pushupWhen we exercise, our bodies typically use a combination of carbohydrate and fat to fuel our performance. Protein (in the form of muscle) can also be used so understanding how the body draws on it’s energy sources is of great importance in achieving body composition goals. There is never a situation where you body is relying on only one nutrient as the sole energy source.

Typically, as we begin any form of exercise, carbohydrates are the first to get used up. In our last post on healthy fats, we discussed how the preferential fuel of the body is fat but it can’t use that when we have sugar (from the carbs we eat) in our system. Getting the sugar out of our bodies is always priority one for our systems.

How intensely we exercise is the primary determining factor of which energy source gets used. At lower intensity levels, such as with walking, fat is the main source of energy. At higher intensities, such as with sprinting, carbohydrates are the main source. If you’re depleted of carbs and try high-intensity exercise, the body turns to your precious muscle as a fuel source. Protein is converted to amino acids and released into the bloodstream. This is a metabolically expensive process though, and it takes up even more energy and that’s why athletes who are going at it hard “hit the wall” or “bonk” when readily available fuel is nowhere to be found.

Unfortunately, it’s not as easy as walking everywhere to become fat free. While a larger proportion of fat calories are used for the lower intensity activity, a higher overall amount of calories are used for the higher intensity activity. The following chart illustrates this point.
fat burning chart

If you walk for 60 minutes, you might burn 500 total calories, of which 250 calories of fat get used up. 250 calories is just under 28 grams of fat. (By way of comparison, a 10-piece chicken McNugget at McDonalds has 29 grams of fat – Now it’s time to work off those fries)

If you perform sprint intervals for 30 minutes, you’ll likely burn twice the amount of calories and though the percentage of those calories will be less than walking, the overall number of fat calories could easily be more.

The benefits of higher intensity training don’t stop there. By getting your heart rate up over 65% of it’s maximum, you’re also raising your overall metabolism. This means you’ll burn more calories for every activity you perform all day long.

So just how does one know the amount of calories burned when exercising? We’ve made it easy for you and created the below calculator to do the math for you.

In using a 2005 published study on fat oxidation, we can make a strong educated guess on the percentage of fats and carbs being used during exercise for a given age and specific gender. The calculator uses the numbers from the study.


Interesting to note is that the study found men to switch from fat fuel to carb fuel faster than women and men always showed lower rates of fat oxidation when they were burning fat.

Also keep in mind that the body has to get rid of excess carbs before burning fat so even a low-intensity activity will be a carbohydrate activity if you stuff your face with twizzlers and a 44oz Mountain Dew before you exercise.

Control the carbs, eat healthy fats and mix in different kinds of cardio for your best fat burning results.

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