Thursday, 23 February 2012

Anaerobics and HIT - the truth about "fat burning" exercise

For my latest Blog post, I decided to pontificate about something other than breathing for a change. The decision was prompted by a peak of irritation that I experienced recently. What or who was I irritated with? “fitness journalists” – not all of them, just the ones who persist in propogating the myth that the key to “fat burning” is low intensity exercise. The REALLY irritating part is that the people doing this should know better, and they do their readers and clients a disservice with their misinterpretation and misundertaning of the science.

The fat burning myth

The fat buring myth goes like this – it claims there’s an optimal exercise intentisty at which you “burn fat” during exercise, thereby reducing your fat stores. The assumption is that exercising at this intensity will optimise your fat loss. The damaging part is that this fat burning silver bullet is low intensity exercise. It’s damaging because it means that millions of people are being misled; they are being directed towards an activity that requires fewer calories and is counter-productive if their objective is to lose fat.

I want to explode the fat burning myth, and to offer an alternative, more effective way to lose fat – I once called it "Anaerobics", but it’s become known as high-intensity-training (HIT). A version of this article was published in 2002 in SportEX Medicine, so in a way, this is very old news, but I think it’s worth repeating, because no one seemed to listen first time around.

Aerobic, anaerobic…what does it all mean?

In the 1980s Jane Fonda brought us “aerobics”, a word that is now firmly entrenched in our popular culture. In the context of exercise and metabolism, the term “aerobic” relates to activity that relies upon energy supplied by oxygen-driven metabolic pathways. Energy can also be produced by “anaerobic” metabolism, which provides energy for short periods of time using metabolic pathways that do not require oxygen.

One factor that distinguishes the two pathways is the rate at which each can supply energy, which in turn determines the intensity at which exercise can be undertaken using each of the pathways:
  • Aerobic metabolism - can only sustain light to moderate intensity exercise, above this intensity
  • Anaerobic metabolism - supplements the aerobic energy production, or may even supply all of the energy (eg. a 100m sprint).

Fat as a source of energy (calories)

The utilisation of fat as a fuel for energy production is complex, but around 95% of the energy liberation from fat occurs in the cell mitochondia, and requires a process called oxidation, i.e., aerobic metabolism. It’s believed that the eventual balance between carbohydrate and fat usage during exercise is determined by the rate of carbohydrate usage. During higher intensity exercise, there is a strong stimulus for use of carbohydrate - muscle glycogen to be broken down by a process called glycolysis to produce a substance called pyruvate. Pyruvate competes with fatty acids for entry into the mitochondria where oxidation takes place (1). Thus, fat oxidation only predominates during low intensity exercise, when there is less pyruvate to compete for entry into the mitochondria.

However, this insight has led to a crucial misconception about how fat stores are reduced by exercise, i.e., people believe that it’s only possible to lose fat during low-to-moderate intensity exercise. The crux of this misconception is the erroneous assumption that fat can only be lost if it is the primary fuel source during exercise. This myth is one that I, as an exercise scientist, feel compelled to explode, because the scientific basis for this simply does not add up.

Why fat burning exercise is a myth

Human metabolism must obey the laws of physics, the most relevant of which relates to the conservation of energy – “energy cannot be created or destroyed, only transformed from one form to another”. This means that the energy budget must balance, irrespective of the fuel source. In other words, if you run 5km, the energy required to do this is the same whether you use fat stores (muscle triglyceride stores and adipose tissue) or carbohydrate stores (muscle and liver glycogen and glucose), or a combination of the two.

The amount of fat lost during exercise is determined by the total calorie expenditure and not by the fuel source. This is because:

  • If glycogen stores have been depleted, subsequent physical activity must resort to fat as a fuel for oxidation.
  • Carbohydrate stores can be replenished by both the carbohydrate that we eat, and by the breakdown of our fat stores (lypolysis).
Not all stored fat can be converted to glucose, but the glycerol portion of stored triglyceride can, and this “gluconeogenesis” is an important role for glycerol when glycogen reserves are depleted. However, this gluconeogenic role of glycerol will only occur if glycogen stores are not immediately and fully replenished by the consumption of dietary carbohydrate – to lose fat you need to be in a state of “negative energy balance” -  expending more calories than you consume. This applies equally whether you use fat as a source of fuel for exercise, or carbohydrate. So you see, it doesn’t matter where the calories come from, if you want to lose fat, what matters is that you expend more calories than you consume.

