This article was first written by me like 16 years ago. I am republishing as the above-mentioned myth never ceases to end (often heard from people who have read somewhere or heard from some half-baked expert).
Here's the Myth:
Many exercises attempt to do at least 20 mins of cardio because they believe that the 'fat-burning' process only starts to kick in after that, when the glycogen (sugar) stores is depleting/depleted and is 'switching to fat for fuel'. They called this 'Fat-burning zone'. Now, this theory is a myth! It is no surprise here that so many people exercising in the 'fat-burning zone' are not getting the results they want.
Here is the fact:
Fat is used as fuel right from the start of the any exercise. In fact, the primary fuel (60-80%, depending on many factors) at rest and during low intensity activities is fat! Our body burns fat 24 hours a day, 7 days a week. To say that fat starts burning after the glycogen is used up after 20 mins is a total myth, because the latter gets depleted only after at least 50-120 mins of hard work
There is no 'switching system' in our body that could turn on fat or sugar at specified time or certain exercise. Our body burns both fat and sugar all the time! It is a matter of predominance of fuel used at different intensities.
What really happen is this:
At rest, we burn about 60-80% fat and 20-40% sugar
At low intensity exercise / easy activity: 50% fat, 50% sugar
At moderate intensity exercise: 40% fat, 60% sugar
At high intensity exercise: 20-30% fat, 70-80% sugar.
What? I am burning up to 80% fat at rest? That sounds great!
From the above, it seems logical to be a couch potatoes or engage in very low intensity exercise to enjoy the wonderful benefit of 'fat-burning'? Hang on there, and don't get all too excited yet, you need to know more.
The key to losing bodyfat is to maximise the calories you burned, rather than selecting fat as the main fuel choice. The more calories you burned the more fat you will burn. Let's do a simple math here, based on the same activity levels above.
Using 15 mins as standard duration, lets see how much fat is being burned at different levels of intensity from the amount of calories used (note: the actual amount of calories burned also depend on the exerciser weight and actual intensity of the exercise):
At rest, for example: using up 12kcal and of which is 70% fat: you burned about 12x 0.7 = a measly 8kcal of fat!
At low intensity exercise / easy activity, for example: using up 50kcal and of which is 50% fat: you burned about 50 x 0.5 = a fair 25kcal of fat.
At moderate intensity exercise, for example: using up 90kcal and of which is 40% fat: you burned about 90 x 0.4 = a good 36kcal of fat.
At high intensity exercise for example: using up 150kcal and of which is only 30% fat: you burned about 150 x 0.3 = a whopping 45kcal of fat! This is almost 6x the amount of fat burned compared to the resting state!
From the examples above, it is obvious that you don't burn a whole lot of fat doing low-intensity workouts. High-intensity exercise is the best way to go if you want to burn the most fat! That is not all! Once you finished training, you'll continue to burn more calories - and most of them all fat calories! This is called excess postexercise oxygen consumption (EPOC), this allows you to burn fat long after you've left the gym, allowing you to potentially burn up to hundreds of unwanted fat calories. Exercising at low-intensity doesn't give you the same advantage. With so many advantages, it is no surprise why HIIT is so popular nowadays
The point I'm putting across to you is this: you don't need to exercise AT LEAST 20mins to 'mobilize the fat' nor 'stay in the fat-burning zone'. You should strive to maximise the calorie expenditure in each workout through high intensity workouts, sometimes taking longer, sometimes shorter than 20 mins. I am also not saying that exercising at low/moderate intensity is bad for you. In fact, for certain group of people, exercising at such lower intensities is the only way to go. It all depends on your objectives, capability and preferences. That is why you should hire a good personal trainer Singapore to help you out on a customised fitness program.
P/s: high intensity may mean different things for different people. For some unfit people, it can be as simple as activities like brisk walking. For those who are superfit, it can be running at 14kph or more. Incorrect application of exercise intensity can be dangerous and could do more harm than good.