The idea of working out in the ‘Fat Burning Zone,’ sounds great in theory – if you want to lose body fat, you need to moderate your exercise intensity so as fat is the predominantly fuel ‘burnt’.
It sounds straight forward enough, however it’s not really the most efficient way to reduce your body fat.
The Fat Burning Zone
So the idea of the ‘Fat burning Zone’ isn’t where this theory is flawed. It is absolutely correct that at low/moderate intensity exercise, you will use more fat to fuel your exercise then if you were to workout at a higher intensity (where glucose/glycogen become the predominant substrate).
In fact, working out at around 50% of your VO2 max you will burn around 50 – 60% fat (and 40 – 50% glycogen/glucose; with a significant allowance for individual variances), compared with working out at around 75% of your VO2 max, which burns around 35% fat (and 65% glycogen/glucose). (1)
Better news still, at rest you burn around 66% fat – so you don’t need to exercise at all, you’ll burn fat in your sleep! Clearly this isn’t helping anyone lose weight.
(Note, this is a very simplified explanation of a complex physiological process)
Increase your intensity
Although while working out at around 50% of your VO2 max you burn a greater percentage of fat, your overall energy expenditure and thus overall fat use, will be far less than when working out at a higher intensity which achieves a greater overall energy expenditure in the same timeframe. (That is, 35% fat from a bigger energy expenditure is more than 50% fat from a smaller energy expenditure.)
It is possible to achieve the same energy expenditure from low/moderate intensity training as higher intensity training however you will need to work out for far longer and unfortunately you will still not reap all the same weight loss benefits of high intensity training.
To give you some figures to put this concept into perspective we can look at a study of the effects of a High Intensity Interval Training (HIIT, think hill sprints, squat jumps, burpees) protocol compared with a continuous aerobic exercise protocol. This study showed that after 15 weeks of HIIT training participants lost 13.9mm of fat (based on skin fold measurements), compared with only 4.5mm after 20 weeks of a continuous aerobic exercise protocol. (2) What is particularly interesting in this study is that participants in the HIIT group lost far more fat, despite expending significantly less energy during their training program (remembering that the HIIT group exercised for 5 weeks less). Once adjustments were made with the data from this study, the overall effect on skin folds (as a marker of body fat) from the HIIT group was 9 times that of the continuous exercise group, which is quite impressive.
So as I said, even with matched energy expenditure, or in the case of this study greater energy expenditure, the same weight loss benefit of high intensity training is not achieved – but why??
HIIT training is more effective for weight loss, even when matched for energy expenditure with low/moderate intensity training because of a little thing called EPOC (Excess Post-exercise Oxygen Consumption). EPOC basically means you have an oxygen debt to pay back to your body after you finish exercise which comes at an additional energy cost.
So although the energy expended during a HIIT session might be less than an aerobic workout in the ‘fat burning’ zone, thanks to EPOC as much as 95% of energy expended can occur after the workout while you repay that oxygen debt. This ‘after burn’ has been reported to last for as long as 48 hours!
In addition to EPOC the metabolic adaptations which occur with ongoing HIIT appears to have long term additional ‘fat burning’ (beta oxidation) benefit. That is, remember how I earlier said to allow for large individual variance in ‘fat burning,’ well this is why – with ongoing HIIT your body become better conditioned to using fat a fuel.
What’s not to love about HIIT!
Don’t get me wrong
This is not to say that low intensity exercise isn’t worthwhile, it’s simply saying that if weight loss is your goal than high intensity training is a better use of your time.
A further clarification still, if you are just starting out on your weight loss/ exercise journey a low intensity program can be very beneficial to ease your body back into exercise and reduce the risk of injury and/or the risk of you quitting the program before you’ve reaped the benefits because it is too hard and you struggle to move for days after.
So by all means, start off at a low intensity and as your fitness, confidence and motivation increases start to build your intensity to get the most out of your workout time (it’s always a good plan to get medical clearance before starting a new exercise regime).
A bit more bang for your buck
In addition to your HITT training (or lower intensity cardio equivalent), adding some strength exercise to your workouts will help give you more ‘fat burning’ bang for your buck.
But strength training, like lifting weights, will make you bulky, right? … Well that’s another myth for another day =)
The Monthly Myth Bust
Using research and evidence to bust and debunk all those Nutrition and Fitness myths you've been wondering about.