Maximising calorie expenditure

In theory, maximal calorie usage is achieved by exercising at a high intensity for very long periods of time. Unfortunately, high intensity exercise requires the use of anaerobic metabolism, which produces lactic acid, a metabolite that is very strongly linked to fatigue. So it can only be sustained for short periods of time.

So how can the calorie expending benefits of high intensity exercise be achieved without premature exhaustion and curtailment of activity? There are two approaches:
  1. Having determined how long a workout is to last, say 30-minutes, the exercise intensity should be fixed as high as is possible in order to sustain exercise for 30-minutes. In other words, the exercise is undertaken above the lactate (anaerobic) threshold, but at an intensity level that produces fatigue gradually over a period of 30-minutes. As “fitness” improves, the exercise can be sustained at progressively higher intensities and more energy is expended in the same 30-minutes.
  2. The second approache is to divide the workout into short, very intense bouts, with periods of recovery between each. This is known as interval training (more recently as HIT), and is explained in more detail below.

The influence of efficiency on calorie expenditure

As well as the straightforward mathematical logic that says you expend more calories during 30 minutes of running at 10kph than you do at 7kph, there is another factor that works in the favour of high intensity exercise for calorie expenditure, and that is the efficiency of movement. Human movement has an efficiency of around 26% (at best).  This is easily illustrated:

Efficiency =   External work done      x 100%
                      Total energy required
to perform the work

Efficiency example
The mechanical work done whilst exercising at 100W on a cycle for 30-minutes is equivalent to 43 kcal (net energy requirement). The total energy required to perform that work is 165 kcal (goss energy enpenditure), giving a gross efficiency of 26% (43/165 x 100%).  In other words, the human machine wastes 64% of the energy it uses, predominately in the form of heat. Unlike a car where you want to get as many miles to the gallon as you can, if you want to maximise calorie expenditure, inefficiency is good news - the less efficient you are the more calories you expend.

Efficiency during high intensity training (HIT)

Producing energy at high rates, to sustain intense exercise, is even less efficient than the pitiful 26% that human beings typically achieve during low intensity exercise.  Whilst this fact has been known for many years, it appears to have been overlooked in the context of maximising “fat burning”.  A 1978 study (2) identified that the efficiency of high intensity work was only 10-15%. This reduction in mechanical efficiency with increasing intensity of work is not fully understood, but is likely to be due to a combination of factors.  These include the contribution made by lactate to energy production, decreases in muscle efficiency and increases in energy expenditure from muscles not directly involved in the locomotor activity (eg. trunk stabilisers, respiratory muscles, cardiac muscle).

Sprinting and weight training are low efficiency activities because they also involved rapid, repeated contractions and relaxations of large muscle masses.  This type of intermittent activity requires more energy than slow, sustained contractions because the actual contraction and relaxation phases of the movement add to the overall energy expenditure (3). Researchers have shown that high intensity interval training has much to offer those interested in weight management (4). All this tends to point to high intensity training (HIT) as the “way to go” for maximising calorie expenditure. There’s also been a lot of recent interst in the benefits of HIT from a health perspective (, but that’s another blog post…

Of course, the problem with HIT is that you cannot sustain exercise for more than a few minutes, or seconds, at a time - precisely how long you can tolerate it will depend upon just how intense the exercise is, how fit you are, and how long you are prepared to tolerate the discomfort. This is where the interval approach comes into its own - you exercise hard for a few minutes (or even seconds) and then rest or take an active recovery at a low intensity. Let’s find out a bit more about the benefits of HIT for fat loss.

