Cardio for fat burning, like many things, seems to come in and out of fashion. Mainstream media tends to dictate this change in favour by either promoting, or condemning cardio, these days often comparing it to High Intensity Training (HIT). The fact that this pendulum effect continues rather than dying out indicates that it does indeed work, at least for some, and doesn’t for others. Part of the problem is the extremist approach we like to adopt, when perhaps a happy medium, or realisation that we are all individuals who respond differently, would be a much more robust stance to take. This article will present a ‘smart’ way of using cardio as a tool to target fat burning.
Note: A free excel document containing a calculator to work out your optimal fat burning intensity is available. Simply click ‘subscribe’ and in the comments section mention this article and you’ll receive a copy of the calculator.
Cardio is generally considered to be an exercise that keeps the heart elevated at a moderate intensity for an extended duration. Forms can be anything including, running, walking, swimming, cycling, skipping and even low intensity weight circuits. Cardio can be divided into two categories. The first is low-moderate intensity, also known as aerobic exercise. This means that throughout the duration of the exercise, your body is able to supply enough oxygen to meet your body’s demands. However not everyone’s ‘cardio’ is purely aerobic. The harder you work during this exercise, the more you begin to use your anaerobic energy system – moderate-high intensity. This means that your body requires more oxygen than you’re currently able to supply it. A by-product of this energy system is lactic acid, which tends to make your legs feel heavy and sore.
High Intensity Training often focuses on the anaerobic system and generally consists of short intense bursts of training with short breaks in between. While most proponents of HIT would never refer to their training style as ‘cardio’, it is ridiculous to claim that this style of training isn’t improving the cardiovascular system. The longer the short bursts continue the more aerobic the nature of the workout becomes anyway. HIT is a tremendous tool for body transformations but will not be discussed within detail in this article.
While strength training, such as weights training, could be considered a form of HIT, when mentioned in this article it will be assumed that a strength training program is already being performed. HIT would instead pertain to a variety of resistance (weights, bands, bodyweight, ect), or high speed exercises (sprints, jumps, bike hill climbs ect) performed in repeated bursts, resulting in short and sharp spikes in heart rate intended entirely to burn fat in addition to the strength-training program.
So to summarise; by altering the intensity of your workouts you also (kind of) change the energy systems being used which has a variety of repercussions.
‘Cardio’ vs ‘HIT’
There is a battle waging between the two conditioning forms and their ardent followers. The argument usually looks something like this.
Proponents of HIT claim that their intense workouts burn more calories compared to ‘classic cardio’.
Cardio lovers hit back saying that due to the lower intensity of their sessions, a greater percentage fat is burnt, as opposed to carbohydrates (which become the dominant energy source as intensity increases).
The HIT rebuttal claim that the workout induced elevated metabolism for the hours following results in a greater amount of fat burnt compared to cardio (Excess Post Oxygen Consumption, EPOC).
While HIT has a host of benefits, it does burn a lower percentage body fat during the workout itself (more glycogen or carbohydrate), and the elevated metabolism following has now been shown to burn far less fat than once assumed. There are other benefits, such as improved muscle glucose sensitivity (pushing carbs to muscle rather than fat) but this is also achieved by adding a strength training program to both a cardio and HIT style of training.
There are other benefits to lower intensity cardio, epecially if combined with a strength program. These include reduced fatigue build up – allowing other forms of training (including HIT or strength), reduced injury risk (asking someone who hasnt been exercising to go straight into sprints for example is a receipe for disaster) and personal preference/enjoyement of types of exercise.
The Ideal Fat Burning Intensity
So the question remains, if you burn the greatest percentage fat at a very low intensity, but a greater number of calories at a higher intensity, what then is the ideal intensity to burn the greatest total amount of body fat? Researchers have looked into this and it appears that the answer can be found by looking at our VO2 Max. VO2 Max refers to the maximum amount of oxygen someone can consume and use during each minute of exercise. This usually requires exercising in a laboratory to measure. Thankfully, there is a way to correlate VO2 max to heart rate, which is a lot easier to measure. A formula will be provided later in the article.
Thorough testing has shown that the ideal percentage VO2 Max is around 65%(+-5%). Training at this intensity burns almost twice as much fat when compared to low and high intensity training.
One Last Factor: Cortisol
Cortisol is a word that causes bodybuilders to shudder. Cortisol is a stress hormone released any time the body is under stress, in this case, physical stress from exercise. It creates a catabolic environment, which encourages the breakdown of muscle. Even if building muscle isn’t your intention, you don’t want to consistently elevate your cortisol levels for a multitude of reasons. Elevated cortisol will, as mentioned, increase muscle breakdown (slowing your metabolism), increase inflammation, decrease your immunity, impair cognitive performance, increase blood pressure and increase stored body fat.
