories than participants using aerobic cardio, and when we look at the numbers we can see why this happened.

Over the course of the full program, the gym workouts of the two groups burned:

  • 28740 kcals (aerobic cardio group)
  • 13830 kcals (HIIT group)
cardio vs hiit calorie burn

At first glance it appears the aerobic cardio group burned more calories, but there was something going on “behind the scenes” that the researchers didn’t expect…

That something is E.P.O.C. (excess post-exercise oxygen consumption), otherwise known as The Afterburn Effect.

Here’s how it works…

Aerobic cardio (regular cardio) uses the aerobic energy system, whereby fat is used as the primary fuel source during exercise. HIIT apears to have more in common with weight training, as it uses the anaerobic energy system. This means you’ll use carbohyrates as the primary fuel source during exercise.

This is a game changer!

You see, when we use the aerobic energy system our calorie burn tends to finish around the same time as the workout itself. But when we use the anaerobic energy system EVERYTHING CHANGES! You will continue burning calories long after your workout ends, and this is how the HIIT group recorded a greater calorie burn in the study above; despite the fact that their workouts were shorter and they didn’t train as frequently!

Despite the fact it uses carbohydrates for fuel during exercise, HIIT actually increases production of HADH, the body’s fat metabolizing enzyme.

It appears that by depleting your body’s glycogen (carbohydrate) reserves in the gym the body’s fat burning mechanisms are temporarily flipped upside down by E.P.O.C., as it protects your remaining carbohydrate reserves and begins the post-workout recovery process by throwing fat into the furnace instead.

And the crazy part is that a 2011 study published in Medicine & Science in Sports & Exercise confirmed that E.P.O.C. can see you burning calories at an accelerated rate for up to 14 hours. (2)

SIDE NOTE: In recent years, many gimmicks trying to cash in on the effectiveness of HIIT have claimed their workouts can turn you into a “fat burning machine” and lead to 18, 24, and even 48 hours of “the afterburn effect”. Don’t buy into this BS. Science shows us it lasts for a maximum of 14 hours. They’re making those figures up without any data in an attempt to beat their competitors, and most of these people couldn’t explain how E.P.O.C. works if you asked!

what is the afterburn effect

shoud you do cardio before or after weights


Many folks believe that performing regular cardio, or even HIIT, prior to lifting weights will leave them feeling “too zapped” to lift weights effectively.

This is incorrect.

A 2012 trial published in the Journal of Strength and Conditioning Research set out to test this theory by putting participants through a workout consisting of heavy bench press and heavy squats. One group of trainees performed an intense 45 minute cardio session beforehand, and the other group did not. They found that the bench press was completely unaffected by the pre-workout cardio, while the squats had a noticable drop off in performance. (6)

should you do cardio before or after lifting weights

This suggests that pre-lifting cardio will only exhaust the muscles used during the cardio (in this case they used a bike, so the trainees’ quadriceps were exhausted). Given that most cardio equipment is lower body dominant, the key advice from this study is simple; cardio before lifting weights is fine, just don’t do it before training legs!

In fact, it’s MORE than fine. If we really get stuck into the science, it’s actually slightly SUPERIOR to do it this way!

I’ll explain.

If we take a look what’s happening inside the body as we train, let’s focus on two enzymes in paticular; AMPK and mTOR.

During a cardio workout our body releases the enzyme AMPK. This helps us to make adaptations which assist us in endurance training, such as helping cells build more mitochondria to process more energy for you to use.

After a weights workout the body releases m-TOR. You’ve probably seen this word referenced on muscle building supplements, as it’s the enzyme which begins the process of post-workout hypertrophy.

But what most people don’t know is that AMPK destroys m-TOR. Therefore, it makes no sense to perform an hour of cardio after a weights workout! (8)

This news should rock bodybuilders to their core, and change the way they train from this day forward, but sadly most are totally unaware of it because the study which broke the news was terribly under-reported in the media at the time.

Following a weights session m-TOR is spiked for about six hours, and we REALLY want to get the most from this prolonged state of enhaned muscle building. This means we should either be performing our cardio before lifting weights, or breaking our session into two. Make sense?

should you do hiit on an empty stomach


People often speak about the importance of POST workout nutrition, but getting your PRE workout nutrition correct should hold just as much importance.

If you were performing regular cardio, I’d advise you that it makes no difference whether you eat before training or not. Fasted cardio will not burn more fat, it literally just depends on what you enjoy.

But with HIIT there’s a definite advantage to eating prior to training.

One of the prerequisites of a good HIIT workout is that we must push ourselves very hard during each burst, and if you’ve ever tried it on an empty stomach you’ll know that’s not possible.

So what kind of food should we eat before a HIIT workout?

This is another area most people get dead wrong. It’s human nature to reach for a piece of fruit or a bowl of oats, but we need to remember your training goal. If you are using HIIT to improve athletic performance you’ll want plenty of carbohydrate reserves to power through your training, but that’s not the case if you are using HIIT for fat loss purposes. Remember, when using HIIT for fat loss the goal is to deplete our carbohydrate reseves and tap into E.P.O.C. By eating carbs immediately before training we are merely buffering those carb reserves and giving ourselves a bit more work to do in the gym.

Instead, opt for protein and/or fat. This will protect lean muscle tissue, provide a temporary metabolism boost, and give you fuel to maintain optimal performance. When you are trying to blast body fat, PROTEIN is your best pal! (9)

best supplements for hiit



The benefits of whey protein are well documented. It’s a true ‘superfood’ and should be part of any serious athlete’s nutrition program. Not only does it have a speedy digestion time (which is awesome after training) but it’s also convenient as f**k. When you are trying to build muscle and lose fat, staying on top of your daily protein intake is KEY, and whey protein makes this very easy.