Interval training…the new fat burning exercise

Interval training normally consists of a series of short duration high intensity bouts of work interspersed with short recovery periods (normally in the ratio 1:1). The recovery period can be either passive or active.

Let’s consider the theoretical energy requirements of a series of one minute intervals of high intensity exercise with one minute of rest between bouts. Each interval might have an energy expenditure of as much as 36 kcal. So our series of four one minute bouts of high intensity exercise could have an energy expenditure (4 x 36 kcal = 144 kcal) that is almost equivalent to 30 minutes of moderate intensity exercise as illustrated by the earlier calculation of expending 165 kcal after cycling for 30 minutes. In theory, you could expend an equivalent number of calories to 30 minutes low-to-moderate intensity cycling in less than 10 minutes of interval training. Or to look at it another way, you could expend three times the number of calories in the same 30 minute visit to the gym. This concept of high intensity exercise having low efficiency is supported by a study (5) that compared the total energy cost of exercise for two theoretically energy equivalent tasks, one of moderate intensity, the other maximal. The first task was a 3.5 minute treadmill walk, the second was a series of three 15 second sprints.  When the actual energy requirements were measured the walk required 39 kcal and the sprints required 65 kcal.

Weight training…the other new fat burning exercise

Weight training can also expend calories very effectively. For example, bench-pressing 30kg for 3 sets of 8 reps has a total gross energy expenditure equivalent to around 25 kcal (assuming gross efficiency of 10%).  Those 25 kcal would take you less than 5-minutes to expend (each set would take around 20-seconds to complete, with 2 periods of 2-minutes rest between each set). The added benefit of weight training is that it also increases muscle mass and raises the basal energy requirements, which means that you expend more calories each 24 hours, even if you’re doing nothing. There is also an acute “calorie burning hangover” from weight training, which has been confirmed empirically in a study showing that up to 16 hours post-training, metabolic rate was 4.2% higher than before the training session, and that there was also a higher level of fat oxidation (6). In other words, weight training elevates post-exercise metabolic rate.

Take care

Clearly, interval and heavy resistance training are not something that can be embarked upon immediately, or by everyone; you need a progressive approach to intensity, and you need to ensure that you are medically fit for HIT.

Furthermore, HIT should form part of a balanced programme of activities that includes some low intensity endurance training. The latter provides a greater stimulus to increasing muscle aerobic capacity, raising the intensity at which fat oxidation can predominate, as well as reducing cardiovascular risk factors.

Take home message

So to summarise, if we compare the energy expended during the active phases of the 3 workout examples given above (excluding time taken for rest periods between intervals or sets) we can easily see the advantages of anaerobic, high intensity exercise for fat loss:

Moderate constant intensity exercise      5.5 kcal per minute
Weight training                                             25 kcal per minute
High intensity intervals                               36 kcal per minute

So now you know the truth about fat burning - the next person who tells you that you need to exercise at low-to-moderate intensities to lose body fat simply doesn’t understand metabolism. You should view their advice with the same scepticism as this old cigarette advert, which claims that smoking is the route to weight loss and a healthy, active lifestyle –

 Thanks to for the image.


  1. Wolfe. Fat metabolism in exercise. Advances in Experimental Biology.  441: 147-56, 1998).
  2. Gladden & Welch. Efficiency of anaerobic work. Journal of Applied Physiology 44:564-70. 1978.
  3. Pahud et al. Energy expended during oxygen deficit period of submaximal exercise in man. Journal of applied Physiology 48:770-5, 1980).
  4. Hunter et al. A role for high intensity exercise in energy balance and weight control. Int. J. Relat. Metab. Disord. 22: 489-93, 1998).
  5. Scott. Interpreting energy expenditure for anaerobic exercise and recovery: an anaerobic hypothesis. Journal of Sports Medicine & Physical Fitness 37: 18-23, 1997.
  6. Osterberg and Melby, Effect of acute resistance exercise on postexercise oxygen consumption and resting metabolic rate in young women. Int J. Sport Nutr. Exerc. Metab. 10: 71-81, 2000.

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