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Question
Posted by: flower | 2004/01/13

When does fat burn during cardio workout?

How long do you have to train (say cardio) to start burning fat? And how would you know if you are strarting to burn fat during a good workout. If you have low blood suger - do you burn fat then to keep you going ? -I mean lose weight?

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Our expert says:
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The two critical things that people always look at for fat burning are the intensity and the duration, and the two are obviously very closely linked – if you train very hard, then you have to train much shorter, whereas when you slow down and take it easy, it is possible to train for longer. The ideal intensity and duration lies somewhere in the middle of a continuum. With regards to heart rate, there is a lot of confusion and misconception. Ever since people noticed that heart rate goes up as intensity goes up, they have used it to gauge exercise intensity. Then, when it was noticed that the fuel that is used also depends on the exercise intensity, it was thought that heart rate monitoring was a good way to train at just the right zone to burn fat. In theory, this is true, but there is more to it than this. What happens is that at low exercise intensities (less than 60% of maximum), the main source of fuel used by the body is fat. As exercise intensity increases above 65 to 70%, progressively more and more of the energy that is produced comes from the carbohydrate stores, meaning that fat contributes a smaller overall percentage of the total energy use. However, and many people forget this, the overall energy use also increases, so that, even though fat might be contributing less as a percentage of the energy, it is still being used in larger quantities. That means that the total amount of fat being burned per minute might actually be higher at higher intensities, which is what you want. So, many people make the mistake of trying to go at a low intensity, to burn fat, which means that they are probably using more fat than carbohydrate, but the overall energy used is so low that the results are barely noticeable.

SO, basically, the body does not have certain ‘zones’ at which it uses just fat and then just carbohydrates – there is no on-off switch, but a gradual change from fat to carbohydrates, which means that you have to find the exact intensity to burn more fat in total, not more fat as a percentage, if you follow my logic. Also, at slightly higher exercise intensities, the total energy that is being used is greater, which means that the total amount of fat that is being used is also greater – so, to answer the question, I’m more in favour of higher intensity training to burn fat. However, a word of caution, this does not mean going out and training hard all the time. You have to find the right balance. Your goal should be to use the greatest total amounts of fat, and this means that the duration of the training must also be long enough to burn more fat. So, it’s not only the intensity, but also the duration that is vital. That’s why you can’t just go out and train at 90% of maximum – you would tire very quickly, meaning that your total fat and energy use would be relatively low.

Therefore, my advice would be to aim for an intensity between 70 and 80% of maximum. In your case, anything between 150 and 175 bpm would probably be about right – the main thing is to be able to finish a session of 45 minutes or so of training feeling like your breathing is elevated, that you’ve had a hard session, but that you are not completely exhausted. The other piece of advice is that you could also exercise at a low intensity (60 to 70%), with short periods of high intensity in between. So, say you are cycling for 45 minutes, every 9 or 10 minutes, you could pick up the intensity for one minute. This type of training is very good for fitness, performance and weight loss.

Now, the question of using fat when you run out of glucose is also fairly complex. The really simple answer is yes, you will use fat if your carb stores are depleted – people often referred to this as entering the fat burning phase. But, as I explained earlier, this is slightly misleading, because there is no switching over, but a gradual progression. Even more importantly, the risks of dropping blood sugar outweigh any benefits, and the very critical thing is that when you do run out of glucose or it gets really low, then you stop – your brain causes you stop long before you could cause harm, so all in all, you won’t really succeed by trying to deplete glucose and then exercise.
Hope this clears it up.

Good luck

The information provided does not constitute a diagnosis of your condition. You should consult a medical practitioner or other appropriate health care professional for a physical exmanication, diagnosis and formal advice. Health24 and the expert accept no responsibility or liability for any damage or personal harm you may suffer resulting from making use of this content.

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Our users say:
Posted by: Jason | 2004/01/14

Fat burns when exercise is being done that keeps the heart rate in the 50 - 60% of Maximum Heart Rate Zone. In order to work out your maximum heart rate, use the formula 220 - Age.

I am 25 therefore my MHR is approximately 195bps. In order to get an accurate MHR reading consult a cardiologist, who will run you on a treadmill till you reach that figure (MHR).

If my MHR is 195, then my fat burning zone would be 97.5 - 117 bps. IE. 50 - 60% of my MHR.

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