If you don’t have any issues with lactose then you’ll get maximum results with something like Total Protein Blend by Myprotein. If you do have digestion issues then switch to a whey isolate formula like Pro JYM or Optimum Nutrition Gold Standard Whey.


DO NOT fuel your HIIT workouts with a bulls**t pre workout that wasn’t designed correctly. What’s the point in using something that doesn’t help your performance?

We want at least 200mg caffeine to boost energy output, alongside at least 2g beta-alanine. In a 2012 study, beta-alanine was shown to significantly increase the buffer time against the build-up of waste product in the muscle, allowing you to keep training harder for longer. We also need at least 4 grams of l-citrulline, which helps widen blood vessels to deliver nutrients to muscles as they work, resulting in faster recovery times. But here’s a twist; make sure your pre workout contains l-citrulline, not citrulline malate. When it comes to cardio-based training, l-citrulline is the superior of the two. (11, 12)

If you reside in the UK you should be using Adapt Nutrition Pre Train v2, and my Stateside readers should be using AML Dopa Rush Pre Workout. Game changers.


Creatine should be a staple of any athlete’s nutrition plan because it has been shown to have incredible effects on explosive strenth and muscle growth. Think about the type of training you’re doing here, it’s literally a perfect match!

Standard creatine monohydrate is optimal for most people, and anyone who experiences digestion issues should switch to creatine HCL.


Many people overlook the benefits of hitting a full spectrum of vitamins and minerals because they are ‘boring’. They might not have flashy names like pre workouts, but these micronutrients are KEY to long-term results because they’ll improve everything from the appearance of your skin and hair, to energy levels and mood!

Ignore under-dosed multivitamin supplements, as they will not give you enough of each key ingredient to make a jot of fifference. Men should use this one and women should use this one.

Team GB athlete Amanda Lightfoot smashes through a HIIT workout in the gym.

hiit vs cardio


Nothing is perfect. Not even HIIT.

In fact, one of the reasons it took so long for the fitness world to accept this form of training is because, well, it sounded too good to be true!

Think about it… I’m telling you that you can train LESS and burn MORE calories. What kind of black magic is that?!

This can give people the wrong idea about HIIT, and there are many charlatans waiting to take advantage of people desperate for weight loss by promising easy, instant results with zero effort. I’ve seen coaches get into social media d**k measuring competitions over whose workouts will give a bigger ‘afterburn effect’, some claiming that their workouts will give you 18, 24, or even 48 hours of additional fat loss after every session. I’ve seen others claiming you only need to train for 4 minutes per day with their method. It’s a jungle of misinformation out there.

Another thing HIIT enthusiasts tend to do is push the notion that regular cardio is bad for you. This is not true. I’m a big fan of HIIT but I’ll never tell you regular aerobic cardio is bad for you; not everyone can do HIIT, and bulls**t advice like that can make the difference between someone starting a fitness routine or quitting on the spot.

So basically the main pitfall of HIIT is that there are a lot of c**kwaffles out there spreading bad information. If you can steer clear of them, you’re golden!

hiit explained


I hope you’ve enjoyed the read.

At this stage, you probably know more about HIIT than 95% of people at your local gym and you know EXACTLY how to structure your routine to suit your current training goals.

This resource will always be here to refer back to in the future, and I’ll add to it as new research is published so it always stays up to date.

  1. Trembalay, A., et al. Impact Of Exercise Intensity On Body Fatness And Skeletal Muscle Metabolism. Metabolism. (1994)
  2. Knab, A. M., et al. A 45 Minute Vigorous Exercise Bout Increases Metabolic Rate For 14 Hours. Med Sci Sports Exerc. (2011)
  3. Lemon, P. W., et al. Run Sprint Interval Training Improves Aerobic Performance But Not Max Cardio Output. Med Sci Sports Exerc. (2011)
  4. Tabata, I., et al. Effects Of Moderate Intensity Endurance And High Intensity Intermittent Training On Anaerobic Capacity And VO2 Max. Med Sci Sports Exerc. (1996)
  5. Driller, M. W., et al. The Effects Of High Intensity Interval Training In Well Trained Rowers. Int J Sports Physiol Perform. (2009)
  6. Schilling, B. K., et al. Acute Neuromuscular And Metabolic Responses To Concurrent Endurance And Resistance Exercise. J Strength Cond Res. (2012)
  7. Wilson, J. M., et al. Concurrent Training: A Meta Analysis Examining Interference Of Aerobic And Resistance Exercise. J Strength Cond Res. (2011)
  8. Hardie, D. G. AMP-Activated/SNF1 Protein Kinases: Conserved Guardians Of Cellular Energy. Nat Rev Mol Cell Biol. (2007)
  9. Hackney, K. J., et al. Timing Protein Intake Increases Energy Expenditure 24 H After Resistance Training. J Am Col Sports Exerc. (2010)
  10. Paoli, A., et al. Exercising Fasting Or Fed To Enhance Fat Loss? Influence Food Intake On Respiratory ratio And Excess Post Exercise Oxygen Consumption After A Bout Of Endurance Training. Int J Sport Nutr Exerc Metab. (2011)
  11. Donovan, T., et al. Beta-alanine improves punch force and frequency in amateur boxers during a simulated contest. Int J Sport Nutr Exerc Metab. (2012)
  12. Pérez-Guisado, J., et al. Citrulline malate enhances athletic anaerobic performance and relieves muscle soreness. J Strength Cond Res. (2010)

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I’m Russ. I’ve been a personal trainer since 2002, and I own russhowepti.com.

My job is to simplify fitness for my readers.

I send out free fitness tips to over 100,000 men and women every week, all in the same no-nonsense style as the article you’ve just read, so if you enjoyed reading it be sure to jump on my email list below.

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