Let’s look at the rationale a little bit. Without getting too technical there are several different energy systems at your disposal. Training at up to around 70% of your VO2max is considered Aerobic – with oxygen – training and your main energy pathways here are fat oxidation and glucose oxidation, with fat oxidation allowing you to train for longer periods, during these low to moderately intense sessions. 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, like in a marathon
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 with a one rep max deadlift, for example.
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.
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:
|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 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, which results in entropy, it's basic thermodynamics. 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 base of aerobic 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 periods, which means that half of those 30-minutes will be recovery, not activity.
What about this so-called after burn effect? Yeah, that’s been massively overstated I’m afraid. A 2017 study by Schleppenbach et al. did show that although 10-minutes of 30:30 sprint intervals burned a similar number of Calories to 30-minutes of moderate exercise, and that HIIT training raised metabolic rate during exercise, there was no difference post-exercise (1). Both forms of exercises resulted in an EPOCH effect, which is a short-term raising of pulse rate and body temperature, to illicit the removal of waste products like lactic acid.
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 at the gym) at 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 favourable body fat distribution to name just two. These workouts can be fun and are time-efficient, so they do have a place. But they are neither more effective or essential for fat loss, if that is your goal. They are especially rubbish for that if you are very overweight and very out of shape. This is something you should build up to. I can't tell you how many previously sedentary people I know who injured themselves within the first two weeks of following Joe Wicks's 'PE' sessions during lockdown.
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. Why this combination? Because resistance training will help to maintain or even build lean body mass like muscle and bone, it will shape your body in a way you like and make you feel more confident, not mention making your stronger and more powerful. Meanwhile, aerobic conditioning is important for heart health. Health should be a goal.
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 @3-8 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:
And, so there you have it. HIT or HIIT training is not a magic bullet that, like everything in life, has pros and cons. Do it if you enjoy, ideally in combination with a progressive resistance training program. If you have never lifted weights before then I strongly advise you to seek a reputable coach or trainer who can teach you safe and effective technique. Don't just go through the motions, train with mindful intent and ALWAYS train towards a goal, ideally a performance-based goal rather than purely for aesthetics, because if you aim for better physical performance it's easier to motivate yourself and to actually enjoy your training.
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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