TIME 4 FAT LOSS SERIES
This is the ninth in a series of original articles we will be publishing looking at the science behind fat loss. Join our mailing list to be informed when we have added the latest article.
What is metabolic training and is it an effective means for reducing body fat?
Although there doesn’t appear to be a universally acknowledged definition of the term ‘metabolic training’, it is generally used to describe forms of exercise which aim to stimulate the metabolism to maximise energy expenditure both during the activity and the recovery period through increased excess post exercise oxygen consumption (EPOC). Typically, metabolic training takes the form of either high intensity interval training (HIIT) or metabolic resistance training (MRT).
High intensity interval training
High intensity interval training (HIIT) involves repeated bouts (<45-240 seconds) of high to maximal intensity exercises followed by equal or longer bouts (60-360 seconds) of light to moderate intensity cardiovascular exercise (1), which may be performed using a variety of modes of training, such as running, cycling, swimming, skipping, etc.
Research has shown HIIT to be a time efficient method for gaining improvements in both aerobic and anaerobic fitness and it provides many other benefits including improvements in blood pressure, cardiovascular health, insulin sensitivity and cholesterol levels (2).
High intensity interval training for fat loss
There is also scientific evidence to support the use of HIIT as an efficient and effective means to reduce body fat, particularly abdominal fat, while preserving muscle mass (2).
The results of a study by Trapp et al., (3), which compared the effects of a 20 minute interval session three times per week, comprising an 8-second sprint followed by12s of low intensity cycling with 40minutes of steady state cycling at 60% VO2max on the body fat of women, showed that participants in the HIIT group lost significantly more fat (2.5kg) than those in the steady state aerobic exercise programme.
Tremblay et al., (4) compared the effects of HITT and endurance training (ET) on body fat and skeletal muscle metabolism in young adults. Subjects participated in a 20-week endurance-training (ET) program or a 15-week high-intensity interval training (HIIT) program.
The average total energy cost of the ET program was 48% greater than the energy expenditure for the HIIT program but the HIIT programme produced a greater reduction in fat compared with the ET program. When corrected for the energy cost of training, the decrease in fat induced by the HIIT program was nine-fold greater than that achieved with the ET program.
There is also evidence to suggest that HIIT is more effective for specifically reducing total abdominal fat than lower intensity exercise (5).
However, not all studies have shown this. Zhang et al. (6) compared the effect of prolonged moderate-intensity continuous training (MICT) on reducing abdominal visceral fat in obese young women with that of work-equivalent (300 kJ/training session) high-intensity interval training (HIIT).
Although the results showed that both methods achieved similar reductions in body fat, the authors concluded that HIIT may be preferable to MICT because it is more time efficient.
Which HIIT programme is the most effective?
As we have seen, various interval formats have been used in the studies, such as 30 seconds of all-out sprint cycling four to six times, separated by four minutes of recovery, which equates to approximately 3-4 minutes of exercise per session (7). Other protocols include an eight second cycle sprint followed by 12 seconds of low intensity cycling for a period of 20minutes (3); a two minute cycle at 95% of VO2max alternating with three minutes at 25% of VO2max (8); and a 15-second cycle sprint followed by 15 seconds of low intensity cycling for a period of 20 minutes (9). As similar results have been reported across the range of HIIT protocols used in the studies (1) it appears that as yet, the most effective has not been determined.
How effective is HIIT in the real world?
While the results of the various studies are compelling, it’s important to remember that they were achieved while the participants were closely supervised and were over a relatively short period, but how effective is HIIT over the long-term and when participants are unsupervised?
The aim of a study by Roy et al., (10) was to determine the effectiveness of an unsupervised HIIT program in overweight/obese adults over a period of 12 months.
Two hundred and fifty overweight/obese adults participated in either HIIT or 30 mins moderate-intensity exercise. At 12 months, there were no differences between exercise groups in weight, although HIIT participants reported greater enjoyment of the physical activity. Adherence to ≥2 sessions per week of unsupervised HIIT declined from 60.8% at baseline to 19.6% by 12 months.
The authors concluded that HIIT was well accepted by overweight adults, and opting for HIIT as an alternative to standard exercise recommendations led to no difference in health outcomes after 12 months. Although regular participation in unsupervised HIIT declined rapidly, those who adhered to regular HIIT demonstrated beneficial weight loss and visceral fat reduction.
Is HIIT safe?
