Is it true you don't start burning fat until 20 minutes into a workout?

So say someone runs for 25 minutes... do they only burn fat for 5 minutes?


Or is it you start burning calories (unused ones) after 20 minutes?Is it true you don't start burning fat until 20 minutes into a workout?
Yes, but the burning goes on for some time after the exercise is over. That is why they ask you to exercise for at least 30 minutesIs it true you don't start burning fat until 20 minutes into a workout?
Oh god, the bad information.. The short answer is 'no'.





The slightly longer answer is: When you exercise, you burn energy (calories are a measure of energy). What form that energy happens to be (fat, carb, protein..), is absolutely irrelevant. All that matters is you use up more energy each day than you take in (ie: eat). If you do that, you lose weight. There is no magical fat burning time frame.





The really long answer is: At rest you burn about 50% fat and 50% carbs (sugar), give or take. When you first start getting active, your body needs extra energy right away, so you switch to something like 70%carbs/30% fats (more like 90% carbs/10%fats during something like a sprint).





The longer you keep it up the more your body switches to burning fat for energy, because it's much more efficient. After about 20-30 minutes of sustained activity you're burning fat as your primary source of energy.





This is all totally irrelevant to weight loss! Why? Your body converts sugar to fat and back again as needed. You eat a bucket of candy (pure carbs), your body will store the excess energy as fat. You run sprints for 10 minutes, burning off your ready to use stores of carbs, your body will start breaking down your fat stores to replace them.
No, you start to burn fat when you reach a certain heart rate. If you're doing a lower intensity workout, you'll take longer to burn fat as compared to if you're doing a workout at a higher intensity, because your heart rate is higher in high intensity workouts. In fact, a 20-minute HIIT session (high-intensity interval training) can burn more fat than a 1-hr low intensity workout.





You can find your target heart rate online, I found this: http://exercise.about.com/cs/cardioworko鈥?/a>
you start burning calories right away, but if you want to lose weight you have to do excercise for more then 20 minutes, that's when your glycogen stores break down, even if you walk for 20 minutes and then run it works


Burning calories is different than burning fat.
yes it is true that is how long it takes to get past all the energy that you already have in your body - then once that is used up you start burning your reserves which is your fat
This is completely false; you start burning fat when your body has runout of carbs so the less carbs you eat the more fat you will burn.
You start burning fat once your heart rate goes up, and when you start sweating.