How can I burn energy from fat instead of carbohydrates when exercising?

When you exercise your body will draw from energy stores to supply the fuel needed to keep your muscles running properly. This energy will come from a variety of sources but primarily from a combination of fat and carbohydrates. Depending on what intensity you work out you will burn a higher percentage of fat vs. carbohydrates or vice versa but the body almost always is burning some combination of the two. Instead of focusing on where the calories are coming from during a workout focus more on burning lots of calories by working out hard. As your body tries to replenish the energy lost from an intense workout it will tap into your fat stores to replace some of that lost energy and over time the total amount of fat you store will change.
BINGO!!!  Great answer Dr. Oz and thank you.  I typically will have my clients do more high intensity cardio for performance reasons and typically will stick to longer durations with lower intensity when trying to burn more fat.  

We are talking about body composition and not performance in this answer.  When increase performance is desired then an increased intensity is important but when burning fat a slow and steady cardio rate is key.

You can also increase the intensity of weight training by circuit training, super sets and multi joint exercises and get a great high intensity cardio bout and strength training all in one.  Plyometric work is also great for improving muscular development and increase the overall cardiovascular conditioning.

But when we are talking about the reduction of body fat the very best thing is to manipulate your fat loss through your caloric intake, build muscle through strength training and do lighter intensity cardio sessions for longer durations.

The easiest way to get stuck at a plateau is to do too much high intensity training with too little calories.  Remember train smarter not harder! 
Dr. Mehmet Oz, MD
Cardiology (Cardiovascular Disease)

It turns out that when the body needs energy during intense exercise it goes for the more easily metabolized foodstuff first: carbohydrates. Carbs are quickly turned into energy and require less energy to metabolize. So to burn the fat, you need to perform exercise more often and less intensely. Sure, intense exercise can burn calories and build muscle mass that will burn energy more efficiently. But it is the steady fire that will help to improve metabolism. The best strategy is to alternate brief intense exercise with slow and steady moderate exercise.

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