Have you heard the one about the 'Fat Burning' zone?

You may have been told about it by a friend or seen it on one of the cardiovascular machines at the gym you attend. For those of you who do not know about this zone it requires you to exercise at around 55-65% of your max heart rate (220-age x 0.55 or 0.65 = target heart rate) for 30-60 minutes. Working at this intensity is reported to burn a higher percentage of calories from fat. Ok, sounds great right? What is there not to like? 

Well before I go on, let’s take a step back. To supply energy (movement) the body requires three main fuel sources:

It has been reported that when exercising in the 'Fat Burning' zone (55-65% max heart rate) we are utilising our fat stores to supply energy allowing us to move. As the intensity increases we start to move into our carbohydrate stores. This can be reported as our respiratory exchange ratio, which is sometimes recorded when people perform VO2max tests. The lower our respiratory exchange ratio (RER) the greater the fat metabolism.

Let’s now go back and take a look at what happens when we exercise at a higher intensity. Say we exercise for 30 minutes at an intensity of 80-85% max heart rate (220-age x 0.8 or 0.85 = target heart rate). 

So looking at the table above exercising at a higher intensity is more beneficial. We burn 2 calories more per minute (60 calories more in total); 0.3 more fat calories per minute;  9 more fat calories; 10% less fat calories. The misleading part of the ‘Fat burning’ zone is expressing the energy derived from fat as a percentage of energy expenditure without considering the total energy expenditure.

You should also consider your energy cost post exercise. Following a high intensity bout of exercise the rate of metabolism is elevated for a slightly longer period of time, when compared to a lower intensity exercise bout and more energy is expanded as your metabolism returns to homeostasis. This is known as excess post exercise oxygen consumption (EPOC). To read more on EPOC look at the following link http://is.gd/4GrMcV

What if you could achieve weight loss exercising for less time but a higher intensity still?

You may have seen two programmes recently showing a new approach to weight loss and a competitor  to the ‘Fat Burning’ Zone. This new approach has been advocated on The One Show http://is.gd/xEGuft and Horizon http://is.gd/7GrX9F.

High Intensity Interval Training (HIIT) refers to short bursts of exercise, for example 30-240s at 90-95% maximum heart rate (220-age x 0.90 or 0.95 = target heart rate) before resting, then repeating the desired number of repetitions. The rest period and workout time will depend on your fitness level and ability to recover. High intensity intervals will have a greater effect on your EPOC compared to steady state training in the 'Fat Burning' zone. 

The potency of HIIT to elicit rapid changes in skeletal muscle oxidative capacity is no doubt related to its high level of muscle fibre recruitment and potential to stress type II muscle fibres in particular (http://is.gd/L7OuFH), but the underlying mechanisms are unclear. Stressing out type II muscle fibres and potentially increasing the size of them has the potential to increase our metabolism at rest and increase the calories we burn whilst exercising.

The best way to describe this is thinking of yourself as a car:

- Engine off = no fuel being used

- Engine on, not moving = fuel being burned (low intensity exercise, burning calories)

- Engine on, moving = more fuel being burned (higher intensity exercise, burning calories)

- Now imagine you are a supped up muscle car (having increased your muscle mass), what miles to the gallon would you get? This will be low meaning you are using more fuel to go the same distance. Your body is the same. 

Below is a table from a review paper from Boutcher (2011). Looking at the results we can see a variety of HIIT protocols have been used to deliver results. I have been unable to find the research papers reported on the Horizon or The One Show. I must also state that I am unaware who the participants were in the studies. Were they males, females, athletes, non-athletes, obese, overweight or diabetics? All of these groups are likely to see a difference in their gains from the same protocol. If you are looking to start performing HIIT protocols and have no medical reason for not exercising at a high intensity, I would recommed the following progressions. Spend 3 weeks starting with one protocol, have week 4 to recover then move onto the next phase in week 5.

Beginner: 4 sets of 4 minutes work with 3 minutes rest

Intermediate: 5 sets of 2 minutes work with 3 minutes rest

Advanced: 6 sets of 30s work with 30s recovery

Extreme: 60 sets of 8s work with 12s recovery (total 20 minutes work)

The 30s testing is reported to have used the Wingate Test http://is.gd/rtVfiO which unfortunately most gym bikes will not be able to replicate. The only way this could occur is if the resistance is set at the appropriate level to replicate this test. 

 The results show:

-      Decreases in body fat percentage

-      Decreases in body mass

-      Decreases in subcutaneous fat

-      Decreases in abdominal fat

-      Decreases in workout time

-      Increases in insulin sensitivity

-      Increase in VO2max

• During your time in the gym you will not miraculously lose weight on the scales. If you weigh yourself before and after, the weight lost will be fluid (sweat), this needs replacing during and after your session (dehydration reduces performance). For every 1 kg of weight lost you need to replace this with 1.5 litres of fluid.