You might be thinking, with all these side effects of intense exercise why would I ever work out?! However, as with most aspects of training, you have to weigh up risk with reward. Resistance training does strain the body, but if allowed a proper recovery you are rewarded with a stronger, healthier body as it adapts. In fact, the this teaches your body to handle and recover from stress outside of the gym as well. Stressful work/life situations can also cause cortisol levels to remain elevated too high for too long. Fitter individuals are found to be more resilient to stress and levels will drop earlier than untrained individuals who experience the same stress.
So how does exercise intensity affect cortisol levels? Again, this has been extensively tested and while cortisol secretion rises with exercise intensity, there appears to be a threshold at 60% VO2 Max before it begins to rise significantly. Any prolonged exercise for an hour or less below this level will not trigger any substantial release of negative stress hormones. I should mention that this is only the case if your blood sugar levels are stable. If you haven’t eaten for a long time or have been eating poor quality foods, your cortisol levels will skyrocket regardless. However if you are already training hard, there is a limit to how much more high intensity work you can handle and still recover from in time for your next workout. This is where moderate intensity cardio work can be useful.
Conveniently, cortisol rises at 60% VO2 max, which is very similar to the ideal fat burning range. This brings our ideal training intensity down from 65% to 60% VO2 Max.
How To Work Out Your Personal ‘Fat Burning Zone’
Research shows that your VO2 Max is closely related to your Heart Rate Reserve, which, thankfully, is much easier to calculate. Heart Rate Reserve is your maximum heart rate subtracted your resting heart rate. Use 220 – (your age) to find your maximum heart rate, and take your resting heart rate after you’ve been sitting for a few minutes and are completely relaxed. Simply find your pulse (a quick google search if you struggle to find it will help), sit still for a few minutes then record the beats in 30s, multiply by 2 for your beats per minute (bpm).
For example, a 25 year old with a resting heart rate of 65 beats per minute would use a formula like this
*I’ve put in bold above and below some of the values that are used in the next step to help show the flow of the formula.
220 – 25(age) = 195 (max heart rate)
195(max heart rate) – 65(resting heart rate) = 130 (heart rate reserve)
Say this individual wants to train at 60% of their VO2 Max. They simply multiply
130(heart rate reserve) x .60(Percentage VO2 Max desired) which = 78
To translate this to a normal heart rate, add the resting heart rate to the above calculation
78 + 65(resting heart rate) = 143 beats per minute (target heart rate for cardio)
How To Use It
Muscle Building Focus: 50-55% VO2 max for 30 minutes, 1-3 times per week to burn fats and improve the efficiency of aerobic energy system to subsequently increase percentage fats burnt at rest.
If you wish to benefit from the increased circulating fat in the blood stream by exercising first thing in the morning on an empty stomach, then reduce the intensity further. Less than 45% VO2 Max (brisk walk) on a fasted stomach to lose weight without fatiguing the body and impairing performance during competition or high intensity training.
Strength/Athletes: 60% VO2 max for 30 minutes 1-2 times per week to burn fats and improve the efficiency of the aerobic energy system to improve competitive performance and recovery. Should always be done after a meal (never on a completely empty stomach).
If you wish to benefit from the increased circulating fat in the blood stream by exercising first thing in the morning on an empty stomach, then reduce the intensity further. Less than 50% VO2 Max (brisk walk), on a fasted stomach first thing in the morning to lose weight without fatiguing the body and impairing performance in competition or high intensity training. This low intensity training can be performed as frequently as desired.
Strictly Fat Loss: 60% VO2 max for 30-60 minutes 3-5 times per week with 1-4 strength based workouts. Consider also performing some fasted cardio on an empty stomach keeping the intensity less than 50% VO2 Max.
Just in case you need any more motivation to include cardio into your week
Cardio softens arteries and prevents them becoming hard and brittle. This improves blood circulation and lowers blood pressure resulting in a host of health improvements.
Consistent cardio has been shown to improve cognitive functioning and prevent cognitive decline as you age. As you age, the part of your brain largely responsible for memory, the hippocampus, literally shrinks and becomes inadequate. Low to moderate intensity exercise causes the release of hormones which act on the hippocampus much the same way steroids act on skeletal muscle. They cause it to grow and develop in both size and function.
Cardio improves the strength of your heart and the effort required to supply blood to your body.
As your fitness levels increase during cardio, you will increase your bodies ability to use fats as a fuel. This means that during the other 23 hours of the day you’re not working out, you’ll burn a greater percentage fats.
By increasing the amounts of fats burnt as fuel,rather than carbohydrates, you save your muscle glycogen stores from depleting, meaning you have more instant energy available for your strength, sport specific workouts or competition.
By now you should see the benefits of cardio for improving general health, the efficiency of the aerobic energy system and to burn fat without hurting your performance in other areas, such as strength based training. Rather than swinging like the pendulum to extremist views (cardio vs HIT vs strength training), start looking at types of training as tools in your toolbox. They serve different functions, are more or less appropriate at different times and for different people. Hopefully you’ve taken away a practical way to increase fat burning during while minimising fatigue.
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