High intensity training or HIT, often referred to as HIIT which actually stands for High Intensity Interval Training has been presented as some kind of magic bullet for fat loss in recent years. I mean, I was even taught this when I first did my level 3 personal trainer diploma. The main reason we are told that HIIT burns more fat than other forms of exercise is because of something called Excess Post-Exercise Oxygen Consumption or EPOC for short. This is often referred to in more simple terms as ‘the afterburn effect’. The theory being that after a bout of very high intensity exercise you burn more Calories while you recover. Some sources have claimed that you could burn an extra 1,000kcals over 24-hours. This is completely false. Let me explain more.
HIIT it Hard
Let’s look at the rationale a little bit. Without getting too technical there are several different energy systems at your disposal. Training at upto around 70% of your VO2max is considered Aerobic – with oxygen – training and your main fuel source here is fat oxidation. That means your body metabolises glycogen from your muscles and fat from your diet for energy and the longer you sustain such efforts the more fat you will oxidise. This is why you can run or ride for very long periods of time, often hours at a time.
The other main pathways are the anaerobic, or alactic energy system – without oxygen or lactic acid – and your main fuel source is Glycogen. These are short bursts of effort of maybe up to a couple of minutes at a time, depending on your fitness. Then, lastly there is the power range or ATP-CP system, where your sole fuel sources are adenosine triphosphate and creatine phosphate. This is for very short bursts of energy like lifting a heavy weight as in with a one rep max deadlift.
HIIT training, as you probably know it, uses the anaerobic system, which means that while you are doing several short bursts of activity you are performing what is known as glycolytic activity because your primary fuel source for such activity is glycogen – the storage form of carbohydrate.
How does HIIT training burn fat?
High intensity training doesn’t burn fat, as I have already shown you, it burns glucose, a sugar. But it’s easy to see where the myth comes from, look at the following table adapted from The Complete Book of Personal Training by Douglas S. Brookes MS:
| EXERCISE |
|Less intense: 30-minutes of exercise at 50% HR Reserve||More intense: 30-minutes of exercise at 75% HR Reserve|
|RESULT|| More fat burned, about 50% |
from fat per kcal
|Less fat burned, about 40% from fat per kcal|
| TOTAL |
|TOTAL FAT CALORIES||113||126|
Looks good, right? Except this is slightly misleading. Yes, those Calories are coming from fat but, as I have already stated, that is dietary fat, NOT body fat. Exercise DOES NOT burn body fat. Calorie deficits over a period of time are where reductions in body fat come from. It’s a simple ratio of energy in versus energy out and if you put less energy into your body than you burn off you will be in an energy deficit. Also, training at 75% of your heart rate reserve is hard and maintaining that for 30-minutes is really tough unless you already have a good level of endurance conditioning. You are more likely to be doing interval training where you do bursts of maybe around 30-seconds of activity followed by short rest period, which means that half of those 30-minutes will be recovery, not activity.
EPOC and the afterburn effect
What about this so-called after burn effect? Yeah, that’s been massively overstated I’m afraid. A 2017 study by Schleppenbach et al showed that although 10-minutes of 30:30 sprint intervals burned a similar number of Calories to 30-minutes of moderate exercise. However, although HIIT training raised metabolic rate during exercise, there was no difference post-exercise (1).
Backing up the previous point about nutrient utilisation during exercise a 2009 study compared 24-hour substrate oxidisation between Obese (top) and Lean (bottom) subjects across low intensity training (LI) high intensity training (HI) and a control group who did no exercise (CON). Basically, lean people are better at carbohydrate oxidation but both groups prioritised that over everything regardless of activity. You can see the results in the following table (2):
Back onto energy expenditure and the whole ‘after burn’ thing. A much-cited paper by LaForge et al – 2006 demonstrated that the EPOC of exercise bouts ranged from 7-13% of net Calories. With the really high intensity training, and we are talking in excess of 85% VO2max here – which is extremely hard and harder than most people train during a typical ‘HIIT’ class – offering the upper end of that scale. But this isn’t a prolonged increase over a 24-hour period, it’s merely an increase in metabolic rate during the recovery intervals which means that a workout session that burns 300kcals, once EPOC during those intervals has been accounted for really burns 340kcals. You see that? Your HIIT sessions burned an extra 40kcals and, as per the above graph, most of those Calories came from Carbohydrates. THAT is why individuals who exercise regularly should be eating plenty of carbs.
There is some good news, however. A 2011 paper by Boutcher et al did note some positive body composition changes after 15-weeks of high intensity interval training. Reductions in visceral and subcutaneous abdominal fat did occur as well as improvements in both aerobic and anaerobic fitness to a significant enough degree to be note-worthy. But these changes were due to hormonal responses to exercise, including improved insulin sensitivity, rather than exercise energy expenditure (4).
So, to sum up this section, I am NOT saying don’t do high intensity exercise. There are benefits to high intensity training that extend far beyond how many Calories you burn. Better metabolic efficiency and body fat distribution to name just two. These workouts can be fun and are time-efficient, so they do have a place. But it ain’t no magic bullet for fat loss.
How should you train?
To answer this question, you need to work out what your goals are. Are you training to be stronger? To get bigger. To compete in an endurance event. To lose weight, etc? Establish what the goal is and why that is important to you. Then train effectively to achieve that goal. But, for most recreational exercisers, it’s to maintain a healthy weight and functional level of fitness. Therefore, a combination of resistance training and aerobic/anaerobic exercise will be best.
I recommend to most typical clients that 3 full body resistance training sessions per week coupled with 1-2 cardio sessions is a good ratio to shoot for. If body composition and fat loss is important to you then resistance plus some high intensity cardio will be the best combo, maybe a 10-minute mini circuit or some sprint intervals at the end of a weight session.
Here is an example of what a typical gym session for the recreational exerciser might look like: Main compound lift (deadlift/squat/bench press) 4-6 sets @3-6 reps plus 3-5 accessory lifts; 2-3 sets @10-15 reps each to add volume. Include some ab/core work at the end and if time allows include a 10-20 minute ‘finisher’. Example:
- Squat 5×5
- Romanian deadlift 3×10
- Dumbbell flat bench chest press 3×10
- Seated row 3×10
- 10-minute bike sprint intervals
Of course, if you train with kettlebells then you can incorporate all of those things into each workout and get really good results in as little as 20-minutes per session.
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- Schleppenbach LN, Ezer AB, Gronemus SA, Widenski KR, Braun SI, Janot JM. Speed- and Circuit-Based High-Intensity Interval Training on Recovery Oxygen Consumption. Int J Exerc Sci. 2017;10(7):942–953. Published 2017 Nov 1
- Melanson, E., MacLean, P. and Hill, J. (2009). Exercise Improves Fat Metabolism in Muscle But Does Not Increase 24-h Fat Oxidation. Exercise and Sport Sciences Reviews, 37(2), pp.93-101.
- La Forge et al. Effects of exercise intensity and duration on the excess post-exercise oxygen consumption.J Sports Sci. 2006 Dec;24(12):1247-64.
- Boutcher SH. High-intensity intermittent exercise and fat loss. J Obes. 2011;2011:868305. doi:10.1155/2011/868305