The body will always burn a mix of carbohydrates and fat when doing exercise but the ratio of these fuels will shift depending on what activity you are doing. For higher intensity exercise like fast pace running your body will rely on more on carbohydrates for energy than fat because the metabolic pathways available to break down carbohydrates for energy are far more efficient than the pathways available to break down fat. For longer, slower, exercise the body relies more on fat for energy. So, in order to burn fat far more efficiently, it’s just knowing the best times to exercise so that the body burns more fat calories than calories from carbohydrates. You need to know that for the first 20 minutes of any exercise, even though you burn a mix of carbohydrates and fat, your body will burn more carbohydrates for quick energy. After 20 minutes the ratio changes. The body doesn’t want to use all the carbohydrate stores so it turns to burning more fat than carbohydrates.
Another question I get asked is when the best time is to do cardio exercise in order to burn fat. Well, First thing in the morning is one of the best times as the blood sugar level is low at this point so the body will rely on fat stores to fuel cardio sessions. Another time would be after your weight training sessions. The reason for this is, again, blood sugar levels are low at this point so the body will rely on fat to fuel the cardio sessions. Remember when I said that for the first 20 minutes of any exercise you burn more carbohydrates to fat ratio then after 20 minutes the ratio changes? Well, if your weight training sessions last an hour the body has already made the switch over to burning more fat than carbs so when you step on that cross trainer for your cardio you will be burning more fat calories to fuel your cardio right from the start.
When you exercise regularly your body becomes more efficient at delivering and extracting oxygen which means this helps your body burn fat a lot more efficiently. It also allows for better circulation which means the fatty acids will move more efficiently through the blood and into muscle. That means fat is readily available for fuelling the body.