Myth: You have to do 40 minutes of cardio before you start burning fat
False: At rest you’re actually mainly burning fat. The problem is that you’re not actually using much energy so you’re not burning much fat. As you exercise energy demand increases and in order to deliver this fast
enough to muscles there is a shift towards carbohydrate, but fat is still being burned.
As exercise progresses and a balance in the metabolism is reached the percentage of energy coming from fat increases and, yes, it is often said that after around 40 minutes this percentage has increased significantly. So what does all this mean?
There are two ways of looking at the problem, both of which are correct in their own way. Firstly our bodyweight and body fat levels are not dictated by what energy source we use in exercise, they are dictated by energy balance across the day, i.e. how much we eat versus how much we burn. With this is mind, the key factor is not how long we exercise for but how many calories we burn whilst we exercise.
More intense exercise performed for shorter periods seems to have a bigger impact upon metabolism so we’re burning more calories when we’re just sitting watching TV in the evening.
On the other hand there is an argument, supported by some evidence, that longer exercise bouts train the fat burning metabolic systems and thus make us a more effective fat burner all the time. The logical conclusion is to do some shorter, high intensity sessions, perhaps in an interval training fashion, and some longer, slower sessions. - Phil Nourse
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