Due to the high level of exertion involved, there is an increased risk associated with HIIT in comparison to less vigorous forms of exercise. Consequently, certain precautions need to be taken prior to participating in a HIIT programme, particularly if you have any health issues or are currently unfit (2)
Even if in the absence of underlying health issues, you should progress to HIIT by first developing a ‘base fitness level’ using the cardiovascular exercise guidelines discussed in part 7 of this series (2).
Note: It is important to remember that exhaustion is not the goal of HIIT; it should be challenging but should not cause post-exercise exhaustion or be a negative experience.
Metabolic resistance training
The term ‘metabolic resistance training’ typically refers to the use of high intensity resistance training, involving the performance of compound exercises with minimal rest between sets to maximise energy expenditure both during the activity and the recovery period, with the ultimate goal of reducing body fat levels.
Does it work?
A number of studies have demonstrated the effectiveness of this approach. A study by Kramer et al., (11) showed that participants who combined metabolic resistance training with a reduced energy intake burned up to 44% more fat than those who restricted energy intake alone.
Schuenke et al., (12), examined the effects of 31 minutes of resistance training, comprising 4 circuits of 10 repetitions of bench press, power cleans and squats performed to failure. Results showed that the participants’ resting metabolic rate was significantly elevated for 38 hours after the training session. In short, for 38 hours after the training session, the participants’ bodies were constantly working to reduce their fat stores while they were resting.
Metabolic resistance training methods
Although there various approaches to metabolic resistance training, they all tend to involve training the entire body using compound exercises, performed at a high intensity and for an extended repetitive effort. One such approach is known as ‘Big circuits’ (13)
Big circuits are sequences that consist of three, four or five compound exercises using relatively heavy loads. They alternate upper body exercises with those for the lower body to ensure each muscle group gets sufficient recovery. This allows participants to perform at a maximal intensity whilst still being able to maintain good form.
By the time a participant performs an exercise for a second time they will have had several minutes to recover. Exercises can be performed for a pre-determined number of repetitions (6-12) or a specified time (25-40 seconds). Minimal recovery should be taken between exercises. 3-5 rounds of each circuit should be performed with a recovery period between each (13).
Example of a big three circuit
- Upper body pulling exercise (e.g., bent over row)
- Lower body exercise (e.g., back squat)
- Upper body pushing exercise (e.g., bench press)
Example of big 5 circuit
- Upper body pulling exercise (e.g., lat pull-down)
- Lower body leg exercise (e.g., dumbbell walking lunges)
- Upper body pushing exercise (e.g., standing cable press)
- Lower body hip exercise (e.g., single leg Romanian dead-lift)
- Core exercise (e.g., stability ball roll- out) (13)
How can we progress big circuits’?
Beginners should start with big three circuits, progressing to big four then big five circuits as their fitness improves.
Progress can also be achieved by:
- Increasing the number of circuits from 3 to 4 up to 5
- Increasing the weight
- Increasing the number of repetitions or time at each station
- Decreasing the recovery period between circuits (13)
How does metabolic training work?
The short answer is that we don’t fully understand the precise mechanisms responsible for the enhanced fat burning capability of metabolic training. There is, of course, the mathematical logic which dictates that you expend more energy running at faster pace (e.g.,16kmph) than you do at a slower one (e.g., 8kmph), but it is more complex than just expending energy at a greater rate during the activity. We also have to consider the inefficiency of the human body. At best we humans have an efficiency of 26%. This means that 74% of the energy our body uses is essentially wasted, as it is given off as heat (14).
For example, cycling at 100 watts for 30 minutes is equivalent to 43 kcal of work. However, due to our efficiency of only 26% the total energy required to perform that work is 165 kcal (43/26 x 100 = 165).
High intensity activities such as sprinting and resistance training have an even lower efficiency of only approximately 10-15%. For example, 3 x15 second sprints have been shown to expend 65 kcal whereas a 3.5 minute walk expends only 39 kcal (14). This is in part, because they involve repeated, rapid contractions and relaxation of large muscle groups.
The influence of metabolic training on the endocrine system may be in part responsible for its fat burning ability, as it has been shown to stimulate the hormones that drive the release and breakdown of fat (7).
The decreased post-exercise appetite and increased EPOC associated with participation in metabolic training are also likely to contribute to the reduction in body fat achieved with metabolic training (7).
Metabolic training both in its form as HIIT and metabolic resistance training has compelling evidence to support its use as an efficient and effective method of reducing body fat. However, it can be extremely demanding and should only be attempted by those who have the requisite level of fitness, skill and good health to allow them to participate safely and who are sufficiently motivated to adhere to this form of training in the long-term